Adding upstream version 5:7.2.4.upstream/5%7.2.4

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 4241 insertions, 0 deletions

diff --git a/src/replication.c b/src/replication.c
new file mode 100644
index 0000000..fbd9b0a
--- /dev/null
+++ b/src/replication.c
@@ -0,0 +1,4241 @@
+/* Asynchronous replication implementation.
+ *
+ * Copyright (c) 2009-2012, Salvatore Sanfilippo <antirez at gmail dot com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *   * Redistributions of source code must retain the above copyright notice,
+ *     this list of conditions and the following disclaimer.
+ *   * Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *   * Neither the name of Redis nor the names of its contributors may be used
+ *     to endorse or promote products derived from this software without
+ *     specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+
+#include "server.h"
+#include "cluster.h"
+#include "bio.h"
+#include "functions.h"
+#include "connection.h"
+
+#include <memory.h>
+#include <sys/time.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+
+void replicationDiscardCachedMaster(void);
+void replicationResurrectCachedMaster(connection *conn);
+void replicationSendAck(void);
+int replicaPutOnline(client *slave);
+void replicaStartCommandStream(client *slave);
+int cancelReplicationHandshake(int reconnect);
+
+/* We take a global flag to remember if this instance generated an RDB
+ * because of replication, so that we can remove the RDB file in case
+ * the instance is configured to have no persistence. */
+int RDBGeneratedByReplication = 0;
+
+/* --------------------------- Utility functions ---------------------------- */
+static ConnectionType *connTypeOfReplication(void) {
+    if (server.tls_replication) {
+        return connectionTypeTls();
+    }
+
+    return connectionTypeTcp();
+}
+
+/* Return the pointer to a string representing the slave ip:listening_port
+ * pair. Mostly useful for logging, since we want to log a slave using its
+ * IP address and its listening port which is more clear for the user, for
+ * example: "Closing connection with replica 10.1.2.3:6380". */
+char *replicationGetSlaveName(client *c) {
+    static char buf[NET_HOST_PORT_STR_LEN];
+    char ip[NET_IP_STR_LEN];
+
+    ip[0] = '\0';
+    buf[0] = '\0';
+    if (c->slave_addr ||
+        connAddrPeerName(c->conn,ip,sizeof(ip),NULL) != -1)
+    {
+        char *addr = c->slave_addr ? c->slave_addr : ip;
+        if (c->slave_listening_port)
+            formatAddr(buf,sizeof(buf),addr,c->slave_listening_port);
+        else
+            snprintf(buf,sizeof(buf),"%s:<unknown-replica-port>",addr);
+    } else {
+        snprintf(buf,sizeof(buf),"client id #%llu",
+            (unsigned long long) c->id);
+    }
+    return buf;
+}
+
+/* Plain unlink() can block for quite some time in order to actually apply
+ * the file deletion to the filesystem. This call removes the file in a
+ * background thread instead. We actually just do close() in the thread,
+ * by using the fact that if there is another instance of the same file open,
+ * the foreground unlink() will only remove the fs name, and deleting the
+ * file's storage space will only happen once the last reference is lost. */
+int bg_unlink(const char *filename) {
+    int fd = open(filename,O_RDONLY|O_NONBLOCK);
+    if (fd == -1) {
+        /* Can't open the file? Fall back to unlinking in the main thread. */
+        return unlink(filename);
+    } else {
+        /* The following unlink() removes the name but doesn't free the
+         * file contents because a process still has it open. */
+        int retval = unlink(filename);
+        if (retval == -1) {
+            /* If we got an unlink error, we just return it, closing the
+             * new reference we have to the file. */
+            int old_errno = errno;
+            close(fd);  /* This would overwrite our errno. So we saved it. */
+            errno = old_errno;
+            return -1;
+        }
+        bioCreateCloseJob(fd, 0, 0);
+        return 0; /* Success. */
+    }
+}
+
+/* ---------------------------------- MASTER -------------------------------- */
+
+void createReplicationBacklog(void) {
+    serverAssert(server.repl_backlog == NULL);
+    server.repl_backlog = zmalloc(sizeof(replBacklog));
+    server.repl_backlog->ref_repl_buf_node = NULL;
+    server.repl_backlog->unindexed_count = 0;
+    server.repl_backlog->blocks_index = raxNew();
+    server.repl_backlog->histlen = 0;
+    /* We don't have any data inside our buffer, but virtually the first
+     * byte we have is the next byte that will be generated for the
+     * replication stream. */
+    server.repl_backlog->offset = server.master_repl_offset+1;
+}
+
+/* This function is called when the user modifies the replication backlog
+ * size at runtime. It is up to the function to resize the buffer and setup it
+ * so that it contains the same data as the previous one (possibly less data,
+ * but the most recent bytes, or the same data and more free space in case the
+ * buffer is enlarged). */
+void resizeReplicationBacklog(void) {
+    if (server.repl_backlog_size < CONFIG_REPL_BACKLOG_MIN_SIZE)
+        server.repl_backlog_size = CONFIG_REPL_BACKLOG_MIN_SIZE;
+    if (server.repl_backlog)
+        incrementalTrimReplicationBacklog(REPL_BACKLOG_TRIM_BLOCKS_PER_CALL);
+}
+
+void freeReplicationBacklog(void) {
+    serverAssert(listLength(server.slaves) == 0);
+    if (server.repl_backlog == NULL) return;
+
+    /* Decrease the start buffer node reference count. */
+    if (server.repl_backlog->ref_repl_buf_node) {
+        replBufBlock *o = listNodeValue(
+            server.repl_backlog->ref_repl_buf_node);
+        serverAssert(o->refcount == 1); /* Last reference. */
+        o->refcount--;
+    }
+
+    /* Replication buffer blocks are completely released when we free the
+     * backlog, since the backlog is released only when there are no replicas
+     * and the backlog keeps the last reference of all blocks. */
+    freeReplicationBacklogRefMemAsync(server.repl_buffer_blocks,
+                            server.repl_backlog->blocks_index);
+    resetReplicationBuffer();
+    zfree(server.repl_backlog);
+    server.repl_backlog = NULL;
+}
+
+/* To make search offset from replication buffer blocks quickly
+ * when replicas ask partial resynchronization, we create one index
+ * block every REPL_BACKLOG_INDEX_PER_BLOCKS blocks. */
+void createReplicationBacklogIndex(listNode *ln) {
+    server.repl_backlog->unindexed_count++;
+    if (server.repl_backlog->unindexed_count >= REPL_BACKLOG_INDEX_PER_BLOCKS) {
+        replBufBlock *o = listNodeValue(ln);
+        uint64_t encoded_offset = htonu64(o->repl_offset);
+        raxInsert(server.repl_backlog->blocks_index,
+                  (unsigned char*)&encoded_offset, sizeof(uint64_t),
+                  ln, NULL);
+        server.repl_backlog->unindexed_count = 0;
+    }
+}
+
+/* Rebase replication buffer blocks' offset since the initial
+ * setting offset starts from 0 when master restart. */
+void rebaseReplicationBuffer(long long base_repl_offset) {
+    raxFree(server.repl_backlog->blocks_index);
+    server.repl_backlog->blocks_index = raxNew();
+    server.repl_backlog->unindexed_count = 0;
+
+    listIter li;
+    listNode *ln;
+    listRewind(server.repl_buffer_blocks, &li);
+    while ((ln = listNext(&li))) {
+        replBufBlock *o = listNodeValue(ln);
+        o->repl_offset += base_repl_offset;
+        createReplicationBacklogIndex(ln);
+    }
+}
+
+void resetReplicationBuffer(void) {
+    server.repl_buffer_mem = 0;
+    server.repl_buffer_blocks = listCreate();
+    listSetFreeMethod(server.repl_buffer_blocks, (void (*)(void*))zfree);
+}
+
+int canFeedReplicaReplBuffer(client *replica) {
+    /* Don't feed replicas that only want the RDB. */
+    if (replica->flags & CLIENT_REPL_RDBONLY) return 0;
+
+    /* Don't feed replicas that are still waiting for BGSAVE to start. */
+    if (replica->replstate == SLAVE_STATE_WAIT_BGSAVE_START) return 0;
+
+    return 1;
+}
+
+/* Similar with 'prepareClientToWrite', note that we must call this function
+ * before feeding replication stream into global replication buffer, since
+ * clientHasPendingReplies in prepareClientToWrite will access the global
+ * replication buffer to make judgements. */
+int prepareReplicasToWrite(void) {
+    listIter li;
+    listNode *ln;
+    int prepared = 0;
+
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        client *slave = ln->value;
+        if (!canFeedReplicaReplBuffer(slave)) continue;
+        if (prepareClientToWrite(slave) == C_ERR) continue;
+        prepared++;
+    }
+
+    return prepared;
+}
+
+/* Wrapper for feedReplicationBuffer() that takes Redis string objects
+ * as input. */
+void feedReplicationBufferWithObject(robj *o) {
+    char llstr[LONG_STR_SIZE];
+    void *p;
+    size_t len;
+
+    if (o->encoding == OBJ_ENCODING_INT) {
+        len = ll2string(llstr,sizeof(llstr),(long)o->ptr);
+        p = llstr;
+    } else {
+        len = sdslen(o->ptr);
+        p = o->ptr;
+    }
+    feedReplicationBuffer(p,len);
+}
+
+/* Generally, we only have one replication buffer block to trim when replication
+ * backlog size exceeds our setting and no replica reference it. But if replica
+ * clients disconnect, we need to free many replication buffer blocks that are
+ * referenced. It would cost much time if there are a lots blocks to free, that
+ * will freeze server, so we trim replication backlog incrementally. */
+void incrementalTrimReplicationBacklog(size_t max_blocks) {
+    serverAssert(server.repl_backlog != NULL);
+
+    size_t trimmed_blocks = 0;
+    while (server.repl_backlog->histlen > server.repl_backlog_size &&
+           trimmed_blocks < max_blocks)
+    {
+        /* We never trim backlog to less than one block. */
+        if (listLength(server.repl_buffer_blocks) <= 1) break;
+
+        /* Replicas increment the refcount of the first replication buffer block
+         * they refer to, in that case, we don't trim the backlog even if
+         * backlog_histlen exceeds backlog_size. This implicitly makes backlog
+         * bigger than our setting, but makes the master accept partial resync as
+         * much as possible. So that backlog must be the last reference of
+         * replication buffer blocks. */
+        listNode *first = listFirst(server.repl_buffer_blocks);
+        serverAssert(first == server.repl_backlog->ref_repl_buf_node);
+        replBufBlock *fo = listNodeValue(first);
+        if (fo->refcount != 1) break;
+
+        /* We don't try trim backlog if backlog valid size will be lessen than
+         * setting backlog size once we release the first repl buffer block. */
+        if (server.repl_backlog->histlen - (long long)fo->size <=
+            server.repl_backlog_size) break;
+
+        /* Decr refcount and release the first block later. */
+        fo->refcount--;
+        trimmed_blocks++;
+        server.repl_backlog->histlen -= fo->size;
+
+        /* Go to use next replication buffer block node. */
+        listNode *next = listNextNode(first);
+        server.repl_backlog->ref_repl_buf_node = next;
+        serverAssert(server.repl_backlog->ref_repl_buf_node != NULL);
+        /* Incr reference count to keep the new head node. */
+        ((replBufBlock *)listNodeValue(next))->refcount++;
+
+        /* Remove the node in recorded blocks. */
+        uint64_t encoded_offset = htonu64(fo->repl_offset);
+        raxRemove(server.repl_backlog->blocks_index,
+            (unsigned char*)&encoded_offset, sizeof(uint64_t), NULL);
+
+        /* Delete the first node from global replication buffer. */
+        serverAssert(fo->refcount == 0 && fo->used == fo->size);
+        server.repl_buffer_mem -= (fo->size +
+            sizeof(listNode) + sizeof(replBufBlock));
+        listDelNode(server.repl_buffer_blocks, first);
+    }
+
+    /* Set the offset of the first byte we have in the backlog. */
+    server.repl_backlog->offset = server.master_repl_offset -
+                              server.repl_backlog->histlen + 1;
+}
+
+/* Free replication buffer blocks that are referenced by this client. */
+void freeReplicaReferencedReplBuffer(client *replica) {
+    if (replica->ref_repl_buf_node != NULL) {
+        /* Decrease the start buffer node reference count. */
+        replBufBlock *o = listNodeValue(replica->ref_repl_buf_node);
+        serverAssert(o->refcount > 0);
+        o->refcount--;
+        incrementalTrimReplicationBacklog(REPL_BACKLOG_TRIM_BLOCKS_PER_CALL);
+    }
+    replica->ref_repl_buf_node = NULL;
+    replica->ref_block_pos = 0;
+}
+
+/* Append bytes into the global replication buffer list, replication backlog and
+ * all replica clients use replication buffers collectively, this function replace
+ * 'addReply*', 'feedReplicationBacklog' for replicas and replication backlog,
+ * First we add buffer into global replication buffer block list, and then
+ * update replica / replication-backlog referenced node and block position. */
+void feedReplicationBuffer(char *s, size_t len) {
+    static long long repl_block_id = 0;
+
+    if (server.repl_backlog == NULL) return;
+
+    while(len > 0) {
+        size_t start_pos = 0; /* The position of referenced block to start sending. */
+        listNode *start_node = NULL; /* Replica/backlog starts referenced node. */
+        int add_new_block = 0; /* Create new block if current block is total used. */
+        listNode *ln = listLast(server.repl_buffer_blocks);
+        replBufBlock *tail = ln ? listNodeValue(ln) : NULL;
+
+        /* Append to tail string when possible. */
+        if (tail && tail->size > tail->used) {
+            start_node = listLast(server.repl_buffer_blocks);
+            start_pos = tail->used;
+            /* Copy the part we can fit into the tail, and leave the rest for a
+             * new node */
+            size_t avail = tail->size - tail->used;
+            size_t copy = (avail >= len) ? len : avail;
+            memcpy(tail->buf + tail->used, s, copy);
+            tail->used += copy;
+            s += copy;
+            len -= copy;
+            server.master_repl_offset += copy;
+            server.repl_backlog->histlen += copy;
+        }
+        if (len) {
+            /* Create a new node, make sure it is allocated to at
+             * least PROTO_REPLY_CHUNK_BYTES */
+            size_t usable_size;
+            /* Avoid creating nodes smaller than PROTO_REPLY_CHUNK_BYTES, so that we can append more data into them,
+             * and also avoid creating nodes bigger than repl_backlog_size / 16, so that we won't have huge nodes that can't
+             * trim when we only still need to hold a small portion from them. */
+            size_t limit = max((size_t)server.repl_backlog_size / 16, (size_t)PROTO_REPLY_CHUNK_BYTES);
+            size_t size = min(max(len, (size_t)PROTO_REPLY_CHUNK_BYTES), limit);
+            tail = zmalloc_usable(size + sizeof(replBufBlock), &usable_size);
+            /* Take over the allocation's internal fragmentation */
+            tail->size = usable_size - sizeof(replBufBlock);
+            size_t copy = (tail->size >= len) ? len : tail->size;
+            tail->used = copy;
+            tail->refcount = 0;
+            tail->repl_offset = server.master_repl_offset + 1;
+            tail->id = repl_block_id++;
+            memcpy(tail->buf, s, copy);
+            listAddNodeTail(server.repl_buffer_blocks, tail);
+            /* We also count the list node memory into replication buffer memory. */
+            server.repl_buffer_mem += (usable_size + sizeof(listNode));
+            add_new_block = 1;
+            if (start_node == NULL) {
+                start_node = listLast(server.repl_buffer_blocks);
+                start_pos = 0;
+            }
+            s += copy;
+            len -= copy;
+            server.master_repl_offset += copy;
+            server.repl_backlog->histlen += copy;
+        }
+
+        /* For output buffer of replicas. */
+        listIter li;
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            client *slave = ln->value;
+            if (!canFeedReplicaReplBuffer(slave)) continue;
+
+            /* Update shared replication buffer start position. */
+            if (slave->ref_repl_buf_node == NULL) {
+                slave->ref_repl_buf_node = start_node;
+                slave->ref_block_pos = start_pos;
+                /* Only increase the start block reference count. */
+                ((replBufBlock *)listNodeValue(start_node))->refcount++;
+            }
+
+            /* Check output buffer limit only when add new block. */
+            if (add_new_block) closeClientOnOutputBufferLimitReached(slave, 1);
+        }
+
+        /* For replication backlog */
+        if (server.repl_backlog->ref_repl_buf_node == NULL) {
+            server.repl_backlog->ref_repl_buf_node = start_node;
+            /* Only increase the start block reference count. */
+            ((replBufBlock *)listNodeValue(start_node))->refcount++;
+
+            /* Replication buffer must be empty before adding replication stream
+             * into replication backlog. */
+            serverAssert(add_new_block == 1 && start_pos == 0);
+        }
+        if (add_new_block) {
+            createReplicationBacklogIndex(listLast(server.repl_buffer_blocks));
+
+            /* It is important to trim after adding replication data to keep the backlog size close to
+             * repl_backlog_size in the common case. We wait until we add a new block to avoid repeated
+             * unnecessary trimming attempts when small amounts of data are added. See comments in
+             * freeMemoryGetNotCountedMemory() for details on replication backlog memory tracking. */
+            incrementalTrimReplicationBacklog(REPL_BACKLOG_TRIM_BLOCKS_PER_CALL);
+        }
+    }
+}
+
+/* Propagate write commands to replication stream.
+ *
+ * This function is used if the instance is a master: we use the commands
+ * received by our clients in order to create the replication stream.
+ * Instead if the instance is a replica and has sub-replicas attached, we use
+ * replicationFeedStreamFromMasterStream() */
+void replicationFeedSlaves(list *slaves, int dictid, robj **argv, int argc) {
+    int j, len;
+    char llstr[LONG_STR_SIZE];
+
+    /* In case we propagate a command that doesn't touch keys (PING, REPLCONF) we
+     * pass dbid=-1 that indicate there is no need to replicate `select` command. */
+    serverAssert(dictid == -1 || (dictid >= 0 && dictid < server.dbnum));
+
+    /* If the instance is not a top level master, return ASAP: we'll just proxy
+     * the stream of data we receive from our master instead, in order to
+     * propagate *identical* replication stream. In this way this slave can
+     * advertise the same replication ID as the master (since it shares the
+     * master replication history and has the same backlog and offsets). */
+    if (server.masterhost != NULL) return;
+
+    /* If there aren't slaves, and there is no backlog buffer to populate,
+     * we can return ASAP. */
+    if (server.repl_backlog == NULL && listLength(slaves) == 0) {
+        /* We increment the repl_offset anyway, since we use that for tracking AOF fsyncs
+         * even when there's no replication active. This code will not be reached if AOF
+         * is also disabled. */
+        server.master_repl_offset += 1;
+        return;
+    }
+
+    /* We can't have slaves attached and no backlog. */
+    serverAssert(!(listLength(slaves) != 0 && server.repl_backlog == NULL));
+
+    /* Must install write handler for all replicas first before feeding
+     * replication stream. */
+    prepareReplicasToWrite();
+
+    /* Send SELECT command to every slave if needed. */
+    if (dictid != -1 && server.slaveseldb != dictid) {
+        robj *selectcmd;
+
+        /* For a few DBs we have pre-computed SELECT command. */
+        if (dictid >= 0 && dictid < PROTO_SHARED_SELECT_CMDS) {
+            selectcmd = shared.select[dictid];
+        } else {
+            int dictid_len;
+
+            dictid_len = ll2string(llstr,sizeof(llstr),dictid);
+            selectcmd = createObject(OBJ_STRING,
+                sdscatprintf(sdsempty(),
+                "*2\r\n$6\r\nSELECT\r\n$%d\r\n%s\r\n",
+                dictid_len, llstr));
+        }
+
+        feedReplicationBufferWithObject(selectcmd);
+
+        if (dictid < 0 || dictid >= PROTO_SHARED_SELECT_CMDS)
+            decrRefCount(selectcmd);
+
+        server.slaveseldb = dictid;
+    }
+
+    /* Write the command to the replication buffer if any. */
+    char aux[LONG_STR_SIZE+3];
+
+    /* Add the multi bulk reply length. */
+    aux[0] = '*';
+    len = ll2string(aux+1,sizeof(aux)-1,argc);
+    aux[len+1] = '\r';
+    aux[len+2] = '\n';
+    feedReplicationBuffer(aux,len+3);
+
+    for (j = 0; j < argc; j++) {
+        long objlen = stringObjectLen(argv[j]);
+
+        /* We need to feed the buffer with the object as a bulk reply
+         * not just as a plain string, so create the $..CRLF payload len
+         * and add the final CRLF */
+        aux[0] = '$';
+        len = ll2string(aux+1,sizeof(aux)-1,objlen);
+        aux[len+1] = '\r';
+        aux[len+2] = '\n';
+        feedReplicationBuffer(aux,len+3);
+        feedReplicationBufferWithObject(argv[j]);
+        feedReplicationBuffer(aux+len+1,2);
+    }
+}
+
+/* This is a debugging function that gets called when we detect something
+ * wrong with the replication protocol: the goal is to peek into the
+ * replication backlog and show a few final bytes to make simpler to
+ * guess what kind of bug it could be. */
+void showLatestBacklog(void) {
+    if (server.repl_backlog == NULL) return;
+    if (listLength(server.repl_buffer_blocks) == 0) return;
+
+    size_t dumplen = 256;
+    if (server.repl_backlog->histlen < (long long)dumplen)
+        dumplen = server.repl_backlog->histlen;
+
+    sds dump = sdsempty();
+    listNode *node = listLast(server.repl_buffer_blocks);
+    while(dumplen) {
+        if (node == NULL) break;
+        replBufBlock *o = listNodeValue(node);
+        size_t thislen = o->used >= dumplen ? dumplen : o->used;
+        sds head = sdscatrepr(sdsempty(), o->buf+o->used-thislen, thislen);
+        sds tmp = sdscatsds(head, dump);
+        sdsfree(dump);
+        dump = tmp;
+        dumplen -= thislen;
+        node = listPrevNode(node);
+    }
+
+    /* Finally log such bytes: this is vital debugging info to
+     * understand what happened. */
+    serverLog(LL_NOTICE,"Latest backlog is: '%s'", dump);
+    sdsfree(dump);
+}
+
+/* This function is used in order to proxy what we receive from our master
+ * to our sub-slaves. */
+#include <ctype.h>
+void replicationFeedStreamFromMasterStream(char *buf, size_t buflen) {
+    /* Debugging: this is handy to see the stream sent from master
+     * to slaves. Disabled with if(0). */
+    if (0) {
+        printf("%zu:",buflen);
+        for (size_t j = 0; j < buflen; j++) {
+            printf("%c", isprint(buf[j]) ? buf[j] : '.');
+        }
+        printf("\n");
+    }
+
+    /* There must be replication backlog if having attached slaves. */
+    if (listLength(server.slaves)) serverAssert(server.repl_backlog != NULL);
+    if (server.repl_backlog) {
+        /* Must install write handler for all replicas first before feeding
+         * replication stream. */
+        prepareReplicasToWrite();
+        feedReplicationBuffer(buf,buflen);
+    }
+}
+
+void replicationFeedMonitors(client *c, list *monitors, int dictid, robj **argv, int argc) {
+    /* Fast path to return if the monitors list is empty or the server is in loading. */
+    if (monitors == NULL || listLength(monitors) == 0 || server.loading) return;
+    listNode *ln;
+    listIter li;
+    int j;
+    sds cmdrepr = sdsnew("+");
+    robj *cmdobj;
+    struct timeval tv;
+
+    gettimeofday(&tv,NULL);
+    cmdrepr = sdscatprintf(cmdrepr,"%ld.%06ld ",(long)tv.tv_sec,(long)tv.tv_usec);
+    if (c->flags & CLIENT_SCRIPT) {
+        cmdrepr = sdscatprintf(cmdrepr,"[%d lua] ",dictid);
+    } else if (c->flags & CLIENT_UNIX_SOCKET) {
+        cmdrepr = sdscatprintf(cmdrepr,"[%d unix:%s] ",dictid,server.unixsocket);
+    } else {
+        cmdrepr = sdscatprintf(cmdrepr,"[%d %s] ",dictid,getClientPeerId(c));
+    }
+
+    for (j = 0; j < argc; j++) {
+        if (argv[j]->encoding == OBJ_ENCODING_INT) {
+            cmdrepr = sdscatprintf(cmdrepr, "\"%ld\"", (long)argv[j]->ptr);
+        } else {
+            cmdrepr = sdscatrepr(cmdrepr,(char*)argv[j]->ptr,
+                        sdslen(argv[j]->ptr));
+        }
+        if (j != argc-1)
+            cmdrepr = sdscatlen(cmdrepr," ",1);
+    }
+    cmdrepr = sdscatlen(cmdrepr,"\r\n",2);
+    cmdobj = createObject(OBJ_STRING,cmdrepr);
+
+    listRewind(monitors,&li);
+    while((ln = listNext(&li))) {
+        client *monitor = ln->value;
+        addReply(monitor,cmdobj);
+        updateClientMemUsageAndBucket(monitor);
+    }
+    decrRefCount(cmdobj);
+}
+
+/* Feed the slave 'c' with the replication backlog starting from the
+ * specified 'offset' up to the end of the backlog. */
+long long addReplyReplicationBacklog(client *c, long long offset) {
+    long long skip;
+
+    serverLog(LL_DEBUG, "[PSYNC] Replica request offset: %lld", offset);
+
+    if (server.repl_backlog->histlen == 0) {
+        serverLog(LL_DEBUG, "[PSYNC] Backlog history len is zero");
+        return 0;
+    }
+
+    serverLog(LL_DEBUG, "[PSYNC] Backlog size: %lld",
+             server.repl_backlog_size);
+    serverLog(LL_DEBUG, "[PSYNC] First byte: %lld",
+             server.repl_backlog->offset);
+    serverLog(LL_DEBUG, "[PSYNC] History len: %lld",
+             server.repl_backlog->histlen);
+
+    /* Compute the amount of bytes we need to discard. */
+    skip = offset - server.repl_backlog->offset;
+    serverLog(LL_DEBUG, "[PSYNC] Skipping: %lld", skip);
+
+    /* Iterate recorded blocks, quickly search the approximate node. */
+    listNode *node = NULL;
+    if (raxSize(server.repl_backlog->blocks_index) > 0) {
+        uint64_t encoded_offset = htonu64(offset);
+        raxIterator ri;
+        raxStart(&ri, server.repl_backlog->blocks_index);
+        raxSeek(&ri, ">", (unsigned char*)&encoded_offset, sizeof(uint64_t));
+        if (raxEOF(&ri)) {
+            /* No found, so search from the last recorded node. */
+            raxSeek(&ri, "$", NULL, 0);
+            raxPrev(&ri);
+            node = (listNode *)ri.data;
+        } else {
+            raxPrev(&ri); /* Skip the sought node. */
+            /* We should search from the prev node since the offset of current
+             * sought node exceeds searching offset. */
+            if (raxPrev(&ri))
+                node = (listNode *)ri.data;
+            else
+                node = server.repl_backlog->ref_repl_buf_node;
+        }
+        raxStop(&ri);
+    } else {
+        /* No recorded blocks, just from the start node to search. */
+        node = server.repl_backlog->ref_repl_buf_node;
+    }
+
+    /* Search the exact node. */
+    while (node != NULL) {
+        replBufBlock *o = listNodeValue(node);
+        if (o->repl_offset + (long long)o->used >= offset) break;
+        node = listNextNode(node);
+    }
+    serverAssert(node != NULL);
+
+    /* Install a writer handler first.*/
+    prepareClientToWrite(c);
+    /* Setting output buffer of the replica. */
+    replBufBlock *o = listNodeValue(node);
+    o->refcount++;
+    c->ref_repl_buf_node = node;
+    c->ref_block_pos = offset - o->repl_offset;
+
+    return server.repl_backlog->histlen - skip;
+}
+
+/* Return the offset to provide as reply to the PSYNC command received
+ * from the slave. The returned value is only valid immediately after
+ * the BGSAVE process started and before executing any other command
+ * from clients. */
+long long getPsyncInitialOffset(void) {
+    return server.master_repl_offset;
+}
+
+/* Send a FULLRESYNC reply in the specific case of a full resynchronization,
+ * as a side effect setup the slave for a full sync in different ways:
+ *
+ * 1) Remember, into the slave client structure, the replication offset
+ *    we sent here, so that if new slaves will later attach to the same
+ *    background RDB saving process (by duplicating this client output
+ *    buffer), we can get the right offset from this slave.
+ * 2) Set the replication state of the slave to WAIT_BGSAVE_END so that
+ *    we start accumulating differences from this point.
+ * 3) Force the replication stream to re-emit a SELECT statement so
+ *    the new slave incremental differences will start selecting the
+ *    right database number.
+ *
+ * Normally this function should be called immediately after a successful
+ * BGSAVE for replication was started, or when there is one already in
+ * progress that we attached our slave to. */
+int replicationSetupSlaveForFullResync(client *slave, long long offset) {
+    char buf[128];
+    int buflen;
+
+    slave->psync_initial_offset = offset;
+    slave->replstate = SLAVE_STATE_WAIT_BGSAVE_END;
+    /* We are going to accumulate the incremental changes for this
+     * slave as well. Set slaveseldb to -1 in order to force to re-emit
+     * a SELECT statement in the replication stream. */
+    server.slaveseldb = -1;
+
+    /* Don't send this reply to slaves that approached us with
+     * the old SYNC command. */
+    if (!(slave->flags & CLIENT_PRE_PSYNC)) {
+        buflen = snprintf(buf,sizeof(buf),"+FULLRESYNC %s %lld\r\n",
+                          server.replid,offset);
+        if (connWrite(slave->conn,buf,buflen) != buflen) {
+            freeClientAsync(slave);
+            return C_ERR;
+        }
+    }
+    return C_OK;
+}
+
+/* This function handles the PSYNC command from the point of view of a
+ * master receiving a request for partial resynchronization.
+ *
+ * On success return C_OK, otherwise C_ERR is returned and we proceed
+ * with the usual full resync. */
+int masterTryPartialResynchronization(client *c, long long psync_offset) {
+    long long psync_len;
+    char *master_replid = c->argv[1]->ptr;
+    char buf[128];
+    int buflen;
+
+    /* Is the replication ID of this master the same advertised by the wannabe
+     * slave via PSYNC? If the replication ID changed this master has a
+     * different replication history, and there is no way to continue.
+     *
+     * Note that there are two potentially valid replication IDs: the ID1
+     * and the ID2. The ID2 however is only valid up to a specific offset. */
+    if (strcasecmp(master_replid, server.replid) &&
+        (strcasecmp(master_replid, server.replid2) ||
+         psync_offset > server.second_replid_offset))
+    {
+        /* Replid "?" is used by slaves that want to force a full resync. */
+        if (master_replid[0] != '?') {
+            if (strcasecmp(master_replid, server.replid) &&
+                strcasecmp(master_replid, server.replid2))
+            {
+                serverLog(LL_NOTICE,"Partial resynchronization not accepted: "
+                    "Replication ID mismatch (Replica asked for '%s', my "
+                    "replication IDs are '%s' and '%s')",
+                    master_replid, server.replid, server.replid2);
+            } else {
+                serverLog(LL_NOTICE,"Partial resynchronization not accepted: "
+                    "Requested offset for second ID was %lld, but I can reply "
+                    "up to %lld", psync_offset, server.second_replid_offset);
+            }
+        } else {
+            serverLog(LL_NOTICE,"Full resync requested by replica %s",
+                replicationGetSlaveName(c));
+        }
+        goto need_full_resync;
+    }
+
+    /* We still have the data our slave is asking for? */
+    if (!server.repl_backlog ||
+        psync_offset < server.repl_backlog->offset ||
+        psync_offset > (server.repl_backlog->offset + server.repl_backlog->histlen))
+    {
+        serverLog(LL_NOTICE,
+            "Unable to partial resync with replica %s for lack of backlog (Replica request was: %lld).", replicationGetSlaveName(c), psync_offset);
+        if (psync_offset > server.master_repl_offset) {
+            serverLog(LL_WARNING,
+                "Warning: replica %s tried to PSYNC with an offset that is greater than the master replication offset.", replicationGetSlaveName(c));
+        }
+        goto need_full_resync;
+    }
+
+    /* If we reached this point, we are able to perform a partial resync:
+     * 1) Set client state to make it a slave.
+     * 2) Inform the client we can continue with +CONTINUE
+     * 3) Send the backlog data (from the offset to the end) to the slave. */
+    c->flags |= CLIENT_SLAVE;
+    c->replstate = SLAVE_STATE_ONLINE;
+    c->repl_ack_time = server.unixtime;
+    c->repl_start_cmd_stream_on_ack = 0;
+    listAddNodeTail(server.slaves,c);
+    /* We can't use the connection buffers since they are used to accumulate
+     * new commands at this stage. But we are sure the socket send buffer is
+     * empty so this write will never fail actually. */
+    if (c->slave_capa & SLAVE_CAPA_PSYNC2) {
+        buflen = snprintf(buf,sizeof(buf),"+CONTINUE %s\r\n", server.replid);
+    } else {
+        buflen = snprintf(buf,sizeof(buf),"+CONTINUE\r\n");
+    }
+    if (connWrite(c->conn,buf,buflen) != buflen) {
+        freeClientAsync(c);
+        return C_OK;
+    }
+    psync_len = addReplyReplicationBacklog(c,psync_offset);
+    serverLog(LL_NOTICE,
+        "Partial resynchronization request from %s accepted. Sending %lld bytes of backlog starting from offset %lld.",
+            replicationGetSlaveName(c),
+            psync_len, psync_offset);
+    /* Note that we don't need to set the selected DB at server.slaveseldb
+     * to -1 to force the master to emit SELECT, since the slave already
+     * has this state from the previous connection with the master. */
+
+    refreshGoodSlavesCount();
+
+    /* Fire the replica change modules event. */
+    moduleFireServerEvent(REDISMODULE_EVENT_REPLICA_CHANGE,
+                          REDISMODULE_SUBEVENT_REPLICA_CHANGE_ONLINE,
+                          NULL);
+
+    return C_OK; /* The caller can return, no full resync needed. */
+
+need_full_resync:
+    /* We need a full resync for some reason... Note that we can't
+     * reply to PSYNC right now if a full SYNC is needed. The reply
+     * must include the master offset at the time the RDB file we transfer
+     * is generated, so we need to delay the reply to that moment. */
+    return C_ERR;
+}
+
+/* Start a BGSAVE for replication goals, which is, selecting the disk or
+ * socket target depending on the configuration, and making sure that
+ * the script cache is flushed before to start.
+ *
+ * The mincapa argument is the bitwise AND among all the slaves capabilities
+ * of the slaves waiting for this BGSAVE, so represents the slave capabilities
+ * all the slaves support. Can be tested via SLAVE_CAPA_* macros.
+ *
+ * Side effects, other than starting a BGSAVE:
+ *
+ * 1) Handle the slaves in WAIT_START state, by preparing them for a full
+ *    sync if the BGSAVE was successfully started, or sending them an error
+ *    and dropping them from the list of slaves.
+ *
+ * 2) Flush the Lua scripting script cache if the BGSAVE was actually
+ *    started.
+ *
+ * Returns C_OK on success or C_ERR otherwise. */
+int startBgsaveForReplication(int mincapa, int req) {
+    int retval;
+    int socket_target = 0;
+    listIter li;
+    listNode *ln;
+
+    /* We use a socket target if slave can handle the EOF marker and we're configured to do diskless syncs.
+     * Note that in case we're creating a "filtered" RDB (functions-only, for example) we also force socket replication
+     * to avoid overwriting the snapshot RDB file with filtered data. */
+    socket_target = (server.repl_diskless_sync || req & SLAVE_REQ_RDB_MASK) && (mincapa & SLAVE_CAPA_EOF);
+    /* `SYNC` should have failed with error if we don't support socket and require a filter, assert this here */
+    serverAssert(socket_target || !(req & SLAVE_REQ_RDB_MASK));
+
+    serverLog(LL_NOTICE,"Starting BGSAVE for SYNC with target: %s",
+        socket_target ? "replicas sockets" : "disk");
+
+    rdbSaveInfo rsi, *rsiptr;
+    rsiptr = rdbPopulateSaveInfo(&rsi);
+    /* Only do rdbSave* when rsiptr is not NULL,
+     * otherwise slave will miss repl-stream-db. */
+    if (rsiptr) {
+        if (socket_target)
+            retval = rdbSaveToSlavesSockets(req,rsiptr);
+        else {
+            /* Keep the page cache since it'll get used soon */
+            retval = rdbSaveBackground(req,server.rdb_filename,rsiptr,RDBFLAGS_KEEP_CACHE);
+        }
+    } else {
+        serverLog(LL_WARNING,"BGSAVE for replication: replication information not available, can't generate the RDB file right now. Try later.");
+        retval = C_ERR;
+    }
+
+    /* If we succeeded to start a BGSAVE with disk target, let's remember
+     * this fact, so that we can later delete the file if needed. Note
+     * that we don't set the flag to 1 if the feature is disabled, otherwise
+     * it would never be cleared: the file is not deleted. This way if
+     * the user enables it later with CONFIG SET, we are fine. */
+    if (retval == C_OK && !socket_target && server.rdb_del_sync_files)
+        RDBGeneratedByReplication = 1;
+
+    /* If we failed to BGSAVE, remove the slaves waiting for a full
+     * resynchronization from the list of slaves, inform them with
+     * an error about what happened, close the connection ASAP. */
+    if (retval == C_ERR) {
+        serverLog(LL_WARNING,"BGSAVE for replication failed");
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            client *slave = ln->value;
+
+            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
+                slave->replstate = REPL_STATE_NONE;
+                slave->flags &= ~CLIENT_SLAVE;
+                listDelNode(server.slaves,ln);
+                addReplyError(slave,
+                    "BGSAVE failed, replication can't continue");
+                slave->flags |= CLIENT_CLOSE_AFTER_REPLY;
+            }
+        }
+        return retval;
+    }
+
+    /* If the target is socket, rdbSaveToSlavesSockets() already setup
+     * the slaves for a full resync. Otherwise for disk target do it now.*/
+    if (!socket_target) {
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            client *slave = ln->value;
+
+            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
+                /* Check slave has the exact requirements */
+                if (slave->slave_req != req)
+                    continue;
+                replicationSetupSlaveForFullResync(slave, getPsyncInitialOffset());
+            }
+        }
+    }
+
+    return retval;
+}
+
+/* SYNC and PSYNC command implementation. */
+void syncCommand(client *c) {
+    /* ignore SYNC if already slave or in monitor mode */
+    if (c->flags & CLIENT_SLAVE) return;
+
+    /* Check if this is a failover request to a replica with the same replid and
+     * become a master if so. */
+    if (c->argc > 3 && !strcasecmp(c->argv[0]->ptr,"psync") && 
+        !strcasecmp(c->argv[3]->ptr,"failover"))
+    {
+        serverLog(LL_NOTICE, "Failover request received for replid %s.",
+            (unsigned char *)c->argv[1]->ptr);
+        if (!server.masterhost) {
+            addReplyError(c, "PSYNC FAILOVER can't be sent to a master.");
+            return;
+        }
+
+        if (!strcasecmp(c->argv[1]->ptr,server.replid)) {
+            replicationUnsetMaster();
+            sds client = catClientInfoString(sdsempty(),c);
+            serverLog(LL_NOTICE,
+                "MASTER MODE enabled (failover request from '%s')",client);
+            sdsfree(client);
+        } else {
+            addReplyError(c, "PSYNC FAILOVER replid must match my replid.");
+            return;            
+        }
+    }
+
+    /* Don't let replicas sync with us while we're failing over */
+    if (server.failover_state != NO_FAILOVER) {
+        addReplyError(c,"-NOMASTERLINK Can't SYNC while failing over");
+        return;
+    }
+
+    /* Refuse SYNC requests if we are a slave but the link with our master
+     * is not ok... */
+    if (server.masterhost && server.repl_state != REPL_STATE_CONNECTED) {
+        addReplyError(c,"-NOMASTERLINK Can't SYNC while not connected with my master");
+        return;
+    }
+
+    /* SYNC can't be issued when the server has pending data to send to
+     * the client about already issued commands. We need a fresh reply
+     * buffer registering the differences between the BGSAVE and the current
+     * dataset, so that we can copy to other slaves if needed. */
+    if (clientHasPendingReplies(c)) {
+        addReplyError(c,"SYNC and PSYNC are invalid with pending output");
+        return;
+    }
+
+    /* Fail sync if slave doesn't support EOF capability but wants a filtered RDB. This is because we force filtered
+     * RDB's to be generated over a socket and not through a file to avoid conflicts with the snapshot files. Forcing
+     * use of a socket is handled, if needed, in `startBgsaveForReplication`. */
+    if (c->slave_req & SLAVE_REQ_RDB_MASK && !(c->slave_capa & SLAVE_CAPA_EOF)) {
+        addReplyError(c,"Filtered replica requires EOF capability");
+        return;
+    }
+
+    serverLog(LL_NOTICE,"Replica %s asks for synchronization",
+        replicationGetSlaveName(c));
+
+    /* Try a partial resynchronization if this is a PSYNC command.
+     * If it fails, we continue with usual full resynchronization, however
+     * when this happens replicationSetupSlaveForFullResync will replied
+     * with:
+     *
+     * +FULLRESYNC <replid> <offset>
+     *
+     * So the slave knows the new replid and offset to try a PSYNC later
+     * if the connection with the master is lost. */
+    if (!strcasecmp(c->argv[0]->ptr,"psync")) {
+        long long psync_offset;
+        if (getLongLongFromObjectOrReply(c, c->argv[2], &psync_offset, NULL) != C_OK) {
+            serverLog(LL_WARNING, "Replica %s asks for synchronization but with a wrong offset",
+                      replicationGetSlaveName(c));
+            return;
+        }
+
+        if (masterTryPartialResynchronization(c, psync_offset) == C_OK) {
+            server.stat_sync_partial_ok++;
+            return; /* No full resync needed, return. */
+        } else {
+            char *master_replid = c->argv[1]->ptr;
+
+            /* Increment stats for failed PSYNCs, but only if the
+             * replid is not "?", as this is used by slaves to force a full
+             * resync on purpose when they are not able to partially
+             * resync. */
+            if (master_replid[0] != '?') server.stat_sync_partial_err++;
+        }
+    } else {
+        /* If a slave uses SYNC, we are dealing with an old implementation
+         * of the replication protocol (like redis-cli --slave). Flag the client
+         * so that we don't expect to receive REPLCONF ACK feedbacks. */
+        c->flags |= CLIENT_PRE_PSYNC;
+    }
+
+    /* Full resynchronization. */
+    server.stat_sync_full++;
+
+    /* Setup the slave as one waiting for BGSAVE to start. The following code
+     * paths will change the state if we handle the slave differently. */
+    c->replstate = SLAVE_STATE_WAIT_BGSAVE_START;
+    if (server.repl_disable_tcp_nodelay)
+        connDisableTcpNoDelay(c->conn); /* Non critical if it fails. */
+    c->repldbfd = -1;
+    c->flags |= CLIENT_SLAVE;
+    listAddNodeTail(server.slaves,c);
+
+    /* Create the replication backlog if needed. */
+    if (listLength(server.slaves) == 1 && server.repl_backlog == NULL) {
+        /* When we create the backlog from scratch, we always use a new
+         * replication ID and clear the ID2, since there is no valid
+         * past history. */
+        changeReplicationId();
+        clearReplicationId2();
+        createReplicationBacklog();
+        serverLog(LL_NOTICE,"Replication backlog created, my new "
+                            "replication IDs are '%s' and '%s'",
+                            server.replid, server.replid2);
+    }
+
+    /* CASE 1: BGSAVE is in progress, with disk target. */
+    if (server.child_type == CHILD_TYPE_RDB &&
+        server.rdb_child_type == RDB_CHILD_TYPE_DISK)
+    {
+        /* Ok a background save is in progress. Let's check if it is a good
+         * one for replication, i.e. if there is another slave that is
+         * registering differences since the server forked to save. */
+        client *slave;
+        listNode *ln;
+        listIter li;
+
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            slave = ln->value;
+            /* If the client needs a buffer of commands, we can't use
+             * a replica without replication buffer. */
+            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END &&
+                (!(slave->flags & CLIENT_REPL_RDBONLY) ||
+                 (c->flags & CLIENT_REPL_RDBONLY)))
+                break;
+        }
+        /* To attach this slave, we check that it has at least all the
+         * capabilities of the slave that triggered the current BGSAVE
+         * and its exact requirements. */
+        if (ln && ((c->slave_capa & slave->slave_capa) == slave->slave_capa) &&
+            c->slave_req == slave->slave_req) {
+            /* Perfect, the server is already registering differences for
+             * another slave. Set the right state, and copy the buffer.
+             * We don't copy buffer if clients don't want. */
+            if (!(c->flags & CLIENT_REPL_RDBONLY))
+                copyReplicaOutputBuffer(c,slave);
+            replicationSetupSlaveForFullResync(c,slave->psync_initial_offset);
+            serverLog(LL_NOTICE,"Waiting for end of BGSAVE for SYNC");
+        } else {
+            /* No way, we need to wait for the next BGSAVE in order to
+             * register differences. */
+            serverLog(LL_NOTICE,"Can't attach the replica to the current BGSAVE. Waiting for next BGSAVE for SYNC");
+        }
+
+    /* CASE 2: BGSAVE is in progress, with socket target. */
+    } else if (server.child_type == CHILD_TYPE_RDB &&
+               server.rdb_child_type == RDB_CHILD_TYPE_SOCKET)
+    {
+        /* There is an RDB child process but it is writing directly to
+         * children sockets. We need to wait for the next BGSAVE
+         * in order to synchronize. */
+        serverLog(LL_NOTICE,"Current BGSAVE has socket target. Waiting for next BGSAVE for SYNC");
+
+    /* CASE 3: There is no BGSAVE is in progress. */
+    } else {
+        if (server.repl_diskless_sync && (c->slave_capa & SLAVE_CAPA_EOF) &&
+            server.repl_diskless_sync_delay)
+        {
+            /* Diskless replication RDB child is created inside
+             * replicationCron() since we want to delay its start a
+             * few seconds to wait for more slaves to arrive. */
+            serverLog(LL_NOTICE,"Delay next BGSAVE for diskless SYNC");
+        } else {
+            /* We don't have a BGSAVE in progress, let's start one. Diskless
+             * or disk-based mode is determined by replica's capacity. */
+            if (!hasActiveChildProcess()) {
+                startBgsaveForReplication(c->slave_capa, c->slave_req);
+            } else {
+                serverLog(LL_NOTICE,
+                    "No BGSAVE in progress, but another BG operation is active. "
+                    "BGSAVE for replication delayed");
+            }
+        }
+    }
+    return;
+}
+
+/* REPLCONF <option> <value> <option> <value> ...
+ * This command is used by a replica in order to configure the replication
+ * process before starting it with the SYNC command.
+ * This command is also used by a master in order to get the replication
+ * offset from a replica.
+ *
+ * Currently we support these options:
+ *
+ * - listening-port <port>
+ * - ip-address <ip>
+ * What is the listening ip and port of the Replica redis instance, so that
+ * the master can accurately lists replicas and their listening ports in the
+ * INFO output.
+ *
+ * - capa <eof|psync2>
+ * What is the capabilities of this instance.
+ * eof: supports EOF-style RDB transfer for diskless replication.
+ * psync2: supports PSYNC v2, so understands +CONTINUE <new repl ID>.
+ *
+ * - ack <offset> [fack <aofofs>]
+ * Replica informs the master the amount of replication stream that it
+ * processed so far, and optionally the replication offset fsynced to the AOF file.
+ * This special pattern doesn't reply to the caller.
+ *
+ * - getack <dummy>
+ * Unlike other subcommands, this is used by master to get the replication
+ * offset from a replica.
+ *
+ * - rdb-only <0|1>
+ * Only wants RDB snapshot without replication buffer.
+ *
+ * - rdb-filter-only <include-filters>
+ * Define "include" filters for the RDB snapshot. Currently we only support
+ * a single include filter: "functions". Passing an empty string "" will
+ * result in an empty RDB. */
+void replconfCommand(client *c) {
+    int j;
+
+    if ((c->argc % 2) == 0) {
+        /* Number of arguments must be odd to make sure that every
+         * option has a corresponding value. */
+        addReplyErrorObject(c,shared.syntaxerr);
+        return;
+    }
+
+    /* Process every option-value pair. */
+    for (j = 1; j < c->argc; j+=2) {
+        if (!strcasecmp(c->argv[j]->ptr,"listening-port")) {
+            long port;
+
+            if ((getLongFromObjectOrReply(c,c->argv[j+1],
+                    &port,NULL) != C_OK))
+                return;
+            c->slave_listening_port = port;
+        } else if (!strcasecmp(c->argv[j]->ptr,"ip-address")) {
+            sds addr = c->argv[j+1]->ptr;
+            if (sdslen(addr) < NET_HOST_STR_LEN) {
+                if (c->slave_addr) sdsfree(c->slave_addr);
+                c->slave_addr = sdsdup(addr);
+            } else {
+                addReplyErrorFormat(c,"REPLCONF ip-address provided by "
+                    "replica instance is too long: %zd bytes", sdslen(addr));
+                return;
+            }
+        } else if (!strcasecmp(c->argv[j]->ptr,"capa")) {
+            /* Ignore capabilities not understood by this master. */
+            if (!strcasecmp(c->argv[j+1]->ptr,"eof"))
+                c->slave_capa |= SLAVE_CAPA_EOF;
+            else if (!strcasecmp(c->argv[j+1]->ptr,"psync2"))
+                c->slave_capa |= SLAVE_CAPA_PSYNC2;
+        } else if (!strcasecmp(c->argv[j]->ptr,"ack")) {
+            /* REPLCONF ACK is used by slave to inform the master the amount
+             * of replication stream that it processed so far. It is an
+             * internal only command that normal clients should never use. */
+            long long offset;
+
+            if (!(c->flags & CLIENT_SLAVE)) return;
+            if ((getLongLongFromObject(c->argv[j+1], &offset) != C_OK))
+                return;
+            if (offset > c->repl_ack_off)
+                c->repl_ack_off = offset;
+            if (c->argc > j+3 && !strcasecmp(c->argv[j+2]->ptr,"fack")) {
+                if ((getLongLongFromObject(c->argv[j+3], &offset) != C_OK))
+                    return;
+                if (offset > c->repl_aof_off)
+                    c->repl_aof_off = offset;
+            }
+            c->repl_ack_time = server.unixtime;
+            /* If this was a diskless replication, we need to really put
+             * the slave online when the first ACK is received (which
+             * confirms slave is online and ready to get more data). This
+             * allows for simpler and less CPU intensive EOF detection
+             * when streaming RDB files.
+             * There's a chance the ACK got to us before we detected that the
+             * bgsave is done (since that depends on cron ticks), so run a
+             * quick check first (instead of waiting for the next ACK. */
+            if (server.child_type == CHILD_TYPE_RDB && c->replstate == SLAVE_STATE_WAIT_BGSAVE_END)
+                checkChildrenDone();
+            if (c->repl_start_cmd_stream_on_ack && c->replstate == SLAVE_STATE_ONLINE)
+                replicaStartCommandStream(c);
+            /* Note: this command does not reply anything! */
+            return;
+        } else if (!strcasecmp(c->argv[j]->ptr,"getack")) {
+            /* REPLCONF GETACK is used in order to request an ACK ASAP
+             * to the slave. */
+            if (server.masterhost && server.master) replicationSendAck();
+            return;
+        } else if (!strcasecmp(c->argv[j]->ptr,"rdb-only")) {
+           /* REPLCONF RDB-ONLY is used to identify the client only wants
+            * RDB snapshot without replication buffer. */
+            long rdb_only = 0;
+            if (getRangeLongFromObjectOrReply(c,c->argv[j+1],
+                    0,1,&rdb_only,NULL) != C_OK)
+                return;
+            if (rdb_only == 1) c->flags |= CLIENT_REPL_RDBONLY;
+            else c->flags &= ~CLIENT_REPL_RDBONLY;
+        } else if (!strcasecmp(c->argv[j]->ptr,"rdb-filter-only")) {
+            /* REPLCONFG RDB-FILTER-ONLY is used to define "include" filters
+             * for the RDB snapshot. Currently we only support a single
+             * include filter: "functions". In the future we may want to add
+             * other filters like key patterns, key types, non-volatile, module
+             * aux fields, ...
+             * We might want to add the complementing "RDB-FILTER-EXCLUDE" to
+             * filter out certain data. */
+            int filter_count, i;
+            sds *filters;
+            if (!(filters = sdssplitargs(c->argv[j+1]->ptr, &filter_count))) {
+                addReplyErrorFormat(c, "Missing rdb-filter-only values");
+                return;
+            }
+            /* By default filter out all parts of the rdb */
+            c->slave_req |= SLAVE_REQ_RDB_EXCLUDE_DATA;
+            c->slave_req |= SLAVE_REQ_RDB_EXCLUDE_FUNCTIONS;
+            for (i = 0; i < filter_count; i++) {
+                if (!strcasecmp(filters[i], "functions"))
+                    c->slave_req &= ~SLAVE_REQ_RDB_EXCLUDE_FUNCTIONS;
+                else {
+                    addReplyErrorFormat(c, "Unsupported rdb-filter-only option: %s", (char*)filters[i]);
+                    sdsfreesplitres(filters, filter_count);
+                    return;
+                }
+            }
+            sdsfreesplitres(filters, filter_count);
+        } else {
+            addReplyErrorFormat(c,"Unrecognized REPLCONF option: %s",
+                (char*)c->argv[j]->ptr);
+            return;
+        }
+    }
+    addReply(c,shared.ok);
+}
+
+/* This function puts a replica in the online state, and should be called just
+ * after a replica received the RDB file for the initial synchronization.
+ *
+ * It does a few things:
+ * 1) Put the slave in ONLINE state.
+ * 2) Update the count of "good replicas".
+ * 3) Trigger the module event.
+ *
+ * the return value indicates that the replica should be disconnected.
+ * */
+int replicaPutOnline(client *slave) {
+    if (slave->flags & CLIENT_REPL_RDBONLY) {
+        slave->replstate = SLAVE_STATE_RDB_TRANSMITTED;
+        /* The client asked for RDB only so we should close it ASAP */
+        serverLog(LL_NOTICE,
+                  "RDB transfer completed, rdb only replica (%s) should be disconnected asap",
+                  replicationGetSlaveName(slave));
+        return 0;
+    }
+    slave->replstate = SLAVE_STATE_ONLINE;
+    slave->repl_ack_time = server.unixtime; /* Prevent false timeout. */
+
+    refreshGoodSlavesCount();
+    /* Fire the replica change modules event. */
+    moduleFireServerEvent(REDISMODULE_EVENT_REPLICA_CHANGE,
+                          REDISMODULE_SUBEVENT_REPLICA_CHANGE_ONLINE,
+                          NULL);
+    serverLog(LL_NOTICE,"Synchronization with replica %s succeeded",
+        replicationGetSlaveName(slave));
+    return 1;
+}
+
+/* This function should be called just after a replica received the RDB file
+ * for the initial synchronization, and we are finally ready to send the
+ * incremental stream of commands.
+ *
+ * It does a few things:
+ * 1) Close the replica's connection async if it doesn't need replication
+ *    commands buffer stream, since it actually isn't a valid replica.
+ * 2) Make sure the writable event is re-installed, since when calling the SYNC
+ *    command we had no replies and it was disabled, and then we could
+ *    accumulate output buffer data without sending it to the replica so it
+ *    won't get mixed with the RDB stream. */
+void replicaStartCommandStream(client *slave) {
+    serverAssert(!(slave->flags & CLIENT_REPL_RDBONLY));
+    slave->repl_start_cmd_stream_on_ack = 0;
+
+    putClientInPendingWriteQueue(slave);
+}
+
+/* We call this function periodically to remove an RDB file that was
+ * generated because of replication, in an instance that is otherwise
+ * without any persistence. We don't want instances without persistence
+ * to take RDB files around, this violates certain policies in certain
+ * environments. */
+void removeRDBUsedToSyncReplicas(void) {
+    /* If the feature is disabled, return ASAP but also clear the
+     * RDBGeneratedByReplication flag in case it was set. Otherwise if the
+     * feature was enabled, but gets disabled later with CONFIG SET, the
+     * flag may remain set to one: then next time the feature is re-enabled
+     * via CONFIG SET we have it set even if no RDB was generated
+     * because of replication recently. */
+    if (!server.rdb_del_sync_files) {
+        RDBGeneratedByReplication = 0;
+        return;
+    }
+
+    if (allPersistenceDisabled() && RDBGeneratedByReplication) {
+        client *slave;
+        listNode *ln;
+        listIter li;
+
+        int delrdb = 1;
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            slave = ln->value;
+            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START ||
+                slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END ||
+                slave->replstate == SLAVE_STATE_SEND_BULK)
+            {
+                delrdb = 0;
+                break; /* No need to check the other replicas. */
+            }
+        }
+        if (delrdb) {
+            struct stat sb;
+            if (lstat(server.rdb_filename,&sb) != -1) {
+                RDBGeneratedByReplication = 0;
+                serverLog(LL_NOTICE,
+                    "Removing the RDB file used to feed replicas "
+                    "in a persistence-less instance");
+                bg_unlink(server.rdb_filename);
+            }
+        }
+    }
+}
+
+/* Close the repldbfd and reclaim the page cache if the client hold
+ * the last reference to replication DB */
+void closeRepldbfd(client *myself) {
+    listNode *ln;
+    listIter li;
+    int reclaim = 1;
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        client *slave = ln->value;
+        if (slave != myself && slave->replstate == SLAVE_STATE_SEND_BULK) {
+            reclaim = 0;
+            break;
+        }
+    }
+
+    if (reclaim) {
+        bioCreateCloseJob(myself->repldbfd, 0, 1);
+    } else {
+        close(myself->repldbfd);
+    }
+    myself->repldbfd = -1;
+}
+
+void sendBulkToSlave(connection *conn) {
+    client *slave = connGetPrivateData(conn);
+    char buf[PROTO_IOBUF_LEN];
+    ssize_t nwritten, buflen;
+
+    /* Before sending the RDB file, we send the preamble as configured by the
+     * replication process. Currently the preamble is just the bulk count of
+     * the file in the form "$<length>\r\n". */
+    if (slave->replpreamble) {
+        nwritten = connWrite(conn,slave->replpreamble,sdslen(slave->replpreamble));
+        if (nwritten == -1) {
+            serverLog(LL_WARNING,
+                "Write error sending RDB preamble to replica: %s",
+                connGetLastError(conn));
+            freeClient(slave);
+            return;
+        }
+        atomicIncr(server.stat_net_repl_output_bytes, nwritten);
+        sdsrange(slave->replpreamble,nwritten,-1);
+        if (sdslen(slave->replpreamble) == 0) {
+            sdsfree(slave->replpreamble);
+            slave->replpreamble = NULL;
+            /* fall through sending data. */
+        } else {
+            return;
+        }
+    }
+
+    /* If the preamble was already transferred, send the RDB bulk data. */
+    lseek(slave->repldbfd,slave->repldboff,SEEK_SET);
+    buflen = read(slave->repldbfd,buf,PROTO_IOBUF_LEN);
+    if (buflen <= 0) {
+        serverLog(LL_WARNING,"Read error sending DB to replica: %s",
+            (buflen == 0) ? "premature EOF" : strerror(errno));
+        freeClient(slave);
+        return;
+    }
+    if ((nwritten = connWrite(conn,buf,buflen)) == -1) {
+        if (connGetState(conn) != CONN_STATE_CONNECTED) {
+            serverLog(LL_WARNING,"Write error sending DB to replica: %s",
+                connGetLastError(conn));
+            freeClient(slave);
+        }
+        return;
+    }
+    slave->repldboff += nwritten;
+    atomicIncr(server.stat_net_repl_output_bytes, nwritten);
+    if (slave->repldboff == slave->repldbsize) {
+        closeRepldbfd(slave);
+        connSetWriteHandler(slave->conn,NULL);
+        if (!replicaPutOnline(slave)) {
+            freeClient(slave);
+            return;
+        }
+        replicaStartCommandStream(slave);
+    }
+}
+
+/* Remove one write handler from the list of connections waiting to be writable
+ * during rdb pipe transfer. */
+void rdbPipeWriteHandlerConnRemoved(struct connection *conn) {
+    if (!connHasWriteHandler(conn))
+        return;
+    connSetWriteHandler(conn, NULL);
+    client *slave = connGetPrivateData(conn);
+    slave->repl_last_partial_write = 0;
+    server.rdb_pipe_numconns_writing--;
+    /* if there are no more writes for now for this conn, or write error: */
+    if (server.rdb_pipe_numconns_writing == 0) {
+        if (aeCreateFileEvent(server.el, server.rdb_pipe_read, AE_READABLE, rdbPipeReadHandler,NULL) == AE_ERR) {
+            serverPanic("Unrecoverable error creating server.rdb_pipe_read file event.");
+        }
+    }
+}
+
+/* Called in diskless master during transfer of data from the rdb pipe, when
+ * the replica becomes writable again. */
+void rdbPipeWriteHandler(struct connection *conn) {
+    serverAssert(server.rdb_pipe_bufflen>0);
+    client *slave = connGetPrivateData(conn);
+    ssize_t nwritten;
+    if ((nwritten = connWrite(conn, server.rdb_pipe_buff + slave->repldboff,
+                              server.rdb_pipe_bufflen - slave->repldboff)) == -1)
+    {
+        if (connGetState(conn) == CONN_STATE_CONNECTED)
+            return; /* equivalent to EAGAIN */
+        serverLog(LL_WARNING,"Write error sending DB to replica: %s",
+            connGetLastError(conn));
+        freeClient(slave);
+        return;
+    } else {
+        slave->repldboff += nwritten;
+        atomicIncr(server.stat_net_repl_output_bytes, nwritten);
+        if (slave->repldboff < server.rdb_pipe_bufflen) {
+            slave->repl_last_partial_write = server.unixtime;
+            return; /* more data to write.. */
+        }
+    }
+    rdbPipeWriteHandlerConnRemoved(conn);
+}
+
+/* Called in diskless master, when there's data to read from the child's rdb pipe */
+void rdbPipeReadHandler(struct aeEventLoop *eventLoop, int fd, void *clientData, int mask) {
+    UNUSED(mask);
+    UNUSED(clientData);
+    UNUSED(eventLoop);
+    int i;
+    if (!server.rdb_pipe_buff)
+        server.rdb_pipe_buff = zmalloc(PROTO_IOBUF_LEN);
+    serverAssert(server.rdb_pipe_numconns_writing==0);
+
+    while (1) {
+        server.rdb_pipe_bufflen = read(fd, server.rdb_pipe_buff, PROTO_IOBUF_LEN);
+        if (server.rdb_pipe_bufflen < 0) {
+            if (errno == EAGAIN || errno == EWOULDBLOCK)
+                return;
+            serverLog(LL_WARNING,"Diskless rdb transfer, read error sending DB to replicas: %s", strerror(errno));
+            for (i=0; i < server.rdb_pipe_numconns; i++) {
+                connection *conn = server.rdb_pipe_conns[i];
+                if (!conn)
+                    continue;
+                client *slave = connGetPrivateData(conn);
+                freeClient(slave);
+                server.rdb_pipe_conns[i] = NULL;
+            }
+            killRDBChild();
+            return;
+        }
+
+        if (server.rdb_pipe_bufflen == 0) {
+            /* EOF - write end was closed. */
+            int stillUp = 0;
+            aeDeleteFileEvent(server.el, server.rdb_pipe_read, AE_READABLE);
+            for (i=0; i < server.rdb_pipe_numconns; i++)
+            {
+                connection *conn = server.rdb_pipe_conns[i];
+                if (!conn)
+                    continue;
+                stillUp++;
+            }
+            serverLog(LL_NOTICE,"Diskless rdb transfer, done reading from pipe, %d replicas still up.", stillUp);
+            /* Now that the replicas have finished reading, notify the child that it's safe to exit. 
+             * When the server detects the child has exited, it can mark the replica as online, and
+             * start streaming the replication buffers. */
+            close(server.rdb_child_exit_pipe);
+            server.rdb_child_exit_pipe = -1;
+            return;
+        }
+
+        int stillAlive = 0;
+        for (i=0; i < server.rdb_pipe_numconns; i++)
+        {
+            ssize_t nwritten;
+            connection *conn = server.rdb_pipe_conns[i];
+            if (!conn)
+                continue;
+
+            client *slave = connGetPrivateData(conn);
+            if ((nwritten = connWrite(conn, server.rdb_pipe_buff, server.rdb_pipe_bufflen)) == -1) {
+                if (connGetState(conn) != CONN_STATE_CONNECTED) {
+                    serverLog(LL_WARNING,"Diskless rdb transfer, write error sending DB to replica: %s",
+                        connGetLastError(conn));
+                    freeClient(slave);
+                    server.rdb_pipe_conns[i] = NULL;
+                    continue;
+                }
+                /* An error and still in connected state, is equivalent to EAGAIN */
+                slave->repldboff = 0;
+            } else {
+                /* Note: when use diskless replication, 'repldboff' is the offset
+                 * of 'rdb_pipe_buff' sent rather than the offset of entire RDB. */
+                slave->repldboff = nwritten;
+                atomicIncr(server.stat_net_repl_output_bytes, nwritten);
+            }
+            /* If we were unable to write all the data to one of the replicas,
+             * setup write handler (and disable pipe read handler, below) */
+            if (nwritten != server.rdb_pipe_bufflen) {
+                slave->repl_last_partial_write = server.unixtime;
+                server.rdb_pipe_numconns_writing++;
+                connSetWriteHandler(conn, rdbPipeWriteHandler);
+            }
+            stillAlive++;
+        }
+
+        if (stillAlive == 0) {
+            serverLog(LL_WARNING,"Diskless rdb transfer, last replica dropped, killing fork child.");
+            killRDBChild();
+        }
+        /*  Remove the pipe read handler if at least one write handler was set. */
+        if (server.rdb_pipe_numconns_writing || stillAlive == 0) {
+            aeDeleteFileEvent(server.el, server.rdb_pipe_read, AE_READABLE);
+            break;
+        }
+    }
+}
+
+/* This function is called at the end of every background saving.
+ *
+ * The argument bgsaveerr is C_OK if the background saving succeeded
+ * otherwise C_ERR is passed to the function.
+ * The 'type' argument is the type of the child that terminated
+ * (if it had a disk or socket target). */
+void updateSlavesWaitingBgsave(int bgsaveerr, int type) {
+    listNode *ln;
+    listIter li;
+
+    /* Note: there's a chance we got here from within the REPLCONF ACK command
+     * so we must avoid using freeClient, otherwise we'll crash on our way up. */
+
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        client *slave = ln->value;
+
+        if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END) {
+            struct redis_stat buf;
+
+            if (bgsaveerr != C_OK) {
+                freeClientAsync(slave);
+                serverLog(LL_WARNING,"SYNC failed. BGSAVE child returned an error");
+                continue;
+            }
+
+            /* If this was an RDB on disk save, we have to prepare to send
+             * the RDB from disk to the slave socket. Otherwise if this was
+             * already an RDB -> Slaves socket transfer, used in the case of
+             * diskless replication, our work is trivial, we can just put
+             * the slave online. */
+            if (type == RDB_CHILD_TYPE_SOCKET) {
+                serverLog(LL_NOTICE,
+                    "Streamed RDB transfer with replica %s succeeded (socket). Waiting for REPLCONF ACK from replica to enable streaming",
+                        replicationGetSlaveName(slave));
+                /* Note: we wait for a REPLCONF ACK message from the replica in
+                 * order to really put it online (install the write handler
+                 * so that the accumulated data can be transferred). However
+                 * we change the replication state ASAP, since our slave
+                 * is technically online now.
+                 *
+                 * So things work like that:
+                 *
+                 * 1. We end transferring the RDB file via socket.
+                 * 2. The replica is put ONLINE but the write handler
+                 *    is not installed.
+                 * 3. The replica however goes really online, and pings us
+                 *    back via REPLCONF ACK commands.
+                 * 4. Now we finally install the write handler, and send
+                 *    the buffers accumulated so far to the replica.
+                 *
+                 * But why we do that? Because the replica, when we stream
+                 * the RDB directly via the socket, must detect the RDB
+                 * EOF (end of file), that is a special random string at the
+                 * end of the RDB (for streamed RDBs we don't know the length
+                 * in advance). Detecting such final EOF string is much
+                 * simpler and less CPU intensive if no more data is sent
+                 * after such final EOF. So we don't want to glue the end of
+                 * the RDB transfer with the start of the other replication
+                 * data. */
+                if (!replicaPutOnline(slave)) {
+                    freeClientAsync(slave);
+                    continue;
+                }
+                slave->repl_start_cmd_stream_on_ack = 1;
+            } else {
+                if ((slave->repldbfd = open(server.rdb_filename,O_RDONLY)) == -1 ||
+                    redis_fstat(slave->repldbfd,&buf) == -1) {
+                    freeClientAsync(slave);
+                    serverLog(LL_WARNING,"SYNC failed. Can't open/stat DB after BGSAVE: %s", strerror(errno));
+                    continue;
+                }
+                slave->repldboff = 0;
+                slave->repldbsize = buf.st_size;
+                slave->replstate = SLAVE_STATE_SEND_BULK;
+                slave->replpreamble = sdscatprintf(sdsempty(),"$%lld\r\n",
+                    (unsigned long long) slave->repldbsize);
+
+                connSetWriteHandler(slave->conn,NULL);
+                if (connSetWriteHandler(slave->conn,sendBulkToSlave) == C_ERR) {
+                    freeClientAsync(slave);
+                    continue;
+                }
+            }
+        }
+    }
+}
+
+/* Change the current instance replication ID with a new, random one.
+ * This will prevent successful PSYNCs between this master and other
+ * slaves, so the command should be called when something happens that
+ * alters the current story of the dataset. */
+void changeReplicationId(void) {
+    getRandomHexChars(server.replid,CONFIG_RUN_ID_SIZE);
+    server.replid[CONFIG_RUN_ID_SIZE] = '\0';
+}
+
+/* Clear (invalidate) the secondary replication ID. This happens, for
+ * example, after a full resynchronization, when we start a new replication
+ * history. */
+void clearReplicationId2(void) {
+    memset(server.replid2,'0',sizeof(server.replid));
+    server.replid2[CONFIG_RUN_ID_SIZE] = '\0';
+    server.second_replid_offset = -1;
+}
+
+/* Use the current replication ID / offset as secondary replication
+ * ID, and change the current one in order to start a new history.
+ * This should be used when an instance is switched from slave to master
+ * so that it can serve PSYNC requests performed using the master
+ * replication ID. */
+void shiftReplicationId(void) {
+    memcpy(server.replid2,server.replid,sizeof(server.replid));
+    /* We set the second replid offset to the master offset + 1, since
+     * the slave will ask for the first byte it has not yet received, so
+     * we need to add one to the offset: for example if, as a slave, we are
+     * sure we have the same history as the master for 50 bytes, after we
+     * are turned into a master, we can accept a PSYNC request with offset
+     * 51, since the slave asking has the same history up to the 50th
+     * byte, and is asking for the new bytes starting at offset 51. */
+    server.second_replid_offset = server.master_repl_offset+1;
+    changeReplicationId();
+    serverLog(LL_NOTICE,"Setting secondary replication ID to %s, valid up to offset: %lld. New replication ID is %s", server.replid2, server.second_replid_offset, server.replid);
+}
+
+/* ----------------------------------- SLAVE -------------------------------- */
+
+/* Returns 1 if the given replication state is a handshake state,
+ * 0 otherwise. */
+int slaveIsInHandshakeState(void) {
+    return server.repl_state >= REPL_STATE_RECEIVE_PING_REPLY &&
+           server.repl_state <= REPL_STATE_RECEIVE_PSYNC_REPLY;
+}
+
+/* Avoid the master to detect the slave is timing out while loading the
+ * RDB file in initial synchronization. We send a single newline character
+ * that is valid protocol but is guaranteed to either be sent entirely or
+ * not, since the byte is indivisible.
+ *
+ * The function is called in two contexts: while we flush the current
+ * data with emptyDb(), and while we load the new data received as an
+ * RDB file from the master. */
+void replicationSendNewlineToMaster(void) {
+    static time_t newline_sent;
+    if (time(NULL) != newline_sent) {
+        newline_sent = time(NULL);
+        /* Pinging back in this stage is best-effort. */
+        if (server.repl_transfer_s) connWrite(server.repl_transfer_s, "\n", 1);
+    }
+}
+
+/* Callback used by emptyDb() while flushing away old data to load
+ * the new dataset received by the master and by discardTempDb()
+ * after loading succeeded or failed. */
+void replicationEmptyDbCallback(dict *d) {
+    UNUSED(d);
+    if (server.repl_state == REPL_STATE_TRANSFER)
+        replicationSendNewlineToMaster();
+}
+
+/* Once we have a link with the master and the synchronization was
+ * performed, this function materializes the master client we store
+ * at server.master, starting from the specified file descriptor. */
+void replicationCreateMasterClient(connection *conn, int dbid) {
+    server.master = createClient(conn);
+    if (conn)
+        connSetReadHandler(server.master->conn, readQueryFromClient);
+
+    /**
+     * Important note:
+     * The CLIENT_DENY_BLOCKING flag is not, and should not, be set here.
+     * For commands like BLPOP, it makes no sense to block the master
+     * connection, and such blocking attempt will probably cause deadlock and
+     * break the replication. We consider such a thing as a bug because
+     * commands as BLPOP should never be sent on the replication link.
+     * A possible use-case for blocking the replication link is if a module wants
+     * to pass the execution to a background thread and unblock after the
+     * execution is done. This is the reason why we allow blocking the replication
+     * connection. */
+    server.master->flags |= CLIENT_MASTER;
+
+    server.master->authenticated = 1;
+    server.master->reploff = server.master_initial_offset;
+    server.master->read_reploff = server.master->reploff;
+    server.master->user = NULL; /* This client can do everything. */
+    memcpy(server.master->replid, server.master_replid,
+        sizeof(server.master_replid));
+    /* If master offset is set to -1, this master is old and is not
+     * PSYNC capable, so we flag it accordingly. */
+    if (server.master->reploff == -1)
+        server.master->flags |= CLIENT_PRE_PSYNC;
+    if (dbid != -1) selectDb(server.master,dbid);
+}
+
+/* This function will try to re-enable the AOF file after the
+ * master-replica synchronization: if it fails after multiple attempts
+ * the replica cannot be considered reliable and exists with an
+ * error. */
+void restartAOFAfterSYNC(void) {
+    unsigned int tries, max_tries = 10;
+    for (tries = 0; tries < max_tries; ++tries) {
+        if (startAppendOnly() == C_OK) break;
+        serverLog(LL_WARNING,
+            "Failed enabling the AOF after successful master synchronization! "
+            "Trying it again in one second.");
+        sleep(1);
+    }
+    if (tries == max_tries) {
+        serverLog(LL_WARNING,
+            "FATAL: this replica instance finished the synchronization with "
+            "its master, but the AOF can't be turned on. Exiting now.");
+        exit(1);
+    }
+}
+
+static int useDisklessLoad(void) {
+    /* compute boolean decision to use diskless load */
+    int enabled = server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB ||
+           (server.repl_diskless_load == REPL_DISKLESS_LOAD_WHEN_DB_EMPTY && dbTotalServerKeyCount()==0);
+
+    if (enabled) {
+        /* Check all modules handle read errors, otherwise it's not safe to use diskless load. */
+        if (!moduleAllDatatypesHandleErrors()) {
+            serverLog(LL_NOTICE,
+                "Skipping diskless-load because there are modules that don't handle read errors.");
+            enabled = 0;
+        }
+        /* Check all modules handle async replication, otherwise it's not safe to use diskless load. */
+        else if (server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB && !moduleAllModulesHandleReplAsyncLoad()) {
+            serverLog(LL_NOTICE,
+                "Skipping diskless-load because there are modules that are not aware of async replication.");
+            enabled = 0;
+        }
+    }
+    return enabled;
+}
+
+/* Helper function for readSyncBulkPayload() to initialize tempDb
+ * before socket-loading the new db from master. The tempDb may be populated
+ * by swapMainDbWithTempDb or freed by disklessLoadDiscardTempDb later. */
+redisDb *disklessLoadInitTempDb(void) {
+    return initTempDb();
+}
+
+/* Helper function for readSyncBulkPayload() to discard our tempDb
+ * when the loading succeeded or failed. */
+void disklessLoadDiscardTempDb(redisDb *tempDb) {
+    discardTempDb(tempDb, replicationEmptyDbCallback);
+}
+
+/* If we know we got an entirely different data set from our master
+ * we have no way to incrementally feed our replicas after that.
+ * We want our replicas to resync with us as well, if we have any sub-replicas.
+ * This is useful on readSyncBulkPayload in places where we just finished transferring db. */
+void replicationAttachToNewMaster(void) { 
+    /* Replica starts to apply data from new master, we must discard the cached
+     * master structure. */
+    serverAssert(server.master == NULL);
+    replicationDiscardCachedMaster();
+
+    disconnectSlaves(); /* Force our replicas to resync with us as well. */
+    freeReplicationBacklog(); /* Don't allow our chained replicas to PSYNC. */
+}
+
+/* Asynchronously read the SYNC payload we receive from a master */
+#define REPL_MAX_WRITTEN_BEFORE_FSYNC (1024*1024*8) /* 8 MB */
+void readSyncBulkPayload(connection *conn) {
+    char buf[PROTO_IOBUF_LEN];
+    ssize_t nread, readlen, nwritten;
+    int use_diskless_load = useDisklessLoad();
+    redisDb *diskless_load_tempDb = NULL;
+    functionsLibCtx* temp_functions_lib_ctx = NULL;
+    int empty_db_flags = server.repl_slave_lazy_flush ? EMPTYDB_ASYNC :
+                                                        EMPTYDB_NO_FLAGS;
+    off_t left;
+
+    /* Static vars used to hold the EOF mark, and the last bytes received
+     * from the server: when they match, we reached the end of the transfer. */
+    static char eofmark[CONFIG_RUN_ID_SIZE];
+    static char lastbytes[CONFIG_RUN_ID_SIZE];
+    static int usemark = 0;
+
+    /* If repl_transfer_size == -1 we still have to read the bulk length
+     * from the master reply. */
+    if (server.repl_transfer_size == -1) {
+        nread = connSyncReadLine(conn,buf,1024,server.repl_syncio_timeout*1000);
+        if (nread == -1) {
+            serverLog(LL_WARNING,
+                "I/O error reading bulk count from MASTER: %s",
+                connGetLastError(conn));
+            goto error;
+        } else {
+            /* nread here is returned by connSyncReadLine(), which calls syncReadLine() and
+             * convert "\r\n" to '\0' so 1 byte is lost. */
+            atomicIncr(server.stat_net_repl_input_bytes, nread+1);
+        }
+
+        if (buf[0] == '-') {
+            serverLog(LL_WARNING,
+                "MASTER aborted replication with an error: %s",
+                buf+1);
+            goto error;
+        } else if (buf[0] == '\0') {
+            /* At this stage just a newline works as a PING in order to take
+             * the connection live. So we refresh our last interaction
+             * timestamp. */
+            server.repl_transfer_lastio = server.unixtime;
+            return;
+        } else if (buf[0] != '$') {
+            serverLog(LL_WARNING,"Bad protocol from MASTER, the first byte is not '$' (we received '%s'), are you sure the host and port are right?", buf);
+            goto error;
+        }
+
+        /* There are two possible forms for the bulk payload. One is the
+         * usual $<count> bulk format. The other is used for diskless transfers
+         * when the master does not know beforehand the size of the file to
+         * transfer. In the latter case, the following format is used:
+         *
+         * $EOF:<40 bytes delimiter>
+         *
+         * At the end of the file the announced delimiter is transmitted. The
+         * delimiter is long and random enough that the probability of a
+         * collision with the actual file content can be ignored. */
+        if (strncmp(buf+1,"EOF:",4) == 0 && strlen(buf+5) >= CONFIG_RUN_ID_SIZE) {
+            usemark = 1;
+            memcpy(eofmark,buf+5,CONFIG_RUN_ID_SIZE);
+            memset(lastbytes,0,CONFIG_RUN_ID_SIZE);
+            /* Set any repl_transfer_size to avoid entering this code path
+             * at the next call. */
+            server.repl_transfer_size = 0;
+            serverLog(LL_NOTICE,
+                "MASTER <-> REPLICA sync: receiving streamed RDB from master with EOF %s",
+                use_diskless_load? "to parser":"to disk");
+        } else {
+            usemark = 0;
+            server.repl_transfer_size = strtol(buf+1,NULL,10);
+            serverLog(LL_NOTICE,
+                "MASTER <-> REPLICA sync: receiving %lld bytes from master %s",
+                (long long) server.repl_transfer_size,
+                use_diskless_load? "to parser":"to disk");
+        }
+        return;
+    }
+
+    if (!use_diskless_load) {
+        /* Read the data from the socket, store it to a file and search
+         * for the EOF. */
+        if (usemark) {
+            readlen = sizeof(buf);
+        } else {
+            left = server.repl_transfer_size - server.repl_transfer_read;
+            readlen = (left < (signed)sizeof(buf)) ? left : (signed)sizeof(buf);
+        }
+
+        nread = connRead(conn,buf,readlen);
+        if (nread <= 0) {
+            if (connGetState(conn) == CONN_STATE_CONNECTED) {
+                /* equivalent to EAGAIN */
+                return;
+            }
+            serverLog(LL_WARNING,"I/O error trying to sync with MASTER: %s",
+                (nread == -1) ? connGetLastError(conn) : "connection lost");
+            cancelReplicationHandshake(1);
+            return;
+        }
+        atomicIncr(server.stat_net_repl_input_bytes, nread);
+
+        /* When a mark is used, we want to detect EOF asap in order to avoid
+         * writing the EOF mark into the file... */
+        int eof_reached = 0;
+
+        if (usemark) {
+            /* Update the last bytes array, and check if it matches our
+             * delimiter. */
+            if (nread >= CONFIG_RUN_ID_SIZE) {
+                memcpy(lastbytes,buf+nread-CONFIG_RUN_ID_SIZE,
+                       CONFIG_RUN_ID_SIZE);
+            } else {
+                int rem = CONFIG_RUN_ID_SIZE-nread;
+                memmove(lastbytes,lastbytes+nread,rem);
+                memcpy(lastbytes+rem,buf,nread);
+            }
+            if (memcmp(lastbytes,eofmark,CONFIG_RUN_ID_SIZE) == 0)
+                eof_reached = 1;
+        }
+
+        /* Update the last I/O time for the replication transfer (used in
+         * order to detect timeouts during replication), and write what we
+         * got from the socket to the dump file on disk. */
+        server.repl_transfer_lastio = server.unixtime;
+        if ((nwritten = write(server.repl_transfer_fd,buf,nread)) != nread) {
+            serverLog(LL_WARNING,
+                "Write error or short write writing to the DB dump file "
+                "needed for MASTER <-> REPLICA synchronization: %s",
+                (nwritten == -1) ? strerror(errno) : "short write");
+            goto error;
+        }
+        server.repl_transfer_read += nread;
+
+        /* Delete the last 40 bytes from the file if we reached EOF. */
+        if (usemark && eof_reached) {
+            if (ftruncate(server.repl_transfer_fd,
+                server.repl_transfer_read - CONFIG_RUN_ID_SIZE) == -1)
+            {
+                serverLog(LL_WARNING,
+                    "Error truncating the RDB file received from the master "
+                    "for SYNC: %s", strerror(errno));
+                goto error;
+            }
+        }
+
+        /* Sync data on disk from time to time, otherwise at the end of the
+         * transfer we may suffer a big delay as the memory buffers are copied
+         * into the actual disk. */
+        if (server.repl_transfer_read >=
+            server.repl_transfer_last_fsync_off + REPL_MAX_WRITTEN_BEFORE_FSYNC)
+        {
+            off_t sync_size = server.repl_transfer_read -
+                              server.repl_transfer_last_fsync_off;
+            rdb_fsync_range(server.repl_transfer_fd,
+                server.repl_transfer_last_fsync_off, sync_size);
+            server.repl_transfer_last_fsync_off += sync_size;
+        }
+
+        /* Check if the transfer is now complete */
+        if (!usemark) {
+            if (server.repl_transfer_read == server.repl_transfer_size)
+                eof_reached = 1;
+        }
+
+        /* If the transfer is yet not complete, we need to read more, so
+         * return ASAP and wait for the handler to be called again. */
+        if (!eof_reached) return;
+    }
+
+    /* We reach this point in one of the following cases:
+     *
+     * 1. The replica is using diskless replication, that is, it reads data
+     *    directly from the socket to the Redis memory, without using
+     *    a temporary RDB file on disk. In that case we just block and
+     *    read everything from the socket.
+     *
+     * 2. Or when we are done reading from the socket to the RDB file, in
+     *    such case we want just to read the RDB file in memory. */
+
+    /* We need to stop any AOF rewriting child before flushing and parsing
+     * the RDB, otherwise we'll create a copy-on-write disaster. */
+    if (server.aof_state != AOF_OFF) stopAppendOnly();
+    /* Also try to stop save RDB child before flushing and parsing the RDB:
+     * 1. Ensure background save doesn't overwrite synced data after being loaded.
+     * 2. Avoid copy-on-write disaster. */
+    if (server.child_type == CHILD_TYPE_RDB) {
+        if (!use_diskless_load) {
+            serverLog(LL_NOTICE,
+                "Replica is about to load the RDB file received from the "
+                "master, but there is a pending RDB child running. "
+                "Killing process %ld and removing its temp file to avoid "
+                "any race",
+                (long) server.child_pid);
+        }
+        killRDBChild();
+    }
+
+    if (use_diskless_load && server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB) {
+        /* Initialize empty tempDb dictionaries. */
+        diskless_load_tempDb = disklessLoadInitTempDb();
+        temp_functions_lib_ctx = functionsLibCtxCreate();
+
+        moduleFireServerEvent(REDISMODULE_EVENT_REPL_ASYNC_LOAD,
+                              REDISMODULE_SUBEVENT_REPL_ASYNC_LOAD_STARTED,
+                              NULL);
+    } else {
+        replicationAttachToNewMaster();
+
+        serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Flushing old data");
+        emptyData(-1,empty_db_flags,replicationEmptyDbCallback);
+    }
+
+    /* Before loading the DB into memory we need to delete the readable
+     * handler, otherwise it will get called recursively since
+     * rdbLoad() will call the event loop to process events from time to
+     * time for non blocking loading. */
+    connSetReadHandler(conn, NULL);
+    
+    serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Loading DB in memory");
+    rdbSaveInfo rsi = RDB_SAVE_INFO_INIT;
+    if (use_diskless_load) {
+        rio rdb;
+        redisDb *dbarray;
+        functionsLibCtx* functions_lib_ctx;
+        int asyncLoading = 0;
+
+        if (server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB) {
+            /* Async loading means we continue serving read commands during full resync, and
+             * "swap" the new db with the old db only when loading is done.
+             * It is enabled only on SWAPDB diskless replication when master replication ID hasn't changed,
+             * because in that state the old content of the db represents a different point in time of the same
+             * data set we're currently receiving from the master. */
+            if (memcmp(server.replid, server.master_replid, CONFIG_RUN_ID_SIZE) == 0) {
+                asyncLoading = 1;
+            }
+            dbarray = diskless_load_tempDb;
+            functions_lib_ctx = temp_functions_lib_ctx;
+        } else {
+            dbarray = server.db;
+            functions_lib_ctx = functionsLibCtxGetCurrent();
+            functionsLibCtxClear(functions_lib_ctx);
+        }
+
+        rioInitWithConn(&rdb,conn,server.repl_transfer_size);
+
+        /* Put the socket in blocking mode to simplify RDB transfer.
+         * We'll restore it when the RDB is received. */
+        connBlock(conn);
+        connRecvTimeout(conn, server.repl_timeout*1000);
+        startLoading(server.repl_transfer_size, RDBFLAGS_REPLICATION, asyncLoading);
+
+        int loadingFailed = 0;
+        rdbLoadingCtx loadingCtx = { .dbarray = dbarray, .functions_lib_ctx = functions_lib_ctx };
+        if (rdbLoadRioWithLoadingCtx(&rdb,RDBFLAGS_REPLICATION,&rsi,&loadingCtx) != C_OK) {
+            /* RDB loading failed. */
+            serverLog(LL_WARNING,
+                      "Failed trying to load the MASTER synchronization DB "
+                      "from socket, check server logs.");
+            loadingFailed = 1;
+        } else if (usemark) {
+            /* Verify the end mark is correct. */
+            if (!rioRead(&rdb, buf, CONFIG_RUN_ID_SIZE) ||
+                memcmp(buf, eofmark, CONFIG_RUN_ID_SIZE) != 0)
+            {
+                serverLog(LL_WARNING, "Replication stream EOF marker is broken");
+                loadingFailed = 1;
+            }
+        }
+
+        if (loadingFailed) {
+            stopLoading(0);
+            cancelReplicationHandshake(1);
+            rioFreeConn(&rdb, NULL);
+
+            if (server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB) {
+                /* Discard potentially partially loaded tempDb. */
+                moduleFireServerEvent(REDISMODULE_EVENT_REPL_ASYNC_LOAD,
+                                      REDISMODULE_SUBEVENT_REPL_ASYNC_LOAD_ABORTED,
+                                      NULL);
+
+                disklessLoadDiscardTempDb(diskless_load_tempDb);
+                functionsLibCtxFree(temp_functions_lib_ctx);
+                serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Discarding temporary DB in background");
+            } else {
+                /* Remove the half-loaded data in case we started with an empty replica. */
+                emptyData(-1,empty_db_flags,replicationEmptyDbCallback);
+            }
+
+            /* Note that there's no point in restarting the AOF on SYNC
+             * failure, it'll be restarted when sync succeeds or the replica
+             * gets promoted. */
+            return;
+        }
+
+        /* RDB loading succeeded if we reach this point. */
+        if (server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB) {
+            /* We will soon swap main db with tempDb and replicas will start
+             * to apply data from new master, we must discard the cached
+             * master structure and force resync of sub-replicas. */
+            replicationAttachToNewMaster();
+
+            serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Swapping active DB with loaded DB");
+            swapMainDbWithTempDb(diskless_load_tempDb);
+
+            /* swap existing functions ctx with the temporary one */
+            functionsLibCtxSwapWithCurrent(temp_functions_lib_ctx);
+
+            moduleFireServerEvent(REDISMODULE_EVENT_REPL_ASYNC_LOAD,
+                        REDISMODULE_SUBEVENT_REPL_ASYNC_LOAD_COMPLETED,
+                        NULL);
+
+            /* Delete the old db as it's useless now. */
+            disklessLoadDiscardTempDb(diskless_load_tempDb);
+            serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Discarding old DB in background");
+        }
+
+        /* Inform about db change, as replication was diskless and didn't cause a save. */
+        server.dirty++;
+
+        stopLoading(1);
+
+        /* Cleanup and restore the socket to the original state to continue
+         * with the normal replication. */
+        rioFreeConn(&rdb, NULL);
+        connNonBlock(conn);
+        connRecvTimeout(conn,0);
+    } else {
+
+        /* Make sure the new file (also used for persistence) is fully synced
+         * (not covered by earlier calls to rdb_fsync_range). */
+        if (fsync(server.repl_transfer_fd) == -1) {
+            serverLog(LL_WARNING,
+                "Failed trying to sync the temp DB to disk in "
+                "MASTER <-> REPLICA synchronization: %s",
+                strerror(errno));
+            cancelReplicationHandshake(1);
+            return;
+        }
+
+        /* Rename rdb like renaming rewrite aof asynchronously. */
+        int old_rdb_fd = open(server.rdb_filename,O_RDONLY|O_NONBLOCK);
+        if (rename(server.repl_transfer_tmpfile,server.rdb_filename) == -1) {
+            serverLog(LL_WARNING,
+                "Failed trying to rename the temp DB into %s in "
+                "MASTER <-> REPLICA synchronization: %s",
+                server.rdb_filename, strerror(errno));
+            cancelReplicationHandshake(1);
+            if (old_rdb_fd != -1) close(old_rdb_fd);
+            return;
+        }
+        /* Close old rdb asynchronously. */
+        if (old_rdb_fd != -1) bioCreateCloseJob(old_rdb_fd, 0, 0);
+
+        /* Sync the directory to ensure rename is persisted */
+        if (fsyncFileDir(server.rdb_filename) == -1) {
+            serverLog(LL_WARNING,
+                "Failed trying to sync DB directory %s in "
+                "MASTER <-> REPLICA synchronization: %s",
+                server.rdb_filename, strerror(errno));
+            cancelReplicationHandshake(1);
+            return;
+        }
+
+        if (rdbLoad(server.rdb_filename,&rsi,RDBFLAGS_REPLICATION) != RDB_OK) {
+            serverLog(LL_WARNING,
+                "Failed trying to load the MASTER synchronization "
+                "DB from disk, check server logs.");
+            cancelReplicationHandshake(1);
+            if (server.rdb_del_sync_files && allPersistenceDisabled()) {
+                serverLog(LL_NOTICE,"Removing the RDB file obtained from "
+                                    "the master. This replica has persistence "
+                                    "disabled");
+                bg_unlink(server.rdb_filename);
+            }
+            /* Note that there's no point in restarting the AOF on sync failure,
+               it'll be restarted when sync succeeds or replica promoted. */
+            return;
+        }
+
+        /* Cleanup. */
+        if (server.rdb_del_sync_files && allPersistenceDisabled()) {
+            serverLog(LL_NOTICE,"Removing the RDB file obtained from "
+                                "the master. This replica has persistence "
+                                "disabled");
+            bg_unlink(server.rdb_filename);
+        }
+
+        zfree(server.repl_transfer_tmpfile);
+        close(server.repl_transfer_fd);
+        server.repl_transfer_fd = -1;
+        server.repl_transfer_tmpfile = NULL;
+    }
+
+    /* Final setup of the connected slave <- master link */
+    replicationCreateMasterClient(server.repl_transfer_s,rsi.repl_stream_db);
+    server.repl_state = REPL_STATE_CONNECTED;
+    server.repl_down_since = 0;
+
+    /* Fire the master link modules event. */
+    moduleFireServerEvent(REDISMODULE_EVENT_MASTER_LINK_CHANGE,
+                          REDISMODULE_SUBEVENT_MASTER_LINK_UP,
+                          NULL);
+
+    /* After a full resynchronization we use the replication ID and
+     * offset of the master. The secondary ID / offset are cleared since
+     * we are starting a new history. */
+    memcpy(server.replid,server.master->replid,sizeof(server.replid));
+    server.master_repl_offset = server.master->reploff;
+    clearReplicationId2();
+
+    /* Let's create the replication backlog if needed. Slaves need to
+     * accumulate the backlog regardless of the fact they have sub-slaves
+     * or not, in order to behave correctly if they are promoted to
+     * masters after a failover. */
+    if (server.repl_backlog == NULL) createReplicationBacklog();
+    serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Finished with success");
+
+    if (server.supervised_mode == SUPERVISED_SYSTEMD) {
+        redisCommunicateSystemd("STATUS=MASTER <-> REPLICA sync: Finished with success. Ready to accept connections in read-write mode.\n");
+    }
+
+    /* Send the initial ACK immediately to put this replica in online state. */
+    if (usemark) replicationSendAck();
+
+    /* Restart the AOF subsystem now that we finished the sync. This
+     * will trigger an AOF rewrite, and when done will start appending
+     * to the new file. */
+    if (server.aof_enabled) restartAOFAfterSYNC();
+    return;
+
+error:
+    cancelReplicationHandshake(1);
+    return;
+}
+
+char *receiveSynchronousResponse(connection *conn) {
+    char buf[256];
+    /* Read the reply from the server. */
+    if (connSyncReadLine(conn,buf,sizeof(buf),server.repl_syncio_timeout*1000) == -1)
+    {
+        serverLog(LL_WARNING, "Failed to read response from the server: %s", connGetLastError(conn));
+        return NULL;
+    }
+    server.repl_transfer_lastio = server.unixtime;
+    return sdsnew(buf);
+}
+
+/* Send a pre-formatted multi-bulk command to the connection. */
+char* sendCommandRaw(connection *conn, sds cmd) {
+    if (connSyncWrite(conn,cmd,sdslen(cmd),server.repl_syncio_timeout*1000) == -1) {
+        return sdscatprintf(sdsempty(),"-Writing to master: %s",
+                connGetLastError(conn));
+    }
+    return NULL;
+}
+
+/* Compose a multi-bulk command and send it to the connection.
+ * Used to send AUTH and REPLCONF commands to the master before starting the
+ * replication.
+ *
+ * Takes a list of char* arguments, terminated by a NULL argument.
+ *
+ * The command returns an sds string representing the result of the
+ * operation. On error the first byte is a "-".
+ */
+char *sendCommand(connection *conn, ...) {
+    va_list ap;
+    sds cmd = sdsempty();
+    sds cmdargs = sdsempty();
+    size_t argslen = 0;
+    char *arg;
+
+    /* Create the command to send to the master, we use redis binary
+     * protocol to make sure correct arguments are sent. This function
+     * is not safe for all binary data. */
+    va_start(ap,conn);
+    while(1) {
+        arg = va_arg(ap, char*);
+        if (arg == NULL) break;
+        cmdargs = sdscatprintf(cmdargs,"$%zu\r\n%s\r\n",strlen(arg),arg);
+        argslen++;
+    }
+
+    cmd = sdscatprintf(cmd,"*%zu\r\n",argslen);
+    cmd = sdscatsds(cmd,cmdargs);
+    sdsfree(cmdargs);
+
+    va_end(ap);
+    char* err = sendCommandRaw(conn, cmd);
+    sdsfree(cmd);
+    if(err)
+        return err;
+    return NULL;
+}
+
+/* Compose a multi-bulk command and send it to the connection. 
+ * Used to send AUTH and REPLCONF commands to the master before starting the
+ * replication.
+ *
+ * argv_lens is optional, when NULL, strlen is used.
+ *
+ * The command returns an sds string representing the result of the
+ * operation. On error the first byte is a "-".
+ */
+char *sendCommandArgv(connection *conn, int argc, char **argv, size_t *argv_lens) {
+    sds cmd = sdsempty();
+    char *arg;
+    int i;
+
+    /* Create the command to send to the master. */
+    cmd = sdscatfmt(cmd,"*%i\r\n",argc);
+    for (i=0; i<argc; i++) {
+        int len;
+        arg = argv[i];
+        len = argv_lens ? argv_lens[i] : strlen(arg);
+        cmd = sdscatfmt(cmd,"$%i\r\n",len);
+        cmd = sdscatlen(cmd,arg,len);
+        cmd = sdscatlen(cmd,"\r\n",2);
+    }
+    char* err = sendCommandRaw(conn, cmd);
+    sdsfree(cmd);
+    if (err)
+        return err;
+    return NULL;
+}
+
+/* Try a partial resynchronization with the master if we are about to reconnect.
+ * If there is no cached master structure, at least try to issue a
+ * "PSYNC ? -1" command in order to trigger a full resync using the PSYNC
+ * command in order to obtain the master replid and the master replication
+ * global offset.
+ *
+ * This function is designed to be called from syncWithMaster(), so the
+ * following assumptions are made:
+ *
+ * 1) We pass the function an already connected socket "fd".
+ * 2) This function does not close the file descriptor "fd". However in case
+ *    of successful partial resynchronization, the function will reuse
+ *    'fd' as file descriptor of the server.master client structure.
+ *
+ * The function is split in two halves: if read_reply is 0, the function
+ * writes the PSYNC command on the socket, and a new function call is
+ * needed, with read_reply set to 1, in order to read the reply of the
+ * command. This is useful in order to support non blocking operations, so
+ * that we write, return into the event loop, and read when there are data.
+ *
+ * When read_reply is 0 the function returns PSYNC_WRITE_ERR if there
+ * was a write error, or PSYNC_WAIT_REPLY to signal we need another call
+ * with read_reply set to 1. However even when read_reply is set to 1
+ * the function may return PSYNC_WAIT_REPLY again to signal there were
+ * insufficient data to read to complete its work. We should re-enter
+ * into the event loop and wait in such a case.
+ *
+ * The function returns:
+ *
+ * PSYNC_CONTINUE: If the PSYNC command succeeded and we can continue.
+ * PSYNC_FULLRESYNC: If PSYNC is supported but a full resync is needed.
+ *                   In this case the master replid and global replication
+ *                   offset is saved.
+ * PSYNC_NOT_SUPPORTED: If the server does not understand PSYNC at all and
+ *                      the caller should fall back to SYNC.
+ * PSYNC_WRITE_ERROR: There was an error writing the command to the socket.
+ * PSYNC_WAIT_REPLY: Call again the function with read_reply set to 1.
+ * PSYNC_TRY_LATER: Master is currently in a transient error condition.
+ *
+ * Notable side effects:
+ *
+ * 1) As a side effect of the function call the function removes the readable
+ *    event handler from "fd", unless the return value is PSYNC_WAIT_REPLY.
+ * 2) server.master_initial_offset is set to the right value according
+ *    to the master reply. This will be used to populate the 'server.master'
+ *    structure replication offset.
+ */
+
+#define PSYNC_WRITE_ERROR 0
+#define PSYNC_WAIT_REPLY 1
+#define PSYNC_CONTINUE 2
+#define PSYNC_FULLRESYNC 3
+#define PSYNC_NOT_SUPPORTED 4
+#define PSYNC_TRY_LATER 5
+int slaveTryPartialResynchronization(connection *conn, int read_reply) {
+    char *psync_replid;
+    char psync_offset[32];
+    sds reply;
+
+    /* Writing half */
+    if (!read_reply) {
+        /* Initially set master_initial_offset to -1 to mark the current
+         * master replid and offset as not valid. Later if we'll be able to do
+         * a FULL resync using the PSYNC command we'll set the offset at the
+         * right value, so that this information will be propagated to the
+         * client structure representing the master into server.master. */
+        server.master_initial_offset = -1;
+
+        if (server.cached_master) {
+            psync_replid = server.cached_master->replid;
+            snprintf(psync_offset,sizeof(psync_offset),"%lld", server.cached_master->reploff+1);
+            serverLog(LL_NOTICE,"Trying a partial resynchronization (request %s:%s).", psync_replid, psync_offset);
+        } else {
+            serverLog(LL_NOTICE,"Partial resynchronization not possible (no cached master)");
+            psync_replid = "?";
+            memcpy(psync_offset,"-1",3);
+        }
+
+        /* Issue the PSYNC command, if this is a master with a failover in
+         * progress then send the failover argument to the replica to cause it
+         * to become a master */
+        if (server.failover_state == FAILOVER_IN_PROGRESS) {
+            reply = sendCommand(conn,"PSYNC",psync_replid,psync_offset,"FAILOVER",NULL);
+        } else {
+            reply = sendCommand(conn,"PSYNC",psync_replid,psync_offset,NULL);
+        }
+
+        if (reply != NULL) {
+            serverLog(LL_WARNING,"Unable to send PSYNC to master: %s",reply);
+            sdsfree(reply);
+            connSetReadHandler(conn, NULL);
+            return PSYNC_WRITE_ERROR;
+        }
+        return PSYNC_WAIT_REPLY;
+    }
+
+    /* Reading half */
+    reply = receiveSynchronousResponse(conn);
+    /* Master did not reply to PSYNC */
+    if (reply == NULL) {
+        connSetReadHandler(conn, NULL);
+        serverLog(LL_WARNING, "Master did not reply to PSYNC, will try later");
+        return PSYNC_TRY_LATER;
+    }
+
+    if (sdslen(reply) == 0) {
+        /* The master may send empty newlines after it receives PSYNC
+         * and before to reply, just to keep the connection alive. */
+        sdsfree(reply);
+        return PSYNC_WAIT_REPLY;
+    }
+
+    connSetReadHandler(conn, NULL);
+
+    if (!strncmp(reply,"+FULLRESYNC",11)) {
+        char *replid = NULL, *offset = NULL;
+
+        /* FULL RESYNC, parse the reply in order to extract the replid
+         * and the replication offset. */
+        replid = strchr(reply,' ');
+        if (replid) {
+            replid++;
+            offset = strchr(replid,' ');
+            if (offset) offset++;
+        }
+        if (!replid || !offset || (offset-replid-1) != CONFIG_RUN_ID_SIZE) {
+            serverLog(LL_WARNING,
+                "Master replied with wrong +FULLRESYNC syntax.");
+            /* This is an unexpected condition, actually the +FULLRESYNC
+             * reply means that the master supports PSYNC, but the reply
+             * format seems wrong. To stay safe we blank the master
+             * replid to make sure next PSYNCs will fail. */
+            memset(server.master_replid,0,CONFIG_RUN_ID_SIZE+1);
+        } else {
+            memcpy(server.master_replid, replid, offset-replid-1);
+            server.master_replid[CONFIG_RUN_ID_SIZE] = '\0';
+            server.master_initial_offset = strtoll(offset,NULL,10);
+            serverLog(LL_NOTICE,"Full resync from master: %s:%lld",
+                server.master_replid,
+                server.master_initial_offset);
+        }
+        sdsfree(reply);
+        return PSYNC_FULLRESYNC;
+    }
+
+    if (!strncmp(reply,"+CONTINUE",9)) {
+        /* Partial resync was accepted. */
+        serverLog(LL_NOTICE,
+            "Successful partial resynchronization with master.");
+
+        /* Check the new replication ID advertised by the master. If it
+         * changed, we need to set the new ID as primary ID, and set
+         * secondary ID as the old master ID up to the current offset, so
+         * that our sub-slaves will be able to PSYNC with us after a
+         * disconnection. */
+        char *start = reply+10;
+        char *end = reply+9;
+        while(end[0] != '\r' && end[0] != '\n' && end[0] != '\0') end++;
+        if (end-start == CONFIG_RUN_ID_SIZE) {
+            char new[CONFIG_RUN_ID_SIZE+1];
+            memcpy(new,start,CONFIG_RUN_ID_SIZE);
+            new[CONFIG_RUN_ID_SIZE] = '\0';
+
+            if (strcmp(new,server.cached_master->replid)) {
+                /* Master ID changed. */
+                serverLog(LL_NOTICE,"Master replication ID changed to %s",new);
+
+                /* Set the old ID as our ID2, up to the current offset+1. */
+                memcpy(server.replid2,server.cached_master->replid,
+                    sizeof(server.replid2));
+                server.second_replid_offset = server.master_repl_offset+1;
+
+                /* Update the cached master ID and our own primary ID to the
+                 * new one. */
+                memcpy(server.replid,new,sizeof(server.replid));
+                memcpy(server.cached_master->replid,new,sizeof(server.replid));
+
+                /* Disconnect all the sub-slaves: they need to be notified. */
+                disconnectSlaves();
+            }
+        }
+
+        /* Setup the replication to continue. */
+        sdsfree(reply);
+        replicationResurrectCachedMaster(conn);
+
+        /* If this instance was restarted and we read the metadata to
+         * PSYNC from the persistence file, our replication backlog could
+         * be still not initialized. Create it. */
+        if (server.repl_backlog == NULL) createReplicationBacklog();
+        return PSYNC_CONTINUE;
+    }
+
+    /* If we reach this point we received either an error (since the master does
+     * not understand PSYNC or because it is in a special state and cannot
+     * serve our request), or an unexpected reply from the master.
+     *
+     * Return PSYNC_NOT_SUPPORTED on errors we don't understand, otherwise
+     * return PSYNC_TRY_LATER if we believe this is a transient error. */
+
+    if (!strncmp(reply,"-NOMASTERLINK",13) ||
+        !strncmp(reply,"-LOADING",8))
+    {
+        serverLog(LL_NOTICE,
+            "Master is currently unable to PSYNC "
+            "but should be in the future: %s", reply);
+        sdsfree(reply);
+        return PSYNC_TRY_LATER;
+    }
+
+    if (strncmp(reply,"-ERR",4)) {
+        /* If it's not an error, log the unexpected event. */
+        serverLog(LL_WARNING,
+            "Unexpected reply to PSYNC from master: %s", reply);
+    } else {
+        serverLog(LL_NOTICE,
+            "Master does not support PSYNC or is in "
+            "error state (reply: %s)", reply);
+    }
+    sdsfree(reply);
+    return PSYNC_NOT_SUPPORTED;
+}
+
+/* This handler fires when the non blocking connect was able to
+ * establish a connection with the master. */
+void syncWithMaster(connection *conn) {
+    char tmpfile[256], *err = NULL;
+    int dfd = -1, maxtries = 5;
+    int psync_result;
+
+    /* If this event fired after the user turned the instance into a master
+     * with SLAVEOF NO ONE we must just return ASAP. */
+    if (server.repl_state == REPL_STATE_NONE) {
+        connClose(conn);
+        return;
+    }
+
+    /* Check for errors in the socket: after a non blocking connect() we
+     * may find that the socket is in error state. */
+    if (connGetState(conn) != CONN_STATE_CONNECTED) {
+        serverLog(LL_WARNING,"Error condition on socket for SYNC: %s",
+                connGetLastError(conn));
+        goto error;
+    }
+
+    /* Send a PING to check the master is able to reply without errors. */
+    if (server.repl_state == REPL_STATE_CONNECTING) {
+        serverLog(LL_NOTICE,"Non blocking connect for SYNC fired the event.");
+        /* Delete the writable event so that the readable event remains
+         * registered and we can wait for the PONG reply. */
+        connSetReadHandler(conn, syncWithMaster);
+        connSetWriteHandler(conn, NULL);
+        server.repl_state = REPL_STATE_RECEIVE_PING_REPLY;
+        /* Send the PING, don't check for errors at all, we have the timeout
+         * that will take care about this. */
+        err = sendCommand(conn,"PING",NULL);
+        if (err) goto write_error;
+        return;
+    }
+
+    /* Receive the PONG command. */
+    if (server.repl_state == REPL_STATE_RECEIVE_PING_REPLY) {
+        err = receiveSynchronousResponse(conn);
+
+        /* The master did not reply */
+        if (err == NULL) goto no_response_error;
+
+        /* We accept only two replies as valid, a positive +PONG reply
+         * (we just check for "+") or an authentication error.
+         * Note that older versions of Redis replied with "operation not
+         * permitted" instead of using a proper error code, so we test
+         * both. */
+        if (err[0] != '+' &&
+            strncmp(err,"-NOAUTH",7) != 0 &&
+            strncmp(err,"-NOPERM",7) != 0 &&
+            strncmp(err,"-ERR operation not permitted",28) != 0)
+        {
+            serverLog(LL_WARNING,"Error reply to PING from master: '%s'",err);
+            sdsfree(err);
+            goto error;
+        } else {
+            serverLog(LL_NOTICE,
+                "Master replied to PING, replication can continue...");
+        }
+        sdsfree(err);
+        err = NULL;
+        server.repl_state = REPL_STATE_SEND_HANDSHAKE;
+    }
+
+    if (server.repl_state == REPL_STATE_SEND_HANDSHAKE) {
+        /* AUTH with the master if required. */
+        if (server.masterauth) {
+            char *args[3] = {"AUTH",NULL,NULL};
+            size_t lens[3] = {4,0,0};
+            int argc = 1;
+            if (server.masteruser) {
+                args[argc] = server.masteruser;
+                lens[argc] = strlen(server.masteruser);
+                argc++;
+            }
+            args[argc] = server.masterauth;
+            lens[argc] = sdslen(server.masterauth);
+            argc++;
+            err = sendCommandArgv(conn, argc, args, lens);
+            if (err) goto write_error;
+        }
+
+        /* Set the slave port, so that Master's INFO command can list the
+         * slave listening port correctly. */
+        {
+            int port;
+            if (server.slave_announce_port)
+                port = server.slave_announce_port;
+            else if (server.tls_replication && server.tls_port)
+                port = server.tls_port;
+            else
+                port = server.port;
+            sds portstr = sdsfromlonglong(port);
+            err = sendCommand(conn,"REPLCONF",
+                    "listening-port",portstr, NULL);
+            sdsfree(portstr);
+            if (err) goto write_error;
+        }
+
+        /* Set the slave ip, so that Master's INFO command can list the
+         * slave IP address port correctly in case of port forwarding or NAT.
+         * Skip REPLCONF ip-address if there is no slave-announce-ip option set. */
+        if (server.slave_announce_ip) {
+            err = sendCommand(conn,"REPLCONF",
+                    "ip-address",server.slave_announce_ip, NULL);
+            if (err) goto write_error;
+        }
+
+        /* Inform the master of our (slave) capabilities.
+         *
+         * EOF: supports EOF-style RDB transfer for diskless replication.
+         * PSYNC2: supports PSYNC v2, so understands +CONTINUE <new repl ID>.
+         *
+         * The master will ignore capabilities it does not understand. */
+        err = sendCommand(conn,"REPLCONF",
+                "capa","eof","capa","psync2",NULL);
+        if (err) goto write_error;
+
+        server.repl_state = REPL_STATE_RECEIVE_AUTH_REPLY;
+        return;
+    }
+
+    if (server.repl_state == REPL_STATE_RECEIVE_AUTH_REPLY && !server.masterauth)
+        server.repl_state = REPL_STATE_RECEIVE_PORT_REPLY;
+
+    /* Receive AUTH reply. */
+    if (server.repl_state == REPL_STATE_RECEIVE_AUTH_REPLY) {
+        err = receiveSynchronousResponse(conn);
+        if (err == NULL) goto no_response_error;
+        if (err[0] == '-') {
+            serverLog(LL_WARNING,"Unable to AUTH to MASTER: %s",err);
+            sdsfree(err);
+            goto error;
+        }
+        sdsfree(err);
+        err = NULL;
+        server.repl_state = REPL_STATE_RECEIVE_PORT_REPLY;
+        return;
+    }
+
+    /* Receive REPLCONF listening-port reply. */
+    if (server.repl_state == REPL_STATE_RECEIVE_PORT_REPLY) {
+        err = receiveSynchronousResponse(conn);
+        if (err == NULL) goto no_response_error;
+        /* Ignore the error if any, not all the Redis versions support
+         * REPLCONF listening-port. */
+        if (err[0] == '-') {
+            serverLog(LL_NOTICE,"(Non critical) Master does not understand "
+                                "REPLCONF listening-port: %s", err);
+        }
+        sdsfree(err);
+        server.repl_state = REPL_STATE_RECEIVE_IP_REPLY;
+        return;
+    }
+
+    if (server.repl_state == REPL_STATE_RECEIVE_IP_REPLY && !server.slave_announce_ip)
+        server.repl_state = REPL_STATE_RECEIVE_CAPA_REPLY;
+
+    /* Receive REPLCONF ip-address reply. */
+    if (server.repl_state == REPL_STATE_RECEIVE_IP_REPLY) {
+        err = receiveSynchronousResponse(conn);
+        if (err == NULL) goto no_response_error;
+        /* Ignore the error if any, not all the Redis versions support
+         * REPLCONF ip-address. */
+        if (err[0] == '-') {
+            serverLog(LL_NOTICE,"(Non critical) Master does not understand "
+                                "REPLCONF ip-address: %s", err);
+        }
+        sdsfree(err);
+        server.repl_state = REPL_STATE_RECEIVE_CAPA_REPLY;
+        return;
+    }
+
+    /* Receive CAPA reply. */
+    if (server.repl_state == REPL_STATE_RECEIVE_CAPA_REPLY) {
+        err = receiveSynchronousResponse(conn);
+        if (err == NULL) goto no_response_error;
+        /* Ignore the error if any, not all the Redis versions support
+         * REPLCONF capa. */
+        if (err[0] == '-') {
+            serverLog(LL_NOTICE,"(Non critical) Master does not understand "
+                                  "REPLCONF capa: %s", err);
+        }
+        sdsfree(err);
+        err = NULL;
+        server.repl_state = REPL_STATE_SEND_PSYNC;
+    }
+
+    /* Try a partial resynchronization. If we don't have a cached master
+     * slaveTryPartialResynchronization() will at least try to use PSYNC
+     * to start a full resynchronization so that we get the master replid
+     * and the global offset, to try a partial resync at the next
+     * reconnection attempt. */
+    if (server.repl_state == REPL_STATE_SEND_PSYNC) {
+        if (slaveTryPartialResynchronization(conn,0) == PSYNC_WRITE_ERROR) {
+            err = sdsnew("Write error sending the PSYNC command.");
+            abortFailover("Write error to failover target");
+            goto write_error;
+        }
+        server.repl_state = REPL_STATE_RECEIVE_PSYNC_REPLY;
+        return;
+    }
+
+    /* If reached this point, we should be in REPL_STATE_RECEIVE_PSYNC_REPLY. */
+    if (server.repl_state != REPL_STATE_RECEIVE_PSYNC_REPLY) {
+        serverLog(LL_WARNING,"syncWithMaster(): state machine error, "
+                             "state should be RECEIVE_PSYNC but is %d",
+                             server.repl_state);
+        goto error;
+    }
+
+    psync_result = slaveTryPartialResynchronization(conn,1);
+    if (psync_result == PSYNC_WAIT_REPLY) return; /* Try again later... */
+
+    /* Check the status of the planned failover. We expect PSYNC_CONTINUE,
+     * but there is nothing technically wrong with a full resync which
+     * could happen in edge cases. */
+    if (server.failover_state == FAILOVER_IN_PROGRESS) {
+        if (psync_result == PSYNC_CONTINUE || psync_result == PSYNC_FULLRESYNC) {
+            clearFailoverState();
+        } else {
+            abortFailover("Failover target rejected psync request");
+            return;
+        }
+    }
+
+    /* If the master is in an transient error, we should try to PSYNC
+     * from scratch later, so go to the error path. This happens when
+     * the server is loading the dataset or is not connected with its
+     * master and so forth. */
+    if (psync_result == PSYNC_TRY_LATER) goto error;
+
+    /* Note: if PSYNC does not return WAIT_REPLY, it will take care of
+     * uninstalling the read handler from the file descriptor. */
+
+    if (psync_result == PSYNC_CONTINUE) {
+        serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Master accepted a Partial Resynchronization.");
+        if (server.supervised_mode == SUPERVISED_SYSTEMD) {
+            redisCommunicateSystemd("STATUS=MASTER <-> REPLICA sync: Partial Resynchronization accepted. Ready to accept connections in read-write mode.\n");
+        }
+        return;
+    }
+
+    /* Fall back to SYNC if needed. Otherwise psync_result == PSYNC_FULLRESYNC
+     * and the server.master_replid and master_initial_offset are
+     * already populated. */
+    if (psync_result == PSYNC_NOT_SUPPORTED) {
+        serverLog(LL_NOTICE,"Retrying with SYNC...");
+        if (connSyncWrite(conn,"SYNC\r\n",6,server.repl_syncio_timeout*1000) == -1) {
+            serverLog(LL_WARNING,"I/O error writing to MASTER: %s",
+                connGetLastError(conn));
+            goto error;
+        }
+    }
+
+    /* Prepare a suitable temp file for bulk transfer */
+    if (!useDisklessLoad()) {
+        while(maxtries--) {
+            snprintf(tmpfile,256,
+                "temp-%d.%ld.rdb",(int)server.unixtime,(long int)getpid());
+            dfd = open(tmpfile,O_CREAT|O_WRONLY|O_EXCL,0644);
+            if (dfd != -1) break;
+            sleep(1);
+        }
+        if (dfd == -1) {
+            serverLog(LL_WARNING,"Opening the temp file needed for MASTER <-> REPLICA synchronization: %s",strerror(errno));
+            goto error;
+        }
+        server.repl_transfer_tmpfile = zstrdup(tmpfile);
+        server.repl_transfer_fd = dfd;
+    }
+
+    /* Setup the non blocking download of the bulk file. */
+    if (connSetReadHandler(conn, readSyncBulkPayload)
+            == C_ERR)
+    {
+        char conninfo[CONN_INFO_LEN];
+        serverLog(LL_WARNING,
+            "Can't create readable event for SYNC: %s (%s)",
+            strerror(errno), connGetInfo(conn, conninfo, sizeof(conninfo)));
+        goto error;
+    }
+
+    server.repl_state = REPL_STATE_TRANSFER;
+    server.repl_transfer_size = -1;
+    server.repl_transfer_read = 0;
+    server.repl_transfer_last_fsync_off = 0;
+    server.repl_transfer_lastio = server.unixtime;
+    return;
+
+no_response_error: /* Handle receiveSynchronousResponse() error when master has no reply */
+    serverLog(LL_WARNING, "Master did not respond to command during SYNC handshake");
+    /* Fall through to regular error handling */
+
+error:
+    if (dfd != -1) close(dfd);
+    connClose(conn);
+    server.repl_transfer_s = NULL;
+    if (server.repl_transfer_fd != -1)
+        close(server.repl_transfer_fd);
+    if (server.repl_transfer_tmpfile)
+        zfree(server.repl_transfer_tmpfile);
+    server.repl_transfer_tmpfile = NULL;
+    server.repl_transfer_fd = -1;
+    server.repl_state = REPL_STATE_CONNECT;
+    return;
+
+write_error: /* Handle sendCommand() errors. */
+    serverLog(LL_WARNING,"Sending command to master in replication handshake: %s", err);
+    sdsfree(err);
+    goto error;
+}
+
+int connectWithMaster(void) {
+    server.repl_transfer_s = connCreate(connTypeOfReplication());
+    if (connConnect(server.repl_transfer_s, server.masterhost, server.masterport,
+                server.bind_source_addr, syncWithMaster) == C_ERR) {
+        serverLog(LL_WARNING,"Unable to connect to MASTER: %s",
+                connGetLastError(server.repl_transfer_s));
+        connClose(server.repl_transfer_s);
+        server.repl_transfer_s = NULL;
+        return C_ERR;
+    }
+
+
+    server.repl_transfer_lastio = server.unixtime;
+    server.repl_state = REPL_STATE_CONNECTING;
+    serverLog(LL_NOTICE,"MASTER <-> REPLICA sync started");
+    return C_OK;
+}
+
+/* This function can be called when a non blocking connection is currently
+ * in progress to undo it.
+ * Never call this function directly, use cancelReplicationHandshake() instead.
+ */
+void undoConnectWithMaster(void) {
+    connClose(server.repl_transfer_s);
+    server.repl_transfer_s = NULL;
+}
+
+/* Abort the async download of the bulk dataset while SYNC-ing with master.
+ * Never call this function directly, use cancelReplicationHandshake() instead.
+ */
+void replicationAbortSyncTransfer(void) {
+    serverAssert(server.repl_state == REPL_STATE_TRANSFER);
+    undoConnectWithMaster();
+    if (server.repl_transfer_fd!=-1) {
+        close(server.repl_transfer_fd);
+        bg_unlink(server.repl_transfer_tmpfile);
+        zfree(server.repl_transfer_tmpfile);
+        server.repl_transfer_tmpfile = NULL;
+        server.repl_transfer_fd = -1;
+    }
+}
+
+/* This function aborts a non blocking replication attempt if there is one
+ * in progress, by canceling the non-blocking connect attempt or
+ * the initial bulk transfer.
+ *
+ * If there was a replication handshake in progress 1 is returned and
+ * the replication state (server.repl_state) set to REPL_STATE_CONNECT.
+ *
+ * Otherwise zero is returned and no operation is performed at all. */
+int cancelReplicationHandshake(int reconnect) {
+    if (server.repl_state == REPL_STATE_TRANSFER) {
+        replicationAbortSyncTransfer();
+        server.repl_state = REPL_STATE_CONNECT;
+    } else if (server.repl_state == REPL_STATE_CONNECTING ||
+               slaveIsInHandshakeState())
+    {
+        undoConnectWithMaster();
+        server.repl_state = REPL_STATE_CONNECT;
+    } else {
+        return 0;
+    }
+
+    if (!reconnect)
+        return 1;
+
+    /* try to re-connect without waiting for replicationCron, this is needed
+     * for the "diskless loading short read" test. */
+    serverLog(LL_NOTICE,"Reconnecting to MASTER %s:%d after failure",
+        server.masterhost, server.masterport);
+    connectWithMaster();
+
+    return 1;
+}
+
+/* Set replication to the specified master address and port. */
+void replicationSetMaster(char *ip, int port) {
+    int was_master = server.masterhost == NULL;
+
+    sdsfree(server.masterhost);
+    server.masterhost = NULL;
+    if (server.master) {
+        freeClient(server.master);
+    }
+    disconnectAllBlockedClients(); /* Clients blocked in master, now slave. */
+
+    /* Setting masterhost only after the call to freeClient since it calls
+     * replicationHandleMasterDisconnection which can trigger a re-connect
+     * directly from within that call. */
+    server.masterhost = sdsnew(ip);
+    server.masterport = port;
+
+    /* Update oom_score_adj */
+    setOOMScoreAdj(-1);
+
+    /* Here we don't disconnect with replicas, since they may hopefully be able
+     * to partially resync with us. We will disconnect with replicas and force
+     * them to resync with us when changing replid on partially resync with new
+     * master, or finishing transferring RDB and preparing loading DB on full
+     * sync with new master. */
+
+    cancelReplicationHandshake(0);
+    /* Before destroying our master state, create a cached master using
+     * our own parameters, to later PSYNC with the new master. */
+    if (was_master) {
+        replicationDiscardCachedMaster();
+        replicationCacheMasterUsingMyself();
+    }
+
+    /* Fire the role change modules event. */
+    moduleFireServerEvent(REDISMODULE_EVENT_REPLICATION_ROLE_CHANGED,
+                          REDISMODULE_EVENT_REPLROLECHANGED_NOW_REPLICA,
+                          NULL);
+
+    /* Fire the master link modules event. */
+    if (server.repl_state == REPL_STATE_CONNECTED)
+        moduleFireServerEvent(REDISMODULE_EVENT_MASTER_LINK_CHANGE,
+                              REDISMODULE_SUBEVENT_MASTER_LINK_DOWN,
+                              NULL);
+
+    server.repl_state = REPL_STATE_CONNECT;
+    serverLog(LL_NOTICE,"Connecting to MASTER %s:%d",
+        server.masterhost, server.masterport);
+    connectWithMaster();
+}
+
+/* Cancel replication, setting the instance as a master itself. */
+void replicationUnsetMaster(void) {
+    if (server.masterhost == NULL) return; /* Nothing to do. */
+
+    /* Fire the master link modules event. */
+    if (server.repl_state == REPL_STATE_CONNECTED)
+        moduleFireServerEvent(REDISMODULE_EVENT_MASTER_LINK_CHANGE,
+                              REDISMODULE_SUBEVENT_MASTER_LINK_DOWN,
+                              NULL);
+
+    /* Clear masterhost first, since the freeClient calls
+     * replicationHandleMasterDisconnection which can attempt to re-connect. */
+    sdsfree(server.masterhost);
+    server.masterhost = NULL;
+    if (server.master) freeClient(server.master);
+    replicationDiscardCachedMaster();
+    cancelReplicationHandshake(0);
+    /* When a slave is turned into a master, the current replication ID
+     * (that was inherited from the master at synchronization time) is
+     * used as secondary ID up to the current offset, and a new replication
+     * ID is created to continue with a new replication history. */
+    shiftReplicationId();
+    /* Disconnecting all the slaves is required: we need to inform slaves
+     * of the replication ID change (see shiftReplicationId() call). However
+     * the slaves will be able to partially resync with us, so it will be
+     * a very fast reconnection. */
+    disconnectSlaves();
+    server.repl_state = REPL_STATE_NONE;
+
+    /* We need to make sure the new master will start the replication stream
+     * with a SELECT statement. This is forced after a full resync, but
+     * with PSYNC version 2, there is no need for full resync after a
+     * master switch. */
+    server.slaveseldb = -1;
+
+    /* Update oom_score_adj */
+    setOOMScoreAdj(-1);
+
+    /* Once we turn from slave to master, we consider the starting time without
+     * slaves (that is used to count the replication backlog time to live) as
+     * starting from now. Otherwise the backlog will be freed after a
+     * failover if slaves do not connect immediately. */
+    server.repl_no_slaves_since = server.unixtime;
+    
+    /* Reset down time so it'll be ready for when we turn into replica again. */
+    server.repl_down_since = 0;
+
+    /* Fire the role change modules event. */
+    moduleFireServerEvent(REDISMODULE_EVENT_REPLICATION_ROLE_CHANGED,
+                          REDISMODULE_EVENT_REPLROLECHANGED_NOW_MASTER,
+                          NULL);
+
+    /* Restart the AOF subsystem in case we shut it down during a sync when
+     * we were still a slave. */
+    if (server.aof_enabled && server.aof_state == AOF_OFF) restartAOFAfterSYNC();
+}
+
+/* This function is called when the slave lose the connection with the
+ * master into an unexpected way. */
+void replicationHandleMasterDisconnection(void) {
+    /* Fire the master link modules event. */
+    if (server.repl_state == REPL_STATE_CONNECTED)
+        moduleFireServerEvent(REDISMODULE_EVENT_MASTER_LINK_CHANGE,
+                              REDISMODULE_SUBEVENT_MASTER_LINK_DOWN,
+                              NULL);
+
+    server.master = NULL;
+    server.repl_state = REPL_STATE_CONNECT;
+    server.repl_down_since = server.unixtime;
+    /* We lost connection with our master, don't disconnect slaves yet,
+     * maybe we'll be able to PSYNC with our master later. We'll disconnect
+     * the slaves only if we'll have to do a full resync with our master. */
+
+    /* Try to re-connect immediately rather than wait for replicationCron
+     * waiting 1 second may risk backlog being recycled. */
+    if (server.masterhost) {
+        serverLog(LL_NOTICE,"Reconnecting to MASTER %s:%d",
+            server.masterhost, server.masterport);
+        connectWithMaster();
+    }
+}
+
+void replicaofCommand(client *c) {
+    /* SLAVEOF is not allowed in cluster mode as replication is automatically
+     * configured using the current address of the master node. */
+    if (server.cluster_enabled) {
+        addReplyError(c,"REPLICAOF not allowed in cluster mode.");
+        return;
+    }
+
+    if (server.failover_state != NO_FAILOVER) {
+        addReplyError(c,"REPLICAOF not allowed while failing over.");
+        return;
+    }
+
+    /* The special host/port combination "NO" "ONE" turns the instance
+     * into a master. Otherwise the new master address is set. */
+    if (!strcasecmp(c->argv[1]->ptr,"no") &&
+        !strcasecmp(c->argv[2]->ptr,"one")) {
+        if (server.masterhost) {
+            replicationUnsetMaster();
+            sds client = catClientInfoString(sdsempty(),c);
+            serverLog(LL_NOTICE,"MASTER MODE enabled (user request from '%s')",
+                client);
+            sdsfree(client);
+        }
+    } else {
+        long port;
+
+        if (c->flags & CLIENT_SLAVE)
+        {
+            /* If a client is already a replica they cannot run this command,
+             * because it involves flushing all replicas (including this
+             * client) */
+            addReplyError(c, "Command is not valid when client is a replica.");
+            return;
+        }
+
+        if (getRangeLongFromObjectOrReply(c, c->argv[2], 0, 65535, &port,
+                                          "Invalid master port") != C_OK)
+            return;
+
+        /* Check if we are already attached to the specified master */
+        if (server.masterhost && !strcasecmp(server.masterhost,c->argv[1]->ptr)
+            && server.masterport == port) {
+            serverLog(LL_NOTICE,"REPLICAOF would result into synchronization "
+                                "with the master we are already connected "
+                                "with. No operation performed.");
+            addReplySds(c,sdsnew("+OK Already connected to specified "
+                                 "master\r\n"));
+            return;
+        }
+        /* There was no previous master or the user specified a different one,
+         * we can continue. */
+        replicationSetMaster(c->argv[1]->ptr, port);
+        sds client = catClientInfoString(sdsempty(),c);
+        serverLog(LL_NOTICE,"REPLICAOF %s:%d enabled (user request from '%s')",
+            server.masterhost, server.masterport, client);
+        sdsfree(client);
+    }
+    addReply(c,shared.ok);
+}
+
+/* ROLE command: provide information about the role of the instance
+ * (master or slave) and additional information related to replication
+ * in an easy to process format. */
+void roleCommand(client *c) {
+    if (server.sentinel_mode) {
+        sentinelRoleCommand(c);
+        return;
+    }
+
+    if (server.masterhost == NULL) {
+        listIter li;
+        listNode *ln;
+        void *mbcount;
+        int slaves = 0;
+
+        addReplyArrayLen(c,3);
+        addReplyBulkCBuffer(c,"master",6);
+        addReplyLongLong(c,server.master_repl_offset);
+        mbcount = addReplyDeferredLen(c);
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            client *slave = ln->value;
+            char ip[NET_IP_STR_LEN], *slaveaddr = slave->slave_addr;
+
+            if (!slaveaddr) {
+                if (connAddrPeerName(slave->conn,ip,sizeof(ip),NULL) == -1)
+                    continue;
+                slaveaddr = ip;
+            }
+            if (slave->replstate != SLAVE_STATE_ONLINE) continue;
+            addReplyArrayLen(c,3);
+            addReplyBulkCString(c,slaveaddr);
+            addReplyBulkLongLong(c,slave->slave_listening_port);
+            addReplyBulkLongLong(c,slave->repl_ack_off);
+            slaves++;
+        }
+        setDeferredArrayLen(c,mbcount,slaves);
+    } else {
+        char *slavestate = NULL;
+
+        addReplyArrayLen(c,5);
+        addReplyBulkCBuffer(c,"slave",5);
+        addReplyBulkCString(c,server.masterhost);
+        addReplyLongLong(c,server.masterport);
+        if (slaveIsInHandshakeState()) {
+            slavestate = "handshake";
+        } else {
+            switch(server.repl_state) {
+            case REPL_STATE_NONE: slavestate = "none"; break;
+            case REPL_STATE_CONNECT: slavestate = "connect"; break;
+            case REPL_STATE_CONNECTING: slavestate = "connecting"; break;
+            case REPL_STATE_TRANSFER: slavestate = "sync"; break;
+            case REPL_STATE_CONNECTED: slavestate = "connected"; break;
+            default: slavestate = "unknown"; break;
+            }
+        }
+        addReplyBulkCString(c,slavestate);
+        addReplyLongLong(c,server.master ? server.master->reploff : -1);
+    }
+}
+
+/* Send a REPLCONF ACK command to the master to inform it about the current
+ * processed offset. If we are not connected with a master, the command has
+ * no effects. */
+void replicationSendAck(void) {
+    client *c = server.master;
+
+    if (c != NULL) {
+        int send_fack = server.fsynced_reploff != -1;
+        c->flags |= CLIENT_MASTER_FORCE_REPLY;
+        addReplyArrayLen(c,send_fack ? 5 : 3);
+        addReplyBulkCString(c,"REPLCONF");
+        addReplyBulkCString(c,"ACK");
+        addReplyBulkLongLong(c,c->reploff);
+        if (send_fack) {
+            addReplyBulkCString(c,"FACK");
+            addReplyBulkLongLong(c,server.fsynced_reploff);
+        }
+        c->flags &= ~CLIENT_MASTER_FORCE_REPLY;
+    }
+}
+
+/* ---------------------- MASTER CACHING FOR PSYNC -------------------------- */
+
+/* In order to implement partial synchronization we need to be able to cache
+ * our master's client structure after a transient disconnection.
+ * It is cached into server.cached_master and flushed away using the following
+ * functions. */
+
+/* This function is called by freeClient() in order to cache the master
+ * client structure instead of destroying it. freeClient() will return
+ * ASAP after this function returns, so every action needed to avoid problems
+ * with a client that is really "suspended" has to be done by this function.
+ *
+ * The other functions that will deal with the cached master are:
+ *
+ * replicationDiscardCachedMaster() that will make sure to kill the client
+ * as for some reason we don't want to use it in the future.
+ *
+ * replicationResurrectCachedMaster() that is used after a successful PSYNC
+ * handshake in order to reactivate the cached master.
+ */
+void replicationCacheMaster(client *c) {
+    serverAssert(server.master != NULL && server.cached_master == NULL);
+    serverLog(LL_NOTICE,"Caching the disconnected master state.");
+
+    /* Unlink the client from the server structures. */
+    unlinkClient(c);
+
+    /* Reset the master client so that's ready to accept new commands:
+     * we want to discard the non processed query buffers and non processed
+     * offsets, including pending transactions, already populated arguments,
+     * pending outputs to the master. */
+    sdsclear(server.master->querybuf);
+    server.master->qb_pos = 0;
+    server.master->repl_applied = 0;
+    server.master->read_reploff = server.master->reploff;
+    if (c->flags & CLIENT_MULTI) discardTransaction(c);
+    listEmpty(c->reply);
+    c->sentlen = 0;
+    c->reply_bytes = 0;
+    c->bufpos = 0;
+    resetClient(c);
+
+    /* Save the master. Server.master will be set to null later by
+     * replicationHandleMasterDisconnection(). */
+    server.cached_master = server.master;
+
+    /* Invalidate the Peer ID cache. */
+    if (c->peerid) {
+        sdsfree(c->peerid);
+        c->peerid = NULL;
+    }
+    /* Invalidate the Sock Name cache. */
+    if (c->sockname) {
+        sdsfree(c->sockname);
+        c->sockname = NULL;
+    }
+
+    /* Caching the master happens instead of the actual freeClient() call,
+     * so make sure to adjust the replication state. This function will
+     * also set server.master to NULL. */
+    replicationHandleMasterDisconnection();
+}
+
+/* This function is called when a master is turned into a slave, in order to
+ * create from scratch a cached master for the new client, that will allow
+ * to PSYNC with the slave that was promoted as the new master after a
+ * failover.
+ *
+ * Assuming this instance was previously the master instance of the new master,
+ * the new master will accept its replication ID, and potential also the
+ * current offset if no data was lost during the failover. So we use our
+ * current replication ID and offset in order to synthesize a cached master. */
+void replicationCacheMasterUsingMyself(void) {
+    serverLog(LL_NOTICE,
+        "Before turning into a replica, using my own master parameters "
+        "to synthesize a cached master: I may be able to synchronize with "
+        "the new master with just a partial transfer.");
+
+    /* This will be used to populate the field server.master->reploff
+     * by replicationCreateMasterClient(). We'll later set the created
+     * master as server.cached_master, so the replica will use such
+     * offset for PSYNC. */
+    server.master_initial_offset = server.master_repl_offset;
+
+    /* The master client we create can be set to any DBID, because
+     * the new master will start its replication stream with SELECT. */
+    replicationCreateMasterClient(NULL,-1);
+
+    /* Use our own ID / offset. */
+    memcpy(server.master->replid, server.replid, sizeof(server.replid));
+
+    /* Set as cached master. */
+    unlinkClient(server.master);
+    server.cached_master = server.master;
+    server.master = NULL;
+}
+
+/* Free a cached master, called when there are no longer the conditions for
+ * a partial resync on reconnection. */
+void replicationDiscardCachedMaster(void) {
+    if (server.cached_master == NULL) return;
+
+    serverLog(LL_NOTICE,"Discarding previously cached master state.");
+    server.cached_master->flags &= ~CLIENT_MASTER;
+    freeClient(server.cached_master);
+    server.cached_master = NULL;
+}
+
+/* Turn the cached master into the current master, using the file descriptor
+ * passed as argument as the socket for the new master.
+ *
+ * This function is called when successfully setup a partial resynchronization
+ * so the stream of data that we'll receive will start from where this
+ * master left. */
+void replicationResurrectCachedMaster(connection *conn) {
+    server.master = server.cached_master;
+    server.cached_master = NULL;
+    server.master->conn = conn;
+    connSetPrivateData(server.master->conn, server.master);
+    server.master->flags &= ~(CLIENT_CLOSE_AFTER_REPLY|CLIENT_CLOSE_ASAP);
+    server.master->authenticated = 1;
+    server.master->lastinteraction = server.unixtime;
+    server.repl_state = REPL_STATE_CONNECTED;
+    server.repl_down_since = 0;
+
+    /* Fire the master link modules event. */
+    moduleFireServerEvent(REDISMODULE_EVENT_MASTER_LINK_CHANGE,
+                          REDISMODULE_SUBEVENT_MASTER_LINK_UP,
+                          NULL);
+
+    /* Re-add to the list of clients. */
+    linkClient(server.master);
+    if (connSetReadHandler(server.master->conn, readQueryFromClient)) {
+        serverLog(LL_WARNING,"Error resurrecting the cached master, impossible to add the readable handler: %s", strerror(errno));
+        freeClientAsync(server.master); /* Close ASAP. */
+    }
+
+    /* We may also need to install the write handler as well if there is
+     * pending data in the write buffers. */
+    if (clientHasPendingReplies(server.master)) {
+        if (connSetWriteHandler(server.master->conn, sendReplyToClient)) {
+            serverLog(LL_WARNING,"Error resurrecting the cached master, impossible to add the writable handler: %s", strerror(errno));
+            freeClientAsync(server.master); /* Close ASAP. */
+        }
+    }
+}
+
+/* ------------------------- MIN-SLAVES-TO-WRITE  --------------------------- */
+
+/* This function counts the number of slaves with lag <= min-slaves-max-lag.
+ * If the option is active, the server will prevent writes if there are not
+ * enough connected slaves with the specified lag (or less). */
+void refreshGoodSlavesCount(void) {
+    listIter li;
+    listNode *ln;
+    int good = 0;
+
+    if (!server.repl_min_slaves_to_write ||
+        !server.repl_min_slaves_max_lag) return;
+
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        client *slave = ln->value;
+        time_t lag = server.unixtime - slave->repl_ack_time;
+
+        if (slave->replstate == SLAVE_STATE_ONLINE &&
+            lag <= server.repl_min_slaves_max_lag) good++;
+    }
+    server.repl_good_slaves_count = good;
+}
+
+/* return true if status of good replicas is OK. otherwise false */
+int checkGoodReplicasStatus(void) {
+    return server.masterhost || /* not a primary status should be OK */
+           !server.repl_min_slaves_max_lag || /* Min slave max lag not configured */
+           !server.repl_min_slaves_to_write || /* Min slave to write not configured */
+           server.repl_good_slaves_count >= server.repl_min_slaves_to_write; /* check if we have enough slaves */
+}
+
+/* ----------------------- SYNCHRONOUS REPLICATION --------------------------
+ * Redis synchronous replication design can be summarized in points:
+ *
+ * - Redis masters have a global replication offset, used by PSYNC.
+ * - Master increment the offset every time new commands are sent to slaves.
+ * - Slaves ping back masters with the offset processed so far.
+ *
+ * So synchronous replication adds a new WAIT command in the form:
+ *
+ *   WAIT <num_replicas> <milliseconds_timeout>
+ *
+ * That returns the number of replicas that processed the query when
+ * we finally have at least num_replicas, or when the timeout was
+ * reached.
+ *
+ * The command is implemented in this way:
+ *
+ * - Every time a client processes a command, we remember the replication
+ *   offset after sending that command to the slaves.
+ * - When WAIT is called, we ask slaves to send an acknowledgement ASAP.
+ *   The client is blocked at the same time (see blocked.c).
+ * - Once we receive enough ACKs for a given offset or when the timeout
+ *   is reached, the WAIT command is unblocked and the reply sent to the
+ *   client.
+ */
+
+/* This just set a flag so that we broadcast a REPLCONF GETACK command
+ * to all the slaves in the beforeSleep() function. Note that this way
+ * we "group" all the clients that want to wait for synchronous replication
+ * in a given event loop iteration, and send a single GETACK for them all. */
+void replicationRequestAckFromSlaves(void) {
+    server.get_ack_from_slaves = 1;
+}
+
+/* Return the number of slaves that already acknowledged the specified
+ * replication offset. */
+int replicationCountAcksByOffset(long long offset) {
+    listIter li;
+    listNode *ln;
+    int count = 0;
+
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        client *slave = ln->value;
+
+        if (slave->replstate != SLAVE_STATE_ONLINE) continue;
+        if (slave->repl_ack_off >= offset) count++;
+    }
+    return count;
+}
+
+/* Return the number of replicas that already acknowledged the specified
+ * replication offset being AOF fsynced. */
+int replicationCountAOFAcksByOffset(long long offset) {
+    listIter li;
+    listNode *ln;
+    int count = 0;
+
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        client *slave = ln->value;
+
+        if (slave->replstate != SLAVE_STATE_ONLINE) continue;
+        if (slave->repl_aof_off >= offset) count++;
+    }
+    return count;
+}
+
+/* WAIT for N replicas to acknowledge the processing of our latest
+ * write command (and all the previous commands). */
+void waitCommand(client *c) {
+    mstime_t timeout;
+    long numreplicas, ackreplicas;
+    long long offset = c->woff;
+
+    if (server.masterhost) {
+        addReplyError(c,"WAIT cannot be used with replica instances. Please also note that since Redis 4.0 if a replica is configured to be writable (which is not the default) writes to replicas are just local and are not propagated.");
+        return;
+    }
+
+    /* Argument parsing. */
+    if (getLongFromObjectOrReply(c,c->argv[1],&numreplicas,NULL) != C_OK)
+        return;
+    if (getTimeoutFromObjectOrReply(c,c->argv[2],&timeout,UNIT_MILLISECONDS)
+        != C_OK) return;
+
+    /* First try without blocking at all. */
+    ackreplicas = replicationCountAcksByOffset(c->woff);
+    if (ackreplicas >= numreplicas || c->flags & CLIENT_DENY_BLOCKING) {
+        addReplyLongLong(c,ackreplicas);
+        return;
+    }
+
+    /* Otherwise block the client and put it into our list of clients
+     * waiting for ack from slaves. */
+    blockForReplication(c,timeout,offset,numreplicas);
+
+    /* Make sure that the server will send an ACK request to all the slaves
+     * before returning to the event loop. */
+    replicationRequestAckFromSlaves();
+}
+
+/* WAIT for N replicas and / or local master to acknowledge our latest
+ * write command got synced to the disk. */
+void waitaofCommand(client *c) {
+    mstime_t timeout;
+    long numreplicas, numlocal, ackreplicas, acklocal;
+
+    /* Argument parsing. */
+    if (getRangeLongFromObjectOrReply(c,c->argv[1],0,1,&numlocal,NULL) != C_OK)
+        return;
+    if (getPositiveLongFromObjectOrReply(c,c->argv[2],&numreplicas,NULL) != C_OK)
+        return;
+    if (getTimeoutFromObjectOrReply(c,c->argv[3],&timeout,UNIT_MILLISECONDS) != C_OK)
+        return;
+
+    if (server.masterhost) {
+        addReplyError(c,"WAITAOF cannot be used with replica instances. Please also note that writes to replicas are just local and are not propagated.");
+        return;
+    }
+    if (numlocal && !server.aof_enabled) {
+        addReplyError(c, "WAITAOF cannot be used when numlocal is set but appendonly is disabled.");
+        return;
+    }
+
+    /* First try without blocking at all. */
+    ackreplicas = replicationCountAOFAcksByOffset(c->woff);
+    acklocal = server.fsynced_reploff >= c->woff;
+    if ((ackreplicas >= numreplicas && acklocal >= numlocal) || c->flags & CLIENT_DENY_BLOCKING) {
+        addReplyArrayLen(c,2);
+        addReplyLongLong(c,acklocal);
+        addReplyLongLong(c,ackreplicas);
+        return;
+    }
+
+    /* Otherwise block the client and put it into our list of clients
+     * waiting for ack from slaves. */
+    blockForAofFsync(c,timeout,c->woff,numlocal,numreplicas);
+
+    /* Make sure that the server will send an ACK request to all the slaves
+     * before returning to the event loop. */
+    replicationRequestAckFromSlaves();
+}
+
+/* This is called by unblockClient() to perform the blocking op type
+ * specific cleanup. We just remove the client from the list of clients
+ * waiting for replica acks. Never call it directly, call unblockClient()
+ * instead. */
+void unblockClientWaitingReplicas(client *c) {
+    listNode *ln = listSearchKey(server.clients_waiting_acks,c);
+    serverAssert(ln != NULL);
+    listDelNode(server.clients_waiting_acks,ln);
+    updateStatsOnUnblock(c, 0, 0, 0);
+}
+
+/* Check if there are clients blocked in WAIT or WAITAOF that can be unblocked
+ * since we received enough ACKs from slaves. */
+void processClientsWaitingReplicas(void) {
+    long long last_offset = 0;
+    long long last_aof_offset = 0;
+    int last_numreplicas = 0;
+    int last_aof_numreplicas = 0;
+
+    listIter li;
+    listNode *ln;
+
+    listRewind(server.clients_waiting_acks,&li);
+    while((ln = listNext(&li))) {
+        int numlocal = 0;
+        int numreplicas = 0;
+
+        client *c = ln->value;
+        int is_wait_aof = c->bstate.btype == BLOCKED_WAITAOF;
+
+        if (is_wait_aof && c->bstate.numlocal && !server.aof_enabled) {
+            addReplyError(c, "WAITAOF cannot be used when numlocal is set but appendonly is disabled.");
+            unblockClient(c, 1);
+            continue;
+        }
+
+        /* Every time we find a client that is satisfied for a given
+         * offset and number of replicas, we remember it so the next client
+         * may be unblocked without calling replicationCountAcksByOffset()
+         * or calling replicationCountAOFAcksByOffset()
+         * if the requested offset / replicas were equal or less. */
+        if (!is_wait_aof && last_offset && last_offset >= c->bstate.reploffset &&
+                           last_numreplicas >= c->bstate.numreplicas)
+        {
+            numreplicas = last_numreplicas;
+        } else if (is_wait_aof && last_aof_offset && last_aof_offset >= c->bstate.reploffset &&
+                    last_aof_numreplicas >= c->bstate.numreplicas)
+        {
+            numreplicas = last_aof_numreplicas;
+        } else {
+            numreplicas = is_wait_aof ?
+                replicationCountAOFAcksByOffset(c->bstate.reploffset) :
+                replicationCountAcksByOffset(c->bstate.reploffset);
+
+            /* Check if the number of replicas is satisfied. */
+            if (numreplicas < c->bstate.numreplicas) continue;
+
+            if (is_wait_aof) {
+                last_aof_offset = c->bstate.reploffset;
+                last_aof_numreplicas = numreplicas;
+            } else {
+                last_offset = c->bstate.reploffset;
+                last_numreplicas = numreplicas;
+            }
+        }
+
+        /* Check if the local constraint of WAITAOF is served */
+        if (is_wait_aof) {
+            numlocal = server.fsynced_reploff >= c->bstate.reploffset;
+            if (numlocal < c->bstate.numlocal) continue;
+        }
+
+        /* Reply before unblocking, because unblock client calls reqresAppendResponse */
+        if (is_wait_aof) {
+            /* WAITAOF has an array reply */
+            addReplyArrayLen(c, 2);
+            addReplyLongLong(c, numlocal);
+            addReplyLongLong(c, numreplicas);
+        } else {
+            addReplyLongLong(c, numreplicas);
+        }
+
+        unblockClient(c, 1);
+    }
+}
+
+/* Return the slave replication offset for this instance, that is
+ * the offset for which we already processed the master replication stream. */
+long long replicationGetSlaveOffset(void) {
+    long long offset = 0;
+
+    if (server.masterhost != NULL) {
+        if (server.master) {
+            offset = server.master->reploff;
+        } else if (server.cached_master) {
+            offset = server.cached_master->reploff;
+        }
+    }
+    /* offset may be -1 when the master does not support it at all, however
+     * this function is designed to return an offset that can express the
+     * amount of data processed by the master, so we return a positive
+     * integer. */
+    if (offset < 0) offset = 0;
+    return offset;
+}
+
+/* --------------------------- REPLICATION CRON  ---------------------------- */
+
+/* Replication cron function, called 1 time per second. */
+void replicationCron(void) {
+    static long long replication_cron_loops = 0;
+
+    /* Check failover status first, to see if we need to start
+     * handling the failover. */
+    updateFailoverStatus();
+
+    /* Non blocking connection timeout? */
+    if (server.masterhost &&
+        (server.repl_state == REPL_STATE_CONNECTING ||
+         slaveIsInHandshakeState()) &&
+         (time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
+    {
+        serverLog(LL_WARNING,"Timeout connecting to the MASTER...");
+        cancelReplicationHandshake(1);
+    }
+
+    /* Bulk transfer I/O timeout? */
+    if (server.masterhost && server.repl_state == REPL_STATE_TRANSFER &&
+        (time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
+    {
+        serverLog(LL_WARNING,"Timeout receiving bulk data from MASTER... If the problem persists try to set the 'repl-timeout' parameter in redis.conf to a larger value.");
+        cancelReplicationHandshake(1);
+    }
+
+    /* Timed out master when we are an already connected slave? */
+    if (server.masterhost && server.repl_state == REPL_STATE_CONNECTED &&
+        (time(NULL)-server.master->lastinteraction) > server.repl_timeout)
+    {
+        serverLog(LL_WARNING,"MASTER timeout: no data nor PING received...");
+        freeClient(server.master);
+    }
+
+    /* Check if we should connect to a MASTER */
+    if (server.repl_state == REPL_STATE_CONNECT) {
+        serverLog(LL_NOTICE,"Connecting to MASTER %s:%d",
+            server.masterhost, server.masterport);
+        connectWithMaster();
+    }
+
+    /* Send ACK to master from time to time.
+     * Note that we do not send periodic acks to masters that don't
+     * support PSYNC and replication offsets. */
+    if (server.masterhost && server.master &&
+        !(server.master->flags & CLIENT_PRE_PSYNC))
+        replicationSendAck();
+
+    /* If we have attached slaves, PING them from time to time.
+     * So slaves can implement an explicit timeout to masters, and will
+     * be able to detect a link disconnection even if the TCP connection
+     * will not actually go down. */
+    listIter li;
+    listNode *ln;
+    robj *ping_argv[1];
+
+    /* First, send PING according to ping_slave_period. */
+    if ((replication_cron_loops % server.repl_ping_slave_period) == 0 &&
+        listLength(server.slaves))
+    {
+        /* Note that we don't send the PING if the clients are paused during
+         * a Redis Cluster manual failover: the PING we send will otherwise
+         * alter the replication offsets of master and slave, and will no longer
+         * match the one stored into 'mf_master_offset' state. */
+        int manual_failover_in_progress =
+            ((server.cluster_enabled &&
+              server.cluster->mf_end) ||
+            server.failover_end_time) &&
+            isPausedActionsWithUpdate(PAUSE_ACTION_REPLICA);
+
+        if (!manual_failover_in_progress) {
+            ping_argv[0] = shared.ping;
+            replicationFeedSlaves(server.slaves, -1,
+                ping_argv, 1);
+        }
+    }
+
+    /* Second, send a newline to all the slaves in pre-synchronization
+     * stage, that is, slaves waiting for the master to create the RDB file.
+     *
+     * Also send the a newline to all the chained slaves we have, if we lost
+     * connection from our master, to keep the slaves aware that their
+     * master is online. This is needed since sub-slaves only receive proxied
+     * data from top-level masters, so there is no explicit pinging in order
+     * to avoid altering the replication offsets. This special out of band
+     * pings (newlines) can be sent, they will have no effect in the offset.
+     *
+     * The newline will be ignored by the slave but will refresh the
+     * last interaction timer preventing a timeout. In this case we ignore the
+     * ping period and refresh the connection once per second since certain
+     * timeouts are set at a few seconds (example: PSYNC response). */
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        client *slave = ln->value;
+
+        int is_presync =
+            (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START ||
+            (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END &&
+             server.rdb_child_type != RDB_CHILD_TYPE_SOCKET));
+
+        if (is_presync) {
+            connWrite(slave->conn, "\n", 1);
+        }
+    }
+
+    /* Disconnect timedout slaves. */
+    if (listLength(server.slaves)) {
+        listIter li;
+        listNode *ln;
+
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            client *slave = ln->value;
+
+            if (slave->replstate == SLAVE_STATE_ONLINE) {
+                if (slave->flags & CLIENT_PRE_PSYNC)
+                    continue;
+                if ((server.unixtime - slave->repl_ack_time) > server.repl_timeout) {
+                    serverLog(LL_WARNING, "Disconnecting timedout replica (streaming sync): %s",
+                          replicationGetSlaveName(slave));
+                    freeClient(slave);
+                    continue;
+                }
+            }
+            /* We consider disconnecting only diskless replicas because disk-based replicas aren't fed
+             * by the fork child so if a disk-based replica is stuck it doesn't prevent the fork child
+             * from terminating. */
+            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END && server.rdb_child_type == RDB_CHILD_TYPE_SOCKET) {
+                if (slave->repl_last_partial_write != 0 &&
+                    (server.unixtime - slave->repl_last_partial_write) > server.repl_timeout)
+                {
+                    serverLog(LL_WARNING, "Disconnecting timedout replica (full sync): %s",
+                          replicationGetSlaveName(slave));
+                    freeClient(slave);
+                    continue;
+                }
+            }
+        }
+    }
+
+    /* If this is a master without attached slaves and there is a replication
+     * backlog active, in order to reclaim memory we can free it after some
+     * (configured) time. Note that this cannot be done for slaves: slaves
+     * without sub-slaves attached should still accumulate data into the
+     * backlog, in order to reply to PSYNC queries if they are turned into
+     * masters after a failover. */
+    if (listLength(server.slaves) == 0 && server.repl_backlog_time_limit &&
+        server.repl_backlog && server.masterhost == NULL)
+    {
+        time_t idle = server.unixtime - server.repl_no_slaves_since;
+
+        if (idle > server.repl_backlog_time_limit) {
+            /* When we free the backlog, we always use a new
+             * replication ID and clear the ID2. This is needed
+             * because when there is no backlog, the master_repl_offset
+             * is not updated, but we would still retain our replication
+             * ID, leading to the following problem:
+             *
+             * 1. We are a master instance.
+             * 2. Our slave is promoted to master. It's repl-id-2 will
+             *    be the same as our repl-id.
+             * 3. We, yet as master, receive some updates, that will not
+             *    increment the master_repl_offset.
+             * 4. Later we are turned into a slave, connect to the new
+             *    master that will accept our PSYNC request by second
+             *    replication ID, but there will be data inconsistency
+             *    because we received writes. */
+            changeReplicationId();
+            clearReplicationId2();
+            freeReplicationBacklog();
+            serverLog(LL_NOTICE,
+                "Replication backlog freed after %d seconds "
+                "without connected replicas.",
+                (int) server.repl_backlog_time_limit);
+        }
+    }
+
+    replicationStartPendingFork();
+
+    /* Remove the RDB file used for replication if Redis is not running
+     * with any persistence. */
+    removeRDBUsedToSyncReplicas();
+
+    /* Sanity check replication buffer, the first block of replication buffer blocks
+     * must be referenced by someone, since it will be freed when not referenced,
+     * otherwise, server will OOM. also, its refcount must not be more than
+     * replicas number + 1(replication backlog). */
+    if (listLength(server.repl_buffer_blocks) > 0) {
+        replBufBlock *o = listNodeValue(listFirst(server.repl_buffer_blocks));
+        serverAssert(o->refcount > 0 &&
+            o->refcount <= (int)listLength(server.slaves)+1);
+    }
+
+    /* Refresh the number of slaves with lag <= min-slaves-max-lag. */
+    refreshGoodSlavesCount();
+    replication_cron_loops++; /* Incremented with frequency 1 HZ. */
+}
+
+int shouldStartChildReplication(int *mincapa_out, int *req_out) {
+    /* We should start a BGSAVE good for replication if we have slaves in
+     * WAIT_BGSAVE_START state.
+     *
+     * In case of diskless replication, we make sure to wait the specified
+     * number of seconds (according to configuration) so that other slaves
+     * have the time to arrive before we start streaming. */
+    if (!hasActiveChildProcess()) {
+        time_t idle, max_idle = 0;
+        int slaves_waiting = 0;
+        int mincapa;
+        int req;
+        int first = 1;
+        listNode *ln;
+        listIter li;
+
+        listRewind(server.slaves,&li);
+        while((ln = listNext(&li))) {
+            client *slave = ln->value;
+            if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
+                if (first) {
+                    /* Get first slave's requirements */
+                    req = slave->slave_req;
+                } else if (req != slave->slave_req) {
+                    /* Skip slaves that don't match */
+                    continue;
+                }
+                idle = server.unixtime - slave->lastinteraction;
+                if (idle > max_idle) max_idle = idle;
+                slaves_waiting++;
+                mincapa = first ? slave->slave_capa : (mincapa & slave->slave_capa);
+                first = 0;
+            }
+        }
+
+        if (slaves_waiting &&
+            (!server.repl_diskless_sync ||
+             (server.repl_diskless_sync_max_replicas > 0 &&
+              slaves_waiting >= server.repl_diskless_sync_max_replicas) ||
+             max_idle >= server.repl_diskless_sync_delay))
+        {
+            if (mincapa_out)
+                *mincapa_out = mincapa;
+            if (req_out)
+                *req_out = req;
+            return 1;
+        }
+    }
+
+    return 0;
+}
+
+void replicationStartPendingFork(void) {
+    int mincapa = -1;
+    int req = -1;
+
+    if (shouldStartChildReplication(&mincapa, &req)) {
+        /* Start the BGSAVE. The called function may start a
+         * BGSAVE with socket target or disk target depending on the
+         * configuration and slaves capabilities and requirements. */
+        startBgsaveForReplication(mincapa, req);
+    }
+}
+
+/* Find replica at IP:PORT from replica list */
+static client *findReplica(char *host, int port) {
+    listIter li;
+    listNode *ln;
+    client *replica;
+
+    listRewind(server.slaves,&li);
+    while((ln = listNext(&li))) {
+        replica = ln->value;
+        char ip[NET_IP_STR_LEN], *replicaip = replica->slave_addr;
+
+        if (!replicaip) {
+            if (connAddrPeerName(replica->conn, ip, sizeof(ip), NULL) == -1)
+                continue;
+            replicaip = ip;
+        }
+
+        if (!strcasecmp(host, replicaip) &&
+                (port == replica->slave_listening_port))
+            return replica;
+    }
+
+    return NULL;
+}
+
+const char *getFailoverStateString(void) {
+    switch(server.failover_state) {
+        case NO_FAILOVER: return "no-failover";
+        case FAILOVER_IN_PROGRESS: return "failover-in-progress";
+        case FAILOVER_WAIT_FOR_SYNC: return "waiting-for-sync";
+        default: return "unknown";
+    }
+}
+
+/* Resets the internal failover configuration, this needs
+ * to be called after a failover either succeeds or fails
+ * as it includes the client unpause. */
+void clearFailoverState(void) {
+    server.failover_end_time = 0;
+    server.force_failover = 0;
+    zfree(server.target_replica_host);
+    server.target_replica_host = NULL;
+    server.target_replica_port = 0;
+    server.failover_state = NO_FAILOVER;
+    unpauseActions(PAUSE_DURING_FAILOVER);
+}
+
+/* Abort an ongoing failover if one is going on. */
+void abortFailover(const char *err) {
+    if (server.failover_state == NO_FAILOVER) return;
+
+    if (server.target_replica_host) {
+        serverLog(LL_NOTICE,"FAILOVER to %s:%d aborted: %s",
+            server.target_replica_host,server.target_replica_port,err);  
+    } else {
+        serverLog(LL_NOTICE,"FAILOVER to any replica aborted: %s",err);  
+    }
+    if (server.failover_state == FAILOVER_IN_PROGRESS) {
+        replicationUnsetMaster();
+    }
+    clearFailoverState();
+}
+
+/* 
+ * FAILOVER [TO <HOST> <PORT> [FORCE]] [ABORT] [TIMEOUT <timeout>]
+ * 
+ * This command will coordinate a failover between the master and one
+ * of its replicas. The happy path contains the following steps:
+ * 1) The master will initiate a client pause write, to stop replication
+ * traffic.
+ * 2) The master will periodically check if any of its replicas has
+ * consumed the entire replication stream through acks. 
+ * 3) Once any replica has caught up, the master will itself become a replica.
+ * 4) The master will send a PSYNC FAILOVER request to the target replica, which
+ * if accepted will cause the replica to become the new master and start a sync.
+ * 
+ * FAILOVER ABORT is the only way to abort a failover command, as replicaof
+ * will be disabled. This may be needed if the failover is unable to progress. 
+ * 
+ * The optional arguments [TO <HOST> <IP>] allows designating a specific replica
+ * to be failed over to.
+ * 
+ * FORCE flag indicates that even if the target replica is not caught up,
+ * failover to it anyway. This must be specified with a timeout and a target
+ * HOST and IP.
+ * 
+ * TIMEOUT <timeout> indicates how long should the primary wait for 
+ * a replica to sync up before aborting. If not specified, the failover
+ * will attempt forever and must be manually aborted.
+ */
+void failoverCommand(client *c) {
+    if (server.cluster_enabled) {
+        addReplyError(c,"FAILOVER not allowed in cluster mode. "
+                        "Use CLUSTER FAILOVER command instead.");
+        return;
+    }
+    
+    /* Handle special case for abort */
+    if ((c->argc == 2) && !strcasecmp(c->argv[1]->ptr,"abort")) {
+        if (server.failover_state == NO_FAILOVER) {
+            addReplyError(c, "No failover in progress.");
+            return;
+        }
+
+        abortFailover("Failover manually aborted");
+        addReply(c,shared.ok);
+        return;
+    }
+
+    long timeout_in_ms = 0;
+    int force_flag = 0;
+    long port = 0;
+    char *host = NULL;
+
+    /* Parse the command for syntax and arguments. */
+    for (int j = 1; j < c->argc; j++) {
+        if (!strcasecmp(c->argv[j]->ptr,"timeout") && (j + 1 < c->argc) &&
+            timeout_in_ms == 0)
+        {
+            if (getLongFromObjectOrReply(c,c->argv[j + 1],
+                        &timeout_in_ms,NULL) != C_OK) return;
+            if (timeout_in_ms <= 0) {
+                addReplyError(c,"FAILOVER timeout must be greater than 0");
+                return;
+            }
+            j++;
+        } else if (!strcasecmp(c->argv[j]->ptr,"to") && (j + 2 < c->argc) &&
+            !host) 
+        {
+            if (getLongFromObjectOrReply(c,c->argv[j + 2],&port,NULL) != C_OK)
+                return;
+            host = c->argv[j + 1]->ptr;
+            j += 2;
+        } else if (!strcasecmp(c->argv[j]->ptr,"force") && !force_flag) {
+            force_flag = 1;
+        } else {
+            addReplyErrorObject(c,shared.syntaxerr);
+            return;
+        }
+    }
+
+    if (server.failover_state != NO_FAILOVER) {
+        addReplyError(c,"FAILOVER already in progress.");
+        return;
+    }
+
+    if (server.masterhost) {
+        addReplyError(c,"FAILOVER is not valid when server is a replica.");
+        return;
+    }
+
+    if (listLength(server.slaves) == 0) {
+        addReplyError(c,"FAILOVER requires connected replicas.");
+        return; 
+    }
+
+    if (force_flag && (!timeout_in_ms || !host)) {
+        addReplyError(c,"FAILOVER with force option requires both a timeout "
+            "and target HOST and IP.");
+        return;     
+    }
+
+    /* If a replica address was provided, validate that it is connected. */
+    if (host) {
+        client *replica = findReplica(host, port);
+
+        if (replica == NULL) {
+            addReplyError(c,"FAILOVER target HOST and PORT is not "
+                            "a replica.");
+            return;
+        }
+
+        /* Check if requested replica is online */
+        if (replica->replstate != SLAVE_STATE_ONLINE) {
+            addReplyError(c,"FAILOVER target replica is not online.");
+            return;
+        }
+
+        server.target_replica_host = zstrdup(host);
+        server.target_replica_port = port;
+        serverLog(LL_NOTICE,"FAILOVER requested to %s:%ld.",host,port);
+    } else {
+        serverLog(LL_NOTICE,"FAILOVER requested to any replica.");
+    }
+
+    mstime_t now = commandTimeSnapshot();
+    if (timeout_in_ms) {
+        server.failover_end_time = now + timeout_in_ms;
+    }
+    
+    server.force_failover = force_flag;
+    server.failover_state = FAILOVER_WAIT_FOR_SYNC;
+    /* Cluster failover will unpause eventually */
+    pauseActions(PAUSE_DURING_FAILOVER,
+                 LLONG_MAX,
+                 PAUSE_ACTIONS_CLIENT_WRITE_SET);
+    addReply(c,shared.ok);
+}
+
+/* Failover cron function, checks coordinated failover state. 
+ *
+ * Implementation note: The current implementation calls replicationSetMaster()
+ * to start the failover request, this has some unintended side effects if the
+ * failover doesn't work like blocked clients will be unblocked and replicas will
+ * be disconnected. This could be optimized further.
+ */
+void updateFailoverStatus(void) {
+    if (server.failover_state != FAILOVER_WAIT_FOR_SYNC) return;
+    mstime_t now = server.mstime;
+
+    /* Check if failover operation has timed out */
+    if (server.failover_end_time && server.failover_end_time <= now) {
+        if (server.force_failover) {
+            serverLog(LL_NOTICE,
+                "FAILOVER to %s:%d time out exceeded, failing over.",
+                server.target_replica_host, server.target_replica_port);
+            server.failover_state = FAILOVER_IN_PROGRESS;
+            /* If timeout has expired force a failover if requested. */
+            replicationSetMaster(server.target_replica_host,
+                server.target_replica_port);
+            return;
+        } else {
+            /* Force was not requested, so timeout. */
+            abortFailover("Replica never caught up before timeout");
+            return;
+        }
+    }
+
+    /* Check to see if the replica has caught up so failover can start */
+    client *replica = NULL;
+    if (server.target_replica_host) {
+        replica = findReplica(server.target_replica_host, 
+            server.target_replica_port);
+    } else {
+        listIter li;
+        listNode *ln;
+
+        listRewind(server.slaves,&li);
+        /* Find any replica that has matched our repl_offset */
+        while((ln = listNext(&li))) {
+            replica = ln->value;
+            if (replica->repl_ack_off == server.master_repl_offset) {
+                char ip[NET_IP_STR_LEN], *replicaaddr = replica->slave_addr;
+
+                if (!replicaaddr) {
+                    if (connAddrPeerName(replica->conn,ip,sizeof(ip),NULL) == -1)
+                        continue;
+                    replicaaddr = ip;
+                }
+
+                /* We are now failing over to this specific node */
+                server.target_replica_host = zstrdup(replicaaddr);
+                server.target_replica_port = replica->slave_listening_port;
+                break;
+            }
+        }
+    }
+
+    /* We've found a replica that is caught up */
+    if (replica && (replica->repl_ack_off == server.master_repl_offset)) {
+        server.failover_state = FAILOVER_IN_PROGRESS;
+        serverLog(LL_NOTICE,
+                "Failover target %s:%d is synced, failing over.",
+                server.target_replica_host, server.target_replica_port);
+        /* Designated replica is caught up, failover to it. */
+        replicationSetMaster(server.target_replica_host,
+            server.target_replica_port);
+    }
+}