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diff --git a/src/master/mail_flow.c b/src/master/mail_flow.c
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+/*++
+/* NAME
+/* mail_flow 3
+/* SUMMARY
+/* global mail flow control
+/* SYNOPSIS
+/* #include <mail_flow.h>
+/*
+/* ssize_t mail_flow_get(count)
+/* ssize_t count;
+/*
+/* ssize_t mail_flow_put(count)
+/* ssize_t count;
+/*
+/* ssize_t mail_flow_count()
+/* DESCRIPTION
+/* This module implements a simple flow control mechanism that
+/* is based on tokens that are consumed by mail receiving processes
+/* and that are produced by mail sending processes.
+/*
+/* mail_flow_get() attempts to read specified number of tokens. The
+/* result is > 0 for success, < 0 for failure. In the latter case,
+/* the process is expected to slow down a little.
+/*
+/* mail_flow_put() produces the specified number of tokens. The
+/* token producing process is expected to produce new tokens
+/* whenever it falls idle and no more tokens are available.
+/*
+/* mail_flow_count() returns the number of available tokens.
+/* BUGS
+/* The producer needs to wake up periodically to ensure that
+/* tokens are not lost due to leakage.
+/* LICENSE
+/* .ad
+/* .fi
+/* The Secure Mailer license must be distributed with this software.
+/* AUTHOR(S)
+/* Wietse Venema
+/* IBM T.J. Watson Research
+/* P.O. Box 704
+/* Yorktown Heights, NY 10598, USA
+/*--*/
+
+/* System library. */
+
+#include <sys_defs.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* Utility library. */
+
+#include <msg.h>
+#include <iostuff.h>
+#include <warn_stat.h>
+
+/* Global library. */
+
+#include <mail_flow.h>
+
+/* Master library. */
+
+#include <master_proto.h>
+
+#define BUFFER_SIZE 1024
+
+/* mail_flow_get - read N tokens */
+
+ssize_t mail_flow_get(ssize_t len)
+{
+ const char *myname = "mail_flow_get";
+ char buf[BUFFER_SIZE];
+ struct stat st;
+ ssize_t count;
+ ssize_t n = 0;
+
+ /*
+ * Sanity check.
+ */
+ if (len <= 0)
+ msg_panic("%s: bad length %ld", myname, (long) len);
+
+ /*
+ * Silence some wild claims.
+ */
+ if (fstat(MASTER_FLOW_WRITE, &st) < 0)
+ msg_fatal("fstat flow pipe write descriptor: %m");
+
+ /*
+ * Read and discard N bytes. XXX AIX read() can return 0 when an open
+ * pipe is empty.
+ */
+ for (count = len; count > 0; count -= n)
+ if ((n = read(MASTER_FLOW_READ, buf, count > BUFFER_SIZE ?
+ BUFFER_SIZE : count)) <= 0)
+ return (-1);
+ if (msg_verbose)
+ msg_info("%s: %ld %ld", myname, (long) len, (long) (len - count));
+ return (len - count);
+}
+
+/* mail_flow_put - put N tokens */
+
+ssize_t mail_flow_put(ssize_t len)
+{
+ const char *myname = "mail_flow_put";
+ char buf[BUFFER_SIZE];
+ ssize_t count;
+ ssize_t n = 0;
+
+ /*
+ * Sanity check.
+ */
+ if (len <= 0)
+ msg_panic("%s: bad length %ld", myname, (long) len);
+
+ /*
+ * Write or discard N bytes.
+ */
+ memset(buf, 0, len > BUFFER_SIZE ? BUFFER_SIZE : len);
+
+ for (count = len; count > 0; count -= n)
+ if ((n = write(MASTER_FLOW_WRITE, buf, count > BUFFER_SIZE ?
+ BUFFER_SIZE : count)) < 0)
+ return (-1);
+ if (msg_verbose)
+ msg_info("%s: %ld %ld", myname, (long) len, (long) (len - count));
+ return (len - count);
+}
+
+/* mail_flow_count - return number of available tokens */
+
+ssize_t mail_flow_count(void)
+{
+ const char *myname = "mail_flow_count";
+ ssize_t count;
+
+ if ((count = peekfd(MASTER_FLOW_READ)) < 0)
+ msg_warn("%s: %m", myname);
+ return (count);
+}