1 files changed, 300 insertions, 0 deletions
diff --git a/src/backend/postmaster/walwriter.c b/src/backend/postmaster/walwriter.c
new file mode 100644
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+/*-------------------------------------------------------------------------
+ *
+ * walwriter.c
+ *
+ * The WAL writer background process is new as of Postgres 8.3.  It attempts
+ * to keep regular backends from having to write out (and fsync) WAL pages.
+ * Also, it guarantees that transaction commit records that weren't synced
+ * to disk immediately upon commit (ie, were "asynchronously committed")
+ * will reach disk within a knowable time --- which, as it happens, is at
+ * most three times the wal_writer_delay cycle time.
+ *
+ * Note that as with the bgwriter for shared buffers, regular backends are
+ * still empowered to issue WAL writes and fsyncs when the walwriter doesn't
+ * keep up. This means that the WALWriter is not an essential process and
+ * can shutdown quickly when requested.
+ *
+ * Because the walwriter's cycle is directly linked to the maximum delay
+ * before async-commit transactions are guaranteed committed, it's probably
+ * unwise to load additional functionality onto it.  For instance, if you've
+ * got a yen to create xlog segments further in advance, that'd be better done
+ * in bgwriter than in walwriter.
+ *
+ * The walwriter is started by the postmaster as soon as the startup subprocess
+ * finishes.  It remains alive until the postmaster commands it to terminate.
+ * Normal termination is by SIGTERM, which instructs the walwriter to exit(0).
+ * Emergency termination is by SIGQUIT; like any backend, the walwriter will
+ * simply abort and exit on SIGQUIT.
+ *
+ * If the walwriter exits unexpectedly, the postmaster treats that the same
+ * as a backend crash: shared memory may be corrupted, so remaining backends
+ * should be killed by SIGQUIT and then a recovery cycle started.
+ *
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/postmaster/walwriter.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <signal.h>
+#include <unistd.h>
+
+#include "access/xlog.h"
+#include "libpq/pqsignal.h"
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "postmaster/interrupt.h"
+#include "postmaster/walwriter.h"
+#include "storage/bufmgr.h"
+#include "storage/condition_variable.h"
+#include "storage/fd.h"
+#include "storage/ipc.h"
+#include "storage/lwlock.h"
+#include "storage/proc.h"
+#include "storage/procsignal.h"
+#include "storage/smgr.h"
+#include "utils/guc.h"
+#include "utils/hsearch.h"
+#include "utils/memutils.h"
+#include "utils/resowner.h"
+
+
+/*
+ * GUC parameters
+ */
+int			WalWriterDelay = 200;
+int			WalWriterFlushAfter = DEFAULT_WAL_WRITER_FLUSH_AFTER;
+
+/*
+ * Number of do-nothing loops before lengthening the delay time, and the
+ * multiplier to apply to WalWriterDelay when we do decide to hibernate.
+ * (Perhaps these need to be configurable?)
+ */
+#define LOOPS_UNTIL_HIBERNATE		50
+#define HIBERNATE_FACTOR			25
+
+/* Prototypes for private functions */
+static void HandleWalWriterInterrupts(void);
+
+/*
+ * Main entry point for walwriter process
+ *
+ * This is invoked from AuxiliaryProcessMain, which has already created the
+ * basic execution environment, but not enabled signals yet.
+ */
+void
+WalWriterMain(void)
+{
+	sigjmp_buf	local_sigjmp_buf;
+	MemoryContext walwriter_context;
+	int			left_till_hibernate;
+	bool		hibernating;
+
+	/*
+	 * Properly accept or ignore signals the postmaster might send us
+	 *
+	 * We have no particular use for SIGINT at the moment, but seems
+	 * reasonable to treat like SIGTERM.
+	 */
+	pqsignal(SIGHUP, SignalHandlerForConfigReload);
+	pqsignal(SIGINT, SignalHandlerForShutdownRequest);
+	pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
+	/* SIGQUIT handler was already set up by InitPostmasterChild */
+	pqsignal(SIGALRM, SIG_IGN);
+	pqsignal(SIGPIPE, SIG_IGN);
+	pqsignal(SIGUSR1, procsignal_sigusr1_handler);
+	pqsignal(SIGUSR2, SIG_IGN); /* not used */
+
+	/*
+	 * Reset some signals that are accepted by postmaster but not here
+	 */
+	pqsignal(SIGCHLD, SIG_DFL);
+
+	/*
+	 * Create a memory context that we will do all our work in.  We do this so
+	 * that we can reset the context during error recovery and thereby avoid
+	 * possible memory leaks.  Formerly this code just ran in
+	 * TopMemoryContext, but resetting that would be a really bad idea.
+	 */
+	walwriter_context = AllocSetContextCreate(TopMemoryContext,
+											  "Wal Writer",
+											  ALLOCSET_DEFAULT_SIZES);
+	MemoryContextSwitchTo(walwriter_context);
+
+	/*
+	 * If an exception is encountered, processing resumes here.
+	 *
+	 * You might wonder why this isn't coded as an infinite loop around a
+	 * PG_TRY construct.  The reason is that this is the bottom of the
+	 * exception stack, and so with PG_TRY there would be no exception handler
+	 * in force at all during the CATCH part.  By leaving the outermost setjmp
+	 * always active, we have at least some chance of recovering from an error
+	 * during error recovery.  (If we get into an infinite loop thereby, it
+	 * will soon be stopped by overflow of elog.c's internal state stack.)
+	 *
+	 * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
+	 * (to wit, BlockSig) will be restored when longjmp'ing to here.  Thus,
+	 * signals other than SIGQUIT will be blocked until we complete error
+	 * recovery.  It might seem that this policy makes the HOLD_INTERRUPTS()
+	 * call redundant, but it is not since InterruptPending might be set
+	 * already.
+	 */
+	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
+	{
+		/* Since not using PG_TRY, must reset error stack by hand */
+		error_context_stack = NULL;
+
+		/* Prevent interrupts while cleaning up */
+		HOLD_INTERRUPTS();
+
+		/* Report the error to the server log */
+		EmitErrorReport();
+
+		/*
+		 * These operations are really just a minimal subset of
+		 * AbortTransaction().  We don't have very many resources to worry
+		 * about in walwriter, but we do have LWLocks, and perhaps buffers?
+		 */
+		LWLockReleaseAll();
+		ConditionVariableCancelSleep();
+		pgstat_report_wait_end();
+		UnlockBuffers();
+		ReleaseAuxProcessResources(false);
+		AtEOXact_Buffers(false);
+		AtEOXact_SMgr();
+		AtEOXact_Files(false);
+		AtEOXact_HashTables(false);
+
+		/*
+		 * Now return to normal top-level context and clear ErrorContext for
+		 * next time.
+		 */
+		MemoryContextSwitchTo(walwriter_context);
+		FlushErrorState();
+
+		/* Flush any leaked data in the top-level context */
+		MemoryContextResetAndDeleteChildren(walwriter_context);
+
+		/* Now we can allow interrupts again */
+		RESUME_INTERRUPTS();
+
+		/*
+		 * Sleep at least 1 second after any error.  A write error is likely
+		 * to be repeated, and we don't want to be filling the error logs as
+		 * fast as we can.
+		 */
+		pg_usleep(1000000L);
+
+		/*
+		 * Close all open files after any error.  This is helpful on Windows,
+		 * where holding deleted files open causes various strange errors.
+		 * It's not clear we need it elsewhere, but shouldn't hurt.
+		 */
+		smgrcloseall();
+	}
+
+	/* We can now handle ereport(ERROR) */
+	PG_exception_stack = &local_sigjmp_buf;
+
+	/*
+	 * Unblock signals (they were blocked when the postmaster forked us)
+	 */
+	sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
+
+	/*
+	 * Reset hibernation state after any error.
+	 */
+	left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
+	hibernating = false;
+	SetWalWriterSleeping(false);
+
+	/*
+	 * Advertise our latch that backends can use to wake us up while we're
+	 * sleeping.
+	 */
+	ProcGlobal->walwriterLatch = &MyProc->procLatch;
+
+	/*
+	 * Loop forever
+	 */
+	for (;;)
+	{
+		long		cur_timeout;
+
+		/*
+		 * Advertise whether we might hibernate in this cycle.  We do this
+		 * before resetting the latch to ensure that any async commits will
+		 * see the flag set if they might possibly need to wake us up, and
+		 * that we won't miss any signal they send us.  (If we discover work
+		 * to do in the last cycle before we would hibernate, the global flag
+		 * will be set unnecessarily, but little harm is done.)  But avoid
+		 * touching the global flag if it doesn't need to change.
+		 */
+		if (hibernating != (left_till_hibernate <= 1))
+		{
+			hibernating = (left_till_hibernate <= 1);
+			SetWalWriterSleeping(hibernating);
+		}
+
+		/* Clear any already-pending wakeups */
+		ResetLatch(MyLatch);
+
+		/* Process any signals received recently */
+		HandleWalWriterInterrupts();
+
+		/*
+		 * Do what we're here for; then, if XLogBackgroundFlush() found useful
+		 * work to do, reset hibernation counter.
+		 */
+		if (XLogBackgroundFlush())
+			left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
+		else if (left_till_hibernate > 0)
+			left_till_hibernate--;
+
+		/* report pending statistics to the cumulative stats system */
+		pgstat_report_wal(false);
+
+		/*
+		 * Sleep until we are signaled or WalWriterDelay has elapsed.  If we
+		 * haven't done anything useful for quite some time, lengthen the
+		 * sleep time so as to reduce the server's idle power consumption.
+		 */
+		if (left_till_hibernate > 0)
+			cur_timeout = WalWriterDelay;	/* in ms */
+		else
+			cur_timeout = WalWriterDelay * HIBERNATE_FACTOR;
+
+		(void) WaitLatch(MyLatch,
+						 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
+						 cur_timeout,
+						 WAIT_EVENT_WAL_WRITER_MAIN);
+	}
+}
+
+/*
+ * Interrupt handler for main loops of WAL writer process.
+ */
+static void
+HandleWalWriterInterrupts(void)
+{
+	if (ProcSignalBarrierPending)
+		ProcessProcSignalBarrier();
+
+	if (ConfigReloadPending)
+	{
+		ConfigReloadPending = false;
+		ProcessConfigFile(PGC_SIGHUP);
+	}
+
+	if (ShutdownRequestPending)
+		proc_exit(0);
+
+	/* Perform logging of memory contexts of this process */
+	if (LogMemoryContextPending)
+		ProcessLogMemoryContextInterrupt();
+}