text: /*-------------------------------------------------------------------------
   *
   * 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) 1996-2020, 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 = 128;
  
  /*
   * 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
  
  /*
   * 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);
  	pqsignal(SIGQUIT, SignalHandlerForCrashExit);
  	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);
  
  	/* We allow SIGQUIT (SignalHandlerForCrashExit) at all times */
  	sigdelset(&BlockSig, SIGQUIT);
  	PG_SETMASK(&BlockSig);
  
  	/*
  	 * 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();
  		AbortBufferIO();
  		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)
  	 */
  	PG_SETMASK(&UnBlockSig);
  
  	/*
  	 * 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);
  
  		HandleMainLoopInterrupts();
  
  		/*
  		 * 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--;
  
  		/*
  		 * 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);
  	}
  }
  