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Functions
void	WalWriterMain (void) pg_attribute_noreturn()

Variables
PGDLLIMPORT int	WalWriterDelay

PGDLLIMPORT int	WalWriterFlushAfter

Function Documentation

◆ WalWriterMain()

void WalWriterMain ( void )

Definition at line 91 of file walwriter.c.

 {
     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();
         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)
      */
     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);
     }
 }