LCOV - code coverage report
Current view: top level - src/backend/postmaster - walwriter.c (source / functions) Hit Total Coverage
Test: PostgreSQL 13devel Lines: 55 76 72.4 %
Date: 2019-09-22 08:06:49 Functions: 4 5 80.0 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : /*-------------------------------------------------------------------------
       2             :  *
       3             :  * walwriter.c
       4             :  *
       5             :  * The WAL writer background process is new as of Postgres 8.3.  It attempts
       6             :  * to keep regular backends from having to write out (and fsync) WAL pages.
       7             :  * Also, it guarantees that transaction commit records that weren't synced
       8             :  * to disk immediately upon commit (ie, were "asynchronously committed")
       9             :  * will reach disk within a knowable time --- which, as it happens, is at
      10             :  * most three times the wal_writer_delay cycle time.
      11             :  *
      12             :  * Note that as with the bgwriter for shared buffers, regular backends are
      13             :  * still empowered to issue WAL writes and fsyncs when the walwriter doesn't
      14             :  * keep up. This means that the WALWriter is not an essential process and
      15             :  * can shutdown quickly when requested.
      16             :  *
      17             :  * Because the walwriter's cycle is directly linked to the maximum delay
      18             :  * before async-commit transactions are guaranteed committed, it's probably
      19             :  * unwise to load additional functionality onto it.  For instance, if you've
      20             :  * got a yen to create xlog segments further in advance, that'd be better done
      21             :  * in bgwriter than in walwriter.
      22             :  *
      23             :  * The walwriter is started by the postmaster as soon as the startup subprocess
      24             :  * finishes.  It remains alive until the postmaster commands it to terminate.
      25             :  * Normal termination is by SIGTERM, which instructs the walwriter to exit(0).
      26             :  * Emergency termination is by SIGQUIT; like any backend, the walwriter will
      27             :  * simply abort and exit on SIGQUIT.
      28             :  *
      29             :  * If the walwriter exits unexpectedly, the postmaster treats that the same
      30             :  * as a backend crash: shared memory may be corrupted, so remaining backends
      31             :  * should be killed by SIGQUIT and then a recovery cycle started.
      32             :  *
      33             :  *
      34             :  * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
      35             :  *
      36             :  *
      37             :  * IDENTIFICATION
      38             :  *    src/backend/postmaster/walwriter.c
      39             :  *
      40             :  *-------------------------------------------------------------------------
      41             :  */
      42             : #include "postgres.h"
      43             : 
      44             : #include <signal.h>
      45             : #include <unistd.h>
      46             : 
      47             : #include "access/xlog.h"
      48             : #include "libpq/pqsignal.h"
      49             : #include "miscadmin.h"
      50             : #include "pgstat.h"
      51             : #include "postmaster/walwriter.h"
      52             : #include "storage/bufmgr.h"
      53             : #include "storage/condition_variable.h"
      54             : #include "storage/fd.h"
      55             : #include "storage/ipc.h"
      56             : #include "storage/lwlock.h"
      57             : #include "storage/proc.h"
      58             : #include "storage/smgr.h"
      59             : #include "utils/guc.h"
      60             : #include "utils/hsearch.h"
      61             : #include "utils/memutils.h"
      62             : #include "utils/resowner.h"
      63             : 
      64             : 
      65             : /*
      66             :  * GUC parameters
      67             :  */
      68             : int         WalWriterDelay = 200;
      69             : int         WalWriterFlushAfter = 128;
      70             : 
      71             : /*
      72             :  * Number of do-nothing loops before lengthening the delay time, and the
      73             :  * multiplier to apply to WalWriterDelay when we do decide to hibernate.
      74             :  * (Perhaps these need to be configurable?)
      75             :  */
      76             : #define LOOPS_UNTIL_HIBERNATE       50
      77             : #define HIBERNATE_FACTOR            25
      78             : 
      79             : /*
      80             :  * Flags set by interrupt handlers for later service in the main loop.
      81             :  */
      82             : static volatile sig_atomic_t got_SIGHUP = false;
      83             : static volatile sig_atomic_t shutdown_requested = false;
      84             : 
      85             : /* Signal handlers */
      86             : static void wal_quickdie(SIGNAL_ARGS);
      87             : static void WalSigHupHandler(SIGNAL_ARGS);
      88             : static void WalShutdownHandler(SIGNAL_ARGS);
      89             : static void walwriter_sigusr1_handler(SIGNAL_ARGS);
      90             : 
      91             : /*
      92             :  * Main entry point for walwriter process
      93             :  *
      94             :  * This is invoked from AuxiliaryProcessMain, which has already created the
      95             :  * basic execution environment, but not enabled signals yet.
      96             :  */
      97             : void
      98         358 : WalWriterMain(void)
      99             : {
     100             :     sigjmp_buf  local_sigjmp_buf;
     101             :     MemoryContext walwriter_context;
     102             :     int         left_till_hibernate;
     103             :     bool        hibernating;
     104             : 
     105             :     /*
     106             :      * Properly accept or ignore signals the postmaster might send us
     107             :      *
     108             :      * We have no particular use for SIGINT at the moment, but seems
     109             :      * reasonable to treat like SIGTERM.
     110             :      */
     111         358 :     pqsignal(SIGHUP, WalSigHupHandler); /* set flag to read config file */
     112         358 :     pqsignal(SIGINT, WalShutdownHandler);   /* request shutdown */
     113         358 :     pqsignal(SIGTERM, WalShutdownHandler);  /* request shutdown */
     114         358 :     pqsignal(SIGQUIT, wal_quickdie);    /* hard crash time */
     115         358 :     pqsignal(SIGALRM, SIG_IGN);
     116         358 :     pqsignal(SIGPIPE, SIG_IGN);
     117         358 :     pqsignal(SIGUSR1, walwriter_sigusr1_handler);
     118         358 :     pqsignal(SIGUSR2, SIG_IGN); /* not used */
     119             : 
     120             :     /*
     121             :      * Reset some signals that are accepted by postmaster but not here
     122             :      */
     123         358 :     pqsignal(SIGCHLD, SIG_DFL);
     124             : 
     125             :     /* We allow SIGQUIT (quickdie) at all times */
     126         358 :     sigdelset(&BlockSig, SIGQUIT);
     127             : 
     128             :     /*
     129             :      * Create a memory context that we will do all our work in.  We do this so
     130             :      * that we can reset the context during error recovery and thereby avoid
     131             :      * possible memory leaks.  Formerly this code just ran in
     132             :      * TopMemoryContext, but resetting that would be a really bad idea.
     133             :      */
     134         358 :     walwriter_context = AllocSetContextCreate(TopMemoryContext,
     135             :                                               "Wal Writer",
     136             :                                               ALLOCSET_DEFAULT_SIZES);
     137         358 :     MemoryContextSwitchTo(walwriter_context);
     138             : 
     139             :     /*
     140             :      * If an exception is encountered, processing resumes here.
     141             :      *
     142             :      * This code is heavily based on bgwriter.c, q.v.
     143             :      */
     144         358 :     if (sigsetjmp(local_sigjmp_buf, 1) != 0)
     145             :     {
     146             :         /* Since not using PG_TRY, must reset error stack by hand */
     147           0 :         error_context_stack = NULL;
     148             : 
     149             :         /* Prevent interrupts while cleaning up */
     150           0 :         HOLD_INTERRUPTS();
     151             : 
     152             :         /* Report the error to the server log */
     153           0 :         EmitErrorReport();
     154             : 
     155             :         /*
     156             :          * These operations are really just a minimal subset of
     157             :          * AbortTransaction().  We don't have very many resources to worry
     158             :          * about in walwriter, but we do have LWLocks, and perhaps buffers?
     159             :          */
     160           0 :         LWLockReleaseAll();
     161           0 :         ConditionVariableCancelSleep();
     162           0 :         pgstat_report_wait_end();
     163           0 :         AbortBufferIO();
     164           0 :         UnlockBuffers();
     165           0 :         ReleaseAuxProcessResources(false);
     166           0 :         AtEOXact_Buffers(false);
     167           0 :         AtEOXact_SMgr();
     168           0 :         AtEOXact_Files(false);
     169           0 :         AtEOXact_HashTables(false);
     170             : 
     171             :         /*
     172             :          * Now return to normal top-level context and clear ErrorContext for
     173             :          * next time.
     174             :          */
     175           0 :         MemoryContextSwitchTo(walwriter_context);
     176           0 :         FlushErrorState();
     177             : 
     178             :         /* Flush any leaked data in the top-level context */
     179           0 :         MemoryContextResetAndDeleteChildren(walwriter_context);
     180             : 
     181             :         /* Now we can allow interrupts again */
     182           0 :         RESUME_INTERRUPTS();
     183             : 
     184             :         /*
     185             :          * Sleep at least 1 second after any error.  A write error is likely
     186             :          * to be repeated, and we don't want to be filling the error logs as
     187             :          * fast as we can.
     188             :          */
     189           0 :         pg_usleep(1000000L);
     190             : 
     191             :         /*
     192             :          * Close all open files after any error.  This is helpful on Windows,
     193             :          * where holding deleted files open causes various strange errors.
     194             :          * It's not clear we need it elsewhere, but shouldn't hurt.
     195             :          */
     196           0 :         smgrcloseall();
     197             :     }
     198             : 
     199             :     /* We can now handle ereport(ERROR) */
     200         358 :     PG_exception_stack = &local_sigjmp_buf;
     201             : 
     202             :     /*
     203             :      * Unblock signals (they were blocked when the postmaster forked us)
     204             :      */
     205         358 :     PG_SETMASK(&UnBlockSig);
     206             : 
     207             :     /*
     208             :      * Reset hibernation state after any error.
     209             :      */
     210         358 :     left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
     211         358 :     hibernating = false;
     212         358 :     SetWalWriterSleeping(false);
     213             : 
     214             :     /*
     215             :      * Advertise our latch that backends can use to wake us up while we're
     216             :      * sleeping.
     217             :      */
     218         358 :     ProcGlobal->walwriterLatch = &MyProc->procLatch;
     219             : 
     220             :     /*
     221             :      * Loop forever
     222             :      */
     223             :     for (;;)
     224       16336 :     {
     225             :         long        cur_timeout;
     226             : 
     227             :         /*
     228             :          * Advertise whether we might hibernate in this cycle.  We do this
     229             :          * before resetting the latch to ensure that any async commits will
     230             :          * see the flag set if they might possibly need to wake us up, and
     231             :          * that we won't miss any signal they send us.  (If we discover work
     232             :          * to do in the last cycle before we would hibernate, the global flag
     233             :          * will be set unnecessarily, but little harm is done.)  But avoid
     234             :          * touching the global flag if it doesn't need to change.
     235             :          */
     236       16694 :         if (hibernating != (left_till_hibernate <= 1))
     237             :         {
     238          40 :             hibernating = (left_till_hibernate <= 1);
     239          40 :             SetWalWriterSleeping(hibernating);
     240             :         }
     241             : 
     242             :         /* Clear any already-pending wakeups */
     243       16694 :         ResetLatch(MyLatch);
     244             : 
     245             :         /*
     246             :          * Process any requests or signals received recently.
     247             :          */
     248       16694 :         if (got_SIGHUP)
     249             :         {
     250           8 :             got_SIGHUP = false;
     251           8 :             ProcessConfigFile(PGC_SIGHUP);
     252             :         }
     253       16694 :         if (shutdown_requested)
     254             :         {
     255             :             /* Normal exit from the walwriter is here */
     256         352 :             proc_exit(0);       /* done */
     257             :         }
     258             : 
     259             :         /*
     260             :          * Do what we're here for; then, if XLogBackgroundFlush() found useful
     261             :          * work to do, reset hibernation counter.
     262             :          */
     263       16342 :         if (XLogBackgroundFlush())
     264        8748 :             left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
     265        7594 :         else if (left_till_hibernate > 0)
     266        7580 :             left_till_hibernate--;
     267             : 
     268             :         /*
     269             :          * Sleep until we are signaled or WalWriterDelay has elapsed.  If we
     270             :          * haven't done anything useful for quite some time, lengthen the
     271             :          * sleep time so as to reduce the server's idle power consumption.
     272             :          */
     273       16342 :         if (left_till_hibernate > 0)
     274       16304 :             cur_timeout = WalWriterDelay;   /* in ms */
     275             :         else
     276          38 :             cur_timeout = WalWriterDelay * HIBERNATE_FACTOR;
     277             : 
     278       16342 :         (void) WaitLatch(MyLatch,
     279             :                          WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
     280             :                          cur_timeout,
     281             :                          WAIT_EVENT_WAL_WRITER_MAIN);
     282             :     }
     283             : }
     284             : 
     285             : 
     286             : /* --------------------------------
     287             :  *      signal handler routines
     288             :  * --------------------------------
     289             :  */
     290             : 
     291             : /*
     292             :  * wal_quickdie() occurs when signalled SIGQUIT by the postmaster.
     293             :  *
     294             :  * Some backend has bought the farm,
     295             :  * so we need to stop what we're doing and exit.
     296             :  */
     297             : static void
     298           0 : wal_quickdie(SIGNAL_ARGS)
     299             : {
     300             :     /*
     301             :      * We DO NOT want to run proc_exit() or atexit() callbacks -- we're here
     302             :      * because shared memory may be corrupted, so we don't want to try to
     303             :      * clean up our transaction.  Just nail the windows shut and get out of
     304             :      * town.  The callbacks wouldn't be safe to run from a signal handler,
     305             :      * anyway.
     306             :      *
     307             :      * Note we do _exit(2) not _exit(0).  This is to force the postmaster into
     308             :      * a system reset cycle if someone sends a manual SIGQUIT to a random
     309             :      * backend.  This is necessary precisely because we don't clean up our
     310             :      * shared memory state.  (The "dead man switch" mechanism in pmsignal.c
     311             :      * should ensure the postmaster sees this as a crash, too, but no harm in
     312             :      * being doubly sure.)
     313             :      */
     314           0 :     _exit(2);
     315             : }
     316             : 
     317             : /* SIGHUP: set flag to re-read config file at next convenient time */
     318             : static void
     319           8 : WalSigHupHandler(SIGNAL_ARGS)
     320             : {
     321           8 :     int         save_errno = errno;
     322             : 
     323           8 :     got_SIGHUP = true;
     324           8 :     SetLatch(MyLatch);
     325             : 
     326           8 :     errno = save_errno;
     327           8 : }
     328             : 
     329             : /* SIGTERM: set flag to exit normally */
     330             : static void
     331         354 : WalShutdownHandler(SIGNAL_ARGS)
     332             : {
     333         354 :     int         save_errno = errno;
     334             : 
     335         354 :     shutdown_requested = true;
     336         354 :     SetLatch(MyLatch);
     337             : 
     338         354 :     errno = save_errno;
     339         354 : }
     340             : 
     341             : /* SIGUSR1: used for latch wakeups */
     342             : static void
     343        7876 : walwriter_sigusr1_handler(SIGNAL_ARGS)
     344             : {
     345        7876 :     int         save_errno = errno;
     346             : 
     347        7876 :     latch_sigusr1_handler();
     348             : 
     349        7876 :     errno = save_errno;
     350        7876 : }

Generated by: LCOV version 1.13