src/backend/postmaster/walwriter.c

   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.
  15  *
  16  * Because the walwriter's cycle is directly linked to the maximum delay
  17  * before async-commit transactions are guaranteed committed, it's probably
  18  * unwise to load additional functionality onto it.  For instance, if you've
  19  * got a yen to create xlog segments further in advance, that'd be better done
  20  * in bgwriter than in walwriter.
  21  *
  22  * The walwriter is started by the postmaster as soon as the startup subprocess
  23  * finishes.  It remains alive until the postmaster commands it to terminate.
  24  * Normal termination is by SIGTERM, which instructs the walwriter to exit(0).
  25  * Emergency termination is by SIGQUIT; like any backend, the walwriter will
  26  * simply abort and exit on SIGQUIT.
  27  *
  28  * If the walwriter exits unexpectedly, the postmaster treats that the same
  29  * as a backend crash: shared memory may be corrupted, so remaining backends
  30  * should be killed by SIGQUIT and then a recovery cycle started.
  31  *
  32  *
  33  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  34  *
  35  *
  36  * IDENTIFICATION
  37  *        $PostgreSQL$
  38  *
  39  *-------------------------------------------------------------------------
  40  */
  41 #include "postgres.h"
  42
  43 #include <signal.h>
  44 #include <sys/time.h>
  45 #include <time.h>
  46 #include <unistd.h>
  47
  48 #include "access/xlog.h"
  49 #include "libpq/pqsignal.h"
  50 #include "miscadmin.h"
  51 #include "postmaster/walwriter.h"
  52 #include "storage/bufmgr.h"
  53 #include "storage/fd.h"
  54 #include "storage/ipc.h"
  55 #include "storage/lwlock.h"
  56 #include "storage/pmsignal.h"
  57 #include "storage/smgr.h"
  58 #include "utils/guc.h"
  59 #include "utils/hsearch.h"
  60 #include "utils/memutils.h"
  61 #include "utils/resowner.h"
  62
  63
  64 /*
  65  * GUC parameters
  66  */
  67 int                     WalWriterDelay = 200;
  68
  69 /*
  70  * Flags set by interrupt handlers for later service in the main loop.
  71  */
  72 static volatile sig_atomic_t got_SIGHUP = false;
  73 static volatile sig_atomic_t shutdown_requested = false;
  74
  75 /* Signal handlers */
  76 static void wal_quickdie(SIGNAL_ARGS);
  77 static void WalSigHupHandler(SIGNAL_ARGS);
  78 static void WalShutdownHandler(SIGNAL_ARGS);
  79
  80
  81 /*
  82  * Main entry point for walwriter process
  83  *
  84  * This is invoked from BootstrapMain, which has already created the basic
  85  * execution environment, but not enabled signals yet.
  86  */
  87 void
  88 WalWriterMain(void)
  89 {
  90         sigjmp_buf      local_sigjmp_buf;
  91         MemoryContext walwriter_context;
  92
  93         /*
  94          * If possible, make this process a group leader, so that the postmaster
  95          * can signal any child processes too.  (walwriter probably never has
  96          * any child processes, but for consistency we make all postmaster
  97          * child processes do this.)
  98          */
  99 #ifdef HAVE_SETSID
 100         if (setsid() < 0)
 101                 elog(FATAL, "setsid() failed: %m");
 102 #endif
 103
 104         /*
 105          * Properly accept or ignore signals the postmaster might send us
 106          *
 107          * We have no particular use for SIGINT at the moment, but seems
 108          * reasonable to treat like SIGTERM.
 109          */
 110         pqsignal(SIGHUP, WalSigHupHandler);     /* set flag to read config file */
 111         pqsignal(SIGINT, WalShutdownHandler);           /* request shutdown */
 112         pqsignal(SIGTERM, WalShutdownHandler);          /* request shutdown */
 113         pqsignal(SIGQUIT, wal_quickdie);                /* hard crash time */
 114         pqsignal(SIGALRM, SIG_IGN);
 115         pqsignal(SIGPIPE, SIG_IGN);
 116         pqsignal(SIGUSR1, SIG_IGN); /* reserve for sinval */
 117         pqsignal(SIGUSR2, SIG_IGN);     /* not used */
 118
 119         /*
 120          * Reset some signals that are accepted by postmaster but not here
 121          */
 122         pqsignal(SIGCHLD, SIG_DFL);
 123         pqsignal(SIGTTIN, SIG_DFL);
 124         pqsignal(SIGTTOU, SIG_DFL);
 125         pqsignal(SIGCONT, SIG_DFL);
 126         pqsignal(SIGWINCH, SIG_DFL);
 127
 128         /* We allow SIGQUIT (quickdie) at all times */
 129 #ifdef HAVE_SIGPROCMASK
 130         sigdelset(&BlockSig, SIGQUIT);
 131 #else
 132         BlockSig &= ~(sigmask(SIGQUIT));
 133 #endif
 134
 135         /*
 136          * Create a resource owner to keep track of our resources (not clear
 137          * that we need this, but may as well have one).
 138          */
 139         CurrentResourceOwner = ResourceOwnerCreate(NULL, "Wal Writer");
 140
 141         /*
 142          * Create a memory context that we will do all our work in.  We do this so
 143          * that we can reset the context during error recovery and thereby avoid
 144          * possible memory leaks.  Formerly this code just ran in
 145          * TopMemoryContext, but resetting that would be a really bad idea.
 146          */
 147         walwriter_context = AllocSetContextCreate(TopMemoryContext,
 148                                                                                           "Wal Writer",
 149                                                                                           ALLOCSET_DEFAULT_MINSIZE,
 150                                                                                           ALLOCSET_DEFAULT_INITSIZE,
 151                                                                                           ALLOCSET_DEFAULT_MAXSIZE);
 152         MemoryContextSwitchTo(walwriter_context);
 153
 154         /*
 155          * If an exception is encountered, processing resumes here.
 156          *
 157          * This code is heavily based on bgwriter.c, q.v.
 158          */
 159         if (sigsetjmp(local_sigjmp_buf, 1) != 0)
 160         {
 161                 /* Since not using PG_TRY, must reset error stack by hand */
 162                 error_context_stack = NULL;
 163
 164                 /* Prevent interrupts while cleaning up */
 165                 HOLD_INTERRUPTS();
 166
 167                 /* Report the error to the server log */
 168                 EmitErrorReport();
 169
 170                 /*
 171                  * These operations are really just a minimal subset of
 172                  * AbortTransaction().  We don't have very many resources to worry
 173                  * about in walwriter, but we do have LWLocks, and perhaps buffers?
 174                  */
 175                 LWLockReleaseAll();
 176                 AbortBufferIO();
 177                 UnlockBuffers();
 178                 /* buffer pins are released here: */
 179                 ResourceOwnerRelease(CurrentResourceOwner,
 180                                                          RESOURCE_RELEASE_BEFORE_LOCKS,
 181                                                          false, true);
 182                 /* we needn't bother with the other ResourceOwnerRelease phases */
 183                 AtEOXact_Buffers(false);
 184                 AtEOXact_Files();
 185                 AtEOXact_HashTables(false);
 186
 187                 /*
 188                  * Now return to normal top-level context and clear ErrorContext for
 189                  * next time.
 190                  */
 191                 MemoryContextSwitchTo(walwriter_context);
 192                 FlushErrorState();
 193
 194                 /* Flush any leaked data in the top-level context */
 195                 MemoryContextResetAndDeleteChildren(walwriter_context);
 196
 197                 /* Now we can allow interrupts again */
 198                 RESUME_INTERRUPTS();
 199
 200                 /*
 201                  * Sleep at least 1 second after any error.  A write error is likely
 202                  * to be repeated, and we don't want to be filling the error logs as
 203                  * fast as we can.
 204                  */
 205                 pg_usleep(1000000L);
 206
 207                 /*
 208                  * Close all open files after any error.  This is helpful on Windows,
 209                  * where holding deleted files open causes various strange errors.
 210                  * It's not clear we need it elsewhere, but shouldn't hurt.
 211                  */
 212                 smgrcloseall();
 213         }
 214
 215         /* We can now handle ereport(ERROR) */
 216         PG_exception_stack = &local_sigjmp_buf;
 217
 218         /*
 219          * Unblock signals (they were blocked when the postmaster forked us)
 220          */
 221         PG_SETMASK(&UnBlockSig);
 222
 223         /*
 224          * Loop forever
 225          */
 226         for (;;)
 227         {
 228                 long            udelay;
 229
 230                 /*
 231                  * Emergency bailout if postmaster has died.  This is to avoid the
 232                  * necessity for manual cleanup of all postmaster children.
 233                  */
 234                 if (!PostmasterIsAlive(true))
 235                         exit(1);
 236
 237                 /*
 238                  * Process any requests or signals received recently.
 239                  */
 240                 if (got_SIGHUP)
 241                 {
 242                         got_SIGHUP = false;
 243                         ProcessConfigFile(PGC_SIGHUP);
 244                 }
 245                 if (shutdown_requested)
 246                 {
 247                         /* Normal exit from the walwriter is here */
 248                         proc_exit(0);           /* done */
 249                 }
 250
 251                 /*
 252                  * Do what we're here for...
 253                  */
 254                 XLogBackgroundFlush();
 255
 256                 /*
 257                  * Delay until time to do something more, but fall out of delay
 258                  * reasonably quickly if signaled.
 259                  */
 260                 udelay = WalWriterDelay * 1000L;
 261                 while (udelay > 999999L)
 262                 {
 263                         if (got_SIGHUP || shutdown_requested)
 264                                 break;
 265                         pg_usleep(1000000L);
 266                         udelay -= 1000000L;
 267                 }
 268                 if (!(got_SIGHUP || shutdown_requested))
 269                         pg_usleep(udelay);
 270         }
 271 }
 272
 273
 274 /* --------------------------------
 275  *              signal handler routines
 276  * --------------------------------
 277  */
 278
 279 /*
 280  * wal_quickdie() occurs when signalled SIGQUIT by the postmaster.
 281  *
 282  * Some backend has bought the farm,
 283  * so we need to stop what we're doing and exit.
 284  */
 285 static void
 286 wal_quickdie(SIGNAL_ARGS)
 287 {
 288         PG_SETMASK(&BlockSig);
 289
 290         /*
 291          * DO NOT proc_exit() -- we're here because shared memory may be
 292          * corrupted, so we don't want to try to clean up our transaction. Just
 293          * nail the windows shut and get out of town.
 294          *
 295          * Note we do exit(2) not exit(0).      This is to force the postmaster into a
 296          * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
 297          * backend.  This is necessary precisely because we don't clean up our
 298          * shared memory state.
 299          */
 300         exit(2);
 301 }
 302
 303 /* SIGHUP: set flag to re-read config file at next convenient time */
 304 static void
 305 WalSigHupHandler(SIGNAL_ARGS)
 306 {
 307         got_SIGHUP = true;
 308 }
 309
 310 /* SIGTERM: set flag to exit normally */
 311 static void
 312 WalShutdownHandler(SIGNAL_ARGS)
 313 {
 314         shutdown_requested = true;
 315 }