src/backend/replication/walsender.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * walsender.c
   4  *
   5  * The WAL sender process (walsender) is new as of Postgres 9.0. It takes
   6  * charge of XLOG streaming sender in the primary server. At first, it is
   7  * started by the postmaster when the walreceiver in the standby server
   8  * connects to the primary server and requests XLOG streaming replication.
   9  * It attempts to keep reading XLOG records from the disk and sending them
  10  * to the standby server, as long as the connection is alive (i.e., like
  11  * any backend, there is a one-to-one relationship between a connection
  12  * and a walsender process).
  13  *
  14  * Normal termination is by SIGTERM, which instructs the walsender to
  15  * close the connection and exit(0) at next convenient moment. Emergency
  16  * termination is by SIGQUIT; like any backend, the walsender will simply
  17  * abort and exit on SIGQUIT. A close of the connection and a FATAL error
  18  * are treated as not a crash but approximately normal termination;
  19  * the walsender will exit quickly without sending any more XLOG records.
  20  *
  21  * If the server is shut down, postmaster sends us SIGUSR2 after all
  22  * regular backends have exited and the shutdown checkpoint has been written.
  23  * This instruct walsender to send any outstanding WAL, including the
  24  * shutdown checkpoint record, and then exit.
  25  *
  26  * Note that there can be more than one walsender process concurrently.
  27  *
  28  * Portions Copyright (c) 2010-2010, PostgreSQL Global Development Group
  29  *
  30  *
  31  * IDENTIFICATION
  32  *        $PostgreSQL: pgsql/src/backend/replication/walsender.c,v 1.25 2010/06/03 22:17:32 tgl Exp $
  33  *
  34  *-------------------------------------------------------------------------
  35  */
  36 #include "postgres.h"
  37
  38 #include <unistd.h>
  39
  40 #include "access/xlog_internal.h"
  41 #include "catalog/pg_type.h"
  42 #include "libpq/libpq.h"
  43 #include "libpq/pqformat.h"
  44 #include "libpq/pqsignal.h"
  45 #include "miscadmin.h"
  46 #include "replication/walprotocol.h"
  47 #include "replication/walsender.h"
  48 #include "storage/fd.h"
  49 #include "storage/ipc.h"
  50 #include "storage/pmsignal.h"
  51 #include "tcop/tcopprot.h"
  52 #include "utils/guc.h"
  53 #include "utils/memutils.h"
  54 #include "utils/ps_status.h"
  55
  56
  57 /* Array of WalSnds in shared memory */
  58 WalSndCtlData *WalSndCtl = NULL;
  59
  60 /* My slot in the shared memory array */
  61 static WalSnd *MyWalSnd = NULL;
  62
  63 /* Global state */
  64 bool            am_walsender = false;           /* Am I a walsender process ? */
  65
  66 /* User-settable parameters for walsender */
  67 int                     max_wal_senders = 0;    /* the maximum number of concurrent walsenders */
  68 int                     WalSndDelay = 200;      /* max sleep time between some actions */
  69
  70 #define NAPTIME_PER_CYCLE 100000L       /* max sleep time between cycles (100ms) */
  71
  72 /*
  73  * These variables are used similarly to openLogFile/Id/Seg/Off,
  74  * but for walsender to read the XLOG.
  75  */
  76 static int      sendFile = -1;
  77 static uint32 sendId = 0;
  78 static uint32 sendSeg = 0;
  79 static uint32 sendOff = 0;
  80
  81 /*
  82  * How far have we sent WAL already? This is also advertised in
  83  * MyWalSnd->sentPtr.  (Actually, this is the next WAL location to send.)
  84  */
  85 static XLogRecPtr sentPtr = {0, 0};
  86
  87 /* Flags set by signal handlers for later service in main loop */
  88 static volatile sig_atomic_t got_SIGHUP = false;
  89 static volatile sig_atomic_t shutdown_requested = false;
  90 static volatile sig_atomic_t ready_to_stop = false;
  91
  92 /* Signal handlers */
  93 static void WalSndSigHupHandler(SIGNAL_ARGS);
  94 static void WalSndShutdownHandler(SIGNAL_ARGS);
  95 static void WalSndQuickDieHandler(SIGNAL_ARGS);
  96
  97 /* Prototypes for private functions */
  98 static int      WalSndLoop(void);
  99 static void InitWalSnd(void);
 100 static void WalSndHandshake(void);
 101 static void WalSndKill(int code, Datum arg);
 102 static void XLogRead(char *buf, XLogRecPtr recptr, Size nbytes);
 103 static bool XLogSend(char *msgbuf, bool *caughtup);
 104 static void CheckClosedConnection(void);
 105
 106
 107 /* Main entry point for walsender process */
 108 int
 109 WalSenderMain(void)
 110 {
 111         MemoryContext walsnd_context;
 112
 113         if (RecoveryInProgress())
 114                 ereport(FATAL,
 115                                 (errcode(ERRCODE_CANNOT_CONNECT_NOW),
 116                                  errmsg("recovery is still in progress, can't accept WAL streaming connections")));
 117
 118         /* Create a per-walsender data structure in shared memory */
 119         InitWalSnd();
 120
 121         /*
 122          * Create a memory context that we will do all our work in.  We do this so
 123          * that we can reset the context during error recovery and thereby avoid
 124          * possible memory leaks.  Formerly this code just ran in
 125          * TopMemoryContext, but resetting that would be a really bad idea.
 126          *
 127          * XXX: we don't actually attempt error recovery in walsender, we just
 128          * close the connection and exit.
 129          */
 130         walsnd_context = AllocSetContextCreate(TopMemoryContext,
 131                                                                                    "Wal Sender",
 132                                                                                    ALLOCSET_DEFAULT_MINSIZE,
 133                                                                                    ALLOCSET_DEFAULT_INITSIZE,
 134                                                                                    ALLOCSET_DEFAULT_MAXSIZE);
 135         MemoryContextSwitchTo(walsnd_context);
 136
 137         /* Unblock signals (they were blocked when the postmaster forked us) */
 138         PG_SETMASK(&UnBlockSig);
 139
 140         /* Tell the standby that walsender is ready for receiving commands */
 141         ReadyForQuery(DestRemote);
 142
 143         /* Handle handshake messages before streaming */
 144         WalSndHandshake();
 145
 146         /* Main loop of walsender */
 147         return WalSndLoop();
 148 }
 149
 150 /*
 151  * Execute commands from walreceiver, until we enter streaming mode.
 152  */
 153 static void
 154 WalSndHandshake(void)
 155 {
 156         StringInfoData input_message;
 157         bool            replication_started = false;
 158
 159         initStringInfo(&input_message);
 160
 161         while (!replication_started)
 162         {
 163                 int                     firstchar;
 164
 165                 /* Wait for a command to arrive */
 166                 firstchar = pq_getbyte();
 167
 168                 /*
 169                  * Emergency bailout if postmaster has died.  This is to avoid the
 170                  * necessity for manual cleanup of all postmaster children.
 171                  */
 172                 if (!PostmasterIsAlive(true))
 173                         exit(1);
 174
 175                 /*
 176                  * Check for any other interesting events that happened while we
 177                  * slept.
 178                  */
 179                 if (got_SIGHUP)
 180                 {
 181                         got_SIGHUP = false;
 182                         ProcessConfigFile(PGC_SIGHUP);
 183                 }
 184
 185                 if (firstchar != EOF)
 186                 {
 187                         /*
 188                          * Read the message contents. This is expected to be done without
 189                          * blocking because we've been able to get message type code.
 190                          */
 191                         if (pq_getmessage(&input_message, 0))
 192                                 firstchar = EOF;        /* suitable message already logged */
 193                 }
 194
 195                 /* Handle the very limited subset of commands expected in this phase */
 196                 switch (firstchar)
 197                 {
 198                         case 'Q':                       /* Query message */
 199                                 {
 200                                         const char *query_string;
 201                                         XLogRecPtr      recptr;
 202
 203                                         query_string = pq_getmsgstring(&input_message);
 204                                         pq_getmsgend(&input_message);
 205
 206                                         if (strcmp(query_string, "IDENTIFY_SYSTEM") == 0)
 207                                         {
 208                                                 StringInfoData buf;
 209                                                 char            sysid[32];
 210                                                 char            tli[11];
 211
 212                                                 /*
 213                                                  * Reply with a result set with one row, two columns.
 214                                                  * First col is system ID, and second is timeline ID
 215                                                  */
 216
 217                                                 snprintf(sysid, sizeof(sysid), UINT64_FORMAT,
 218                                                                  GetSystemIdentifier());
 219                                                 snprintf(tli, sizeof(tli), "%u", ThisTimeLineID);
 220
 221                                                 /* Send a RowDescription message */
 222                                                 pq_beginmessage(&buf, 'T');
 223                                                 pq_sendint(&buf, 2, 2); /* 2 fields */
 224
 225                                                 /* first field */
 226                                                 pq_sendstring(&buf, "systemid");                /* col name */
 227                                                 pq_sendint(&buf, 0, 4); /* table oid */
 228                                                 pq_sendint(&buf, 0, 2); /* attnum */
 229                                                 pq_sendint(&buf, TEXTOID, 4);   /* type oid */
 230                                                 pq_sendint(&buf, -1, 2);                /* typlen */
 231                                                 pq_sendint(&buf, 0, 4); /* typmod */
 232                                                 pq_sendint(&buf, 0, 2); /* format code */
 233
 234                                                 /* second field */
 235                                                 pq_sendstring(&buf, "timeline");                /* col name */
 236                                                 pq_sendint(&buf, 0, 4); /* table oid */
 237                                                 pq_sendint(&buf, 0, 2); /* attnum */
 238                                                 pq_sendint(&buf, INT4OID, 4);   /* type oid */
 239                                                 pq_sendint(&buf, 4, 2); /* typlen */
 240                                                 pq_sendint(&buf, 0, 4); /* typmod */
 241                                                 pq_sendint(&buf, 0, 2); /* format code */
 242                                                 pq_endmessage(&buf);
 243
 244                                                 /* Send a DataRow message */
 245                                                 pq_beginmessage(&buf, 'D');
 246                                                 pq_sendint(&buf, 2, 2); /* # of columns */
 247                                                 pq_sendint(&buf, strlen(sysid), 4);             /* col1 len */
 248                                                 pq_sendbytes(&buf, (char *) &sysid, strlen(sysid));
 249                                                 pq_sendint(&buf, strlen(tli), 4);               /* col2 len */
 250                                                 pq_sendbytes(&buf, (char *) tli, strlen(tli));
 251                                                 pq_endmessage(&buf);
 252
 253                                                 /* Send CommandComplete and ReadyForQuery messages */
 254                                                 EndCommand("SELECT", DestRemote);
 255                                                 ReadyForQuery(DestRemote);
 256                                                 /* ReadyForQuery did pq_flush for us */
 257                                         }
 258                                         else if (sscanf(query_string, "START_REPLICATION %X/%X",
 259                                                                         &recptr.xlogid, &recptr.xrecoff) == 2)
 260                                         {
 261                                                 StringInfoData buf;
 262
 263                                                 /*
 264                                                  * Check that we're logging enough information in the
 265                                                  * WAL for log-shipping.
 266                                                  *
 267                                                  * NOTE: This only checks the current value of
 268                                                  * wal_level. Even if the current setting is not
 269                                                  * 'minimal', there can be old WAL in the pg_xlog
 270                                                  * directory that was created with 'minimal'.
 271                                                  * So this is not bulletproof, the purpose is
 272                                                  * just to give a user-friendly error message that
 273                                                  * hints how to configure the system correctly.
 274                                                  */
 275                                                 if (wal_level == WAL_LEVEL_MINIMAL)
 276                                                         ereport(FATAL,
 277                                                                         (errcode(ERRCODE_CANNOT_CONNECT_NOW),
 278                                                                          errmsg("standby connections not allowed because wal_level=minimal")));
 279
 280                                                 /* Send a CopyOutResponse message, and start streaming */
 281                                                 pq_beginmessage(&buf, 'H');
 282                                                 pq_sendbyte(&buf, 0);
 283                                                 pq_sendint(&buf, 0, 2);
 284                                                 pq_endmessage(&buf);
 285                                                 pq_flush();
 286
 287                                                 /*
 288                                                  * Initialize position to the received one, then the
 289                                                  * xlog records begin to be shipped from that position
 290                                                  */
 291                                                 sentPtr = recptr;
 292
 293                                                 /* break out of the loop */
 294                                                 replication_started = true;
 295                                         }
 296                                         else
 297                                         {
 298                                                 ereport(FATAL,
 299                                                                 (errcode(ERRCODE_PROTOCOL_VIOLATION),
 300                                                                  errmsg("invalid standby query string: %s", query_string)));
 301                                         }
 302                                         break;
 303                                 }
 304
 305                         case 'X':
 306                                 /* standby is closing the connection */
 307                                 proc_exit(0);
 308
 309                         case EOF:
 310                                 /* standby disconnected unexpectedly */
 311                                 ereport(COMMERROR,
 312                                                 (errcode(ERRCODE_PROTOCOL_VIOLATION),
 313                                                  errmsg("unexpected EOF on standby connection")));
 314                                 proc_exit(0);
 315
 316                         default:
 317                                 ereport(FATAL,
 318                                                 (errcode(ERRCODE_PROTOCOL_VIOLATION),
 319                                                  errmsg("invalid standby handshake message type %d", firstchar)));
 320                 }
 321         }
 322 }
 323
 324 /*
 325  * Check if the remote end has closed the connection.
 326  */
 327 static void
 328 CheckClosedConnection(void)
 329 {
 330         unsigned char firstchar;
 331         int                     r;
 332
 333         r = pq_getbyte_if_available(&firstchar);
 334         if (r < 0)
 335         {
 336                 /* unexpected error or EOF */
 337                 ereport(COMMERROR,
 338                                 (errcode(ERRCODE_PROTOCOL_VIOLATION),
 339                                  errmsg("unexpected EOF on standby connection")));
 340                 proc_exit(0);
 341         }
 342         if (r == 0)
 343         {
 344                 /* no data available without blocking */
 345                 return;
 346         }
 347
 348         /* Handle the very limited subset of commands expected in this phase */
 349         switch (firstchar)
 350         {
 351                         /*
 352                          * 'X' means that the standby is closing down the socket.
 353                          */
 354                 case 'X':
 355                         proc_exit(0);
 356
 357                 default:
 358                         ereport(FATAL,
 359                                         (errcode(ERRCODE_PROTOCOL_VIOLATION),
 360                                          errmsg("invalid standby closing message type %d",
 361                                                         firstchar)));
 362         }
 363 }
 364
 365 /* Main loop of walsender process */
 366 static int
 367 WalSndLoop(void)
 368 {
 369         char       *output_message;
 370         bool            caughtup = false;
 371
 372         /*
 373          * Allocate buffer that will be used for each output message.  We do this
 374          * just once to reduce palloc overhead.  The buffer must be made large
 375          * enough for maximum-sized messages.
 376          */
 377         output_message = palloc(1 + sizeof(WalDataMessageHeader) + MAX_SEND_SIZE);
 378
 379         /* Loop forever, unless we get an error */
 380         for (;;)
 381         {
 382                 long    remain;         /* remaining time (us) */
 383
 384                 /*
 385                  * Emergency bailout if postmaster has died.  This is to avoid the
 386                  * necessity for manual cleanup of all postmaster children.
 387                  */
 388                 if (!PostmasterIsAlive(true))
 389                         exit(1);
 390
 391                 /* Process any requests or signals received recently */
 392                 if (got_SIGHUP)
 393                 {
 394                         got_SIGHUP = false;
 395                         ProcessConfigFile(PGC_SIGHUP);
 396                 }
 397
 398                 /*
 399                  * When SIGUSR2 arrives, we send all outstanding logs up to the
 400                  * shutdown checkpoint record (i.e., the latest record) and exit.
 401                  */
 402                 if (ready_to_stop)
 403                 {
 404                         if (!XLogSend(output_message, &caughtup))
 405                                 break;
 406                         if (caughtup)
 407                                 shutdown_requested = true;
 408                 }
 409
 410                 /* Normal exit from the walsender is here */
 411                 if (shutdown_requested)
 412                 {
 413                         /* Inform the standby that XLOG streaming was done */
 414                         pq_puttextmessage('C', "COPY 0");
 415                         pq_flush();
 416
 417                         proc_exit(0);
 418                 }
 419
 420                 /*
 421                  * If we had sent all accumulated WAL in last round, nap for the
 422                  * configured time before retrying.
 423                  *
 424                  * On some platforms, signals won't interrupt the sleep.  To ensure we
 425                  * respond reasonably promptly when someone signals us, break down the
 426                  * sleep into NAPTIME_PER_CYCLE increments, and check for
 427                  * interrupts after each nap.
 428                  */
 429                 if (caughtup)
 430                 {
 431                         remain = WalSndDelay * 1000L;
 432                         while (remain > 0)
 433                         {
 434                                 /* Check for interrupts */
 435                                 if (got_SIGHUP || shutdown_requested || ready_to_stop)
 436                                         break;
 437
 438                                 /* Sleep and check that the connection is still alive */
 439                                 pg_usleep(remain > NAPTIME_PER_CYCLE ? NAPTIME_PER_CYCLE : remain);
 440                                 CheckClosedConnection();
 441
 442                                 remain -= NAPTIME_PER_CYCLE;
 443                         }
 444                 }
 445
 446                 /* Attempt to send the log once every loop */
 447                 if (!XLogSend(output_message, &caughtup))
 448                         break;
 449         }
 450
 451         /*
 452          * Get here on send failure.  Clean up and exit.
 453          *
 454          * Reset whereToSendOutput to prevent ereport from attempting to send any
 455          * more messages to the standby.
 456          */
 457         if (whereToSendOutput == DestRemote)
 458                 whereToSendOutput = DestNone;
 459
 460         proc_exit(0);
 461         return 1;                                       /* keep the compiler quiet */
 462 }
 463
 464 /* Initialize a per-walsender data structure for this walsender process */
 465 static void
 466 InitWalSnd(void)
 467 {
 468         int                     i;
 469
 470         /*
 471          * WalSndCtl should be set up already (we inherit this by fork() or
 472          * EXEC_BACKEND mechanism from the postmaster).
 473          */
 474         Assert(WalSndCtl != NULL);
 475         Assert(MyWalSnd == NULL);
 476
 477         /*
 478          * Find a free walsender slot and reserve it. If this fails, we must be
 479          * out of WalSnd structures.
 480          */
 481         for (i = 0; i < max_wal_senders; i++)
 482         {
 483                 /* use volatile pointer to prevent code rearrangement */
 484                 volatile WalSnd *walsnd = &WalSndCtl->walsnds[i];
 485
 486                 SpinLockAcquire(&walsnd->mutex);
 487
 488                 if (walsnd->pid != 0)
 489                 {
 490                         SpinLockRelease(&walsnd->mutex);
 491                         continue;
 492                 }
 493                 else
 494                 {
 495                         /* found */
 496                         MyWalSnd = (WalSnd *) walsnd;
 497                         walsnd->pid = MyProcPid;
 498                         MemSet(&MyWalSnd->sentPtr, 0, sizeof(XLogRecPtr));
 499                         SpinLockRelease(&walsnd->mutex);
 500                         break;
 501                 }
 502         }
 503         if (MyWalSnd == NULL)
 504                 ereport(FATAL,
 505                                 (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 506                                  errmsg("number of requested standby connections "
 507                                         "exceeds max_wal_senders (currently %d)",
 508                                         max_wal_senders)));
 509
 510         /* Arrange to clean up at walsender exit */
 511         on_shmem_exit(WalSndKill, 0);
 512 }
 513
 514 /* Destroy the per-walsender data structure for this walsender process */
 515 static void
 516 WalSndKill(int code, Datum arg)
 517 {
 518         Assert(MyWalSnd != NULL);
 519
 520         /*
 521          * Mark WalSnd struct no longer in use. Assume that no lock is required
 522          * for this.
 523          */
 524         MyWalSnd->pid = 0;
 525
 526         /* WalSnd struct isn't mine anymore */
 527         MyWalSnd = NULL;
 528 }
 529
 530 /*
 531  * Read 'nbytes' bytes from WAL into 'buf', starting at location 'recptr'
 532  *
 533  * XXX probably this should be improved to suck data directly from the
 534  * WAL buffers when possible.
 535  */
 536 static void
 537 XLogRead(char *buf, XLogRecPtr recptr, Size nbytes)
 538 {
 539         XLogRecPtr      startRecPtr = recptr;
 540         char            path[MAXPGPATH];
 541         uint32          lastRemovedLog;
 542         uint32          lastRemovedSeg;
 543         uint32          log;
 544         uint32          seg;
 545
 546         while (nbytes > 0)
 547         {
 548                 uint32          startoff;
 549                 int                     segbytes;
 550                 int                     readbytes;
 551
 552                 startoff = recptr.xrecoff % XLogSegSize;
 553
 554                 if (sendFile < 0 || !XLByteInSeg(recptr, sendId, sendSeg))
 555                 {
 556                         /* Switch to another logfile segment */
 557                         if (sendFile >= 0)
 558                                 close(sendFile);
 559
 560                         XLByteToSeg(recptr, sendId, sendSeg);
 561                         XLogFilePath(path, ThisTimeLineID, sendId, sendSeg);
 562
 563                         sendFile = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
 564                         if (sendFile < 0)
 565                         {
 566                                 /*
 567                                  * If the file is not found, assume it's because the
 568                                  * standby asked for a too old WAL segment that has already
 569                                  * been removed or recycled.
 570                                  */
 571                                 if (errno == ENOENT)
 572                                 {
 573                                         char filename[MAXFNAMELEN];
 574                                         XLogFileName(filename, ThisTimeLineID, sendId, sendSeg);
 575                                         ereport(ERROR,
 576                                                         (errcode_for_file_access(),
 577                                                          errmsg("requested WAL segment %s has already been removed",
 578                                                                         filename)));
 579                                 }
 580                                 else
 581                                         ereport(ERROR,
 582                                                         (errcode_for_file_access(),
 583                                                          errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
 584                                                                         path, sendId, sendSeg)));
 585                         }
 586                         sendOff = 0;
 587                 }
 588
 589                 /* Need to seek in the file? */
 590                 if (sendOff != startoff)
 591                 {
 592                         if (lseek(sendFile, (off_t) startoff, SEEK_SET) < 0)
 593                                 ereport(ERROR,
 594                                                 (errcode_for_file_access(),
 595                                                  errmsg("could not seek in log file %u, segment %u to offset %u: %m",
 596                                                                 sendId, sendSeg, startoff)));
 597                         sendOff = startoff;
 598                 }
 599
 600                 /* How many bytes are within this segment? */
 601                 if (nbytes > (XLogSegSize - startoff))
 602                         segbytes = XLogSegSize - startoff;
 603                 else
 604                         segbytes = nbytes;
 605
 606                 readbytes = read(sendFile, buf, segbytes);
 607                 if (readbytes <= 0)
 608                         ereport(ERROR,
 609                                         (errcode_for_file_access(),
 610                         errmsg("could not read from log file %u, segment %u, offset %u, "
 611                                    "length %lu: %m",
 612                                    sendId, sendSeg, sendOff, (unsigned long) segbytes)));
 613
 614                 /* Update state for read */
 615                 XLByteAdvance(recptr, readbytes);
 616
 617                 sendOff += readbytes;
 618                 nbytes -= readbytes;
 619                 buf += readbytes;
 620         }
 621
 622         /*
 623          * After reading into the buffer, check that what we read was valid.
 624          * We do this after reading, because even though the segment was present
 625          * when we opened it, it might get recycled or removed while we read it.
 626          * The read() succeeds in that case, but the data we tried to read might
 627          * already have been overwritten with new WAL records.
 628          */
 629         XLogGetLastRemoved(&lastRemovedLog, &lastRemovedSeg);
 630         XLByteToSeg(startRecPtr, log, seg);
 631         if (log < lastRemovedLog ||
 632                 (log == lastRemovedLog && seg <= lastRemovedSeg))
 633         {
 634                 char filename[MAXFNAMELEN];
 635                 XLogFileName(filename, ThisTimeLineID, log, seg);
 636                 ereport(ERROR,
 637                                 (errcode_for_file_access(),
 638                                  errmsg("requested WAL segment %s has already been removed",
 639                                                 filename)));
 640         }
 641 }
 642
 643 /*
 644  * Read up to MAX_SEND_SIZE bytes of WAL that's been written (and flushed),
 645  * but not yet sent to the client, and send it.
 646  *
 647  * msgbuf is a work area in which the output message is constructed.  It's
 648  * passed in just so we can avoid re-palloc'ing the buffer on each cycle.
 649  * It must be of size 1 + sizeof(WalDataMessageHeader) + MAX_SEND_SIZE.
 650  *
 651  * If there is no unsent WAL remaining, *caughtup is set to true, otherwise
 652  * *caughtup is set to false.
 653  *
 654  * Returns true if OK, false if trouble.
 655  */
 656 static bool
 657 XLogSend(char *msgbuf, bool *caughtup)
 658 {
 659         XLogRecPtr      SendRqstPtr;
 660         XLogRecPtr      startptr;
 661         XLogRecPtr      endptr;
 662         Size            nbytes;
 663         WalDataMessageHeader msghdr;
 664
 665         /* Attempt to send all records flushed to the disk already */
 666         SendRqstPtr = GetWriteRecPtr();
 667
 668         /* Quick exit if nothing to do */
 669         if (XLByteLE(SendRqstPtr, sentPtr))
 670         {
 671                 *caughtup = true;
 672                 return true;
 673         }
 674
 675         /*
 676          * Figure out how much to send in one message. If there's no more than
 677          * MAX_SEND_SIZE bytes to send, send everything. Otherwise send
 678          * MAX_SEND_SIZE bytes, but round to logfile or page boundary.
 679          *
 680          * The rounding is not only for performance reasons. Walreceiver
 681          * relies on the fact that we never split a WAL record across two
 682          * messages. Since a long WAL record is split at page boundary into
 683          * continuation records, page boundary is always a safe cut-off point.
 684          * We also assume that SendRqstPtr never points to the middle of a WAL
 685          * record.
 686          */
 687         startptr = sentPtr;
 688         if (startptr.xrecoff >= XLogFileSize)
 689         {
 690                 /*
 691                  * crossing a logid boundary, skip the non-existent last log
 692                  * segment in previous logical log file.
 693                  */
 694                 startptr.xlogid += 1;
 695                 startptr.xrecoff = 0;
 696         }
 697
 698         endptr = startptr;
 699         XLByteAdvance(endptr, MAX_SEND_SIZE);
 700         if (endptr.xlogid != startptr.xlogid)
 701         {
 702                 /* Don't cross a logfile boundary within one message */
 703                 Assert(endptr.xlogid == startptr.xlogid + 1);
 704                 endptr.xlogid = startptr.xlogid;
 705                 endptr.xrecoff = XLogFileSize;
 706         }
 707
 708         /* if we went beyond SendRqstPtr, back off */
 709         if (XLByteLE(SendRqstPtr, endptr))
 710         {
 711                 endptr = SendRqstPtr;
 712                 *caughtup = true;
 713         }
 714         else
 715         {
 716                 /* round down to page boundary. */
 717                 endptr.xrecoff -= (endptr.xrecoff % XLOG_BLCKSZ);
 718                 *caughtup = false;
 719         }
 720
 721         nbytes = endptr.xrecoff - startptr.xrecoff;
 722         Assert(nbytes <= MAX_SEND_SIZE);
 723
 724         /*
 725          * OK to read and send the slice.
 726          */
 727         msgbuf[0] = 'w';
 728
 729         /*
 730          * Read the log directly into the output buffer to avoid extra memcpy
 731          * calls.
 732          */
 733         XLogRead(msgbuf + 1 + sizeof(WalDataMessageHeader), startptr, nbytes);
 734
 735         /*
 736          * We fill the message header last so that the send timestamp is taken
 737          * as late as possible.
 738          */
 739         msghdr.dataStart = startptr;
 740         msghdr.walEnd = SendRqstPtr;
 741         msghdr.sendTime = GetCurrentTimestamp();
 742
 743         memcpy(msgbuf + 1, &msghdr, sizeof(WalDataMessageHeader));
 744
 745         pq_putmessage('d', msgbuf, 1 + sizeof(WalDataMessageHeader) + nbytes);
 746
 747         /* Flush pending output */
 748         if (pq_flush())
 749                 return false;
 750
 751         sentPtr = endptr;
 752
 753         /* Update shared memory status */
 754         {
 755                 /* use volatile pointer to prevent code rearrangement */
 756                 volatile WalSnd *walsnd = MyWalSnd;
 757
 758                 SpinLockAcquire(&walsnd->mutex);
 759                 walsnd->sentPtr = sentPtr;
 760                 SpinLockRelease(&walsnd->mutex);
 761         }
 762
 763         /* Report progress of XLOG streaming in PS display */
 764         if (update_process_title)
 765         {
 766                 char            activitymsg[50];
 767
 768                 snprintf(activitymsg, sizeof(activitymsg), "streaming %X/%X",
 769                                  sentPtr.xlogid, sentPtr.xrecoff);
 770                 set_ps_display(activitymsg, false);
 771         }
 772
 773         return true;
 774 }
 775
 776 /* SIGHUP: set flag to re-read config file at next convenient time */
 777 static void
 778 WalSndSigHupHandler(SIGNAL_ARGS)
 779 {
 780         got_SIGHUP = true;
 781 }
 782
 783 /* SIGTERM: set flag to shut down */
 784 static void
 785 WalSndShutdownHandler(SIGNAL_ARGS)
 786 {
 787         shutdown_requested = true;
 788 }
 789
 790 /*
 791  * WalSndQuickDieHandler() occurs when signalled SIGQUIT by the postmaster.
 792  *
 793  * Some backend has bought the farm,
 794  * so we need to stop what we're doing and exit.
 795  */
 796 static void
 797 WalSndQuickDieHandler(SIGNAL_ARGS)
 798 {
 799         PG_SETMASK(&BlockSig);
 800
 801         /*
 802          * We DO NOT want to run proc_exit() callbacks -- we're here because
 803          * shared memory may be corrupted, so we don't want to try to clean up our
 804          * transaction.  Just nail the windows shut and get out of town.  Now that
 805          * there's an atexit callback to prevent third-party code from breaking
 806          * things by calling exit() directly, we have to reset the callbacks
 807          * explicitly to make this work as intended.
 808          */
 809         on_exit_reset();
 810
 811         /*
 812          * Note we do exit(2) not exit(0).      This is to force the postmaster into a
 813          * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
 814          * backend.  This is necessary precisely because we don't clean up our
 815          * shared memory state.  (The "dead man switch" mechanism in pmsignal.c
 816          * should ensure the postmaster sees this as a crash, too, but no harm in
 817          * being doubly sure.)
 818          */
 819         exit(2);
 820 }
 821
 822 /* SIGUSR2: set flag to do a last cycle and shut down afterwards */
 823 static void
 824 WalSndLastCycleHandler(SIGNAL_ARGS)
 825 {
 826         ready_to_stop = true;
 827 }
 828
 829 /* Set up signal handlers */
 830 void
 831 WalSndSignals(void)
 832 {
 833         /* Set up signal handlers */
 834         pqsignal(SIGHUP, WalSndSigHupHandler);          /* set flag to read config
 835                                                                                                  * file */
 836         pqsignal(SIGINT, SIG_IGN);      /* not used */
 837         pqsignal(SIGTERM, WalSndShutdownHandler);       /* request shutdown */
 838         pqsignal(SIGQUIT, WalSndQuickDieHandler);       /* hard crash time */
 839         pqsignal(SIGALRM, SIG_IGN);
 840         pqsignal(SIGPIPE, SIG_IGN);
 841         pqsignal(SIGUSR1, SIG_IGN); /* not used */
 842         pqsignal(SIGUSR2, WalSndLastCycleHandler);      /* request a last cycle and
 843                                                                                                  * shutdown */
 844
 845         /* Reset some signals that are accepted by postmaster but not here */
 846         pqsignal(SIGCHLD, SIG_DFL);
 847         pqsignal(SIGTTIN, SIG_DFL);
 848         pqsignal(SIGTTOU, SIG_DFL);
 849         pqsignal(SIGCONT, SIG_DFL);
 850         pqsignal(SIGWINCH, SIG_DFL);
 851 }
 852
 853 /* Report shared-memory space needed by WalSndShmemInit */
 854 Size
 855 WalSndShmemSize(void)
 856 {
 857         Size            size = 0;
 858
 859         size = offsetof(WalSndCtlData, walsnds);
 860         size = add_size(size, mul_size(max_wal_senders, sizeof(WalSnd)));
 861
 862         return size;
 863 }
 864
 865 /* Allocate and initialize walsender-related shared memory */
 866 void
 867 WalSndShmemInit(void)
 868 {
 869         bool            found;
 870         int                     i;
 871
 872         WalSndCtl = (WalSndCtlData *)
 873                 ShmemInitStruct("Wal Sender Ctl", WalSndShmemSize(), &found);
 874
 875         if (!found)
 876         {
 877                 /* First time through, so initialize */
 878                 MemSet(WalSndCtl, 0, WalSndShmemSize());
 879
 880                 for (i = 0; i < max_wal_senders; i++)
 881                 {
 882                         WalSnd     *walsnd = &WalSndCtl->walsnds[i];
 883
 884                         SpinLockInit(&walsnd->mutex);
 885                 }
 886         }
 887 }
 888
 889 /*
 890  * This isn't currently used for anything. Monitoring tools might be
 891  * interested in the future, and we'll need something like this in the
 892  * future for synchronous replication.
 893  */
 894 #ifdef NOT_USED
 895 /*
 896  * Returns the oldest Send position among walsenders. Or InvalidXLogRecPtr
 897  * if none.
 898  */
 899 XLogRecPtr
 900 GetOldestWALSendPointer(void)
 901 {
 902         XLogRecPtr      oldest = {0, 0};
 903         int                     i;
 904         bool            found = false;
 905
 906         for (i = 0; i < max_wal_senders; i++)
 907         {
 908                 /* use volatile pointer to prevent code rearrangement */
 909                 volatile WalSnd *walsnd = &WalSndCtl->walsnds[i];
 910                 XLogRecPtr      recptr;
 911
 912                 if (walsnd->pid == 0)
 913                         continue;
 914
 915                 SpinLockAcquire(&walsnd->mutex);
 916                 recptr = walsnd->sentPtr;
 917                 SpinLockRelease(&walsnd->mutex);
 918
 919                 if (recptr.xlogid == 0 && recptr.xrecoff == 0)
 920                         continue;
 921
 922                 if (!found || XLByteLT(recptr, oldest))
 923                         oldest = recptr;
 924                 found = true;
 925         }
 926         return oldest;
 927 }
 928 #endif