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bgworker.c
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1 /*--------------------------------------------------------------------
2  * bgworker.c
3  * POSTGRES pluggable background workers implementation
4  *
5  * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
6  *
7  * IDENTIFICATION
8  * src/backend/postmaster/bgworker.c
9  *
10  *-------------------------------------------------------------------------
11  */
12 
13 #include "postgres.h"
14 
15 #include "access/parallel.h"
16 #include "libpq/pqsignal.h"
17 #include "miscadmin.h"
18 #include "pgstat.h"
19 #include "port/atomics.h"
21 #include "postmaster/interrupt.h"
22 #include "postmaster/postmaster.h"
25 #include "storage/dsm.h"
26 #include "storage/ipc.h"
27 #include "storage/latch.h"
28 #include "storage/lwlock.h"
29 #include "storage/pg_shmem.h"
30 #include "storage/pmsignal.h"
31 #include "storage/proc.h"
32 #include "storage/procsignal.h"
33 #include "storage/shmem.h"
34 #include "tcop/tcopprot.h"
35 #include "utils/ascii.h"
36 #include "utils/ps_status.h"
37 #include "utils/timeout.h"
38 
39 /*
40  * The postmaster's list of registered background workers, in private memory.
41  */
43 
44 /*
45  * BackgroundWorkerSlots exist in shared memory and can be accessed (via
46  * the BackgroundWorkerArray) by both the postmaster and by regular backends.
47  * However, the postmaster cannot take locks, even spinlocks, because this
48  * might allow it to crash or become wedged if shared memory gets corrupted.
49  * Such an outcome is intolerable. Therefore, we need a lockless protocol
50  * for coordinating access to this data.
51  *
52  * The 'in_use' flag is used to hand off responsibility for the slot between
53  * the postmaster and the rest of the system. When 'in_use' is false,
54  * the postmaster will ignore the slot entirely, except for the 'in_use' flag
55  * itself, which it may read. In this state, regular backends may modify the
56  * slot. Once a backend sets 'in_use' to true, the slot becomes the
57  * responsibility of the postmaster. Regular backends may no longer modify it,
58  * but the postmaster may examine it. Thus, a backend initializing a slot
59  * must fully initialize the slot - and insert a write memory barrier - before
60  * marking it as in use.
61  *
62  * As an exception, however, even when the slot is in use, regular backends
63  * may set the 'terminate' flag for a slot, telling the postmaster not
64  * to restart it. Once the background worker is no longer running, the slot
65  * will be released for reuse.
66  *
67  * In addition to coordinating with the postmaster, backends modifying this
68  * data structure must coordinate with each other. Since they can take locks,
69  * this is straightforward: any backend wishing to manipulate a slot must
70  * take BackgroundWorkerLock in exclusive mode. Backends wishing to read
71  * data that might get concurrently modified by other backends should take
72  * this lock in shared mode. No matter what, backends reading this data
73  * structure must be able to tolerate concurrent modifications by the
74  * postmaster.
75  */
76 typedef struct BackgroundWorkerSlot
77 {
78  bool in_use;
79  bool terminate;
80  pid_t pid; /* InvalidPid = not started yet; 0 = dead */
81  uint64 generation; /* incremented when slot is recycled */
84 
85 /*
86  * In order to limit the total number of parallel workers (according to
87  * max_parallel_workers GUC), we maintain the number of active parallel
88  * workers. Since the postmaster cannot take locks, two variables are used for
89  * this purpose: the number of registered parallel workers (modified by the
90  * backends, protected by BackgroundWorkerLock) and the number of terminated
91  * parallel workers (modified only by the postmaster, lockless). The active
92  * number of parallel workers is the number of registered workers minus the
93  * terminated ones. These counters can of course overflow, but it's not
94  * important here since the subtraction will still give the right number.
95  */
96 typedef struct BackgroundWorkerArray
97 {
103 
105 {
106  int slot;
107  uint64 generation;
108 };
109 
111 
112 /*
113  * List of internal background worker entry points. We need this for
114  * reasons explained in LookupBackgroundWorkerFunction(), below.
115  */
116 static const struct
117 {
118  const char *fn_name;
120 } InternalBGWorkers[] =
121 
122 {
123  {
124  "ParallelWorkerMain", ParallelWorkerMain
125  },
126  {
127  "ApplyLauncherMain", ApplyLauncherMain
128  },
129  {
130  "ApplyWorkerMain", ApplyWorkerMain
131  }
132 };
133 
134 /* Private functions. */
135 static bgworker_main_type LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname);
136 
137 
138 /*
139  * Calculate shared memory needed.
140  */
141 Size
143 {
144  Size size;
145 
146  /* Array of workers is variably sized. */
147  size = offsetof(BackgroundWorkerArray, slot);
149  sizeof(BackgroundWorkerSlot)));
150 
151  return size;
152 }
153 
154 /*
155  * Initialize shared memory.
156  */
157 void
159 {
160  bool found;
161 
162  BackgroundWorkerData = ShmemInitStruct("Background Worker Data",
164  &found);
165  if (!IsUnderPostmaster)
166  {
167  slist_iter siter;
168  int slotno = 0;
169 
170  BackgroundWorkerData->total_slots = max_worker_processes;
171  BackgroundWorkerData->parallel_register_count = 0;
172  BackgroundWorkerData->parallel_terminate_count = 0;
173 
174  /*
175  * Copy contents of worker list into shared memory. Record the shared
176  * memory slot assigned to each worker. This ensures a 1-to-1
177  * correspondence between the postmaster's private list and the array
178  * in shared memory.
179  */
180  slist_foreach(siter, &BackgroundWorkerList)
181  {
182  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
183  RegisteredBgWorker *rw;
184 
185  rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
186  Assert(slotno < max_worker_processes);
187  slot->in_use = true;
188  slot->terminate = false;
189  slot->pid = InvalidPid;
190  slot->generation = 0;
191  rw->rw_shmem_slot = slotno;
192  rw->rw_worker.bgw_notify_pid = 0; /* might be reinit after crash */
193  memcpy(&slot->worker, &rw->rw_worker, sizeof(BackgroundWorker));
194  ++slotno;
195  }
196 
197  /*
198  * Mark any remaining slots as not in use.
199  */
200  while (slotno < max_worker_processes)
201  {
202  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
203 
204  slot->in_use = false;
205  ++slotno;
206  }
207  }
208  else
209  Assert(found);
210 }
211 
212 /*
213  * Search the postmaster's backend-private list of RegisteredBgWorker objects
214  * for the one that maps to the given slot number.
215  */
216 static RegisteredBgWorker *
218 {
219  slist_iter siter;
220 
221  slist_foreach(siter, &BackgroundWorkerList)
222  {
223  RegisteredBgWorker *rw;
224 
225  rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
226  if (rw->rw_shmem_slot == slotno)
227  return rw;
228  }
229 
230  return NULL;
231 }
232 
233 /*
234  * Notice changes to shared memory made by other backends.
235  * Accept new worker requests only if allow_new_workers is true.
236  *
237  * This code runs in the postmaster, so we must be very careful not to assume
238  * that shared memory contents are sane. Otherwise, a rogue backend could
239  * take out the postmaster.
240  */
241 void
242 BackgroundWorkerStateChange(bool allow_new_workers)
243 {
244  int slotno;
245 
246  /*
247  * The total number of slots stored in shared memory should match our
248  * notion of max_worker_processes. If it does not, something is very
249  * wrong. Further down, we always refer to this value as
250  * max_worker_processes, in case shared memory gets corrupted while we're
251  * looping.
252  */
253  if (max_worker_processes != BackgroundWorkerData->total_slots)
254  {
255  ereport(LOG,
256  (errmsg("inconsistent background worker state (max_worker_processes=%d, total_slots=%d)",
258  BackgroundWorkerData->total_slots)));
259  return;
260  }
261 
262  /*
263  * Iterate through slots, looking for newly-registered workers or workers
264  * who must die.
265  */
266  for (slotno = 0; slotno < max_worker_processes; ++slotno)
267  {
268  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
269  RegisteredBgWorker *rw;
270 
271  if (!slot->in_use)
272  continue;
273 
274  /*
275  * Make sure we don't see the in_use flag before the updated slot
276  * contents.
277  */
278  pg_read_barrier();
279 
280  /* See whether we already know about this worker. */
282  if (rw != NULL)
283  {
284  /*
285  * In general, the worker data can't change after it's initially
286  * registered. However, someone can set the terminate flag.
287  */
288  if (slot->terminate && !rw->rw_terminate)
289  {
290  rw->rw_terminate = true;
291  if (rw->rw_pid != 0)
292  kill(rw->rw_pid, SIGTERM);
293  else
294  {
295  /* Report never-started, now-terminated worker as dead. */
297  }
298  }
299  continue;
300  }
301 
302  /*
303  * If we aren't allowing new workers, then immediately mark it for
304  * termination; the next stanza will take care of cleaning it up.
305  * Doing this ensures that any process waiting for the worker will get
306  * awoken, even though the worker will never be allowed to run.
307  */
308  if (!allow_new_workers)
309  slot->terminate = true;
310 
311  /*
312  * If the worker is marked for termination, we don't need to add it to
313  * the registered workers list; we can just free the slot. However, if
314  * bgw_notify_pid is set, the process that registered the worker may
315  * need to know that we've processed the terminate request, so be sure
316  * to signal it.
317  */
318  if (slot->terminate)
319  {
320  int notify_pid;
321 
322  /*
323  * We need a memory barrier here to make sure that the load of
324  * bgw_notify_pid and the update of parallel_terminate_count
325  * complete before the store to in_use.
326  */
327  notify_pid = slot->worker.bgw_notify_pid;
328  if ((slot->worker.bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
329  BackgroundWorkerData->parallel_terminate_count++;
330  slot->pid = 0;
331 
333  slot->in_use = false;
334 
335  if (notify_pid != 0)
336  kill(notify_pid, SIGUSR1);
337 
338  continue;
339  }
340 
341  /*
342  * Copy the registration data into the registered workers list.
343  */
344  rw = malloc(sizeof(RegisteredBgWorker));
345  if (rw == NULL)
346  {
347  ereport(LOG,
348  (errcode(ERRCODE_OUT_OF_MEMORY),
349  errmsg("out of memory")));
350  return;
351  }
352 
353  /*
354  * Copy strings in a paranoid way. If shared memory is corrupted, the
355  * source data might not even be NUL-terminated.
356  */
358  slot->worker.bgw_name, BGW_MAXLEN);
360  slot->worker.bgw_type, BGW_MAXLEN);
365 
366  /*
367  * Copy various fixed-size fields.
368  *
369  * flags, start_time, and restart_time are examined by the postmaster,
370  * but nothing too bad will happen if they are corrupted. The
371  * remaining fields will only be examined by the child process. It
372  * might crash, but we won't.
373  */
374  rw->rw_worker.bgw_flags = slot->worker.bgw_flags;
378  memcpy(rw->rw_worker.bgw_extra, slot->worker.bgw_extra, BGW_EXTRALEN);
379 
380  /*
381  * Copy the PID to be notified about state changes, but only if the
382  * postmaster knows about a backend with that PID. It isn't an error
383  * if the postmaster doesn't know about the PID, because the backend
384  * that requested the worker could have died (or been killed) just
385  * after doing so. Nonetheless, at least until we get some experience
386  * with how this plays out in the wild, log a message at a relative
387  * high debug level.
388  */
391  {
392  elog(DEBUG1, "worker notification PID %ld is not valid",
393  (long) rw->rw_worker.bgw_notify_pid);
394  rw->rw_worker.bgw_notify_pid = 0;
395  }
396 
397  /* Initialize postmaster bookkeeping. */
398  rw->rw_backend = NULL;
399  rw->rw_pid = 0;
400  rw->rw_child_slot = 0;
401  rw->rw_crashed_at = 0;
402  rw->rw_shmem_slot = slotno;
403  rw->rw_terminate = false;
404 
405  /* Log it! */
406  ereport(DEBUG1,
407  (errmsg_internal("registering background worker \"%s\"",
408  rw->rw_worker.bgw_name)));
409 
410  slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
411  }
412 }
413 
414 /*
415  * Forget about a background worker that's no longer needed.
416  *
417  * The worker must be identified by passing an slist_mutable_iter that
418  * points to it. This convention allows deletion of workers during
419  * searches of the worker list, and saves having to search the list again.
420  *
421  * Caller is responsible for notifying bgw_notify_pid, if appropriate.
422  *
423  * This function must be invoked only in the postmaster.
424  */
425 void
427 {
428  RegisteredBgWorker *rw;
429  BackgroundWorkerSlot *slot;
430 
431  rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
432 
434  slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
435  Assert(slot->in_use);
436 
437  /*
438  * We need a memory barrier here to make sure that the update of
439  * parallel_terminate_count completes before the store to in_use.
440  */
442  BackgroundWorkerData->parallel_terminate_count++;
443 
445  slot->in_use = false;
446 
447  ereport(DEBUG1,
448  (errmsg_internal("unregistering background worker \"%s\"",
449  rw->rw_worker.bgw_name)));
450 
452  free(rw);
453 }
454 
455 /*
456  * Report the PID of a newly-launched background worker in shared memory.
457  *
458  * This function should only be called from the postmaster.
459  */
460 void
462 {
463  BackgroundWorkerSlot *slot;
464 
466  slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
467  slot->pid = rw->rw_pid;
468 
469  if (rw->rw_worker.bgw_notify_pid != 0)
471 }
472 
473 /*
474  * Report that the PID of a background worker is now zero because a
475  * previously-running background worker has exited.
476  *
477  * This function should only be called from the postmaster.
478  */
479 void
481 {
482  RegisteredBgWorker *rw;
483  BackgroundWorkerSlot *slot;
484  int notify_pid;
485 
486  rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
487 
489  slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
490  slot->pid = rw->rw_pid;
491  notify_pid = rw->rw_worker.bgw_notify_pid;
492 
493  /*
494  * If this worker is slated for deregistration, do that before notifying
495  * the process which started it. Otherwise, if that process tries to
496  * reuse the slot immediately, it might not be available yet. In theory
497  * that could happen anyway if the process checks slot->pid at just the
498  * wrong moment, but this makes the window narrower.
499  */
500  if (rw->rw_terminate ||
503 
504  if (notify_pid != 0)
505  kill(notify_pid, SIGUSR1);
506 }
507 
508 /*
509  * Cancel SIGUSR1 notifications for a PID belonging to an exiting backend.
510  *
511  * This function should only be called from the postmaster.
512  */
513 void
515 {
516  slist_iter siter;
517 
518  slist_foreach(siter, &BackgroundWorkerList)
519  {
520  RegisteredBgWorker *rw;
521 
522  rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
523  if (rw->rw_worker.bgw_notify_pid == pid)
524  rw->rw_worker.bgw_notify_pid = 0;
525  }
526 }
527 
528 /*
529  * Cancel any not-yet-started worker requests that have waiting processes.
530  *
531  * This is called during a normal ("smart" or "fast") database shutdown.
532  * After this point, no new background workers will be started, so anything
533  * that might be waiting for them needs to be kicked off its wait. We do
534  * that by cancelling the bgworker registration entirely, which is perhaps
535  * overkill, but since we're shutting down it does not matter whether the
536  * registration record sticks around.
537  *
538  * This function should only be called from the postmaster.
539  */
540 void
542 {
543  slist_mutable_iter iter;
544 
545  slist_foreach_modify(iter, &BackgroundWorkerList)
546  {
547  RegisteredBgWorker *rw;
548  BackgroundWorkerSlot *slot;
549 
550  rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
552  slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
553 
554  /* If it's not yet started, and there's someone waiting ... */
555  if (slot->pid == InvalidPid &&
556  rw->rw_worker.bgw_notify_pid != 0)
557  {
558  /* ... then zap it, and notify the waiter */
559  int notify_pid = rw->rw_worker.bgw_notify_pid;
560 
561  ForgetBackgroundWorker(&iter);
562  if (notify_pid != 0)
563  kill(notify_pid, SIGUSR1);
564  }
565  }
566 }
567 
568 /*
569  * Reset background worker crash state.
570  *
571  * We assume that, after a crash-and-restart cycle, background workers without
572  * the never-restart flag should be restarted immediately, instead of waiting
573  * for bgw_restart_time to elapse. On the other hand, workers with that flag
574  * should be forgotten immediately, since we won't ever restart them.
575  *
576  * This function should only be called from the postmaster.
577  */
578 void
580 {
581  slist_mutable_iter iter;
582 
583  slist_foreach_modify(iter, &BackgroundWorkerList)
584  {
585  RegisteredBgWorker *rw;
586 
587  rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
588 
590  {
591  /*
592  * Workers marked BGW_NEVER_RESTART shouldn't get relaunched after
593  * the crash, so forget about them. (If we wait until after the
594  * crash to forget about them, and they are parallel workers,
595  * parallel_terminate_count will get incremented after we've
596  * already zeroed parallel_register_count, which would be bad.)
597  */
598  ForgetBackgroundWorker(&iter);
599  }
600  else
601  {
602  /*
603  * The accounting which we do via parallel_register_count and
604  * parallel_terminate_count would get messed up if a worker marked
605  * parallel could survive a crash and restart cycle. All such
606  * workers should be marked BGW_NEVER_RESTART, and thus control
607  * should never reach this branch.
608  */
610 
611  /*
612  * Allow this worker to be restarted immediately after we finish
613  * resetting.
614  */
615  rw->rw_crashed_at = 0;
616 
617  /*
618  * If there was anyone waiting for it, they're history.
619  */
620  rw->rw_worker.bgw_notify_pid = 0;
621  }
622  }
623 }
624 
625 #ifdef EXEC_BACKEND
626 /*
627  * In EXEC_BACKEND mode, workers use this to retrieve their details from
628  * shared memory.
629  */
631 BackgroundWorkerEntry(int slotno)
632 {
633  static BackgroundWorker myEntry;
634  BackgroundWorkerSlot *slot;
635 
636  Assert(slotno < BackgroundWorkerData->total_slots);
637  slot = &BackgroundWorkerData->slot[slotno];
638  Assert(slot->in_use);
639 
640  /* must copy this in case we don't intend to retain shmem access */
641  memcpy(&myEntry, &slot->worker, sizeof myEntry);
642  return &myEntry;
643 }
644 #endif
645 
646 /*
647  * Complain about the BackgroundWorker definition using error level elevel.
648  * Return true if it looks ok, false if not (unless elevel >= ERROR, in
649  * which case we won't return at all in the not-OK case).
650  */
651 static bool
653 {
654  /* sanity check for flags */
655 
656  /*
657  * We used to support workers not connected to shared memory, but don't
658  * anymore. Thus this is a required flag now. We're not removing the flag
659  * for compatibility reasons and because the flag still provides some
660  * signal when reading code.
661  */
662  if (!(worker->bgw_flags & BGWORKER_SHMEM_ACCESS))
663  {
664  ereport(elevel,
665  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
666  errmsg("background worker \"%s\": background worker without shared memory access are not supported",
667  worker->bgw_name)));
668  return false;
669  }
670 
672  {
674  {
675  ereport(elevel,
676  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
677  errmsg("background worker \"%s\": cannot request database access if starting at postmaster start",
678  worker->bgw_name)));
679  return false;
680  }
681 
682  /* XXX other checks? */
683  }
684 
685  if ((worker->bgw_restart_time < 0 &&
686  worker->bgw_restart_time != BGW_NEVER_RESTART) ||
687  (worker->bgw_restart_time > USECS_PER_DAY / 1000))
688  {
689  ereport(elevel,
690  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
691  errmsg("background worker \"%s\": invalid restart interval",
692  worker->bgw_name)));
693  return false;
694  }
695 
696  /*
697  * Parallel workers may not be configured for restart, because the
698  * parallel_register_count/parallel_terminate_count accounting can't
699  * handle parallel workers lasting through a crash-and-restart cycle.
700  */
701  if (worker->bgw_restart_time != BGW_NEVER_RESTART &&
702  (worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
703  {
704  ereport(elevel,
705  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
706  errmsg("background worker \"%s\": parallel workers may not be configured for restart",
707  worker->bgw_name)));
708  return false;
709  }
710 
711  /*
712  * If bgw_type is not filled in, use bgw_name.
713  */
714  if (strcmp(worker->bgw_type, "") == 0)
715  strcpy(worker->bgw_type, worker->bgw_name);
716 
717  return true;
718 }
719 
720 /*
721  * Standard SIGTERM handler for background workers
722  */
723 static void
725 {
727 
728  ereport(FATAL,
729  (errcode(ERRCODE_ADMIN_SHUTDOWN),
730  errmsg("terminating background worker \"%s\" due to administrator command",
732 }
733 
734 /*
735  * Start a new background worker
736  *
737  * This is the main entry point for background worker, to be called from
738  * postmaster.
739  */
740 void
742 {
743  sigjmp_buf local_sigjmp_buf;
745  bgworker_main_type entrypt;
746 
747  if (worker == NULL)
748  elog(FATAL, "unable to find bgworker entry");
749 
750  IsBackgroundWorker = true;
751 
753  init_ps_display(worker->bgw_name);
754 
756 
757  /* Apply PostAuthDelay */
758  if (PostAuthDelay > 0)
759  pg_usleep(PostAuthDelay * 1000000L);
760 
761  /*
762  * Set up signal handlers.
763  */
765  {
766  /*
767  * SIGINT is used to signal canceling the current action
768  */
772 
773  /* XXX Any other handlers needed here? */
774  }
775  else
776  {
777  pqsignal(SIGINT, SIG_IGN);
779  pqsignal(SIGFPE, SIG_IGN);
780  }
781  pqsignal(SIGTERM, bgworker_die);
782  /* SIGQUIT handler was already set up by InitPostmasterChild */
784 
785  InitializeTimeouts(); /* establishes SIGALRM handler */
786 
790 
791  /*
792  * If an exception is encountered, processing resumes here.
793  *
794  * We just need to clean up, report the error, and go away.
795  */
796  if (sigsetjmp(local_sigjmp_buf, 1) != 0)
797  {
798  /* Since not using PG_TRY, must reset error stack by hand */
799  error_context_stack = NULL;
800 
801  /* Prevent interrupts while cleaning up */
802  HOLD_INTERRUPTS();
803 
804  /*
805  * sigsetjmp will have blocked all signals, but we may need to accept
806  * signals while communicating with our parallel leader. Once we've
807  * done HOLD_INTERRUPTS() it should be safe to unblock signals.
808  */
810 
811  /* Report the error to the parallel leader and the server log */
812  EmitErrorReport();
813 
814  /*
815  * Do we need more cleanup here? For shmem-connected bgworkers, we
816  * will call InitProcess below, which will install ProcKill as exit
817  * callback. That will take care of releasing locks, etc.
818  */
819 
820  /* and go away */
821  proc_exit(1);
822  }
823 
824  /* We can now handle ereport(ERROR) */
825  PG_exception_stack = &local_sigjmp_buf;
826 
827  /*
828  * Create a per-backend PGPROC struct in shared memory, except in the
829  * EXEC_BACKEND case where this was done in SubPostmasterMain. We must
830  * do this before we can use LWLocks (and in the EXEC_BACKEND case we
831  * already had to do some stuff with LWLocks).
832  */
833 #ifndef EXEC_BACKEND
834  InitProcess();
835 #endif
836 
837  /*
838  * Early initialization.
839  */
840  BaseInit();
841 
842  /*
843  * Look up the entry point function, loading its library if necessary.
844  */
846  worker->bgw_function_name);
847 
848  /*
849  * Note that in normal processes, we would call InitPostgres here. For a
850  * worker, however, we don't know what database to connect to, yet; so we
851  * need to wait until the user code does it via
852  * BackgroundWorkerInitializeConnection().
853  */
854 
855  /*
856  * Now invoke the user-defined worker code
857  */
858  entrypt(worker->bgw_main_arg);
859 
860  /* ... and if it returns, we're done */
861  proc_exit(0);
862 }
863 
864 /*
865  * Register a new static background worker.
866  *
867  * This can only be called directly from postmaster or in the _PG_init
868  * function of a module library that's loaded by shared_preload_libraries;
869  * otherwise it will have no effect.
870  */
871 void
873 {
874  RegisteredBgWorker *rw;
875  static int numworkers = 0;
876 
877  if (!IsUnderPostmaster)
878  ereport(DEBUG1,
879  (errmsg_internal("registering background worker \"%s\"", worker->bgw_name)));
880 
882  strcmp(worker->bgw_library_name, "postgres") != 0)
883  {
884  if (!IsUnderPostmaster)
885  ereport(LOG,
886  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
887  errmsg("background worker \"%s\": must be registered in shared_preload_libraries",
888  worker->bgw_name)));
889  return;
890  }
891 
892  if (!SanityCheckBackgroundWorker(worker, LOG))
893  return;
894 
895  if (worker->bgw_notify_pid != 0)
896  {
897  ereport(LOG,
898  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
899  errmsg("background worker \"%s\": only dynamic background workers can request notification",
900  worker->bgw_name)));
901  return;
902  }
903 
904  /*
905  * Enforce maximum number of workers. Note this is overly restrictive: we
906  * could allow more non-shmem-connected workers, because these don't count
907  * towards the MAX_BACKENDS limit elsewhere. For now, it doesn't seem
908  * important to relax this restriction.
909  */
910  if (++numworkers > max_worker_processes)
911  {
912  ereport(LOG,
913  (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
914  errmsg("too many background workers"),
915  errdetail_plural("Up to %d background worker can be registered with the current settings.",
916  "Up to %d background workers can be registered with the current settings.",
919  errhint("Consider increasing the configuration parameter \"max_worker_processes\".")));
920  return;
921  }
922 
923  /*
924  * Copy the registration data into the registered workers list.
925  */
926  rw = malloc(sizeof(RegisteredBgWorker));
927  if (rw == NULL)
928  {
929  ereport(LOG,
930  (errcode(ERRCODE_OUT_OF_MEMORY),
931  errmsg("out of memory")));
932  return;
933  }
934 
935  rw->rw_worker = *worker;
936  rw->rw_backend = NULL;
937  rw->rw_pid = 0;
938  rw->rw_child_slot = 0;
939  rw->rw_crashed_at = 0;
940  rw->rw_terminate = false;
941 
942  slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
943 }
944 
945 /*
946  * Register a new background worker from a regular backend.
947  *
948  * Returns true on success and false on failure. Failure typically indicates
949  * that no background worker slots are currently available.
950  *
951  * If handle != NULL, we'll set *handle to a pointer that can subsequently
952  * be used as an argument to GetBackgroundWorkerPid(). The caller can
953  * free this pointer using pfree(), if desired.
954  */
955 bool
957  BackgroundWorkerHandle **handle)
958 {
959  int slotno;
960  bool success = false;
961  bool parallel;
962  uint64 generation = 0;
963 
964  /*
965  * We can't register dynamic background workers from the postmaster. If
966  * this is a standalone backend, we're the only process and can't start
967  * any more. In a multi-process environment, it might be theoretically
968  * possible, but we don't currently support it due to locking
969  * considerations; see comments on the BackgroundWorkerSlot data
970  * structure.
971  */
972  if (!IsUnderPostmaster)
973  return false;
974 
975  if (!SanityCheckBackgroundWorker(worker, ERROR))
976  return false;
977 
978  parallel = (worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0;
979 
980  LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
981 
982  /*
983  * If this is a parallel worker, check whether there are already too many
984  * parallel workers; if so, don't register another one. Our view of
985  * parallel_terminate_count may be slightly stale, but that doesn't really
986  * matter: we would have gotten the same result if we'd arrived here
987  * slightly earlier anyway. There's no help for it, either, since the
988  * postmaster must not take locks; a memory barrier wouldn't guarantee
989  * anything useful.
990  */
991  if (parallel && (BackgroundWorkerData->parallel_register_count -
992  BackgroundWorkerData->parallel_terminate_count) >=
994  {
995  Assert(BackgroundWorkerData->parallel_register_count -
996  BackgroundWorkerData->parallel_terminate_count <=
998  LWLockRelease(BackgroundWorkerLock);
999  return false;
1000  }
1001 
1002  /*
1003  * Look for an unused slot. If we find one, grab it.
1004  */
1005  for (slotno = 0; slotno < BackgroundWorkerData->total_slots; ++slotno)
1006  {
1007  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
1008 
1009  if (!slot->in_use)
1010  {
1011  memcpy(&slot->worker, worker, sizeof(BackgroundWorker));
1012  slot->pid = InvalidPid; /* indicates not started yet */
1013  slot->generation++;
1014  slot->terminate = false;
1015  generation = slot->generation;
1016  if (parallel)
1017  BackgroundWorkerData->parallel_register_count++;
1018 
1019  /*
1020  * Make sure postmaster doesn't see the slot as in use before it
1021  * sees the new contents.
1022  */
1023  pg_write_barrier();
1024 
1025  slot->in_use = true;
1026  success = true;
1027  break;
1028  }
1029  }
1030 
1031  LWLockRelease(BackgroundWorkerLock);
1032 
1033  /* If we found a slot, tell the postmaster to notice the change. */
1034  if (success)
1036 
1037  /*
1038  * If we found a slot and the user has provided a handle, initialize it.
1039  */
1040  if (success && handle)
1041  {
1042  *handle = palloc(sizeof(BackgroundWorkerHandle));
1043  (*handle)->slot = slotno;
1044  (*handle)->generation = generation;
1045  }
1046 
1047  return success;
1048 }
1049 
1050 /*
1051  * Get the PID of a dynamically-registered background worker.
1052  *
1053  * If the worker is determined to be running, the return value will be
1054  * BGWH_STARTED and *pidp will get the PID of the worker process. If the
1055  * postmaster has not yet attempted to start the worker, the return value will
1056  * be BGWH_NOT_YET_STARTED. Otherwise, the return value is BGWH_STOPPED.
1057  *
1058  * BGWH_STOPPED can indicate either that the worker is temporarily stopped
1059  * (because it is configured for automatic restart and exited non-zero),
1060  * or that the worker is permanently stopped (because it exited with exit
1061  * code 0, or was not configured for automatic restart), or even that the
1062  * worker was unregistered without ever starting (either because startup
1063  * failed and the worker is not configured for automatic restart, or because
1064  * TerminateBackgroundWorker was used before the worker was successfully
1065  * started).
1066  */
1069 {
1070  BackgroundWorkerSlot *slot;
1071  pid_t pid;
1072 
1073  Assert(handle->slot < max_worker_processes);
1074  slot = &BackgroundWorkerData->slot[handle->slot];
1075 
1076  /*
1077  * We could probably arrange to synchronize access to data using memory
1078  * barriers only, but for now, let's just keep it simple and grab the
1079  * lock. It seems unlikely that there will be enough traffic here to
1080  * result in meaningful contention.
1081  */
1082  LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
1083 
1084  /*
1085  * The generation number can't be concurrently changed while we hold the
1086  * lock. The pid, which is updated by the postmaster, can change at any
1087  * time, but we assume such changes are atomic. So the value we read
1088  * won't be garbage, but it might be out of date by the time the caller
1089  * examines it (but that's unavoidable anyway).
1090  *
1091  * The in_use flag could be in the process of changing from true to false,
1092  * but if it is already false then it can't change further.
1093  */
1094  if (handle->generation != slot->generation || !slot->in_use)
1095  pid = 0;
1096  else
1097  pid = slot->pid;
1098 
1099  /* All done. */
1100  LWLockRelease(BackgroundWorkerLock);
1101 
1102  if (pid == 0)
1103  return BGWH_STOPPED;
1104  else if (pid == InvalidPid)
1105  return BGWH_NOT_YET_STARTED;
1106  *pidp = pid;
1107  return BGWH_STARTED;
1108 }
1109 
1110 /*
1111  * Wait for a background worker to start up.
1112  *
1113  * This is like GetBackgroundWorkerPid(), except that if the worker has not
1114  * yet started, we wait for it to do so; thus, BGWH_NOT_YET_STARTED is never
1115  * returned. However, if the postmaster has died, we give up and return
1116  * BGWH_POSTMASTER_DIED, since it that case we know that startup will not
1117  * take place.
1118  *
1119  * The caller *must* have set our PID as the worker's bgw_notify_pid,
1120  * else we will not be awoken promptly when the worker's state changes.
1121  */
1124 {
1126  int rc;
1127 
1128  for (;;)
1129  {
1130  pid_t pid;
1131 
1133 
1134  status = GetBackgroundWorkerPid(handle, &pid);
1135  if (status == BGWH_STARTED)
1136  *pidp = pid;
1137  if (status != BGWH_NOT_YET_STARTED)
1138  break;
1139 
1140  rc = WaitLatch(MyLatch,
1143 
1144  if (rc & WL_POSTMASTER_DEATH)
1145  {
1146  status = BGWH_POSTMASTER_DIED;
1147  break;
1148  }
1149 
1151  }
1152 
1153  return status;
1154 }
1155 
1156 /*
1157  * Wait for a background worker to stop.
1158  *
1159  * If the worker hasn't yet started, or is running, we wait for it to stop
1160  * and then return BGWH_STOPPED. However, if the postmaster has died, we give
1161  * up and return BGWH_POSTMASTER_DIED, because it's the postmaster that
1162  * notifies us when a worker's state changes.
1163  *
1164  * The caller *must* have set our PID as the worker's bgw_notify_pid,
1165  * else we will not be awoken promptly when the worker's state changes.
1166  */
1169 {
1171  int rc;
1172 
1173  for (;;)
1174  {
1175  pid_t pid;
1176 
1178 
1179  status = GetBackgroundWorkerPid(handle, &pid);
1180  if (status == BGWH_STOPPED)
1181  break;
1182 
1183  rc = WaitLatch(MyLatch,
1186 
1187  if (rc & WL_POSTMASTER_DEATH)
1188  {
1189  status = BGWH_POSTMASTER_DIED;
1190  break;
1191  }
1192 
1194  }
1195 
1196  return status;
1197 }
1198 
1199 /*
1200  * Instruct the postmaster to terminate a background worker.
1201  *
1202  * Note that it's safe to do this without regard to whether the worker is
1203  * still running, or even if the worker may already have exited and been
1204  * unregistered.
1205  */
1206 void
1208 {
1209  BackgroundWorkerSlot *slot;
1210  bool signal_postmaster = false;
1211 
1212  Assert(handle->slot < max_worker_processes);
1213  slot = &BackgroundWorkerData->slot[handle->slot];
1214 
1215  /* Set terminate flag in shared memory, unless slot has been reused. */
1216  LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
1217  if (handle->generation == slot->generation)
1218  {
1219  slot->terminate = true;
1220  signal_postmaster = true;
1221  }
1222  LWLockRelease(BackgroundWorkerLock);
1223 
1224  /* Make sure the postmaster notices the change to shared memory. */
1225  if (signal_postmaster)
1227 }
1228 
1229 /*
1230  * Look up (and possibly load) a bgworker entry point function.
1231  *
1232  * For functions contained in the core code, we use library name "postgres"
1233  * and consult the InternalBGWorkers array. External functions are
1234  * looked up, and loaded if necessary, using load_external_function().
1235  *
1236  * The point of this is to pass function names as strings across process
1237  * boundaries. We can't pass actual function addresses because of the
1238  * possibility that the function has been loaded at a different address
1239  * in a different process. This is obviously a hazard for functions in
1240  * loadable libraries, but it can happen even for functions in the core code
1241  * on platforms using EXEC_BACKEND (e.g., Windows).
1242  *
1243  * At some point it might be worthwhile to get rid of InternalBGWorkers[]
1244  * in favor of applying load_external_function() for core functions too;
1245  * but that raises portability issues that are not worth addressing now.
1246  */
1247 static bgworker_main_type
1248 LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname)
1249 {
1250  /*
1251  * If the function is to be loaded from postgres itself, search the
1252  * InternalBGWorkers array.
1253  */
1254  if (strcmp(libraryname, "postgres") == 0)
1255  {
1256  int i;
1257 
1258  for (i = 0; i < lengthof(InternalBGWorkers); i++)
1259  {
1260  if (strcmp(InternalBGWorkers[i].fn_name, funcname) == 0)
1261  return InternalBGWorkers[i].fn_addr;
1262  }
1263 
1264  /* We can only reach this by programming error. */
1265  elog(ERROR, "internal function \"%s\" not found", funcname);
1266  }
1267 
1268  /* Otherwise load from external library. */
1269  return (bgworker_main_type)
1270  load_external_function(libraryname, funcname, true, NULL);
1271 }
1272 
1273 /*
1274  * Given a PID, get the bgw_type of the background worker. Returns NULL if
1275  * not a valid background worker.
1276  *
1277  * The return value is in static memory belonging to this function, so it has
1278  * to be used before calling this function again. This is so that the caller
1279  * doesn't have to worry about the background worker locking protocol.
1280  */
1281 const char *
1283 {
1284  int slotno;
1285  bool found = false;
1286  static char result[BGW_MAXLEN];
1287 
1288  LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
1289 
1290  for (slotno = 0; slotno < BackgroundWorkerData->total_slots; slotno++)
1291  {
1292  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
1293 
1294  if (slot->pid > 0 && slot->pid == pid)
1295  {
1296  strcpy(result, slot->worker.bgw_type);
1297  found = true;
1298  break;
1299  }
1300  }
1301 
1302  LWLockRelease(BackgroundWorkerLock);
1303 
1304  if (!found)
1305  return NULL;
1306 
1307  return result;
1308 }
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Definition: bgworker.c:242
#define MAX_PARALLEL_WORKER_LIMIT
void procsignal_sigusr1_handler(SIGNAL_ARGS)
Definition: procsignal.c:642
static void static void status(const char *fmt,...) pg_attribute_printf(1
Definition: pg_regress.c:227
BackgroundWorker rw_worker
static bool success
Definition: initdb.c:165
BackgroundWorkerSlot slot[FLEXIBLE_ARRAY_MEMBER]
Definition: bgworker.c:101
char bgw_library_name[BGW_MAXLEN]
Definition: bgworker.h:96
#define BGW_EXTRALEN
Definition: bgworker.h:87
#define WL_LATCH_SET
Definition: latch.h:125
void(* bgworker_main_type)(Datum main_arg)
Definition: bgworker.h:72
#define offsetof(type, field)
Definition: c.h:727
BgwHandleStatus GetBackgroundWorkerPid(BackgroundWorkerHandle *handle, pid_t *pidp)
Definition: bgworker.c:1068
void ApplyWorkerMain(Datum main_arg)
Definition: worker.c:3329
#define InvalidPid
Definition: miscadmin.h:32
void BackgroundWorkerUnblockSignals(void)
Definition: postmaster.c:5787