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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) 1996-2017, PostgreSQL Global Development Group
6  *
7  * IDENTIFICATION
8  * src/backend/postmaster/bgworker.c
9  *
10  *-------------------------------------------------------------------------
11  */
12 
13 #include "postgres.h"
14 
15 #include <unistd.h>
16 
17 #include "libpq/pqsignal.h"
18 #include "access/parallel.h"
19 #include "miscadmin.h"
20 #include "pgstat.h"
21 #include "port/atomics.h"
23 #include "postmaster/postmaster.h"
26 #include "storage/dsm.h"
27 #include "storage/ipc.h"
28 #include "storage/latch.h"
29 #include "storage/lwlock.h"
30 #include "storage/pg_shmem.h"
31 #include "storage/pmsignal.h"
32 #include "storage/proc.h"
33 #include "storage/procsignal.h"
34 #include "storage/shmem.h"
35 #include "tcop/tcopprot.h"
36 #include "utils/ascii.h"
37 #include "utils/ps_status.h"
38 #include "utils/timeout.h"
39 
40 /*
41  * The postmaster's list of registered background workers, in private memory.
42  */
44 
45 /*
46  * BackgroundWorkerSlots exist in shared memory and can be accessed (via
47  * the BackgroundWorkerArray) by both the postmaster and by regular backends.
48  * However, the postmaster cannot take locks, even spinlocks, because this
49  * might allow it to crash or become wedged if shared memory gets corrupted.
50  * Such an outcome is intolerable. Therefore, we need a lockless protocol
51  * for coordinating access to this data.
52  *
53  * The 'in_use' flag is used to hand off responsibility for the slot between
54  * the postmaster and the rest of the system. When 'in_use' is false,
55  * the postmaster will ignore the slot entirely, except for the 'in_use' flag
56  * itself, which it may read. In this state, regular backends may modify the
57  * slot. Once a backend sets 'in_use' to true, the slot becomes the
58  * responsibility of the postmaster. Regular backends may no longer modify it,
59  * but the postmaster may examine it. Thus, a backend initializing a slot
60  * must fully initialize the slot - and insert a write memory barrier - before
61  * marking it as in use.
62  *
63  * As an exception, however, even when the slot is in use, regular backends
64  * may set the 'terminate' flag for a slot, telling the postmaster not
65  * to restart it. Once the background worker is no longer running, the slot
66  * will be released for reuse.
67  *
68  * In addition to coordinating with the postmaster, backends modifying this
69  * data structure must coordinate with each other. Since they can take locks,
70  * this is straightforward: any backend wishing to manipulate a slot must
71  * take BackgroundWorkerLock in exclusive mode. Backends wishing to read
72  * data that might get concurrently modified by other backends should take
73  * this lock in shared mode. No matter what, backends reading this data
74  * structure must be able to tolerate concurrent modifications by the
75  * postmaster.
76  */
77 typedef struct BackgroundWorkerSlot
78 {
79  bool in_use;
80  bool terminate;
81  pid_t pid; /* InvalidPid = not started yet; 0 = dead */
82  uint64 generation; /* incremented when slot is recycled */
85 
86 /*
87  * In order to limit the total number of parallel workers (according to
88  * max_parallel_workers GUC), we maintain the number of active parallel
89  * workers. Since the postmaster cannot take locks, two variables are used for
90  * this purpose: the number of registered parallel workers (modified by the
91  * backends, protected by BackgroundWorkerLock) and the number of terminated
92  * parallel workers (modified only by the postmaster, lockless). The active
93  * number of parallel workers is the number of registered workers minus the
94  * terminated ones. These counters can of course overflow, but it's not
95  * important here since the subtraction will still give the right number.
96  */
97 typedef struct BackgroundWorkerArray
98 {
102  BackgroundWorkerSlot slot[FLEXIBLE_ARRAY_MEMBER];
104 
106 {
107  int slot;
108  uint64 generation;
109 };
110 
112 
113 /*
114  * List of internal background worker entry points. We need this for
115  * reasons explained in LookupBackgroundWorkerFunction(), below.
116  */
117 static const struct
118 {
119  const char *fn_name;
121 } InternalBGWorkers[] =
122 
123 {
124  {
125  "ParallelWorkerMain", ParallelWorkerMain
126  },
127  {
128  "ApplyLauncherMain", ApplyLauncherMain
129  },
130  {
131  "ApplyWorkerMain", ApplyWorkerMain
132  }
133 };
134 
135 /* Private functions. */
136 static bgworker_main_type LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname);
137 
138 
139 /*
140  * Calculate shared memory needed.
141  */
142 Size
144 {
145  Size size;
146 
147  /* Array of workers is variably sized. */
148  size = offsetof(BackgroundWorkerArray, slot);
150  sizeof(BackgroundWorkerSlot)));
151 
152  return size;
153 }
154 
155 /*
156  * Initialize shared memory.
157  */
158 void
160 {
161  bool found;
162 
163  BackgroundWorkerData = ShmemInitStruct("Background Worker Data",
165  &found);
166  if (!IsUnderPostmaster)
167  {
168  slist_iter siter;
169  int slotno = 0;
170 
171  BackgroundWorkerData->total_slots = max_worker_processes;
172  BackgroundWorkerData->parallel_register_count = 0;
173  BackgroundWorkerData->parallel_terminate_count = 0;
174 
175  /*
176  * Copy contents of worker list into shared memory. Record the shared
177  * memory slot assigned to each worker. This ensures a 1-to-1
178  * correspondence between the postmaster's private list and the array
179  * in shared memory.
180  */
181  slist_foreach(siter, &BackgroundWorkerList)
182  {
183  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
184  RegisteredBgWorker *rw;
185 
186  rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
187  Assert(slotno < max_worker_processes);
188  slot->in_use = true;
189  slot->terminate = false;
190  slot->pid = InvalidPid;
191  slot->generation = 0;
192  rw->rw_shmem_slot = slotno;
193  rw->rw_worker.bgw_notify_pid = 0; /* might be reinit after crash */
194  memcpy(&slot->worker, &rw->rw_worker, sizeof(BackgroundWorker));
195  ++slotno;
196  }
197 
198  /*
199  * Mark any remaining slots as not in use.
200  */
201  while (slotno < max_worker_processes)
202  {
203  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
204 
205  slot->in_use = false;
206  ++slotno;
207  }
208  }
209  else
210  Assert(found);
211 }
212 
213 /*
214  * Search the postmaster's backend-private list of RegisteredBgWorker objects
215  * for the one that maps to the given slot number.
216  */
217 static RegisteredBgWorker *
219 {
220  slist_iter siter;
221 
222  slist_foreach(siter, &BackgroundWorkerList)
223  {
224  RegisteredBgWorker *rw;
225 
226  rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
227  if (rw->rw_shmem_slot == slotno)
228  return rw;
229  }
230 
231  return NULL;
232 }
233 
234 /*
235  * Notice changes to shared memory made by other backends. This code
236  * runs in the postmaster, so we must be very careful not to assume that
237  * shared memory contents are sane. Otherwise, a rogue backend could take
238  * out the postmaster.
239  */
240 void
242 {
243  int slotno;
244 
245  /*
246  * The total number of slots stored in shared memory should match our
247  * notion of max_worker_processes. If it does not, something is very
248  * wrong. Further down, we always refer to this value as
249  * max_worker_processes, in case shared memory gets corrupted while we're
250  * looping.
251  */
252  if (max_worker_processes != BackgroundWorkerData->total_slots)
253  {
254  elog(LOG,
255  "inconsistent background worker state (max_worker_processes=%d, total_slots=%d",
257  BackgroundWorkerData->total_slots);
258  return;
259  }
260 
261  /*
262  * Iterate through slots, looking for newly-registered workers or workers
263  * who must die.
264  */
265  for (slotno = 0; slotno < max_worker_processes; ++slotno)
266  {
267  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
268  RegisteredBgWorker *rw;
269 
270  if (!slot->in_use)
271  continue;
272 
273  /*
274  * Make sure we don't see the in_use flag before the updated slot
275  * contents.
276  */
277  pg_read_barrier();
278 
279  /* See whether we already know about this worker. */
281  if (rw != NULL)
282  {
283  /*
284  * In general, the worker data can't change after it's initially
285  * registered. However, someone can set the terminate flag.
286  */
287  if (slot->terminate && !rw->rw_terminate)
288  {
289  rw->rw_terminate = true;
290  if (rw->rw_pid != 0)
291  kill(rw->rw_pid, SIGTERM);
292  else
293  {
294  /* Report never-started, now-terminated worker as dead. */
296  }
297  }
298  continue;
299  }
300 
301  /*
302  * If the worker is marked for termination, we don't need to add it to
303  * the registered workers list; we can just free the slot. However, if
304  * bgw_notify_pid is set, the process that registered the worker may
305  * need to know that we've processed the terminate request, so be sure
306  * to signal it.
307  */
308  if (slot->terminate)
309  {
310  int notify_pid;
311 
312  /*
313  * We need a memory barrier here to make sure that the load of
314  * bgw_notify_pid and the update of parallel_terminate_count
315  * complete before the store to in_use.
316  */
317  notify_pid = slot->worker.bgw_notify_pid;
318  if ((slot->worker.bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
319  BackgroundWorkerData->parallel_terminate_count++;
321  slot->pid = 0;
322  slot->in_use = false;
323  if (notify_pid != 0)
324  kill(notify_pid, SIGUSR1);
325 
326  continue;
327  }
328 
329  /*
330  * Copy the registration data into the registered workers list.
331  */
332  rw = malloc(sizeof(RegisteredBgWorker));
333  if (rw == NULL)
334  {
335  ereport(LOG,
336  (errcode(ERRCODE_OUT_OF_MEMORY),
337  errmsg("out of memory")));
338  return;
339  }
340 
341  /*
342  * Copy strings in a paranoid way. If shared memory is corrupted, the
343  * source data might not even be NUL-terminated.
344  */
346  slot->worker.bgw_name, BGW_MAXLEN);
348  slot->worker.bgw_type, BGW_MAXLEN);
353 
354  /*
355  * Copy various fixed-size fields.
356  *
357  * flags, start_time, and restart_time are examined by the postmaster,
358  * but nothing too bad will happen if they are corrupted. The
359  * remaining fields will only be examined by the child process. It
360  * might crash, but we won't.
361  */
362  rw->rw_worker.bgw_flags = slot->worker.bgw_flags;
366  memcpy(rw->rw_worker.bgw_extra, slot->worker.bgw_extra, BGW_EXTRALEN);
367 
368  /*
369  * Copy the PID to be notified about state changes, but only if the
370  * postmaster knows about a backend with that PID. It isn't an error
371  * if the postmaster doesn't know about the PID, because the backend
372  * that requested the worker could have died (or been killed) just
373  * after doing so. Nonetheless, at least until we get some experience
374  * with how this plays out in the wild, log a message at a relative
375  * high debug level.
376  */
379  {
380  elog(DEBUG1, "worker notification PID %lu is not valid",
381  (long) rw->rw_worker.bgw_notify_pid);
382  rw->rw_worker.bgw_notify_pid = 0;
383  }
384 
385  /* Initialize postmaster bookkeeping. */
386  rw->rw_backend = NULL;
387  rw->rw_pid = 0;
388  rw->rw_child_slot = 0;
389  rw->rw_crashed_at = 0;
390  rw->rw_shmem_slot = slotno;
391  rw->rw_terminate = false;
392 
393  /* Log it! */
394  ereport(DEBUG1,
395  (errmsg("registering background worker \"%s\"",
396  rw->rw_worker.bgw_name)));
397 
398  slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
399  }
400 }
401 
402 /*
403  * Forget about a background worker that's no longer needed.
404  *
405  * The worker must be identified by passing an slist_mutable_iter that
406  * points to it. This convention allows deletion of workers during
407  * searches of the worker list, and saves having to search the list again.
408  *
409  * This function must be invoked only in the postmaster.
410  */
411 void
413 {
414  RegisteredBgWorker *rw;
415  BackgroundWorkerSlot *slot;
416 
417  rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
418 
420  slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
422  BackgroundWorkerData->parallel_terminate_count++;
423 
424  slot->in_use = false;
425 
426  ereport(DEBUG1,
427  (errmsg("unregistering background worker \"%s\"",
428  rw->rw_worker.bgw_name)));
429 
431  free(rw);
432 }
433 
434 /*
435  * Report the PID of a newly-launched background worker in shared memory.
436  *
437  * This function should only be called from the postmaster.
438  */
439 void
441 {
442  BackgroundWorkerSlot *slot;
443 
445  slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
446  slot->pid = rw->rw_pid;
447 
448  if (rw->rw_worker.bgw_notify_pid != 0)
449  kill(rw->rw_worker.bgw_notify_pid, SIGUSR1);
450 }
451 
452 /*
453  * Report that the PID of a background worker is now zero because a
454  * previously-running background worker has exited.
455  *
456  * This function should only be called from the postmaster.
457  */
458 void
460 {
461  RegisteredBgWorker *rw;
462  BackgroundWorkerSlot *slot;
463  int notify_pid;
464 
465  rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
466 
468  slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
469  slot->pid = rw->rw_pid;
470  notify_pid = rw->rw_worker.bgw_notify_pid;
471 
472  /*
473  * If this worker is slated for deregistration, do that before notifying
474  * the process which started it. Otherwise, if that process tries to
475  * reuse the slot immediately, it might not be available yet. In theory
476  * that could happen anyway if the process checks slot->pid at just the
477  * wrong moment, but this makes the window narrower.
478  */
479  if (rw->rw_terminate ||
482 
483  if (notify_pid != 0)
484  kill(notify_pid, SIGUSR1);
485 }
486 
487 /*
488  * Cancel SIGUSR1 notifications for a PID belonging to an exiting backend.
489  *
490  * This function should only be called from the postmaster.
491  */
492 void
494 {
495  slist_iter siter;
496 
497  slist_foreach(siter, &BackgroundWorkerList)
498  {
499  RegisteredBgWorker *rw;
500 
501  rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
502  if (rw->rw_worker.bgw_notify_pid == pid)
503  rw->rw_worker.bgw_notify_pid = 0;
504  }
505 }
506 
507 /*
508  * Reset background worker crash state.
509  *
510  * We assume that, after a crash-and-restart cycle, background workers without
511  * the never-restart flag should be restarted immediately, instead of waiting
512  * for bgw_restart_time to elapse.
513  */
514 void
516 {
517  slist_mutable_iter iter;
518 
519  slist_foreach_modify(iter, &BackgroundWorkerList)
520  {
521  RegisteredBgWorker *rw;
522 
523  rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
524 
526  {
527  /*
528  * Workers marked BGW_NVER_RESTART shouldn't get relaunched after
529  * the crash, so forget about them. (If we wait until after the
530  * crash to forget about them, and they are parallel workers,
531  * parallel_terminate_count will get incremented after we've
532  * already zeroed parallel_register_count, which would be bad.)
533  */
534  ForgetBackgroundWorker(&iter);
535  }
536  else
537  {
538  /*
539  * The accounting which we do via parallel_register_count and
540  * parallel_terminate_count would get messed up if a worker marked
541  * parallel could survive a crash and restart cycle. All such
542  * workers should be marked BGW_NEVER_RESTART, and thus control
543  * should never reach this branch.
544  */
546 
547  /*
548  * Allow this worker to be restarted immediately after we finish
549  * resetting.
550  */
551  rw->rw_crashed_at = 0;
552  }
553  }
554 }
555 
556 #ifdef EXEC_BACKEND
557 /*
558  * In EXEC_BACKEND mode, workers use this to retrieve their details from
559  * shared memory.
560  */
562 BackgroundWorkerEntry(int slotno)
563 {
564  static BackgroundWorker myEntry;
565  BackgroundWorkerSlot *slot;
566 
567  Assert(slotno < BackgroundWorkerData->total_slots);
568  slot = &BackgroundWorkerData->slot[slotno];
569  Assert(slot->in_use);
570 
571  /* must copy this in case we don't intend to retain shmem access */
572  memcpy(&myEntry, &slot->worker, sizeof myEntry);
573  return &myEntry;
574 }
575 #endif
576 
577 /*
578  * Complain about the BackgroundWorker definition using error level elevel.
579  * Return true if it looks ok, false if not (unless elevel >= ERROR, in
580  * which case we won't return at all in the not-OK case).
581  */
582 static bool
584 {
585  /* sanity check for flags */
587  {
588  if (!(worker->bgw_flags & BGWORKER_SHMEM_ACCESS))
589  {
590  ereport(elevel,
591  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
592  errmsg("background worker \"%s\": must attach to shared memory in order to request a database connection",
593  worker->bgw_name)));
594  return false;
595  }
596 
598  {
599  ereport(elevel,
600  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
601  errmsg("background worker \"%s\": cannot request database access if starting at postmaster start",
602  worker->bgw_name)));
603  return false;
604  }
605 
606  /* XXX other checks? */
607  }
608 
609  if ((worker->bgw_restart_time < 0 &&
610  worker->bgw_restart_time != BGW_NEVER_RESTART) ||
611  (worker->bgw_restart_time > USECS_PER_DAY / 1000))
612  {
613  ereport(elevel,
614  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
615  errmsg("background worker \"%s\": invalid restart interval",
616  worker->bgw_name)));
617  return false;
618  }
619 
620  /*
621  * Parallel workers may not be configured for restart, because the
622  * parallel_register_count/parallel_terminate_count accounting can't
623  * handle parallel workers lasting through a crash-and-restart cycle.
624  */
625  if (worker->bgw_restart_time != BGW_NEVER_RESTART &&
626  (worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
627  {
628  ereport(elevel,
629  (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
630  errmsg("background worker \"%s\": parallel workers may not be configured for restart",
631  worker->bgw_name)));
632  return false;
633  }
634 
635  /*
636  * If bgw_type is not filled in, use bgw_name.
637  */
638  if (strcmp(worker->bgw_type, "") == 0)
639  strcpy(worker->bgw_type, worker->bgw_name);
640 
641  return true;
642 }
643 
644 static void
646 {
647  sigaddset(&BlockSig, SIGQUIT); /* prevent nested calls */
649 
650  /*
651  * We DO NOT want to run proc_exit() callbacks -- we're here because
652  * shared memory may be corrupted, so we don't want to try to clean up our
653  * transaction. Just nail the windows shut and get out of town. Now that
654  * there's an atexit callback to prevent third-party code from breaking
655  * things by calling exit() directly, we have to reset the callbacks
656  * explicitly to make this work as intended.
657  */
658  on_exit_reset();
659 
660  /*
661  * Note we do exit(2) not exit(0). This is to force the postmaster into a
662  * system reset cycle if some idiot DBA sends a manual SIGQUIT to a random
663  * backend. This is necessary precisely because we don't clean up our
664  * shared memory state. (The "dead man switch" mechanism in pmsignal.c
665  * should ensure the postmaster sees this as a crash, too, but no harm in
666  * being doubly sure.)
667  */
668  exit(2);
669 }
670 
671 /*
672  * Standard SIGTERM handler for background workers
673  */
674 static void
676 {
678 
679  ereport(FATAL,
680  (errcode(ERRCODE_ADMIN_SHUTDOWN),
681  errmsg("terminating background worker \"%s\" due to administrator command",
683 }
684 
685 /*
686  * Standard SIGUSR1 handler for unconnected workers
687  *
688  * Here, we want to make sure an unconnected worker will at least heed
689  * latch activity.
690  */
691 static void
693 {
694  int save_errno = errno;
695 
697 
698  errno = save_errno;
699 }
700 
701 /*
702  * Start a new background worker
703  *
704  * This is the main entry point for background worker, to be called from
705  * postmaster.
706  */
707 void
709 {
710  sigjmp_buf local_sigjmp_buf;
712  bgworker_main_type entrypt;
713 
714  if (worker == NULL)
715  elog(FATAL, "unable to find bgworker entry");
716 
717  IsBackgroundWorker = true;
718 
719  /* Identify myself via ps */
720  init_ps_display(worker->bgw_name, "", "", "");
721 
722  /*
723  * If we're not supposed to have shared memory access, then detach from
724  * shared memory. If we didn't request shared memory access, the
725  * postmaster won't force a cluster-wide restart if we exit unexpectedly,
726  * so we'd better make sure that we don't mess anything up that would
727  * require that sort of cleanup.
728  */
729  if ((worker->bgw_flags & BGWORKER_SHMEM_ACCESS) == 0)
730  {
731  dsm_detach_all();
733  }
734 
736 
737  /* Apply PostAuthDelay */
738  if (PostAuthDelay > 0)
739  pg_usleep(PostAuthDelay * 1000000L);
740 
741  /*
742  * Set up signal handlers.
743  */
745  {
746  /*
747  * SIGINT is used to signal canceling the current action
748  */
752 
753  /* XXX Any other handlers needed here? */
754  }
755  else
756  {
757  pqsignal(SIGINT, SIG_IGN);
759  pqsignal(SIGFPE, SIG_IGN);
760  }
761  pqsignal(SIGTERM, bgworker_die);
763 
765  InitializeTimeouts(); /* establishes SIGALRM handler */
766 
770 
771  /*
772  * If an exception is encountered, processing resumes here.
773  *
774  * See notes in postgres.c about the design of this coding.
775  */
776  if (sigsetjmp(local_sigjmp_buf, 1) != 0)
777  {
778  /* Since not using PG_TRY, must reset error stack by hand */
779  error_context_stack = NULL;
780 
781  /* Prevent interrupts while cleaning up */
782  HOLD_INTERRUPTS();
783 
784  /* Report the error to the server log */
785  EmitErrorReport();
786 
787  /*
788  * Do we need more cleanup here? For shmem-connected bgworkers, we
789  * will call InitProcess below, which will install ProcKill as exit
790  * callback. That will take care of releasing locks, etc.
791  */
792 
793  /* and go away */
794  proc_exit(1);
795  }
796 
797  /* We can now handle ereport(ERROR) */
798  PG_exception_stack = &local_sigjmp_buf;
799 
800  /*
801  * If the background worker request shared memory access, set that up now;
802  * else, detach all shared memory segments.
803  */
804  if (worker->bgw_flags & BGWORKER_SHMEM_ACCESS)
805  {
806  /*
807  * Early initialization. Some of this could be useful even for
808  * background workers that aren't using shared memory, but they can
809  * call the individual startup routines for those subsystems if
810  * needed.
811  */
812  BaseInit();
813 
814  /*
815  * Create a per-backend PGPROC struct in shared memory, except in the
816  * EXEC_BACKEND case where this was done in SubPostmasterMain. We must
817  * do this before we can use LWLocks (and in the EXEC_BACKEND case we
818  * already had to do some stuff with LWLocks).
819  */
820 #ifndef EXEC_BACKEND
821  InitProcess();
822 #endif
823  }
824 
825  /*
826  * Look up the entry point function, loading its library if necessary.
827  */
829  worker->bgw_function_name);
830 
831  /*
832  * Note that in normal processes, we would call InitPostgres here. For a
833  * worker, however, we don't know what database to connect to, yet; so we
834  * need to wait until the user code does it via
835  * BackgroundWorkerInitializeConnection().
836  */
837 
838  /*
839  * Now invoke the user-defined worker code
840  */
841  entrypt(worker->bgw_main_arg);
842 
843  /* ... and if it returns, we're done */
844  proc_exit(0);
845 }
846 
847 /*
848  * Register a new static background worker.
849  *
850  * This can only be called directly from postmaster or in the _PG_init
851  * function of a module library that's loaded by shared_preload_libraries;
852  * otherwise it will have no effect.
853  */
854 void
856 {
857  RegisteredBgWorker *rw;
858  static int numworkers = 0;
859 
860  if (!IsUnderPostmaster)
861  ereport(DEBUG1,
862  (errmsg("registering background worker \"%s\"", worker->bgw_name)));
863 
865  strcmp(worker->bgw_library_name, "postgres") != 0)
866  {
867  if (!IsUnderPostmaster)
868  ereport(LOG,
869  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
870  errmsg("background worker \"%s\": must be registered in shared_preload_libraries",
871  worker->bgw_name)));
872  return;
873  }
874 
875  if (!SanityCheckBackgroundWorker(worker, LOG))
876  return;
877 
878  if (worker->bgw_notify_pid != 0)
879  {
880  ereport(LOG,
881  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
882  errmsg("background worker \"%s\": only dynamic background workers can request notification",
883  worker->bgw_name)));
884  return;
885  }
886 
887  /*
888  * Enforce maximum number of workers. Note this is overly restrictive: we
889  * could allow more non-shmem-connected workers, because these don't count
890  * towards the MAX_BACKENDS limit elsewhere. For now, it doesn't seem
891  * important to relax this restriction.
892  */
893  if (++numworkers > max_worker_processes)
894  {
895  ereport(LOG,
896  (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
897  errmsg("too many background workers"),
898  errdetail_plural("Up to %d background worker can be registered with the current settings.",
899  "Up to %d background workers can be registered with the current settings.",
902  errhint("Consider increasing the configuration parameter \"max_worker_processes\".")));
903  return;
904  }
905 
906  /*
907  * Copy the registration data into the registered workers list.
908  */
909  rw = malloc(sizeof(RegisteredBgWorker));
910  if (rw == NULL)
911  {
912  ereport(LOG,
913  (errcode(ERRCODE_OUT_OF_MEMORY),
914  errmsg("out of memory")));
915  return;
916  }
917 
918  rw->rw_worker = *worker;
919  rw->rw_backend = NULL;
920  rw->rw_pid = 0;
921  rw->rw_child_slot = 0;
922  rw->rw_crashed_at = 0;
923  rw->rw_terminate = false;
924 
925  slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
926 }
927 
928 /*
929  * Register a new background worker from a regular backend.
930  *
931  * Returns true on success and false on failure. Failure typically indicates
932  * that no background worker slots are currently available.
933  *
934  * If handle != NULL, we'll set *handle to a pointer that can subsequently
935  * be used as an argument to GetBackgroundWorkerPid(). The caller can
936  * free this pointer using pfree(), if desired.
937  */
938 bool
940  BackgroundWorkerHandle **handle)
941 {
942  int slotno;
943  bool success = false;
944  bool parallel;
945  uint64 generation = 0;
946 
947  /*
948  * We can't register dynamic background workers from the postmaster. If
949  * this is a standalone backend, we're the only process and can't start
950  * any more. In a multi-process environment, it might be theoretically
951  * possible, but we don't currently support it due to locking
952  * considerations; see comments on the BackgroundWorkerSlot data
953  * structure.
954  */
955  if (!IsUnderPostmaster)
956  return false;
957 
958  if (!SanityCheckBackgroundWorker(worker, ERROR))
959  return false;
960 
961  parallel = (worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0;
962 
963  LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
964 
965  /*
966  * If this is a parallel worker, check whether there are already too many
967  * parallel workers; if so, don't register another one. Our view of
968  * parallel_terminate_count may be slightly stale, but that doesn't really
969  * matter: we would have gotten the same result if we'd arrived here
970  * slightly earlier anyway. There's no help for it, either, since the
971  * postmaster must not take locks; a memory barrier wouldn't guarantee
972  * anything useful.
973  */
974  if (parallel && (BackgroundWorkerData->parallel_register_count -
975  BackgroundWorkerData->parallel_terminate_count) >=
977  {
978  Assert(BackgroundWorkerData->parallel_register_count -
979  BackgroundWorkerData->parallel_terminate_count <=
981  LWLockRelease(BackgroundWorkerLock);
982  return false;
983  }
984 
985  /*
986  * Look for an unused slot. If we find one, grab it.
987  */
988  for (slotno = 0; slotno < BackgroundWorkerData->total_slots; ++slotno)
989  {
990  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
991 
992  if (!slot->in_use)
993  {
994  memcpy(&slot->worker, worker, sizeof(BackgroundWorker));
995  slot->pid = InvalidPid; /* indicates not started yet */
996  slot->generation++;
997  slot->terminate = false;
998  generation = slot->generation;
999  if (parallel)
1000  BackgroundWorkerData->parallel_register_count++;
1001 
1002  /*
1003  * Make sure postmaster doesn't see the slot as in use before it
1004  * sees the new contents.
1005  */
1006  pg_write_barrier();
1007 
1008  slot->in_use = true;
1009  success = true;
1010  break;
1011  }
1012  }
1013 
1014  LWLockRelease(BackgroundWorkerLock);
1015 
1016  /* If we found a slot, tell the postmaster to notice the change. */
1017  if (success)
1019 
1020  /*
1021  * If we found a slot and the user has provided a handle, initialize it.
1022  */
1023  if (success && handle)
1024  {
1025  *handle = palloc(sizeof(BackgroundWorkerHandle));
1026  (*handle)->slot = slotno;
1027  (*handle)->generation = generation;
1028  }
1029 
1030  return success;
1031 }
1032 
1033 /*
1034  * Get the PID of a dynamically-registered background worker.
1035  *
1036  * If the worker is determined to be running, the return value will be
1037  * BGWH_STARTED and *pidp will get the PID of the worker process.
1038  * Otherwise, the return value will be BGWH_NOT_YET_STARTED if the worker
1039  * hasn't been started yet, and BGWH_STOPPED if the worker was previously
1040  * running but is no longer.
1041  *
1042  * In the latter case, the worker may be stopped temporarily (if it is
1043  * configured for automatic restart and exited non-zero) or gone for
1044  * good (if it exited with code 0 or if it is configured not to restart).
1045  */
1048 {
1049  BackgroundWorkerSlot *slot;
1050  pid_t pid;
1051 
1052  Assert(handle->slot < max_worker_processes);
1053  slot = &BackgroundWorkerData->slot[handle->slot];
1054 
1055  /*
1056  * We could probably arrange to synchronize access to data using memory
1057  * barriers only, but for now, let's just keep it simple and grab the
1058  * lock. It seems unlikely that there will be enough traffic here to
1059  * result in meaningful contention.
1060  */
1061  LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
1062 
1063  /*
1064  * The generation number can't be concurrently changed while we hold the
1065  * lock. The pid, which is updated by the postmaster, can change at any
1066  * time, but we assume such changes are atomic. So the value we read
1067  * won't be garbage, but it might be out of date by the time the caller
1068  * examines it (but that's unavoidable anyway).
1069  */
1070  if (handle->generation != slot->generation)
1071  pid = 0;
1072  else
1073  pid = slot->pid;
1074 
1075  /* All done. */
1076  LWLockRelease(BackgroundWorkerLock);
1077 
1078  if (pid == 0)
1079  return BGWH_STOPPED;
1080  else if (pid == InvalidPid)
1081  return BGWH_NOT_YET_STARTED;
1082  *pidp = pid;
1083  return BGWH_STARTED;
1084 }
1085 
1086 /*
1087  * Wait for a background worker to start up.
1088  *
1089  * This is like GetBackgroundWorkerPid(), except that if the worker has not
1090  * yet started, we wait for it to do so; thus, BGWH_NOT_YET_STARTED is never
1091  * returned. However, if the postmaster has died, we give up and return
1092  * BGWH_POSTMASTER_DIED, since it that case we know that startup will not
1093  * take place.
1094  */
1097 {
1099  int rc;
1100 
1101  for (;;)
1102  {
1103  pid_t pid;
1104 
1106 
1107  status = GetBackgroundWorkerPid(handle, &pid);
1108  if (status == BGWH_STARTED)
1109  *pidp = pid;
1110  if (status != BGWH_NOT_YET_STARTED)
1111  break;
1112 
1113  rc = WaitLatch(MyLatch,
1116 
1117  if (rc & WL_POSTMASTER_DEATH)
1118  {
1119  status = BGWH_POSTMASTER_DIED;
1120  break;
1121  }
1122 
1124  }
1125 
1126  return status;
1127 }
1128 
1129 /*
1130  * Wait for a background worker to stop.
1131  *
1132  * If the worker hasn't yet started, or is running, we wait for it to stop
1133  * and then return BGWH_STOPPED. However, if the postmaster has died, we give
1134  * up and return BGWH_POSTMASTER_DIED, because it's the postmaster that
1135  * notifies us when a worker's state changes.
1136  */
1139 {
1141  int rc;
1142 
1143  for (;;)
1144  {
1145  pid_t pid;
1146 
1148 
1149  status = GetBackgroundWorkerPid(handle, &pid);
1150  if (status == BGWH_STOPPED)
1151  break;
1152 
1153  rc = WaitLatch(MyLatch,
1156 
1157  if (rc & WL_POSTMASTER_DEATH)
1158  {
1159  status = BGWH_POSTMASTER_DIED;
1160  break;
1161  }
1162 
1164  }
1165 
1166  return status;
1167 }
1168 
1169 /*
1170  * Instruct the postmaster to terminate a background worker.
1171  *
1172  * Note that it's safe to do this without regard to whether the worker is
1173  * still running, or even if the worker may already have existed and been
1174  * unregistered.
1175  */
1176 void
1178 {
1179  BackgroundWorkerSlot *slot;
1180  bool signal_postmaster = false;
1181 
1182  Assert(handle->slot < max_worker_processes);
1183  slot = &BackgroundWorkerData->slot[handle->slot];
1184 
1185  /* Set terminate flag in shared memory, unless slot has been reused. */
1186  LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
1187  if (handle->generation == slot->generation)
1188  {
1189  slot->terminate = true;
1190  signal_postmaster = true;
1191  }
1192  LWLockRelease(BackgroundWorkerLock);
1193 
1194  /* Make sure the postmaster notices the change to shared memory. */
1195  if (signal_postmaster)
1197 }
1198 
1199 /*
1200  * Look up (and possibly load) a bgworker entry point function.
1201  *
1202  * For functions contained in the core code, we use library name "postgres"
1203  * and consult the InternalBGWorkers array. External functions are
1204  * looked up, and loaded if necessary, using load_external_function().
1205  *
1206  * The point of this is to pass function names as strings across process
1207  * boundaries. We can't pass actual function addresses because of the
1208  * possibility that the function has been loaded at a different address
1209  * in a different process. This is obviously a hazard for functions in
1210  * loadable libraries, but it can happen even for functions in the core code
1211  * on platforms using EXEC_BACKEND (e.g., Windows).
1212  *
1213  * At some point it might be worthwhile to get rid of InternalBGWorkers[]
1214  * in favor of applying load_external_function() for core functions too;
1215  * but that raises portability issues that are not worth addressing now.
1216  */
1217 static bgworker_main_type
1218 LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname)
1219 {
1220  /*
1221  * If the function is to be loaded from postgres itself, search the
1222  * InternalBGWorkers array.
1223  */
1224  if (strcmp(libraryname, "postgres") == 0)
1225  {
1226  int i;
1227 
1228  for (i = 0; i < lengthof(InternalBGWorkers); i++)
1229  {
1230  if (strcmp(InternalBGWorkers[i].fn_name, funcname) == 0)
1231  return InternalBGWorkers[i].fn_addr;
1232  }
1233 
1234  /* We can only reach this by programming error. */
1235  elog(ERROR, "internal function \"%s\" not found", funcname);
1236  }
1237 
1238  /* Otherwise load from external library. */
1239  return (bgworker_main_type)
1240  load_external_function(libraryname, funcname, true, NULL);
1241 }
1242 
1243 /*
1244  * Given a PID, get the bgw_type of the background worker. Returns NULL if
1245  * not a valid background worker.
1246  *
1247  * The return value is in static memory belonging to this function, so it has
1248  * to be used before calling this function again. This is so that the caller
1249  * doesn't have to worry about the background worker locking protocol.
1250  */
1251 const char *
1253 {
1254  int slotno;
1255  bool found = false;
1256  static char result[BGW_MAXLEN];
1257 
1258  LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
1259 
1260  for (slotno = 0; slotno < BackgroundWorkerData->total_slots; slotno++)
1261  {
1262  BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
1263 
1264  if (slot->pid > 0 && slot->pid == pid)
1265  {
1266  strcpy(result, slot->worker.bgw_type);
1267  found = true;
1268  break;
1269  }
1270  }
1271 
1272  LWLockRelease(BackgroundWorkerLock);
1273 
1274  if (!found)
1275  return NULL;
1276 
1277  return result;
1278 }
char bgw_extra[BGW_EXTRALEN]
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