When a background worker exists with code 0, unregister it.

The previous behavior was to restart immediately, which was generally
viewed as less useful.

Petr Jelinek, with some adjustments by me.

Author: robertmhaas

doc/src/sgml/bgworker.sgml

@@ -166,10 +166,16 @@ typedef struct BackgroundWorker
   </para>
 
   <para>
-   Background workers are expected to be continuously running; if they exit
-   cleanly, <command>postgres</> will restart them immediately.  Consider doing
-   interruptible sleep when they have nothing to do; this can be achieved by
-   calling <function>WaitLatch()</function>.  Make sure the
+   If <structfield>bgw_restart_time</structfield> for a background worker is
+   configured as <literal>BGW_NEVER_RESTART</>, or if it exits with an exit
+   code of 0 or is terminated by <function>TerminateBackgroundWorker</>,
+   it will be automatically unregistered by the postmaster on exit.
+   Otherwise, it will be restarted after the time period configured via
+   <structfield>bgw_restart_time</>, or immediately if the postmaster
+   reinitializes the cluster due to a backend failure.  Backends which need
+   to suspend execution only temporarily should use an interruptible sleep
+   rather than exiting; this can be achieved by calling
+   <function>WaitLatch()</function>. Make sure the
    <literal>WL_POSTMASTER_DEATH</> flag is set when calling that function, and
    verify the return code for a prompt exit in the emergency case that
    <command>postgres</> itself has terminated.

src/backend/postmaster/bgworker.c

@@ -884,8 +884,8 @@ RegisterDynamicBackgroundWorker(BackgroundWorker *worker,
  * running but is no longer.
  *
  * In the latter case, the worker may be stopped temporarily (if it is
- * configured for automatic restart, or if it exited with code 0) or gone
- * for good (if it is configured not to restart and exited with code 1).
+ * configured for automatic restart and exited non-zero) or gone for
+ * good (if it exited with code 0 or if it is configured not to restart).
  */
 BgwHandleStatus
 GetBackgroundWorkerPid(BackgroundWorkerHandle *handle, pid_t *pidp)

src/backend/postmaster/postmaster.c

@@ -2845,11 +2845,17 @@ CleanupBackgroundWorker(int pid,
 		snprintf(namebuf, MAXPGPATH, "%s: %s", _("worker process"),
 				 rw->rw_worker.bgw_name);
 
-		/* Delay restarting any bgworker that exits with a nonzero status. */
 		if (!EXIT_STATUS_0(exitstatus))
+		{
+			/* Record timestamp, so we know when to restart the worker. */
 			rw->rw_crashed_at = GetCurrentTimestamp();
+		}
 		else
+		{
+			/* Zero exit status means terminate */
 			rw->rw_crashed_at = 0;
+			rw->rw_terminate = true;
+		}
 
 		/*
 		 * Additionally, for shared-memory-connected workers, just like a

src/include/postmaster/bgworker.h

@@ -16,10 +16,10 @@
  * that the failure can only be transient (fork failure due to high load,
  * memory pressure, too many processes, etc); more permanent problems, like
  * failure to connect to a database, are detected later in the worker and dealt
- * with just by having the worker exit normally.  A worker which exits with a
- * return code of 0 will be immediately restarted by the postmaster.  A worker
- * which exits with a return code of 1 will be restarted after the configured
- * restart interval, or never if that interval is set to BGW_NEVER_RESTART.
+ * with just by having the worker exit normally. A worker which exits with
+ * a return code of 0 will never be restarted and will be removed from worker
+ * list. A worker which exits with a return code of 1 will be restarted after
+ * the configured restart interval (unless that interval is BGW_NEVER_RESTART).
  * The TerminateBackgroundWorker() function can be used to terminate a
  * dynamically registered background worker; the worker will be sent a SIGTERM
  * and will not be restarted after it exits.  Whenever the postmaster knows