Fix SIGUSR1 handling by unconnected bgworkers

Latch activity was not being detected by non-database-connected workers; the
SIGUSR1 signal handler which is normally in charge of that was set to SIG_IGN.
Create a simple handler to call latch_sigusr1_handler instead.

Robert Haas (bug report and suggested fix)

src/backend/postmaster/postmaster.c

@@ -5354,6 +5354,22 @@ bgworker_die(SIGNAL_ARGS)
 					MyBgworkerEntry->bgw_name)));
 }
 
+/*
+ * Standard SIGUSR1 handler for unconnected workers
+ *
+ * Here, we want to make sure an unconnected worker will at least heed
+ * latch activity.
+ */
+static void
+bgworker_sigusr1_handler(SIGNAL_ARGS)
+{
+	int			save_errno = errno;
+
+	latch_sigusr1_handler();
+
+	errno = save_errno;
+}
+
 static void
 do_start_bgworker(void)
 {
@@ -5410,7 +5426,7 @@ do_start_bgworker(void)
 	else
 	{
 		pqsignal(SIGINT, SIG_IGN);
-		pqsignal(SIGUSR1, SIG_IGN);
+		pqsignal(SIGUSR1, bgworker_sigusr1_handler);
 		pqsignal(SIGFPE, SIG_IGN);
 	}