check-for-enough-child-slots-for-new-bgworkers.patch

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index eb9e022..7947c96 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -409,7 +409,7 @@ static void SendNegotiateProtocolVersion(List *unrecognized_protocol_options);
 static void processCancelRequest(Port *port, void *pkt);
 static int	initMasks(fd_set *rmask);
 static void report_fork_failure_to_client(Port *port, int errnum);
-static CAC_state canAcceptConnections(void);
+static CAC_state canAcceptConnections(int backend_type);
 static bool RandomCancelKey(int32 *cancel_key);
 static void signal_child(pid_t pid, int signal);
 static bool SignalSomeChildren(int signal, int targets);
@@ -2401,13 +2401,15 @@ processCancelRequest(Port *port, void *pkt)
  * canAcceptConnections --- check to see if database state allows connections.
  */
 static CAC_state
-canAcceptConnections(void)
+canAcceptConnections(int backend_type)
 {
 	CAC_state	result = CAC_OK;
 
 	/*
 	 * Can't start backends when in startup/shutdown/inconsistent recovery
-	 * state.
+	 * state.  We treat autovac workers the same as user backends for this
+	 * purpose.  However, bgworkers are excluded from this test; we expect
+	 * bgworker_should_start_now() decided whether the DB state allows them.
 	 *
 	 * In state PM_WAIT_BACKUP only superusers can connect (this must be
 	 * allowed so that a superuser can end online backup mode); we return
@@ -2415,7 +2417,8 @@ canAcceptConnections(void)
 	 * that neither CAC_OK nor CAC_WAITBACKUP can safely be returned until we
 	 * have checked for too many children.
 	 */
-	if (pmState != PM_RUN)
+	if (pmState != PM_RUN &&
+		backend_type != BACKEND_TYPE_BGWORKER)
 	{
 		if (pmState == PM_WAIT_BACKUP)
 			result = CAC_WAITBACKUP;	/* allow superusers only */
@@ -2435,9 +2438,9 @@ canAcceptConnections(void)
 	/*
 	 * Don't start too many children.
 	 *
-	 * We allow more connections than we can have backends here because some
+	 * We allow more connections here than we can have backends because some
 	 * might still be authenticating; they might fail auth, or some existing
-	 * backend might exit before the auth cycle is completed. The exact
+	 * backend might exit before the auth cycle is completed.  The exact
 	 * MaxBackends limit is enforced when a new backend tries to join the
 	 * shared-inval backend array.
 	 *
@@ -4114,7 +4117,7 @@ BackendStartup(Port *port)
 	bn->cancel_key = MyCancelKey;
 
 	/* Pass down canAcceptConnections state */
-	port->canAcceptConnections = canAcceptConnections();
+	port->canAcceptConnections = canAcceptConnections(BACKEND_TYPE_NORMAL);
 	bn->dead_end = (port->canAcceptConnections != CAC_OK &&
 					port->canAcceptConnections != CAC_WAITBACKUP);
 
@@ -5486,7 +5489,7 @@ StartAutovacuumWorker(void)
 	 * we have to check to avoid race-condition problems during DB state
 	 * changes.
 	 */
-	if (canAcceptConnections() == CAC_OK)
+	if (canAcceptConnections(BACKEND_TYPE_AUTOVAC) == CAC_OK)
 	{
 		/*
 		 * Compute the cancel key that will be assigned to this session. We
@@ -5863,6 +5866,19 @@ assign_backendlist_entry(RegisteredBgWorker *rw)
 	Backend    *bn;
 
 	/*
+	 * Check that database state allows another connection.  Currently the
+	 * only possible failure is CAC_TOOMANY, so we just log an error message
+	 * based on that rather than checking the error code precisely.
+	 */
+	if (canAcceptConnections(BACKEND_TYPE_BGWORKER) != CAC_OK)
+	{
+		ereport(LOG,
+				(errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
+				 errmsg("no slot available for new worker process")));
+		return false;
+	}
+
+	/*
 	 * Compute the cancel key that will be assigned to this session. We
 	 * probably don't need cancel keys for background workers, but we'd better
 	 * have something random in the field to prevent unfriendly people from