Fix background workers for EXEC_BACKEND

Commit da07a1e was broken for EXEC_BACKEND because I failed to realize
that the MaxBackends recomputation needed to be duplicated by
subprocesses in SubPostmasterMain.  However, instead of having the value
be recomputed at all, it's better to assign the correct value at
postmaster initialization time, and have it be propagated to exec'ed
backends via BackendParameters.

MaxBackends stays as zero until after modules in
shared_preload_libraries have had a chance to register bgworkers, since
the value is going to be untrustworthy till that's finished.

Heikki Linnakangas and Álvaro Herrera

Author: alvherre

src/backend/postmaster/postmaster.c

@@ -499,6 +499,7 @@ typedef struct
 	bool		redirection_done;
 	bool		IsBinaryUpgrade;
 	int			max_safe_fds;
+	int			MaxBackends;
 #ifdef WIN32
 	HANDLE		PostmasterHandle;
 	HANDLE		initial_signal_pipe;
@@ -897,15 +898,14 @@ PostmasterMain(int argc, char *argv[])
 	process_shared_preload_libraries();
 
 	/*
-	 * If loadable modules have added background workers, MaxBackends needs to
-	 * be updated.	Do so now by forcing a no-op update of max_connections.
-	 * XXX This is a pretty ugly way to do it, but it doesn't seem worth
-	 * introducing a new entry point in guc.c to do it in a cleaner fashion.
+	 * Now that loadable modules have had their chance to register background
+	 * workers, calculate MaxBackends.  Add one for the autovacuum launcher.
 	 */
-	if (GetNumShmemAttachedBgworkers() > 0)
-		SetConfigOption("max_connections",
-						GetConfigOption("max_connections", false, false),
-						PGC_POSTMASTER, PGC_S_OVERRIDE);
+	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
+		GetNumShmemAttachedBgworkers();
+	/* internal error because the values were all checked previously */
+	if (MaxBackends > MAX_BACKENDS)
+		elog(ERROR, "too many backends configured");
 
 	/*
 	 * Establish input sockets.
@@ -5152,6 +5152,8 @@ RegisterBackgroundWorker(BackgroundWorker *worker)
 {
 	RegisteredBgWorker *rw;
 	int			namelen = strlen(worker->bgw_name);
+	static int	maxworkers;
+	static int	numworkers = 0;
 
 #ifdef EXEC_BACKEND
 
@@ -5162,6 +5164,11 @@ RegisterBackgroundWorker(BackgroundWorker *worker)
 	static int	BackgroundWorkerCookie = 1;
 #endif
 
+	/* initialize upper limit on first call */
+	if (numworkers == 0)
+		maxworkers = MAX_BACKENDS -
+			(MaxConnections + autovacuum_max_workers + 1);
+
 	if (!IsUnderPostmaster)
 		ereport(LOG,
 			(errmsg("registering background worker: %s", worker->bgw_name)));
@@ -5214,6 +5221,23 @@ RegisterBackgroundWorker(BackgroundWorker *worker)
 		return;
 	}
 
+	/*
+	 * Enforce maximum number of workers.  Note this is overly restrictive:
+	 * we could allow more non-shmem-connected workers, because these don't
+	 * count towards the MAX_BACKENDS limit elsewhere.  This doesn't really
+	 * matter for practical purposes; several million processes would need to
+	 * run on a single server.
+	 */
+	if (++numworkers > maxworkers)
+	{
+		ereport(LOG,
+				(errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
+				 errmsg("too many background workers"),
+				 errdetail("Up to %d background workers can be registered with the current settings.",
+						   maxworkers)));
+		return;
+	}
+
 	/*
 	 * Copy the registration data into the registered workers list.
 	 */
@@ -5836,6 +5860,8 @@ save_backend_variables(BackendParameters *param, Port *port,
 	param->IsBinaryUpgrade = IsBinaryUpgrade;
 	param->max_safe_fds = max_safe_fds;
 
+	param->MaxBackends = MaxBackends;
+
 #ifdef WIN32
 	param->PostmasterHandle = PostmasterHandle;
 	if (!write_duplicated_handle(&param->initial_signal_pipe,
@@ -6061,6 +6087,8 @@ restore_backend_variables(BackendParameters *param, Port *port)
 	IsBinaryUpgrade = param->IsBinaryUpgrade;
 	max_safe_fds = param->max_safe_fds;
 
+	MaxBackends = param->MaxBackends;
+
 #ifdef WIN32
 	PostmasterHandle = param->PostmasterHandle;
 	pgwin32_initial_signal_pipe = param->initial_signal_pipe;

src/backend/utils/init/globals.c

@@ -103,13 +103,14 @@ int			work_mem = 1024;
 int			maintenance_work_mem = 16384;
 
 /*
- * Primary determinants of sizes of shared-memory structures.  MaxBackends is
- * MaxConnections + autovacuum_max_workers + 1 (it is computed by the GUC
- * assign hooks for those variables):
+ * Primary determinants of sizes of shared-memory structures.
+ *
+ * MaxBackends is computed by PostmasterMain after modules have had a chance to
+ * register background workers.
  */
 int			NBuffers = 1000;
-int			MaxBackends = 100;
 int			MaxConnections = 90;
+int			MaxBackends = 0;
 
 int			VacuumCostPageHit = 1;		/* GUC parameters for vacuum */
 int			VacuumCostPageMiss = 10;

src/backend/utils/misc/guc.c

@@ -103,17 +103,6 @@
 #define MAX_KILOBYTES	(INT_MAX / 1024)
 #endif
 
-/*
- * Note: MAX_BACKENDS is limited to 2^23-1 because inval.c stores the
- * backend ID as a 3-byte signed integer.  Even if that limitation were
- * removed, we still could not exceed INT_MAX/4 because some places compute
- * 4*MaxBackends without any overflow check.  This is rechecked in
- * check_maxconnections, since MaxBackends is computed as MaxConnections
- * plus the number of bgworkers plus autovacuum_max_workers plus one (for the
- * autovacuum launcher).
- */
-#define MAX_BACKENDS	0x7fffff
-
 #define KB_PER_MB (1024)
 #define KB_PER_GB (1024*1024)
 
@@ -199,9 +188,7 @@ static const char *show_tcp_keepalives_idle(void);
 static const char *show_tcp_keepalives_interval(void);
 static const char *show_tcp_keepalives_count(void);
 static bool check_maxconnections(int *newval, void **extra, GucSource source);
-static void assign_maxconnections(int newval, void *extra);
 static bool check_autovacuum_max_workers(int *newval, void **extra, GucSource source);
-static void assign_autovacuum_max_workers(int newval, void *extra);
 static bool check_effective_io_concurrency(int *newval, void **extra, GucSource source);
 static void assign_effective_io_concurrency(int newval, void *extra);
 static void assign_pgstat_temp_directory(const char *newval, void *extra);
@@ -1615,7 +1602,7 @@ static struct config_int ConfigureNamesInt[] =
 		},
 		&MaxConnections,
 		100, 1, MAX_BACKENDS,
-		check_maxconnections, assign_maxconnections, NULL
+		check_maxconnections, NULL, NULL
 	},
 
 	{
@@ -2290,7 +2277,7 @@ static struct config_int ConfigureNamesInt[] =
 		},
 		&autovacuum_max_workers,
 		3, 1, MAX_BACKENDS,
-		check_autovacuum_max_workers, assign_autovacuum_max_workers, NULL
+		check_autovacuum_max_workers, NULL, NULL
 	},
 
 	{
@@ -8636,13 +8623,6 @@ check_maxconnections(int *newval, void **extra, GucSource source)
 	return true;
 }
 
-static void
-assign_maxconnections(int newval, void *extra)
-{
-	MaxBackends = newval + autovacuum_max_workers + 1 +
-		GetNumShmemAttachedBgworkers();
-}
-
 static bool
 check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 {
@@ -8652,12 +8632,6 @@ check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 	return true;
 }
 
-static void
-assign_autovacuum_max_workers(int newval, void *extra)
-{
-	MaxBackends = MaxConnections + newval + 1 + GetNumShmemAttachedBgworkers();
-}
-
 static bool
 check_effective_io_concurrency(int *newval, void **extra, GucSource source)
 {

src/include/postmaster/postmaster.h

@@ -61,4 +61,13 @@ extern Size ShmemBackendArraySize(void);
 extern void ShmemBackendArrayAllocation(void);
 #endif
 
+/*
+ * Note: MAX_BACKENDS is limited to 2^23-1 because inval.c stores the
+ * backend ID as a 3-byte signed integer.  Even if that limitation were
+ * removed, we still could not exceed INT_MAX/4 because some places compute
+ * 4*MaxBackends without any overflow check.  This is rechecked in the relevant
+ * GUC check hooks and in RegisterBackgroundWorker().
+ */
+#define MAX_BACKENDS	0x7fffff
+
 #endif   /* _POSTMASTER_H */