Move max_wal_senders out of max_connections for connection slot handling

Since its introduction, max_wal_senders is counted as part of
max_connections when it comes to define how many connection slots can be
used for replication connections with a WAL sender context.  This can
lead to confusion for some users, as it could be possible to block a
base backup or replication from happening because other backend sessions
are already taken for other purposes by an application, and
superuser-only connection slots are not a correct solution to handle
that case.

This commit makes max_wal_senders independent of max_connections for its
handling of PGPROC entries in ProcGlobal, meaning that connection slots
for WAL senders are handled using their own free queue, like autovacuum
workers and bgworkers.

One compatibility issue that this change creates is that a standby now
requires to have a value of max_wal_senders at least equal to its
primary.  So, if a standby created enforces the value of
max_wal_senders to be lower than that, then this could break failovers.
Normally this should not be an issue though, as any settings of a
standby are inherited from its primary as postgresql.conf gets normally
copied as part of a base backup, so parameters would be consistent.

Author: Alexander Kukushkin
Reviewed-by: Kyotaro Horiguchi, Petr Jelínek, Masahiko Sawada, Oleksii
Kliukin
Discussion: https://postgr.es/m/CAFh8B=nBzHQeYAu0b8fjK-AF1X4+_p6GRtwG+cCgs6Vci2uRuQ@mail.gmail.com

2 files changed, 22 insertions, 12 deletions

diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index c0b62314580..a5ee209f910 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -527,7 +527,7 @@ InitializeMaxBackends(void)
 
 	/* the extra unit accounts for the autovacuum launcher */
 	MaxBackends = MaxConnections + autovacuum_max_workers + 1 +
-		max_worker_processes;
+		max_worker_processes + max_wal_senders;
 
 	/* internal error because the values were all checked previously */
 	if (MaxBackends > MAX_BACKENDS)
@@ -811,12 +811,11 @@ InitPostgres(const char *in_dbname, Oid dboid, const char *username,
 	}
 
 	/*
-	 * The last few connection slots are reserved for superusers.  Although
-	 * replication connections currently require superuser privileges, we
-	 * don't allow them to consume the reserved slots, which are intended for
-	 * interactive use.
+	 * The last few connection slots are reserved for superusers.  Replication
+	 * connections are drawn from slots reserved with max_wal_senders and not
+	 * limited by max_connections or superuser_reserved_connections.
 	 */
-	if ((!am_superuser || am_walsender) &&
+	if (!am_superuser && !am_walsender &&
 		ReservedBackends > 0 &&
 		!HaveNFreeProcs(ReservedBackends))
 		ereport(FATAL,
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index ea5444c6f15..41d477165cd 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -187,6 +187,7 @@ static const char *show_tcp_keepalives_count(void);
 static bool check_maxconnections(int *newval, void **extra, GucSource source);
 static bool check_max_worker_processes(int *newval, void **extra, GucSource source);
 static bool check_autovacuum_max_workers(int *newval, void **extra, GucSource source);
+static bool check_max_wal_senders(int *newval, void **extra, GucSource source);
 static bool check_autovacuum_work_mem(int *newval, void **extra, GucSource source);
 static bool check_effective_io_concurrency(int *newval, void **extra, GucSource source);
 static void assign_effective_io_concurrency(int newval, void *extra);
@@ -2090,7 +2091,7 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		/* see max_connections and max_wal_senders */
+		/* see max_connections */
 		{"superuser_reserved_connections", PGC_POSTMASTER, CONN_AUTH_SETTINGS,
 			gettext_noop("Sets the number of connection slots reserved for superusers."),
 			NULL
@@ -2608,14 +2609,13 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		/* see max_connections and superuser_reserved_connections */
 		{"max_wal_senders", PGC_POSTMASTER, REPLICATION_SENDING,
 			gettext_noop("Sets the maximum number of simultaneously running WAL sender processes."),
 			NULL
 		},
 		&max_wal_senders,
 		10, 0, MAX_BACKENDS,
-		NULL, NULL, NULL
+		check_max_wal_senders, NULL, NULL
 	},
 
 	{
@@ -10911,7 +10911,7 @@ static bool
 check_maxconnections(int *newval, void **extra, GucSource source)
 {
 	if (*newval + autovacuum_max_workers + 1 +
-		max_worker_processes > MAX_BACKENDS)
+		max_worker_processes + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
 }
@@ -10919,7 +10919,17 @@ check_maxconnections(int *newval, void **extra, GucSource source)
 static bool
 check_autovacuum_max_workers(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + *newval + 1 + max_worker_processes > MAX_BACKENDS)
+	if (MaxConnections + *newval + 1 +
+		max_worker_processes + max_wal_senders > MAX_BACKENDS)
+		return false;
+	return true;
+}
+
+static bool
+check_max_wal_senders(int *newval, void **extra, GucSource source)
+{
+	if (MaxConnections + autovacuum_max_workers + 1 +
+		max_worker_processes + *newval > MAX_BACKENDS)
 		return false;
 	return true;
 }
@@ -10950,7 +10960,8 @@ check_autovacuum_work_mem(int *newval, void **extra, GucSource source)
 static bool
 check_max_worker_processes(int *newval, void **extra, GucSource source)
 {
-	if (MaxConnections + autovacuum_max_workers + 1 + *newval > MAX_BACKENDS)
+	if (MaxConnections + autovacuum_max_workers + 1 +
+		*newval + max_wal_senders > MAX_BACKENDS)
 		return false;
 	return true;
 }