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

Author: michaelpq

doc/src/sgml/config.sgml

@@ -697,8 +697,7 @@ include_dir 'conf.d'
 
        <para>
         The default value is three connections. The value must be less
-        than <varname>max_connections</varname> minus
-        <xref linkend="guc-max-wal-senders"/>.
+        than <varname>max_connections</varname>.
         This parameter can only be set at server start.
        </para>
       </listitem>
@@ -3495,24 +3494,25 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
        </term>
        <listitem>
        <para>
-        Specifies the maximum number of concurrent connections from
-        standby servers or streaming base backup clients (i.e., the
-        maximum number of simultaneously running WAL sender
-        processes). The default is 10. The value 0 means replication is
-        disabled. WAL sender processes count towards the total number
-        of connections, so this parameter's value must be less than
-        <xref linkend="guc-max-connections"/> minus
-        <xref linkend="guc-superuser-reserved-connections"/>.
-        Abrupt streaming client disconnection might leave an orphaned
-        connection slot behind until
-        a timeout is reached, so this parameter should be set slightly
-        higher than the maximum number of expected clients so disconnected
-        clients can immediately reconnect.  This parameter can only
-        be set at server start.
-        Also, <varname>wal_level</varname> must be set to
+        Specifies the maximum number of concurrent connections from standby
+        servers or streaming base backup clients (i.e., the maximum number of
+        simultaneously running WAL sender processes). The default is
+        <literal>10</literal>.  The value <literal>0</literal> means
+        replication is disabled.  Abrupt streaming client disconnection might
+        leave an orphaned connection slot behind until a timeout is reached,
+        so this parameter should be set slightly higher than the maximum
+        number of expected clients so disconnected clients can immediately
+        reconnect.  This parameter can only be set at server start.  Also,
+        <varname>wal_level</varname> must be set to
         <literal>replica</literal> or higher to allow connections from standby
         servers.
        </para>
+
+       <para>
+         When running a standby server, you must set this parameter to the
+         same or higher value than on the master server. Otherwise, queries
+         will not be allowed in the standby server.
+        </para>
        </listitem>
       </varlistentry>
 

doc/src/sgml/high-availability.sgml

@@ -2177,6 +2177,11 @@ LOG:  database system is ready to accept read only connections
          <varname>max_locks_per_transaction</varname>
         </para>
        </listitem>
+       <listitem>
+        <para>
+         <varname>max_wal_senders</varname>
+        </para>
+       </listitem>
        <listitem>
         <para>
          <varname>max_worker_processes</varname>

doc/src/sgml/runtime.sgml

@@ -720,13 +720,13 @@ psql: could not connect to server: No such file or directory
        <row>
         <entry><varname>SEMMNI</varname></entry>
         <entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
-        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
+        <entry>at least <literal>ceil((max_connections + autovacuum_max_workers + wax_wal_senders + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
        </row>
 
        <row>
         <entry><varname>SEMMNS</varname></entry>
         <entry>Maximum number of semaphores system-wide</entry>
-        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
+        <entry><literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
        </row>
 
        <row>
@@ -785,13 +785,13 @@ psql: could not connect to server: No such file or directory
     other applications. The maximum number of semaphores in the system
     is set by <varname>SEMMNS</varname>, which consequently must be at least
     as high as <varname>max_connections</varname> plus
-    <varname>autovacuum_max_workers</varname> plus <varname>max_worker_processes</varname>,
-    plus one extra for each 16
+    <varname>autovacuum_max_workers</varname> plus <varname>max_wal_senders</varname>,
+    plus <varname>max_worker_processes</varname>, plus one extra for each 16
     allowed connections plus workers (see the formula in <xref
     linkend="sysvipc-parameters"/>).  The parameter <varname>SEMMNI</varname>
     determines the limit on the number of semaphore sets that can
     exist on the system at one time.  Hence this parameter must be at
-    least <literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)</literal>.
+    least <literal>ceil((max_connections + autovacuum_max_workers + max_wal_senders + max_worker_processes + 5) / 16)</literal>.
     Lowering the number
     of allowed connections is a temporary workaround for failures,
     which are usually confusingly worded <quote>No space

src/backend/access/rmgrdesc/xlogdesc.c

@@ -110,10 +110,11 @@ xlog_desc(StringInfo buf, XLogReaderState *record)
 		}
 
 		appendStringInfo(buf, "max_connections=%d max_worker_processes=%d "
-						 "max_prepared_xacts=%d max_locks_per_xact=%d "
-						 "wal_level=%s wal_log_hints=%s "
-						 "track_commit_timestamp=%s",
+						 "max_wal_senders=%d max_prepared_xacts=%d "
+						 "max_locks_per_xact=%d wal_level=%s "
+						 "wal_log_hints=%s track_commit_timestamp=%s",
 						 xlrec.MaxConnections,
+						 xlrec.max_wal_senders,
 						 xlrec.max_worker_processes,
 						 xlrec.max_prepared_xacts,
 						 xlrec.max_locks_per_xact,

src/backend/access/transam/xlog.c

@@ -5257,6 +5257,7 @@ BootStrapXLOG(void)
 	/* Set important parameter values for use when replaying WAL */
 	ControlFile->MaxConnections = MaxConnections;
 	ControlFile->max_worker_processes = max_worker_processes;
+	ControlFile->max_wal_senders = max_wal_senders;
 	ControlFile->max_prepared_xacts = max_prepared_xacts;
 	ControlFile->max_locks_per_xact = max_locks_per_xact;
 	ControlFile->wal_level = wal_level;
@@ -6170,6 +6171,9 @@ CheckRequiredParameterValues(void)
 		RecoveryRequiresIntParameter("max_worker_processes",
 									 max_worker_processes,
 									 ControlFile->max_worker_processes);
+		RecoveryRequiresIntParameter("max_wal_senders",
+									 max_wal_senders,
+									 ControlFile->max_wal_senders);
 		RecoveryRequiresIntParameter("max_prepared_transactions",
 									 max_prepared_xacts,
 									 ControlFile->max_prepared_xacts);
@@ -9460,6 +9464,7 @@ XLogReportParameters(void)
 		wal_log_hints != ControlFile->wal_log_hints ||
 		MaxConnections != ControlFile->MaxConnections ||
 		max_worker_processes != ControlFile->max_worker_processes ||
+		max_wal_senders != ControlFile->max_wal_senders ||
 		max_prepared_xacts != ControlFile->max_prepared_xacts ||
 		max_locks_per_xact != ControlFile->max_locks_per_xact ||
 		track_commit_timestamp != ControlFile->track_commit_timestamp)
@@ -9478,6 +9483,7 @@ XLogReportParameters(void)
 
 			xlrec.MaxConnections = MaxConnections;
 			xlrec.max_worker_processes = max_worker_processes;
+			xlrec.max_wal_senders = max_wal_senders;
 			xlrec.max_prepared_xacts = max_prepared_xacts;
 			xlrec.max_locks_per_xact = max_locks_per_xact;
 			xlrec.wal_level = wal_level;
@@ -9493,6 +9499,7 @@ XLogReportParameters(void)
 
 		ControlFile->MaxConnections = MaxConnections;
 		ControlFile->max_worker_processes = max_worker_processes;
+		ControlFile->max_wal_senders = max_wal_senders;
 		ControlFile->max_prepared_xacts = max_prepared_xacts;
 		ControlFile->max_locks_per_xact = max_locks_per_xact;
 		ControlFile->wal_level = wal_level;
@@ -9896,6 +9903,7 @@ xlog_redo(XLogReaderState *record)
 		LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
 		ControlFile->MaxConnections = xlrec.MaxConnections;
 		ControlFile->max_worker_processes = xlrec.max_worker_processes;
+		ControlFile->max_wal_senders = xlrec.max_wal_senders;
 		ControlFile->max_prepared_xacts = xlrec.max_prepared_xacts;
 		ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact;
 		ControlFile->wal_level = xlrec.wal_level;
@@ -9927,7 +9935,7 @@ xlog_redo(XLogReaderState *record)
 		UpdateControlFile();
 		LWLockRelease(ControlFileLock);
 
-		/* Check to see if any changes to max_connections give problems */
+		/* Check to see if any parameter change gives a problem on recovery */
 		CheckRequiredParameterValues();
 	}
 	else if (info == XLOG_FPW_CHANGE)

src/backend/postmaster/postmaster.c

@@ -885,11 +885,11 @@ PostmasterMain(int argc, char *argv[])
 	/*
 	 * Check for invalid combinations of GUC settings.
 	 */
-	if (ReservedBackends + max_wal_senders >= MaxConnections)
+	if (ReservedBackends >= MaxConnections)
 	{
-		write_stderr("%s: superuser_reserved_connections (%d) plus max_wal_senders (%d) must be less than max_connections (%d)\n",
+		write_stderr("%s: superuser_reserved_connections (%d) must be less than max_connections (%d)\n",
 					 progname,
-					 ReservedBackends, max_wal_senders, MaxConnections);
+					 ReservedBackends, MaxConnections);
 		ExitPostmaster(1);
 	}
 	if (XLogArchiveMode > ARCHIVE_MODE_OFF && wal_level == WAL_LEVEL_MINIMAL)
@@ -5532,7 +5532,7 @@ int
 MaxLivePostmasterChildren(void)
 {
 	return 2 * (MaxConnections + autovacuum_max_workers + 1 +
-				max_worker_processes);
+				max_wal_senders + max_worker_processes);
 }
 
 /*

src/backend/replication/walsender.c

@@ -2273,8 +2273,8 @@ InitWalSenderSlot(void)
 	Assert(MyWalSnd == NULL);
 
 	/*
-	 * Find a free walsender slot and reserve it. If this fails, we must be
-	 * out of WalSnd structures.
+	 * Find a free walsender slot and reserve it. This must not fail due to
+	 * the prior check for free WAL senders in InitProcess().
 	 */
 	for (i = 0; i < max_wal_senders; i++)
 	{
@@ -2310,12 +2310,8 @@ InitWalSenderSlot(void)
 			break;
 		}
 	}
-	if (MyWalSnd == NULL)
-		ereport(FATAL,
-				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
-				 errmsg("number of requested standby connections "
-						"exceeds max_wal_senders (currently %d)",
-						max_wal_senders)));
+
+	Assert(MyWalSnd != NULL);
 
 	/* Arrange to clean up at walsender exit */
 	on_shmem_exit(WalSndKill, 0);

src/backend/storage/lmgr/proc.c

@@ -43,6 +43,7 @@
 #include "postmaster/autovacuum.h"
 #include "replication/slot.h"
 #include "replication/syncrep.h"
+#include "replication/walsender.h"
 #include "storage/condition_variable.h"
 #include "storage/standby.h"
 #include "storage/ipc.h"
@@ -147,8 +148,9 @@ ProcGlobalSemas(void)
  *	  running out when trying to start another backend is a common failure.
  *	  So, now we grab enough semaphores to support the desired max number
  *	  of backends immediately at initialization --- if the sysadmin has set
- *	  MaxConnections, max_worker_processes, or autovacuum_max_workers higher
- *	  than his kernel will support, he'll find out sooner rather than later.
+ *	  MaxConnections, max_worker_processes, max_wal_senders, or
+ *	  autovacuum_max_workers higher than his kernel will support, he'll
+ *	  find out sooner rather than later.
  *
  *	  Another reason for creating semaphores here is that the semaphore
  *	  implementation typically requires us to create semaphores in the
@@ -180,6 +182,7 @@ InitProcGlobal(void)
 	ProcGlobal->freeProcs = NULL;
 	ProcGlobal->autovacFreeProcs = NULL;
 	ProcGlobal->bgworkerFreeProcs = NULL;
+	ProcGlobal->walsenderFreeProcs = NULL;
 	ProcGlobal->startupProc = NULL;
 	ProcGlobal->startupProcPid = 0;
 	ProcGlobal->startupBufferPinWaitBufId = -1;
@@ -253,13 +256,20 @@ InitProcGlobal(void)
 			ProcGlobal->autovacFreeProcs = &procs[i];
 			procs[i].procgloballist = &ProcGlobal->autovacFreeProcs;
 		}
-		else if (i < MaxBackends)
+		else if (i < MaxConnections + autovacuum_max_workers + 1 + max_worker_processes)
 		{
 			/* PGPROC for bgworker, add to bgworkerFreeProcs list */
 			procs[i].links.next = (SHM_QUEUE *) ProcGlobal->bgworkerFreeProcs;
 			ProcGlobal->bgworkerFreeProcs = &procs[i];
 			procs[i].procgloballist = &ProcGlobal->bgworkerFreeProcs;
 		}
+		else if (i < MaxBackends)
+		{
+			/* PGPROC for walsender, add to walsenderFreeProcs list */
+			procs[i].links.next = (SHM_QUEUE *) ProcGlobal->walsenderFreeProcs;
+			ProcGlobal->walsenderFreeProcs = &procs[i];
+			procs[i].procgloballist = &ProcGlobal->walsenderFreeProcs;
+		}
 
 		/* Initialize myProcLocks[] shared memory queues. */
 		for (j = 0; j < NUM_LOCK_PARTITIONS; j++)
@@ -311,6 +321,8 @@ InitProcess(void)
 		procgloballist = &ProcGlobal->autovacFreeProcs;
 	else if (IsBackgroundWorker)
 		procgloballist = &ProcGlobal->bgworkerFreeProcs;
+	else if (am_walsender)
+		procgloballist = &ProcGlobal->walsenderFreeProcs;
 	else
 		procgloballist = &ProcGlobal->freeProcs;
 
@@ -341,6 +353,11 @@ InitProcess(void)
 		 * in the autovacuum case?
 		 */
 		SpinLockRelease(ProcStructLock);
+		if (am_walsender)
+			ereport(FATAL,
+					(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
+					 errmsg("number of requested standby connections exceeds max_wal_senders (currently %d)",
+							max_wal_senders)));
 		ereport(FATAL,
 				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
 				 errmsg("sorry, too many clients already")));

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,

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
 	},
 
 	{
@@ -10911,15 +10911,25 @@ 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;
 }
 
 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;
 }

src/bin/pg_controldata/pg_controldata.c

@@ -304,6 +304,8 @@ main(int argc, char *argv[])
 		   ControlFile->MaxConnections);
 	printf(_("max_worker_processes setting:         %d\n"),
 		   ControlFile->max_worker_processes);
+	printf(_("max_wal_senders setting:              %d\n"),
+		   ControlFile->max_wal_senders);
 	printf(_("max_prepared_xacts setting:           %d\n"),
 		   ControlFile->max_prepared_xacts);
 	printf(_("max_locks_per_xact setting:           %d\n"),