Subject: [PATCH v11 3/3] Rework wal_receiver_create_temp_slot using new infrastructure

From: Alvaro Herrera <[email-masked]>
Date: Wed, 25 Mar 2020 20:48:44 -0300

---
 doc/src/sgml/config.sgml                      | 10 ++---
 src/backend/access/transam/xlog.c             |  3 +-
 src/backend/postmaster/startup.c              | 10 ++++-
 src/backend/replication/walreceiver.c         | 40 +++++--------------
 src/backend/replication/walreceiverfuncs.c    | 28 ++++++++++---
 src/backend/utils/misc/guc.c                  |  2 +-
 src/backend/utils/misc/postgresql.conf.sample |  3 +-
 src/include/access/xlog.h                     |  1 +
 src/include/replication/walreceiver.h         |  4 +-
 9 files changed, 55 insertions(+), 46 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 1cf88e953d..c5160fe907 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -4173,11 +4173,11 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         Specifies whether a WAL receiver should create a temporary replication
         slot on the remote instance when no permanent replication slot to use
         has been configured (using <xref linkend="guc-primary-slot-name"/>).
-        The default is on.  The only reason to turn this off would be if the
-        remote instance is currently out of available replication slots.  This
-        parameter can only be set in the <filename>postgresql.conf</filename>
-        file or on the server command line.  Changes only take effect when the
-        WAL receiver process starts a new connection.
+        The default is off.  This parameter can only be set in the 
+        <filename>postgresql.conf</filename> file or on the server command line.
+       </para>
+       <para>
+        The WAL receiver is restarted after an update of <varname>wal_receiver_create_temp_slot</varname>.
        </para>
       </listitem>
      </varlistentry>
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index bbf8d4eee5..2ecb319438 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -290,6 +290,7 @@ bool		StandbyModeRequested = false;
 char	   *PrimaryConnInfo = NULL;
 char	   *PrimarySlotName = NULL;
 char	   *PromoteTriggerFile = NULL;
+bool		wal_receiver_create_temp_slot = false;
 
 /* are we currently in standby mode? */
 bool		StandbyMode = false;
@@ -12157,7 +12158,7 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
 						}
 						curFileTLI = tli;
 						RequestXLogStreaming(tli, ptr, PrimaryConnInfo,
-											 PrimarySlotName);
+											 PrimarySlotName, wal_receiver_create_temp_slot);
 						receivedUpto = 0;
 					}
 
diff --git a/src/backend/postmaster/startup.c b/src/backend/postmaster/startup.c
index f35b5adf7d..fd9ac35dac 100644
--- a/src/backend/postmaster/startup.c
+++ b/src/backend/postmaster/startup.c
@@ -107,18 +107,26 @@ StartupRereadConfig(void)
 {
 	char	   *conninfo = pstrdup(PrimaryConnInfo);
 	char	   *slotname = pstrdup(PrimarySlotName);
+	bool		tempSlot = wal_receiver_create_temp_slot;
 	bool		conninfoChanged;
 	bool		slotnameChanged;
+	bool		tempSlotChanged = false;
 
 	ProcessConfigFile(PGC_SIGHUP);
 
 	conninfoChanged = strcmp(conninfo, PrimaryConnInfo) != 0;
 	slotnameChanged = strcmp(slotname, PrimarySlotName) != 0;
 
+	/*
+	 * wal_receiver_create_temp_slot is used only when we have no slot
+	 * configured.  We do not need to track this change if it has no effect.
+	 */
+	if (!slotnameChanged && strcmp(PrimarySlotName, "") == 0)
+		tempSlotChanged = tempSlot != wal_receiver_create_temp_slot;
 	pfree(conninfo);
 	pfree(slotname);
 
-	if (conninfoChanged || slotnameChanged)
+	if (conninfoChanged || slotnameChanged || tempSlotChanged)
 		StartupRequestWalReceiverRestart();
 }
 
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index b471a49d12..0aa0c52c49 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -80,7 +80,6 @@

 /* GUC variables */
-bool		wal_receiver_create_temp_slot;
 int			wal_receiver_status_interval;
 int			wal_receiver_timeout;
 bool		hot_standby_feedback;
@@ -355,42 +354,23 @@ WalReceiverMain(void)
 		WalRcvFetchTimeLineHistoryFiles(startpointTLI, primaryTLI);
 
 		/*
-		 * Create temporary replication slot if no slot name is configured or
-		 * the slot from the previous run was temporary, unless
-		 * wal_receiver_create_temp_slot is disabled.  We also need to handle
-		 * the case where the previous run used a temporary slot but
-		 * wal_receiver_create_temp_slot was changed in the meantime.  In that
-		 * case, we delete the old slot name in shared memory.  (This would
+		 * Create temporary replication slot if requested.  In that
+		 * case, we update slot name in shared memory.  (This would
 		 * all be a bit easier if we just didn't copy the slot name into
 		 * shared memory, since we won't need it again later, but then we
 		 * can't see the slot name in the stats views.)
 		 */
-		if (slotname[0] == '\0' || is_temp_slot)
+		if (is_temp_slot)
 		{
-			bool		changed = false;
+			snprintf(slotname, sizeof(slotname),
+					 "pg_walreceiver_%lld",
+					 (long long int) walrcv_get_backend_pid(wrconn));
 
-			if (wal_receiver_create_temp_slot)
-			{
-				snprintf(slotname, sizeof(slotname),
-						 "pg_walreceiver_%lld",
-						 (long long int) walrcv_get_backend_pid(wrconn));
+			walrcv_create_slot(wrconn, slotname, true, 0, NULL);
 
-				walrcv_create_slot(wrconn, slotname, true, 0, NULL);
-				changed = true;
-			}
-			else if (slotname[0] != '\0')
-			{
-				slotname[0] = '\0';
-				changed = true;
-			}
-
-			if (changed)
-			{
-				SpinLockAcquire(&walrcv->mutex);
-				strlcpy(walrcv->slotname, slotname, NAMEDATALEN);
-				walrcv->is_temp_slot = wal_receiver_create_temp_slot;
-				SpinLockRelease(&walrcv->mutex);
-			}
+			SpinLockAcquire(&walrcv->mutex);
+			strlcpy(walrcv->slotname, slotname, NAMEDATALEN);
+			SpinLockRelease(&walrcv->mutex);
 		}
 
 		/*
diff --git a/src/backend/replication/walreceiverfuncs.c b/src/backend/replication/walreceiverfuncs.c
index 89c903e45a..21d1823607 100644
--- a/src/backend/replication/walreceiverfuncs.c
+++ b/src/backend/replication/walreceiverfuncs.c
@@ -215,13 +215,19 @@ ShutdownWalRcv(void)
 /*
  * Request postmaster to start walreceiver.
  *
- * recptr indicates the position where streaming should begin, conninfo
- * is a libpq connection string to use, and slotname is, optionally, the name
- * of a replication slot to acquire.
+ * "recptr" indicates the position where streaming should begin.  "conninfo"
+ * is a libpq connection string to use.  "slotname" is, optionally, the name
+ * of a replication slot to acquire.  "create_temp_slot" indicates to create
+ * a temporary slot when no "slotname" is given.
+ *
+ * WAL receivers do not directly load GUC parameters used for the connection
+ * to the primary, and rely on the values passed down by the caller of this
+ * routine instead.  Hence, the addition of any new parameters should happen
+ * through this code path.
  */
 void
 RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo,
-					 const char *slotname)
+					 const char *slotname, bool create_temp_slot)
 {
 	WalRcvData *walrcv = WalRcv;
 	bool		launch = false;
@@ -248,10 +254,22 @@ RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr, const char *conninfo,
 	else
 		walrcv->conninfo[0] = '\0';
 
-	if (slotname != NULL)
+	/*
+	 * Use configured replication slot if present, and ignore the value
+	 * of create_temp_slot as the slot name should be persistent.  Otherwise,
+	 * use create_temp_slot to determine whether this WAL receiver should
+	 * create a temporary slot by itself and use it, or not.
+	 */
+	if (slotname != NULL && slotname[0] != '\0')
+	{
 		strlcpy((char *) walrcv->slotname, slotname, NAMEDATALEN);
+		walrcv->is_temp_slot = false;
+	}
 	else
+	{
 		walrcv->slotname[0] = '\0';
+		walrcv->is_temp_slot = create_temp_slot;
+	}
 
 	if (walrcv->walRcvState == WALRCV_STOPPED)
 	{
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index c0b2b75407..53665971f5 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -2054,7 +2054,7 @@ static struct config_bool ConfigureNamesBool[] =
 			gettext_noop("Sets whether a WAL receiver should create a temporary replication slot if no permanent slot is configured."),
 		},
 		&wal_receiver_create_temp_slot,
-		true,
+		false,
 		NULL, NULL, NULL
 	},
 
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index c4f2f881c9..f01e43b818 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -319,7 +319,8 @@
 #max_standby_streaming_delay = 30s	# max delay before canceling queries
 					# when reading streaming WAL;
 					# -1 allows indefinite delay
-#wal_receiver_create_temp_slot = on	# create temp slot if primary_slot_name not set
+#wal_receiver_create_temp_slot = off	# Create temp slot if primary_slot_name
+					# is not set.
 #wal_receiver_status_interval = 10s	# send replies at least this often
 					# 0 disables
 #hot_standby_feedback = off		# send info from standby to prevent
diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 42c4df0262..997db214cc 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -129,6 +129,7 @@ extern int	recoveryTargetAction;
 extern int	recovery_min_apply_delay;
 extern char *PrimaryConnInfo;
 extern char *PrimarySlotName;
+extern bool wal_receiver_create_temp_slot;
 
 /* indirectly set via GUC system */
 extern TransactionId recoveryTargetXid;
diff --git a/src/include/replication/walreceiver.h b/src/include/replication/walreceiver.h
index e08afc6548..cf3e43128c 100644
--- a/src/include/replication/walreceiver.h
+++ b/src/include/replication/walreceiver.h
@@ -23,7 +23,6 @@
 #include "utils/tuplestore.h"
 
 /* user-settable parameters */
-extern bool wal_receiver_create_temp_slot;
 extern int	wal_receiver_status_interval;
 extern int	wal_receiver_timeout;
 extern bool hot_standby_feedback;
@@ -321,7 +320,8 @@ extern void ShutdownWalRcv(void);
 extern bool WalRcvStreaming(void);
 extern bool WalRcvRunning(void);
 extern void RequestXLogStreaming(TimeLineID tli, XLogRecPtr recptr,
-								 const char *conninfo, const char *slotname);
+								 const char *conninfo, const char *slotname,
+								 bool create_temp_slot);
 extern XLogRecPtr GetWalRcvWriteRecPtr(XLogRecPtr *latestChunkStart, TimeLineID *receiveTLI);
 extern int	GetReplicationApplyDelay(void);
 extern int	GetReplicationTransferLatency(void);
-- 
2.20.1

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