From 9e76c6bef0350718d36a6db5993baa4cdeacc5e3 Mon Sep 17 00:00:00 2001
From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Date: Wed, 15 Nov 2023 05:21:45 +0000
Subject: [PATCH v16] Log messages for replication slot acquisition and
 release.

This commit log messages (at LOG level when log_replication_commands is
set, otherwise at DEBUG1 level) when walsenders acquire and release
replication slots. These messages help to know the lifetime of a
replication slot - one can know how long a streaming standby, logical
subscriber, or replication slot consumer is down. These messages will be
useful on production servers to debug and analyze inactive replication
slots.

Note that these messages are emitted only for walsenders but not for
backends. This is because walsenders are the ones that typically hold
replication slots for longer durations, unlike backends which hold them
for executing replication related functions.

Author: Bharath Rupireddy
Reviewed-by: Peter Smith, Amit Kapila
Discussion: http://postgr.es/m/CALj2ACX17G7F-jeLt+7KhJ6YxVeRwR8Zk0rDh4VnT546o0UpTQ@mail.gmail.com
---
 doc/src/sgml/config.sgml       | 11 ++++++-----
 src/backend/replication/slot.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index fc35a46e5e..9398afbcbd 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -7510,11 +7510,12 @@ log_line_prefix = '%m [%p] %q%u@%d/%a '
       </term>
       <listitem>
        <para>
-        Causes each replication command to be logged in the server log.
-        See <xref linkend="protocol-replication"/> for more information about
-        replication command. The default value is <literal>off</literal>.
-        Only superusers and users with the appropriate <literal>SET</literal>
-        privilege can change this setting.
+        Causes each replication command and <literal>walsender</literal>
+        process's replication slot acquisition/release to be logged in the
+        server log. See <xref linkend="protocol-replication"/> for more
+        information about replication command. The default value is
+        <literal>off</literal>. Only superusers and users with the appropriate
+        <literal>SET</literal> privilege can change this setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 781aa43cc4..6f014d0df5 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -537,6 +537,15 @@ retry:
 	 */
 	if (SlotIsLogical(s))
 		pgstat_acquire_replslot(s);
+
+	if (am_walsender)
+	{
+		ereport(log_replication_commands ? LOG : DEBUG1,
+		/* translator: first %s is logical or physical slot */
+				errmsg("acquired %s replication slot \"%s\"",
+					   SlotIsLogical(s) ? "logical" : "physical",
+					   NameStr(s->data.name)));
+	}
 }
 
 /*
@@ -549,9 +558,17 @@ void
 ReplicationSlotRelease(void)
 {
 	ReplicationSlot *slot = MyReplicationSlot;
+	char	   *slotname = NULL;	/* keep compiler quiet */
+	bool		is_logical = false; /* keep compiler quiet */
 
 	Assert(slot != NULL && slot->active_pid != 0);
 
+	if (am_walsender)
+	{
+		slotname = pstrdup(NameStr(slot->data.name));
+		is_logical = SlotIsLogical(slot);
+	}
+
 	if (slot->data.persistency == RS_EPHEMERAL)
 	{
 		/*
@@ -596,6 +613,17 @@ ReplicationSlotRelease(void)
 	MyProc->statusFlags &= ~PROC_IN_LOGICAL_DECODING;
 	ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
 	LWLockRelease(ProcArrayLock);
+
+	if (am_walsender)
+	{
+		ereport(log_replication_commands ? LOG : DEBUG1,
+		/* translator: first %s is logical or physical slot */
+				errmsg("released %s replication slot \"%s\"",
+					   is_logical ? "logical" : "physical",
+					   slotname));
+
+		pfree(slotname);
+	}
 }
 
 /*
-- 
2.28.0.windows.1