From 3cf4416fc01cd27bf90059e136dc008b8b5b726f 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 v15] Log messages for replication slot acquisition and release

This commit adds 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 or logical subscriber or replication slot
consumer is down. In other words, one can know how long a
replication slot is inactive - the time between released and
acquired slot messages is the inactive replication slot duration.
These messages will be extremely useful on production servers to
debug and analyze inactive replication slot issues.

Note that these messages are emitted only for walsenders when
replication slot is acquired and released, but not for backends.
This is because walsenders are the ones that typically hold
replication slots for longer durations unlike backends which hold
replication slots for executing some replication related
functions.
---
 doc/src/sgml/config.sgml       | 11 ++++++-----
 src/backend/replication/slot.c | 36 ++++++++++++++++++++++++++++++++++
 2 files changed, 42 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..bac21244bd 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -537,6 +537,19 @@ retry:
 	 */
 	if (SlotIsLogical(s))
 		pgstat_acquire_replslot(s);
+
+	if (am_walsender)
+	{
+		ereport(log_replication_commands ? LOG : DEBUG1,
+
+		/*
+		 * translator: first %s is replication slot type, eg logical or
+		 * physical
+		 */
+				errmsg("acquired %s replication slot \"%s\"",
+					   SlotIsLogical(s) ? "logical" : "physical",
+					   NameStr(s->data.name)));
+	}
 }
 
 /*
@@ -549,9 +562,17 @@ 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 +617,21 @@ 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 replication slot type, eg logical or
+		 * physical
+		 */
+				errmsg("released %s replication slot \"%s\"",
+					   is_logical ? "logical" : "physical",
+					   slotname));
+
+		pfree(slotname);
+	}
 }
 
 /*
-- 
2.34.1