Add GUC to enable logging of replication commands.

Previously replication commands like IDENTIFY_COMMAND were not logged
even when log_statements is set to all. Some users who want to audit
all types of statements were not satisfied with this situation. To
address the problem, this commit adds new GUC log_replication_commands.
If it's enabled, all replication commands are logged in the server log.

There are many ways to allow us to enable that logging. For example,
we can extend log_statement so that replication commands are logged
when it's set to all. But per discussion in the community, we reached
the consensus to add separate GUC for that.

Reviewed by Ian Barwick, Robert Haas and Heikki Linnakangas.

doc/src/sgml/config.sgml

@@ -4672,6 +4672,22 @@ FROM pg_stat_activity;
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-log-replication-commands" xreflabel="log_replication_commands">
+      <term><varname>log_replication_commands</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>log_replication_commands</> configuration parameter</primary>
+      </indexterm>
+      </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</>.
+        Only superusers can change this setting.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-log-temp-files" xreflabel="log_temp_files">
       <term><varname>log_temp_files</varname> (<type>integer</type>)
       <indexterm>

doc/src/sgml/protocol.sgml

@@ -1306,6 +1306,8 @@ To initiate streaming replication, the frontend sends the
 of <literal>true</> tells the backend to go into walsender mode, wherein a
 small set of replication commands can be issued instead of SQL statements. Only
 the simple query protocol can be used in walsender mode.
+Replication commands are logged in the server log when
+<xref linkend="guc-log-replication-commands"> is enabled.
 Passing <literal>database</> as the value instructs walsender to connect to
 the database specified in the <literal>dbname</> parameter, which will allow
 the connection to be used for logical replication from that database.

src/backend/replication/walsender.c

@@ -108,6 +108,7 @@ bool		am_db_walsender = false;	/* Connected to a database? */
 int			max_wal_senders = 0;	/* the maximum number of concurrent walsenders */
 int			wal_sender_timeout = 60 * 1000;		/* maximum time to send one
 												 * WAL data message */
+bool		log_replication_commands = false;
 
 /*
  * State for WalSndWakeupRequest
@@ -1267,14 +1268,20 @@ exec_replication_command(const char *cmd_string)
 	MemoryContext cmd_context;
 	MemoryContext old_context;
 
+	/*
+	 * Log replication command if log_replication_commands is enabled.
+	 * Even when it's disabled, log the command with DEBUG1 level for
+	 * backward compatibility.
+	 */
+	ereport(log_replication_commands ? LOG : DEBUG1,
+			(errmsg("received replication command: %s", cmd_string)));
+
 	/*
 	 * CREATE_REPLICATION_SLOT ... LOGICAL exports a snapshot until the next
 	 * command arrives. Clean up the old stuff if there's anything.
 	 */
 	SnapBuildClearExportedSnapshot();
 
-	elog(DEBUG1, "received replication command: %s", cmd_string);
-
 	CHECK_FOR_INTERRUPTS();
 
 	cmd_context = AllocSetContextCreate(CurrentMemoryContext,

src/backend/utils/misc/guc.c

@@ -924,6 +924,15 @@ static struct config_bool ConfigureNamesBool[] =
 		false,
 		NULL, NULL, NULL
 	},
+	{
+		{"log_replication_commands", PGC_SUSET, LOGGING_WHAT,
+			gettext_noop("Logs each replication command."),
+			NULL
+		},
+		&log_replication_commands,
+		false,
+		NULL, NULL, NULL
+	},
 	{
 		{"debug_assertions", PGC_INTERNAL, PRESET_OPTIONS,
 			gettext_noop("Shows whether the running server has assertion checks enabled."),

src/backend/utils/misc/postgresql.conf.sample

@@ -431,6 +431,7 @@
 					# e.g. '<%u%%%d> '
 #log_lock_waits = off			# log lock waits >= deadlock_timeout
 #log_statement = 'none'			# none, ddl, mod, all
+#log_replication_commands = off
 #log_temp_files = -1			# log temporary files equal or larger
 					# than the specified size in kilobytes;
 					# -1 disables, 0 logs all temp files

src/include/replication/walsender.h

@@ -25,6 +25,7 @@ extern bool wake_wal_senders;
 /* user-settable parameters */
 extern int	max_wal_senders;
 extern int	wal_sender_timeout;
+extern bool	log_replication_commands;
 
 extern void InitWalSender(void);
 extern void exec_replication_command(const char *query_string);