Add value 'current' for recovery_target_timeline

This value represents the default behavior of using the current
timeline.  Previously, this was represented by an empty string.

(Before the removal of recovery.conf, this setting could not be chosen
explicitly but was used when recovery_target_timeline was not
mentioned at all.)

Discussion: https://www.postgresql.org/message-id/flat/6dd2c23a-4162-8469-410f-bfe146e28c0c@2ndquadrant.com/Reviewed-by: David Steele <david@pgmasters.net>
Reviewed-by: Michael Paquier <michael@paquier.xyz>

doc/src/sgml/config.sgml

@@ -3350,9 +3350,11 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
       </term>
       <listitem>
        <para>
-        Specifies recovering into a particular timeline.  The default is
-        to recover along the same timeline that was current when the
-        base backup was taken. Setting this to <literal>latest</literal> recovers
+        Specifies recovering into a particular timeline.  The value can be a
+        numeric timeline ID or a special value.  The value
+        <literal>current</literal> recovers along the same timeline that was
+        current when the base backup was taken.  That is the default.  The
+        value <literal>latest</literal> recovers
         to the latest timeline found in the archive, which is useful in
         a standby server. Other than that you only need to set this parameter
         in complex re-recovery situations, where you need to return to

src/backend/utils/misc/guc.c

@@ -3387,7 +3387,7 @@ static struct config_string ConfigureNamesString[] =
 			NULL
 		},
 		&recovery_target_timeline_string,
-		"",
+		"current",
 		check_recovery_target_timeline, assign_recovery_target_timeline, NULL
 	},
 
@@ -11031,7 +11031,7 @@ check_recovery_target_timeline(char **newval, void **extra, GucSource source)
 	RecoveryTargetTimeLineGoal rttg = RECOVERY_TARGET_TIMELINE_CONTROLFILE;
 	RecoveryTargetTimeLineGoal *myextra;
 
-	if (strcmp(*newval, "") == 0)
+	if (strcmp(*newval, "current") == 0)
 		rttg = RECOVERY_TARGET_TIMELINE_CONTROLFILE;
 	else if (strcmp(*newval, "latest") == 0)
 		rttg = RECOVERY_TARGET_TIMELINE_LATEST;

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

@@ -261,8 +261,7 @@
 				# just after the specified recovery target (on)
 				# just before the recovery target (off)
 				# (change requires restart)
-#recovery_target_timeline = ''	# unset means read from control file (default),
-				# or set to 'latest' or timeline ID
+#recovery_target_timeline = 'current'	# 'current', 'latest', or timeline ID
 				# (change requires restart)
 #recovery_target_action = 'pause'	# 'pause', 'promote', 'shutdown'
 				# (change requires restart)