Add archive_mode='always' option.

In 'always' mode, the standby independently archives all files it receives
from the primary.

Original patch by Fujii Masao, docs and review by me.

doc/src/sgml/config.sgml

@@ -2521,7 +2521,7 @@ include_dir 'conf.d'
 
     <variablelist>
      <varlistentry id="guc-archive-mode" xreflabel="archive_mode">
-      <term><varname>archive_mode</varname> (<type>boolean</type>)
+      <term><varname>archive_mode</varname> (<type>enum</type>)
       <indexterm>
        <primary><varname>archive_mode</> configuration parameter</primary>
       </indexterm>
@@ -2530,7 +2530,16 @@ include_dir 'conf.d'
        <para>
         When <varname>archive_mode</> is enabled, completed WAL segments
         are sent to archive storage by setting
-        <xref linkend="guc-archive-command">.
+        <xref linkend="guc-archive-command">. In addition to <literal>off</>,
+        to disable, there are two modes: <literal>on</>, and
+        <literal>always</>. During normal operation, there is no
+        difference between the two modes, but when set to <literal>always</>
+        the WAL archiver is enabled also during archive recovery or standby
+        mode. In <literal>always</> mode, all files restored from the archive
+        or streamed with streaming replication will be archived (again). See
+        <xref linkend="continuous-archiving-in-standby"> for details.
+       </para>  
+       <para>
         <varname>archive_mode</> and <varname>archive_command</> are
         separate variables so that <varname>archive_command</> can be
         changed without leaving archiving mode.

doc/src/sgml/high-availability.sgml

@@ -1220,6 +1220,45 @@ primary_slot_name = 'node_a_slot'
 
    </sect3>
   </sect2>
+
+  <sect2 id="continuous-archiving-in-standby">
+   <title>Continuous archiving in standby</title>
+
+   <indexterm>
+     <primary>continuous archiving</primary>
+     <secondary>in standby</secondary>
+   </indexterm>
+
+   <para>
+     When continuous WAL archiving is used in a standby, there are two
+     different scenarios: the WAL archive can be shared between the primary
+     and the standby, or the standby can have its own WAL archive. When
+     the standby has its own WAL archive, set <varname>archive_mode</varname>
+     to <literal>always</literal>, and the standby will call the archive
+     command for every WAL segment it receives, whether it's by restoring
+     from the archive or by streaming replication. The shared archive can
+     be handled similarly, but the archive_command must test if the file
+     being archived exists already, and if the existing file has identical
+     contents. This requires more care in the archive_command, as it must
+     be careful to not overwrite an existing file with different contents,
+     but return success if the exactly same file is archived twice. And
+     all that must be done free of race conditions, if two servers attempt
+     to archive the same file at the same time.
+   </para>
+
+   </para>
+     If <varname>archive_mode</varname> is set to <literal>on</>, the
+     archiver is not enabled during recovery or standby mode. If the standby
+     server is promoted, it will start archiving after the promotion, but
+     will not archive any WAL it did not generate itself. To get a complete
+     series of WAL files in the archive, you must ensure that all WAL is
+     archived, before it reaches the standby. This is inherently true with
+     file-based log shipping, as the standby can only restore files that
+     are found in the archive, but not if streaming replication is enabled.
+     When a server is not in recovery mode, there is no difference between
+     <literal>on</literal> and <literal>always</literal> modes.
+   </para>
+  </sect2>
   </sect1>
 
   <sect1 id="warm-standby-failover">

src/backend/access/transam/xlog.c

@@ -86,7 +86,7 @@ int			min_wal_size = 5;		/* 80 MB */
 int			wal_keep_segments = 0;
 int			XLOGbuffers = -1;
 int			XLogArchiveTimeout = 0;
-bool		XLogArchiveMode = false;
+int			XLogArchiveMode = ARCHIVE_MODE_OFF;
 char	   *XLogArchiveCommand = NULL;
 bool		EnableHotStandby = false;
 bool		fullPageWrites = true;
@@ -140,6 +140,24 @@ const struct config_enum_entry sync_method_options[] = {
 	{NULL, 0, false}
 };
 
+
+/*
+ * Although only "on", "off", and "always" are documented,
+ * we accept all the likely variants of "on" and "off".
+ */
+const struct config_enum_entry archive_mode_options[] = {
+	{"always", ARCHIVE_MODE_ALWAYS, false},
+	{"on", ARCHIVE_MODE_ON, false},
+	{"off", ARCHIVE_MODE_OFF, false},
+	{"true", ARCHIVE_MODE_ON, true},
+	{"false", ARCHIVE_MODE_OFF, true},
+	{"yes", ARCHIVE_MODE_ON, true},
+	{"no", ARCHIVE_MODE_OFF, true},
+	{"1", ARCHIVE_MODE_ON, true},
+	{"0", ARCHIVE_MODE_OFF, true},
+	{NULL, 0, false}
+};
+
 /*
  * Statistics for current checkpoint are collected in this global struct.
  * Because only the checkpointer or a stand-alone backend can perform
@@ -767,7 +785,7 @@ static MemoryContext walDebugCxt = NULL;
 #endif
 
 static void readRecoveryCommandFile(void);
-static void exitArchiveRecovery(TimeLineID endTLI, XLogSegNo endLogSegNo);
+static void exitArchiveRecovery(TimeLineID endTLI, XLogRecPtr endOfLog);
 static bool recoveryStopsBefore(XLogReaderState *record);
 static bool recoveryStopsAfter(XLogReaderState *record);
 static void recoveryPausesHere(void);

src/backend/access/transam/xlogarchive.c

@@ -480,7 +480,10 @@ KeepFileRestoredFromArchive(char *path, char *xlogfname)
 	 * Create .done file forcibly to prevent the restored segment from being
 	 * archived again later.
 	 */
-	XLogArchiveForceDone(xlogfname);
+	if (XLogArchiveMode != ARCHIVE_MODE_ALWAYS)
+		XLogArchiveForceDone(xlogfname);
+	else
+		XLogArchiveNotify(xlogfname);
 
 	/*
 	 * If the existing file was replaced, since walsenders might have it open,

src/backend/postmaster/postmaster.c

@@ -828,9 +828,9 @@ PostmasterMain(int argc, char *argv[])
 		write_stderr("%s: max_wal_senders must be less than max_connections\n", progname);
 		ExitPostmaster(1);
 	}
-	if (XLogArchiveMode && wal_level == WAL_LEVEL_MINIMAL)
+	if (XLogArchiveMode > ARCHIVE_MODE_OFF && wal_level == WAL_LEVEL_MINIMAL)
 		ereport(ERROR,
-				(errmsg("WAL archival (archive_mode=on) requires wal_level \"archive\", \"hot_standby\", or \"logical\"")));
+				(errmsg("WAL archival cannot be enabled when wal_level is \"minimal\"")));
 	if (max_wal_senders > 0 && wal_level == WAL_LEVEL_MINIMAL)
 		ereport(ERROR,
 				(errmsg("WAL streaming (max_wal_senders > 0) requires wal_level \"archive\", \"hot_standby\", or \"logical\"")));
@@ -1645,13 +1645,21 @@ ServerLoop(void)
 				start_autovac_launcher = false; /* signal processed */
 		}
 
-		/* If we have lost the archiver, try to start a new one */
-		if (XLogArchivingActive() && PgArchPID == 0 && pmState == PM_RUN)
-			PgArchPID = pgarch_start();
-
-		/* If we have lost the stats collector, try to start a new one */
-		if (PgStatPID == 0 && pmState == PM_RUN)
-			PgStatPID = pgstat_start();
+		/*
+		 * If we have lost the archiver, try to start a new one.
+		 *
+		 * If WAL archiving is enabled always, we try to start a new archiver
+		 * even during recovery.
+		 */
+		if (PgArchPID == 0 && wal_level >= WAL_LEVEL_ARCHIVE)
+		{
+			if ((pmState == PM_RUN && XLogArchiveMode > ARCHIVE_MODE_OFF) ||
+				((pmState == PM_RECOVERY || pmState == PM_HOT_STANDBY) &&
+				 XLogArchiveMode == ARCHIVE_MODE_ALWAYS))
+			{
+				PgArchPID = pgarch_start();
+			}
+		}
 
 		/* If we need to signal the autovacuum launcher, do so now */
 		if (avlauncher_needs_signal)
@@ -4807,6 +4815,17 @@ sigusr1_handler(SIGNAL_ARGS)
 		Assert(BgWriterPID == 0);
 		BgWriterPID = StartBackgroundWriter();
 
+		/*
+		 * Start the archiver if we're responsible for (re-)archiving received
+		 * files.
+		 */
+		Assert(PgArchPID == 0);
+		if (wal_level >= WAL_LEVEL_ARCHIVE &&
+			XLogArchiveMode == ARCHIVE_MODE_ALWAYS)
+		{
+			PgArchPID = pgarch_start();
+		}
+
 		pmState = PM_RECOVERY;
 	}
 	if (CheckPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY) &&

src/backend/replication/walreceiver.c

@@ -540,7 +540,10 @@ WalReceiverMain(void)
 			 * being archived later.
 			 */
 			XLogFileName(xlogfname, recvFileTLI, recvSegNo);
-			XLogArchiveForceDone(xlogfname);
+			if (XLogArchiveMode != ARCHIVE_MODE_ALWAYS)
+				XLogArchiveForceDone(xlogfname);
+			else
+				XLogArchiveNotify(xlogfname);
 		}
 		recvFile = -1;
 
@@ -897,7 +900,10 @@ XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr)
 				 * from being archived later.
 				 */
 				XLogFileName(xlogfname, recvFileTLI, recvSegNo);
-				XLogArchiveForceDone(xlogfname);
+				if (XLogArchiveMode != ARCHIVE_MODE_ALWAYS)
+					XLogArchiveForceDone(xlogfname);
+				else
+					XLogArchiveNotify(xlogfname);
 			}
 			recvFile = -1;
 

src/backend/utils/misc/guc.c

@@ -396,6 +396,7 @@ static const struct config_enum_entry row_security_options[] = {
  * Options for enum values stored in other modules
  */
 extern const struct config_enum_entry wal_level_options[];
+extern const struct config_enum_entry archive_mode_options[];
 extern const struct config_enum_entry sync_method_options[];
 extern const struct config_enum_entry dynamic_shared_memory_options[];
 
@@ -1529,16 +1530,6 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
-	{
-		{"archive_mode", PGC_POSTMASTER, WAL_ARCHIVING,
-			gettext_noop("Allows archiving of WAL files using archive_command."),
-			NULL
-		},
-		&XLogArchiveMode,
-		false,
-		NULL, NULL, NULL
-	},
-
 	{
 		{"hot_standby", PGC_POSTMASTER, REPLICATION_STANDBY,
 			gettext_noop("Allows connections and queries during recovery."),
@@ -3551,6 +3542,16 @@ static struct config_enum ConfigureNamesEnum[] =
 		NULL, assign_synchronous_commit, NULL
 	},
 
+	{
+		{"archive_mode", PGC_POSTMASTER, WAL_ARCHIVING,
+			gettext_noop("Allows archiving of WAL files using archive_command."),
+			NULL
+		},
+		&XLogArchiveMode,
+		ARCHIVE_MODE_OFF, archive_mode_options,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"trace_recovery_messages", PGC_SIGHUP, DEVELOPER_OPTIONS,
 			gettext_noop("Enables logging of recovery-related debugging information."),

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

@@ -206,7 +206,7 @@
 
 # - Archiving -
 
-#archive_mode = off		# allows archiving to be done
+#archive_mode = off		# enables archiving; off, on, or always
 				# (change requires restart)
 #archive_command = ''		# command to use to archive a logfile segment
 				# placeholders: %p = path of file to archive

src/include/access/xlog.h

@@ -98,7 +98,6 @@ extern int	wal_keep_segments;
 extern int	XLOGbuffers;
 extern int	XLogArchiveTimeout;
 extern int	wal_retrieve_retry_interval;
-extern bool XLogArchiveMode;
 extern char *XLogArchiveCommand;
 extern bool EnableHotStandby;
 extern bool fullPageWrites;
@@ -108,6 +107,15 @@ extern bool log_checkpoints;
 
 extern int	CheckPointSegments;
 
+/* Archive modes */
+typedef enum ArchiveMode
+{
+	ARCHIVE_MODE_OFF = 0,	/* disabled */
+	ARCHIVE_MODE_ON,		/* enabled while server is running normally */
+	ARCHIVE_MODE_ALWAYS		/* enabled always (even during recovery) */
+} ArchiveMode;
+extern int	XLogArchiveMode;
+
 /* WAL levels */
 typedef enum WalLevel
 {
@@ -118,7 +126,8 @@ typedef enum WalLevel
 } WalLevel;
 extern int	wal_level;
 
-#define XLogArchivingActive()	(XLogArchiveMode && wal_level >= WAL_LEVEL_ARCHIVE)
+#define XLogArchivingActive() \
+	(XLogArchiveMode > ARCHIVE_MODE_OFF && wal_level >= WAL_LEVEL_ARCHIVE)
 #define XLogArchiveCommandSet() (XLogArchiveCommand[0] != '\0')
 
 /*