Make pg_basebackup use temporary replication slots

Temporary replication slots will be used by default when wal streaming
is used and no slot name is specified with -S. If a slot name is
specified, then a permanent slot with that name is used. If --no-slot is
specified, then no permanent or temporary slot will be used.

Temporary slots are only used on 10.0 and newer, of course.

doc/src/sgml/ref/pg_basebackup.sgml

@@ -240,6 +240,31 @@ PostgreSQL documentation
         the server does not remove any necessary WAL data in the time between
         the end of the base backup and the start of streaming replication.
        </para>
+       <para>
+        If this option is not specified and the server supports temporary
+        replication slots (version 10 and later), then a temporary replication
+        slot is automatically used for WAL streaming.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
+      <term><option>--no-slot</option></term>
+      <listitem>
+       <para>
+        This option prevents the creation of a temporary replication slot
+        during the backup even if it's supported by the server.
+       </para>
+       <para>
+        Temporary replication slots are created by default if no slot name
+        is given with the option <option>-S</option> when using log streaming.
+       </para>
+       <para>
+        The main purpose of this option is to allow taking a base backup when
+        the server is out of free replication slots.  Using replication slots
+        is almost always preferred, because it prevents needed WAL from being
+        removed by the server during the backup.
+       </para>
       </listitem>
      </varlistentry>
 

src/bin/pg_basebackup/pg_basebackup.c

@@ -61,6 +61,11 @@ typedef struct TablespaceList
  */
 #define MINIMUM_VERSION_FOR_PG_WAL	100000
 
+/*
+ * Temporary replication slots are supported from version 10.
+ */
+#define MINIMUM_VERSION_FOR_TEMP_SLOTS 100000
+
 /*
  * Different ways to include WAL
  */
@@ -88,6 +93,8 @@ static bool do_sync = true;
 static int	standby_message_timeout = 10 * 1000;		/* 10 sec = default */
 static pg_time_t last_progress_report = 0;
 static int32 maxrate = 0;		/* no limit by default */
+static char *replication_slot = NULL;
+static bool temp_replication_slot = true;
 
 static bool success = false;
 static bool made_new_pgdata = false;
@@ -332,6 +339,7 @@ usage(void)
 	printf(_("  -R, --write-recovery-conf\n"
 			 "                         write recovery.conf after backup\n"));
 	printf(_("  -S, --slot=SLOTNAME    replication slot to use\n"));
+	printf(_("      --no-slot          prevent creation of temporary replication slot\n"));
 	printf(_("  -T, --tablespace-mapping=OLDDIR=NEWDIR\n"
 	  "                         relocate tablespace in OLDDIR to NEWDIR\n"));
 	printf(_("  -X, --xlog-method=none|fetch|stream\n"
@@ -460,6 +468,7 @@ typedef struct
 	char		xlog[MAXPGPATH];	/* directory or tarfile depending on mode */
 	char	   *sysidentifier;
 	int			timeline;
+	bool		temp_slot;
 } logstreamer_param;
 
 static int
@@ -479,6 +488,10 @@ LogStreamerMain(logstreamer_param *param)
 	stream.do_sync = do_sync;
 	stream.mark_done = true;
 	stream.partial_suffix = NULL;
+	stream.replication_slot = replication_slot;
+	stream.temp_slot = param->temp_slot;
+	if (stream.temp_slot && !stream.replication_slot)
+		stream.replication_slot = psprintf("pg_basebackup_%d", (int) getpid());
 
 	if (format == 'p')
 		stream.walmethod = CreateWalDirectoryMethod(param->xlog, do_sync);
@@ -565,6 +578,11 @@ StartLogStreamer(char *startpos, uint32 timeline, char *sysidentifier)
 			 PQserverVersion(conn) < MINIMUM_VERSION_FOR_PG_WAL ?
 				"pg_xlog" : "pg_wal");
 
+	/* Temporary replication slots are only supported in 10 and newer */
+	if (PQserverVersion(conn) < MINIMUM_VERSION_FOR_TEMP_SLOTS)
+		param->temp_slot = false;
+	else
+		param->temp_slot = temp_replication_slot;
 
 	if (format == 'p')
 	{
@@ -2063,11 +2081,13 @@ main(int argc, char **argv)
 		{"verbose", no_argument, NULL, 'v'},
 		{"progress", no_argument, NULL, 'P'},
 		{"xlogdir", required_argument, NULL, 1},
+		{"no-slot", no_argument, NULL, 2},
 		{NULL, 0, NULL, 0}
 	};
 	int			c;
 
 	int			option_index;
+	bool		no_slot = false;
 
 	progname = get_progname(argv[0]);
 	set_pglocale_pgservice(argv[0], PG_TEXTDOMAIN("pg_basebackup"));
@@ -2117,7 +2137,16 @@ main(int argc, char **argv)
 				writerecoveryconf = true;
 				break;
 			case 'S':
+
+				/*
+				 * When specifying replication slot name, use a permanent
+				 * slot.
+				 */
 				replication_slot = pg_strdup(optarg);
+				temp_replication_slot = false;
+				break;
+			case 2:
+				no_slot = true;
 				break;
 			case 'T':
 				tablespace_list_append(optarg);
@@ -2277,7 +2306,7 @@ main(int argc, char **argv)
 		exit(1);
 	}
 
-	if (replication_slot && includewal != STREAM_WAL)
+	if ((replication_slot || no_slot) && includewal != STREAM_WAL)
 	{
 		fprintf(stderr,
 			_("%s: replication slots can only be used with WAL streaming\n"),
@@ -2287,6 +2316,20 @@ main(int argc, char **argv)
 		exit(1);
 	}
 
+	if (no_slot)
+	{
+		if (replication_slot)
+		{
+			fprintf(stderr,
+					_("%s: --no-slot cannot be used with slot name\n"),
+					progname);
+			fprintf(stderr, _("Try \"%s --help\" for more information.\n"),
+					progname);
+			exit(1);
+		}
+		temp_replication_slot = false;
+	}
+
 	if (strcmp(xlog_dir, "") != 0)
 	{
 		if (format != 'p')

src/bin/pg_basebackup/pg_receivexlog.c

@@ -41,6 +41,7 @@ static bool do_create_slot = false;
 static bool slot_exists_ok = false;
 static bool do_drop_slot = false;
 static bool synchronous = false;
+static char *replication_slot = NULL;
 
 
 static void usage(void);
@@ -340,6 +341,8 @@ StreamLog(void)
 	stream.mark_done = false;
 	stream.walmethod = CreateWalDirectoryMethod(basedir, stream.do_sync);
 	stream.partial_suffix = ".partial";
+	stream.replication_slot = replication_slot;
+	stream.temp_slot = false;
 
 	ReceiveXlogStream(conn, &stream);
 

src/bin/pg_basebackup/pg_recvlogical.c

@@ -45,6 +45,7 @@ static bool do_create_slot = false;
 static bool slot_exists_ok = false;
 static bool do_start_slot = false;
 static bool do_drop_slot = false;
+static char *replication_slot = NULL;
 
 /* filled pairwise with option, value. value may be NULL */
 static char **options;

src/bin/pg_basebackup/receivelog.c

@@ -455,10 +455,10 @@ ReceiveXlogStream(PGconn *conn, StreamCtl *stream)
 	 * synchronous_standby_names, but we've protected them against it so
 	 * far, so let's continue to do so unless specifically requested.
 	 */
-	if (replication_slot != NULL)
+	if (stream->replication_slot != NULL)
 	{
 		reportFlushPosition = true;
-		sprintf(slotcmd, "SLOT \"%s\" ", replication_slot);
+		sprintf(slotcmd, "SLOT \"%s\" ", stream->replication_slot);
 	}
 	else
 	{
@@ -508,6 +508,24 @@ ReceiveXlogStream(PGconn *conn, StreamCtl *stream)
 		PQclear(res);
 	}
 
+	/*
+	 * Create temporary replication slot if one is needed
+	 */
+	if (stream->temp_slot)
+	{
+		snprintf(query, sizeof(query),
+			 "CREATE_REPLICATION_SLOT \"%s\" TEMPORARY PHYSICAL RESERVE_WAL",
+				 stream->replication_slot);
+		res = PQexec(conn, query);
+		if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		{
+			fprintf(stderr, _("%s: could not create temporary replication slot \"%s\": %s"),
+					progname, stream->replication_slot, PQerrorMessage(conn));
+			PQclear(res);
+			return false;
+		}
+	}
+
 	/*
 	 * initialize flush position to starting point, it's the caller's
 	 * responsibility that that's sane.

src/bin/pg_basebackup/receivelog.h

@@ -37,13 +37,15 @@ typedef struct StreamCtl
 												 * often */
 	bool		synchronous;	/* Flush immediately WAL data on write */
 	bool		mark_done;		/* Mark segment as done in generated archive */
-	bool		do_sync;		/* Flush to disk to ensure consistent state
-								 * of data */
+	bool		do_sync;		/* Flush to disk to ensure consistent state of
+								 * data */
 
 	stream_stop_callback stream_stop;	/* Stop streaming when returns true */
 
 	WalWriteMethod *walmethod;	/* How to write the WAL */
 	char	   *partial_suffix; /* Suffix appended to partially received files */
+	char	   *replication_slot;		/* Replication slot to use, or NULL */
+	bool		temp_slot;		/* Create temporary replication slot */
 } StreamCtl;
 
 

src/bin/pg_basebackup/streamutil.c

@@ -38,7 +38,6 @@ char	   *connection_string = NULL;
 char	   *dbhost = NULL;
 char	   *dbuser = NULL;
 char	   *dbport = NULL;
-char	   *replication_slot = NULL;
 char	   *dbname = NULL;
 int			dbgetpassword = 0;	/* 0=auto, -1=never, 1=always */
 static bool have_password = false;

src/bin/pg_basebackup/streamutil.h

@@ -23,7 +23,6 @@ extern char *dbuser;
 extern char *dbport;
 extern char *dbname;
 extern int	dbgetpassword;
-extern char *replication_slot;
 
 /* Connection kept global so we can disconnect easily */
 extern PGconn *conn;

src/bin/pg_basebackup/t/010_pg_basebackup.pl

@@ -4,7 +4,7 @@ use Cwd;
 use Config;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 71;
+use Test::More tests => 72;
 
 program_help_ok('pg_basebackup');
 program_version_ok('pg_basebackup');
@@ -244,6 +244,9 @@ $node->command_ok(
 	[ 'pg_basebackup', '-D', "$tempdir/backupxst", '-X', 'stream', '-Ft' ],
 	'pg_basebackup -X stream runs in tar mode');
 ok(-f "$tempdir/backupxst/pg_wal.tar", "tar file was created");
+$node->command_ok(
+	[ 'pg_basebackup', '-D', "$tempdir/backupnoslot", '-X', 'stream', '--no-slot' ],
+	'pg_basebackup -X stream runs with --no-slot');
 
 $node->command_fails(
 	[ 'pg_basebackup', '-D', "$tempdir/fail", '-S', 'slot1' ],