Optionally don't error out due to preexisting slots in commandline utilities.

pg_receivexlog and pg_recvlogical error out when --create-slot is
specified and a slot with the same name already exists. In some cases,
especially with pg_receivexlog, that's rather annoying and requires
additional scripting.

Backpatch to 9.5 as slot control functions have newly been added to
pg_receivexlog, and there doesn't seem much point leaving it in a less
useful state.

Discussion: 20150619144755.GG29350@alap3.anarazel.de

doc/src/sgml/ref/pg_receivexlog.sgml

@@ -92,6 +92,16 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--if-not-exists</option></term>
+      <listitem>
+       <para>
+        Do not not error out when <option>--create-slot</option> is specified
+        and a slot with the specified name already exists.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-n</option></term>
       <term><option>--no-loop</option></term>

doc/src/sgml/ref/pg_recvlogical.sgml

@@ -154,6 +154,16 @@ PostgreSQL documentation
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><option>--if-not-exists</option></term>
+      <listitem>
+       <para>
+        Do not not error out when <option>--create-slot</option> is specified
+        and a slot with the specified name already exists.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-n</option></term>
       <term><option>--no-loop</option></term>

src/bin/pg_basebackup/pg_receivexlog.c

@@ -38,6 +38,7 @@ static int	noloop = 0;
 static int	standby_message_timeout = 10 * 1000;		/* 10 sec = default */
 static volatile bool time_to_abort = false;
 static bool do_create_slot = false;
+static bool slot_exists_ok = false;
 static bool do_drop_slot = false;
 static bool synchronous = false;
 
@@ -66,6 +67,7 @@ usage(void)
 	printf(_("  %s [OPTION]...\n"), progname);
 	printf(_("\nOptions:\n"));
 	printf(_("  -D, --directory=DIR    receive transaction log files into this directory\n"));
+	printf(_("      --if-not-exists    do not treat naming conflicts as an error when creating a slot\n"));
 	printf(_("  -n, --no-loop          do not loop on connection lost\n"));
 	printf(_("  -s, --status-interval=SECS\n"
 			 "                         time between status packets sent to server (default: %d)\n"), (standby_message_timeout / 1000));
@@ -371,7 +373,8 @@ main(int argc, char **argv)
 /* action */
 		{"create-slot", no_argument, NULL, 1},
 		{"drop-slot", no_argument, NULL, 2},
-		{"synchronous", no_argument, NULL, 3},
+		{"if-not-exists", no_argument, NULL, 3},
+		{"synchronous", no_argument, NULL, 4},
 		{NULL, 0, NULL, 0}
 	};
 
@@ -455,6 +458,9 @@ main(int argc, char **argv)
 				do_drop_slot = true;
 				break;
 			case 3:
+				slot_exists_ok = true;
+				break;
+			case 4:
 				synchronous = true;
 				break;
 			default:
@@ -575,7 +581,8 @@ main(int argc, char **argv)
 					_("%s: creating replication slot \"%s\"\n"),
 					progname, replication_slot);
 
-		if (!CreateReplicationSlot(conn, replication_slot, NULL, NULL, true))
+		if (!CreateReplicationSlot(conn, replication_slot, NULL, true,
+								   slot_exists_ok))
 			disconnect_and_exit(1);
 	}
 

src/bin/pg_basebackup/pg_recvlogical.c

@@ -38,6 +38,7 @@ static int	standby_message_timeout = 10 * 1000;		/* 10 sec = default */
 static int	fsync_interval = 10 * 1000; /* 10 sec = default */
 static XLogRecPtr startpos = InvalidXLogRecPtr;
 static bool do_create_slot = false;
+static bool slot_exists_ok = false;
 static bool do_start_slot = false;
 static bool do_drop_slot = false;
 
@@ -75,6 +76,7 @@ usage(void)
 	printf(_("  -f, --file=FILE        receive log into this file, - for stdout\n"));
 	printf(_("  -F  --fsync-interval=SECS\n"
 			 "                         time between fsyncs to the output file (default: %d)\n"), (fsync_interval / 1000));
+	printf(_("      --if-not-exists    do not treat naming conflicts as an error when creating a slot\n"));
 	printf(_("  -I, --startpos=LSN     where in an existing slot should the streaming start\n"));
 	printf(_("  -n, --no-loop          do not loop on connection lost\n"));
 	printf(_("  -o, --option=NAME[=VALUE]\n"
@@ -633,6 +635,7 @@ main(int argc, char **argv)
 		{"create-slot", no_argument, NULL, 1},
 		{"start", no_argument, NULL, 2},
 		{"drop-slot", no_argument, NULL, 3},
+		{"if-not-exists", no_argument, NULL, 4},
 		{NULL, 0, NULL, 0}
 	};
 	int			c;
@@ -764,6 +767,9 @@ main(int argc, char **argv)
 			case 3:
 				do_drop_slot = true;
 				break;
+			case 4:
+				slot_exists_ok = true;
+				break;
 
 			default:
 
@@ -891,8 +897,9 @@ main(int argc, char **argv)
 					progname, replication_slot);
 
 		if (!CreateReplicationSlot(conn, replication_slot, plugin,
-								   &startpos, false))
+								   false, slot_exists_ok))
 			disconnect_and_exit(1);
+		startpos = InvalidXLogRecPtr;
 	}
 
 	if (!do_start_slot)

src/bin/pg_basebackup/streamutil.c

@@ -31,6 +31,8 @@
 #include "common/fe_memutils.h"
 #include "datatype/timestamp.h"
 
+#define ERRCODE_DUPLICATE_OBJECT  "42710"
+
 const char *progname;
 char	   *connection_string = NULL;
 char	   *dbhost = NULL;
@@ -314,7 +316,7 @@ RunIdentifySystem(PGconn *conn, char **sysid, TimeLineID *starttli,
  */
 bool
 CreateReplicationSlot(PGconn *conn, const char *slot_name, const char *plugin,
-					  XLogRecPtr *startpos, bool is_physical)
+					  bool is_physical, bool slot_exists_ok)
 {
 	PQExpBuffer query;
 	PGresult   *res;
@@ -335,6 +337,16 @@ CreateReplicationSlot(PGconn *conn, const char *slot_name, const char *plugin,
 
 	res = PQexec(conn, query->data);
 	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+	{
+		const char *sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
+
+		if (slot_exists_ok && strcmp(sqlstate, ERRCODE_DUPLICATE_OBJECT) == 0)
+		{
+			destroyPQExpBuffer(query);
+			PQclear(res);
+			return true;
+		}
+		else
 		{
 			fprintf(stderr, _("%s: could not send replication command \"%s\": %s"),
 					progname, query->data, PQerrorMessage(conn));
@@ -343,6 +355,7 @@ CreateReplicationSlot(PGconn *conn, const char *slot_name, const char *plugin,
 			PQclear(res);
 			return false;
 		}
+	}
 
 	if (PQntuples(res) != 1 || PQnfields(res) != 4)
 	{
@@ -356,25 +369,6 @@ CreateReplicationSlot(PGconn *conn, const char *slot_name, const char *plugin,
 		return false;
 	}
 
-	/* Get LSN start position if necessary */
-	if (startpos != NULL)
-	{
-		uint32		hi,
-					lo;
-
-		if (sscanf(PQgetvalue(res, 0, 1), "%X/%X", &hi, &lo) != 2)
-		{
-			fprintf(stderr,
-				  _("%s: could not parse transaction log location \"%s\"\n"),
-					progname, PQgetvalue(res, 0, 1));
-
-			destroyPQExpBuffer(query);
-			PQclear(res);
-			return false;
-		}
-		*startpos = ((uint64) hi) << 32 | lo;
-	}
-
 	destroyPQExpBuffer(query);
 	PQclear(res);
 	return true;

src/bin/pg_basebackup/streamutil.h

@@ -32,8 +32,8 @@ extern PGconn *GetConnection(void);
 
 /* Replication commands */
 extern bool CreateReplicationSlot(PGconn *conn, const char *slot_name,
-					  const char *plugin, XLogRecPtr *startpos,
+					  const char *plugin, bool is_physical,
-					  bool is_physical);
+					  bool slot_exists_ok);
 extern bool DropReplicationSlot(PGconn *conn, const char *slot_name);
 extern bool RunIdentifySystem(PGconn *conn, char **sysid,
 				  TimeLineID *starttli,