Make pg_dump error cleanly with -j against hot standby

Getting a synchronized snapshot is not supported on a hot standby node,
and is by default taken when using -j with multiple sessions. Trying to
do so still failed, but with a server error that would also go in the
log. Instead, proprely detect this case and give a better error message.

Author: mhagander

src/bin/pg_dump/pg_backup.h

@@ -173,6 +173,7 @@ typedef struct Archive
 	int			verbose;
 	char	   *remoteVersionStr;		/* server's version string */
 	int			remoteVersion;	/* same in numeric form */
+	bool		isStandby;		/* is server a standby node */
 
 	int			minRemoteVersion;		/* allowable range */
 	int			maxRemoteVersion;

src/bin/pg_dump/pg_backup_db.c

@@ -37,6 +37,7 @@ _check_database_version(ArchiveHandle *AH)
 {
 	const char *remoteversion_str;
 	int			remoteversion;
+	PGresult   *res;
 
 	remoteversion_str = PQparameterStatus(AH->connection, "server_version");
 	remoteversion = PQserverVersion(AH->connection);
@@ -56,6 +57,20 @@ _check_database_version(ArchiveHandle *AH)
 				  remoteversion_str, progname, PG_VERSION);
 		exit_horribly(NULL, "aborting because of server version mismatch\n");
 	}
+
+	/*
+	 * When running against 9.0 or later, check if we are in recovery mode,
+	 * which means we are on a hot standby.
+	 */
+	if (remoteversion >= 90000)
+	{
+		res = ExecuteSqlQueryForSingleRow((Archive *) AH, "SELECT pg_catalog.pg_is_in_recovery()");
+
+		AH->public.isStandby = (strcmp(PQgetvalue(res, 0, 0), "t") == 0);
+		PQclear(res);
+	}
+	else
+		AH->public.isStandby = false;
 }
 
 /*
@@ -388,6 +403,29 @@ ExecuteSqlQuery(Archive *AHX, const char *query, ExecStatusType status)
 	return res;
 }
 
+/*
+ * Execute an SQL query and verify that we got exactly one row back.
+ */
+PGresult *
+ExecuteSqlQueryForSingleRow(Archive *fout, char *query)
+{
+	PGresult   *res;
+	int			ntups;
+
+	res = ExecuteSqlQuery(fout, query, PGRES_TUPLES_OK);
+
+	/* Expecting a single result only */
+	ntups = PQntuples(res);
+	if (ntups != 1)
+		exit_horribly(NULL,
+					  ngettext("query returned %d row instead of one: %s\n",
+							   "query returned %d rows instead of one: %s\n",
+							   ntups),
+					  ntups, query);
+
+	return res;
+}
+
 /*
  * Convenience function to send a query.
  * Monitors result to detect COPY statements

src/bin/pg_dump/pg_backup_db.h

@@ -16,6 +16,7 @@ extern int	ExecuteSqlCommandBuf(Archive *AHX, const char *buf, size_t bufLen);
 extern void ExecuteSqlStatement(Archive *AHX, const char *query);
 extern PGresult *ExecuteSqlQuery(Archive *AHX, const char *query,
 				ExecStatusType status);
+extern PGresult *ExecuteSqlQueryForSingleRow(Archive *fout, char *query);
 
 extern void EndDBCopyMode(Archive *AHX, const char *tocEntryTag);
 

src/bin/pg_dump/pg_dump.c

@@ -264,7 +264,6 @@ static bool nonemptyReloptions(const char *reloptions);
 static void appendReloptionsArrayAH(PQExpBuffer buffer, const char *reloptions,
 						const char *prefix, Archive *fout);
 static char *get_synchronized_snapshot(Archive *fout);
-static PGresult *ExecuteSqlQueryForSingleRow(Archive *fout, char *query);
 static void setupDumpWorker(Archive *AHX);
 
 
@@ -657,23 +656,11 @@ main(int argc, char **argv)
 		dopt.no_security_labels = 1;
 
 	/*
-	 * When running against 9.0 or later, check if we are in recovery mode,
-	 * which means we are on a hot standby.
+	 * On hot standby slaves, never try to dump unlogged table data, since it
+	 * will just throw an error.
 	 */
-	if (fout->remoteVersion >= 90000)
-	{
-		PGresult   *res = ExecuteSqlQueryForSingleRow(fout, "SELECT pg_catalog.pg_is_in_recovery()");
-
-		if (strcmp(PQgetvalue(res, 0, 0), "t") == 0)
-		{
-			/*
-			 * On hot standby slaves, never try to dump unlogged table data,
-			 * since it will just throw an error.
-			 */
-			dopt.no_unlogged_table_data = true;
-		}
-		PQclear(res);
-	}
+	if (fout->isStandby)
+		dopt.no_unlogged_table_data = true;
 
 	/* Select the appropriate subquery to convert user IDs to names */
 	if (fout->remoteVersion >= 80100)
@@ -1105,7 +1092,16 @@ setup_connection(Archive *AH, const char *dumpencoding,
 	else if (AH->numWorkers > 1 &&
 			 AH->remoteVersion >= 90200 &&
 			 !dopt->no_synchronized_snapshots)
+	{
+		if (AH->isStandby)
+			exit_horribly(NULL,
+			 "Synchronized snapshots are not supported on standby servers.\n"
+						  "Run with --no-synchronized-snapshots instead if you do not need\n"
+						  "synchronized snapshots.\n");
+
+
 		AH->sync_snapshot_id = get_synchronized_snapshot(AH);
+	}
 }
 
 static void
@@ -17855,26 +17851,3 @@ appendReloptionsArrayAH(PQExpBuffer buffer, const char *reloptions,
 	if (!res)
 		write_msg(NULL, "WARNING: could not parse reloptions array\n");
 }
-
-/*
- * Execute an SQL query and verify that we got exactly one row back.
- */
-static PGresult *
-ExecuteSqlQueryForSingleRow(Archive *fout, char *query)
-{
-	PGresult   *res;
-	int			ntups;
-
-	res = ExecuteSqlQuery(fout, query, PGRES_TUPLES_OK);
-
-	/* Expecting a single result only */
-	ntups = PQntuples(res);
-	if (ntups != 1)
-		exit_horribly(NULL,
-					  ngettext("query returned %d row instead of one: %s\n",
-							   "query returned %d rows instead of one: %s\n",
-							   ntups),
-					  ntups, query);
-
-	return res;
-}