pg_dump: Lock all relations, not just plain tables

Now that LOCK TABLE can take any relation type, acquire lock on all
relations that are to be dumped.  This prevents schema changes or
deadlock errors that could cause a dump to fail after expending much
effort.  The server is tested to have the capability and the feature
disabled if it doesn't, so that a patched pg_dump doesn't fail when
connecting to an unpatched server.

Backpatch to 9.5.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reported-by: Wells Oliver <wells.oliver@gmail.com>
Discussion: https://postgr.es/m/20201021200659.GA32358@alvherre.pgsql

4 files changed, 79 insertions, 7 deletions

diff --git a/src/bin/pg_dump/pg_backup.h b/src/bin/pg_dump/pg_backup.h
index 9a9b1e1d51b..92e6bae63e2 100644
--- a/src/bin/pg_dump/pg_backup.h
+++ b/src/bin/pg_dump/pg_backup.h
@@ -200,6 +200,8 @@ typedef struct Archive
 	int			minRemoteVersion;	/* allowable range */
 	int			maxRemoteVersion;
 
+	bool		hasGenericLockTable;	/* can LOCK TABLE do non-table rels */
+
 	int			numWorkers;		/* number of parallel processes */
 	char	   *sync_snapshot_id;	/* sync snapshot id for parallel operation */
 
diff --git a/src/bin/pg_dump/pg_backup_db.c b/src/bin/pg_dump/pg_backup_db.c
index 3c3a313180c..7fbf425ff9a 100644
--- a/src/bin/pg_dump/pg_backup_db.c
+++ b/src/bin/pg_dump/pg_backup_db.c
@@ -537,6 +537,71 @@ EndDBCopyMode(Archive *AHX, const char *tocEntryTag)
 	}
 }
 
+/*
+ * Does LOCK TABLE work on non-table relations on this server?
+ *
+ * Note: assumes it is called out of any transaction
+ */
+bool
+IsLockTableGeneric(Archive *AHX)
+{
+	ArchiveHandle *AH = (ArchiveHandle *) AHX;
+	PGresult *res;
+	char	 *sqlstate;
+	bool	retval;
+
+	if (AHX->remoteVersion >= 140000)
+		return true;
+	else if (AHX->remoteVersion < 90500)
+		return false;
+
+	StartTransaction(AHX);
+
+	/*
+	 * Try a LOCK TABLE on a well-known non-table catalog; WRONG_OBJECT_TYPE
+	 * tells us that this server doesn't support locking non-table rels, while
+	 * LOCK_NOT_AVAILABLE and INSUFFICIENT_PRIVILEGE tell us that it does.
+	 * Report anything else as a fatal problem.
+	 */
+#define ERRCODE_INSUFFICIENT_PRIVILEGE	"42501"
+#define ERRCODE_WRONG_OBJECT_TYPE	"42809"
+#define ERRCODE_LOCK_NOT_AVAILABLE	"55P03"
+	res = PQexec(AH->connection,
+				 "LOCK TABLE pg_catalog.pg_class_tblspc_relfilenode_index IN ACCESS SHARE MODE NOWAIT");
+	switch (PQresultStatus(res))
+	{
+		case PGRES_COMMAND_OK:
+			retval = true;
+			break;
+		case PGRES_FATAL_ERROR:
+			sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
+			if (strcmp(sqlstate, ERRCODE_WRONG_OBJECT_TYPE) == 0)
+			{
+				retval = false;
+				break;
+			}
+			else if (strcmp(sqlstate, ERRCODE_LOCK_NOT_AVAILABLE) == 0 ||
+					 strcmp(sqlstate, ERRCODE_INSUFFICIENT_PRIVILEGE) == 0)
+			{
+				retval = true;
+				break;
+			}
+			/* else, falls through */
+		default:
+			warn_or_exit_horribly(AH, modulename,
+								  "LOCK TABLE failed for \"%s\": %s",
+								  "pg_catalog.pg_class_tblspc_relfilenode_index",
+								  PQerrorMessage(AH->connection));
+			retval = false;		/* not reached */
+			break;
+	}
+	PQclear(res);
+
+	CommitTransaction(AHX);
+
+	return retval;
+}
+
 void
 StartTransaction(Archive *AHX)
 {
diff --git a/src/bin/pg_dump/pg_backup_db.h b/src/bin/pg_dump/pg_backup_db.h
index a79f5283fe5..0943f0700ed 100644
--- a/src/bin/pg_dump/pg_backup_db.h
+++ b/src/bin/pg_dump/pg_backup_db.h
@@ -20,6 +20,8 @@ extern PGresult *ExecuteSqlQueryForSingleRow(Archive *fout, const char *query);
 
 extern void EndDBCopyMode(Archive *AHX, const char *tocEntryTag);
 
+extern bool IsLockTableGeneric(Archive *AHX);
+
 extern void StartTransaction(Archive *AHX);
 extern void CommitTransaction(Archive *AHX);
 
diff --git a/src/bin/pg_dump/pg_dump.c b/src/bin/pg_dump/pg_dump.c
index ad7c0376c9b..8c2fef29903 100644
--- a/src/bin/pg_dump/pg_dump.c
+++ b/src/bin/pg_dump/pg_dump.c
@@ -1115,6 +1115,9 @@ setup_connection(Archive *AH, const char *dumpencoding,
 			ExecuteSqlStatement(AH, "SET row_security = off");
 	}
 
+	/* Detect whether LOCK TABLE can handle non-table relations */
+	AH->hasGenericLockTable = IsLockTableGeneric(AH);
+
 	/*
 	 * Start transaction-snapshot mode transaction to dump consistent data.
 	 */
@@ -6662,16 +6665,16 @@ getTables(Archive *fout, int *numTables)
 		 * assume our lock on the child is enough to prevent schema
 		 * alterations to parent tables.
 		 *
-		 * NOTE: it'd be kinda nice to lock other relations too, not only
-		 * plain or partitioned tables, but the backend doesn't presently
-		 * allow that.
-		 *
-		 * We only need to lock the table for certain components; see
+		 * We only need to lock the relation for certain components; see
 		 * pg_dump.h
+		 *
+		 * On server versions that support it, we lock all relations not just
+		 * plain tables.
 		 */
 		if (tblinfo[i].dobj.dump &&
-			(tblinfo[i].relkind == RELKIND_RELATION ||
-			 tblinfo->relkind == RELKIND_PARTITIONED_TABLE) &&
+			(fout->hasGenericLockTable ||
+			 tblinfo[i].relkind == RELKIND_PARTITIONED_TABLE ||
+			 tblinfo[i].relkind == RELKIND_RELATION) &&
 			(tblinfo[i].dobj.dump & DUMP_COMPONENTS_REQUIRING_LOCK))
 		{
 			resetPQExpBuffer(query);