pg_upgrade: Preserve database OIDs.

Commit 9a974cbcba arranged to preserve
relfilenodes and tablespace OIDs. For similar reasons, also arrange
to preserve database OIDs.

One problem is that, up until now, the OIDs assigned to the template0
and postgres databases have not been fixed. This could be a problem
when upgrading, because pg_upgrade might try to migrate a database
from the old cluster to the new cluster while keeping the OID and find
a different database with that OID, resulting in a failure. If it finds
a database with the same name and the same OID that's OK: it will be
dropped and recreated. But the same OID and a different name is a
problem.

To prevent that, fix the OIDs for postgres and template0 to specific
values less than 16384. To avoid running afoul of this rule, these
values should not be changed in future releases. It's not a problem
that these OIDs aren't fixed in existing releases, because the OIDs
that we're assigning here weren't used for either of these databases
in any previous release. Thus, there's no chance that an upgrade of
a cluster from any previous release will collide with the OIDs we're
assigning here. And going forward, the OIDs will always be fixed, so
the only potential collision is with a system database having the
same name and the same OID, which is OK.

This patch lets users assign a specific OID to a database as well,
provided however that it can't be less than 16384. I (rhaas) thought
it might be better not to expose this capability to users, but the
consensus was otherwise, so the syntax is documented. Letting users
assign OIDs below 16384 would not be OK, though, because a
user-created database with a low-numbered OID might collide with a
system-created database in a future release. We therefore prohibit
that.

Shruthi KC, based on an earlier patch from Antonin Houska, reviewed
and with some adjustments by me.

Discussion: http://postgr.es/m/CA+TgmoYgTwYcUmB=e8+hRHOFA0kkS6Kde85+UNdon6q7bt1niQ@mail.gmail.com
Discussion: http://postgr.es/m/CAASxf_Mnwm1Dh2vd5FAhVX6S1nwNSZUB1z12VddYtM++H2+p7w@mail.gmail.com

doc/src/sgml/ref/create_database.sgml

@@ -31,7 +31,8 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
            [ TABLESPACE [=] <replaceable class="parameter">tablespace_name</replaceable> ]
            [ ALLOW_CONNECTIONS [=] <replaceable class="parameter">allowconn</replaceable> ]
            [ CONNECTION LIMIT [=] <replaceable class="parameter">connlimit</replaceable> ]
-           [ IS_TEMPLATE [=] <replaceable class="parameter">istemplate</replaceable> ] ]
+           [ IS_TEMPLATE [=] <replaceable class="parameter">istemplate</replaceable> ]
+           [ OID [=] <replaceable class="parameter">oid</replaceable> ] ]
 </synopsis>
  </refsynopsisdiv>
 
@@ -203,6 +204,21 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
         </para>
        </listitem>
       </varlistentry>
+
+      <varlistentry>
+       <term><replaceable class="parameter">oid</replaceable></term>
+       <listitem>
+        <para>
+         The object identifier to be used for the new database. If this
+         parameter is not specified, the database will choose a suitable
+         OID automatically. This parameter is primarily intended for internal
+         use by <application>pg_upgrade</application>, and only
+         <application>pg_upgrade</application> can specify a value less
+         than 16384.
+        </para>
+       </listitem>
+      </varlistentry>
+
     </variablelist>
 
   <para>

src/backend/commands/dbcommands.c

@@ -115,7 +115,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	HeapTuple	tuple;
 	Datum		new_record[Natts_pg_database];
 	bool		new_record_nulls[Natts_pg_database];
-	Oid			dboid;
+	Oid			dboid = InvalidOid;
 	Oid			datdba;
 	ListCell   *option;
 	DefElem    *dtablespacename = NULL;
@@ -215,6 +215,30 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 					 errhint("Consider using tablespaces instead."),
 					 parser_errposition(pstate, defel->location)));
 		}
+		else if (strcmp(defel->defname, "oid") == 0)
+		{
+			dboid = defGetInt32(defel);
+
+			/*
+			 * We don't normally permit new databases to be created with
+			 * system-assigned OIDs. pg_upgrade tries to preserve database
+			 * OIDs, so we can't allow any database to be created with an
+			 * OID that might be in use in a freshly-initialized cluster
+			 * created by some future version. We assume all such OIDs will
+			 * be from the system-managed OID range.
+			 *
+			 * As an exception, however, we permit any OID to be assigned when
+			 * allow_system_table_mods=on (so that initdb can assign system
+			 * OIDs to template0 and postgres) or when performing a binary
+			 * upgrade (so that pg_upgrade can preserve whatever OIDs it finds
+			 * in the source cluster).
+			 */
+			if (dboid < FirstNormalObjectId &&
+				!allowSystemTableMods && !IsBinaryUpgrade)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_PARAMETER_VALUE)),
+						errmsg("OIDs less than %u are reserved for system objects", FirstNormalObjectId));
+		}
 		else
 			ereport(ERROR,
 					(errcode(ERRCODE_SYNTAX_ERROR),
@@ -502,11 +526,34 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 	 */
 	pg_database_rel = table_open(DatabaseRelationId, RowExclusiveLock);
 
-	do
+	/*
+	 * If database OID is configured, check if the OID is already in use or
+	 * data directory already exists.
+	 */
+	if (OidIsValid(dboid))
 	{
-		dboid = GetNewOidWithIndex(pg_database_rel, DatabaseOidIndexId,
-								   Anum_pg_database_oid);
-	} while (check_db_file_conflict(dboid));
+		char	   *existing_dbname = get_database_name(dboid);
+
+		if (existing_dbname != NULL)
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE)),
+					errmsg("database OID %u is already in use by database \"%s\"",
+						   dboid, existing_dbname));
+
+		if (check_db_file_conflict(dboid))
+			ereport(ERROR,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE)),
+					errmsg("data directory with the specified OID %u already exists", dboid));
+	}
+	else
+	{
+		/* Select an OID for the new database if is not explicitly configured. */
+		do
+		{
+			dboid = GetNewOidWithIndex(pg_database_rel, DatabaseOidIndexId,
+									   Anum_pg_database_oid);
+		} while (check_db_file_conflict(dboid));
+	}
 
 	/*
 	 * Insert a new tuple into pg_database.  This establishes our ownership of

src/bin/initdb/initdb.c

@@ -59,6 +59,7 @@
 #include "sys/mman.h"
 #endif
 
+#include "access/transam.h"
 #include "access/xlog_internal.h"
 #include "catalog/pg_authid_d.h"
 #include "catalog/pg_class_d.h" /* pgrminclude ignore */
@@ -1838,8 +1839,23 @@ static void
 make_template0(FILE *cmdfd)
 {
 	const char *const *line;
+
+	/*
+	 * pg_upgrade tries to preserve database OIDs across upgrades. It's smart
+	 * enough to drop and recreate a conflicting database with the same name,
+	 * but if the same OID were used for one system-created database in the
+	 * old cluster and a different system-created database in the new cluster,
+	 * it would fail. To avoid that, assign a fixed OID to template0 rather
+	 * than letting the server choose one.
+	 *
+	 * (Note that, while the user could have dropped and recreated these
+	 * objects in the old cluster, the problem scenario only exists if the OID
+	 * that is in use in the old cluster is also used in the new cluster - and
+	 * the new cluster should be the result of a fresh initdb.)
+	 */
 	static const char *const template0_setup[] = {
-		"CREATE DATABASE template0 IS_TEMPLATE = true ALLOW_CONNECTIONS = false;\n\n",
+		"CREATE DATABASE template0 IS_TEMPLATE = true ALLOW_CONNECTIONS = false OID = "
+		CppAsString2(Template0ObjectId) ";\n\n",
 
 		/*
 		 * Explicitly revoke public create-schema and create-temp-table
@@ -1869,8 +1885,10 @@ static void
 make_postgres(FILE *cmdfd)
 {
 	const char *const *line;
+
+	/* Assign a fixed OID to postgres, for the same reasons as template0 */
 	static const char *const postgres_setup[] = {
-		"CREATE DATABASE postgres;\n\n",
+		"CREATE DATABASE postgres OID = " CppAsString2(PostgresObjectId) ";\n\n",
 		"COMMENT ON DATABASE postgres IS 'default administrative connection database';\n\n",
 		NULL
 	};

src/bin/pg_dump/pg_dump.c

@@ -2838,8 +2838,16 @@ dumpDatabase(Archive *fout)
 	 * are left to the DATABASE PROPERTIES entry, so that they can be applied
 	 * after reconnecting to the target DB.
 	 */
-	appendPQExpBuffer(creaQry, "CREATE DATABASE %s WITH TEMPLATE = template0",
-					  qdatname);
+	if (dopt->binary_upgrade)
+	{
+		appendPQExpBuffer(creaQry, "CREATE DATABASE %s WITH TEMPLATE = template0 OID = %u",
+						  qdatname, dbCatId.oid);
+	}
+	else
+	{
+		appendPQExpBuffer(creaQry, "CREATE DATABASE %s WITH TEMPLATE = template0",
+						  qdatname);
+	}
 	if (strlen(encoding) > 0)
 	{
 		appendPQExpBufferStr(creaQry, " ENCODING = ");

src/bin/pg_upgrade/IMPLEMENTATION

@@ -86,7 +86,7 @@ by pg_dumpall --- this script effectively creates the complete
 user-defined metadata from the old cluster to the new cluster.  It
 preserves the relfilenode numbers so TOAST and other references
 to relfilenodes in user data is preserved.  (See binary-upgrade usage
-in pg_dump).
+in pg_dump). We choose to preserve tablespace and database OIDs as well.
 
 Finally, pg_upgrade links or copies each user-defined table and its
 supporting indexes and toast tables from the old cluster to the new

src/bin/pg_upgrade/info.c

@@ -190,10 +190,8 @@ create_rel_filename_map(const char *old_data, const char *new_data,
 		map->new_tablespace_suffix = new_cluster.tablespace_suffix;
 	}
 
-	map->old_db_oid = old_db->db_oid;
-	map->new_db_oid = new_db->db_oid;
-
-	/* relfilenode is preserved across old and new cluster */
+	/* DB oid and relfilenodes are preserved between old and new cluster */
+	map->db_oid = old_db->db_oid;
 	map->relfilenode = old_rel->relfilenode;
 
 	/* used only for logging and error reporting, old/new are identical */
@@ -324,8 +322,7 @@ get_db_infos(ClusterInfo *cluster)
 			 " LEFT OUTER JOIN pg_catalog.pg_tablespace t "
 			 " ON d.dattablespace = t.oid "
 			 "WHERE d.datallowconn = true "
-	/* we don't preserve pg_database.oid so we sort by name */
-			 "ORDER BY 2");
+			 "ORDER BY 1");
 
 	res = executeQueryOrDie(conn, "%s", query);
 

src/bin/pg_upgrade/pg_upgrade.h

@@ -145,8 +145,7 @@ typedef struct
 	const char *new_tablespace;
 	const char *old_tablespace_suffix;
 	const char *new_tablespace_suffix;
-	Oid			old_db_oid;
-	Oid			new_db_oid;
+	Oid			db_oid;
 	Oid			relfilenode;
 	/* the rest are used only for logging and error reporting */
 	char	   *nspname;		/* namespaces */

src/bin/pg_upgrade/relfilenode.c

@@ -193,14 +193,14 @@ transfer_relfile(FileNameMap *map, const char *type_suffix, bool vm_must_add_fro
 		snprintf(old_file, sizeof(old_file), "%s%s/%u/%u%s%s",
 				 map->old_tablespace,
 				 map->old_tablespace_suffix,
-				 map->old_db_oid,
+				 map->db_oid,
 				 map->relfilenode,
 				 type_suffix,
 				 extent_suffix);
 		snprintf(new_file, sizeof(new_file), "%s%s/%u/%u%s%s",
 				 map->new_tablespace,
 				 map->new_tablespace_suffix,
-				 map->new_db_oid,
+				 map->db_oid,
 				 map->relfilenode,
 				 type_suffix,
 				 extent_suffix);

src/bin/psql/tab-complete.c

@@ -2633,7 +2633,7 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH("OWNER", "TEMPLATE", "ENCODING", "TABLESPACE",
 					  "IS_TEMPLATE",
 					  "ALLOW_CONNECTIONS", "CONNECTION LIMIT",
-					  "LC_COLLATE", "LC_CTYPE", "LOCALE");
+					  "LC_COLLATE", "LC_CTYPE", "LOCALE", "OID");
 
 	else if (Matches("CREATE", "DATABASE", MatchAny, "TEMPLATE"))
 		COMPLETE_WITH_QUERY(Query_for_list_of_template_databases);

src/include/access/transam.h

@@ -196,6 +196,10 @@ FullTransactionIdAdvance(FullTransactionId *dest)
 #define FirstUnpinnedObjectId	12000
 #define FirstNormalObjectId		16384
 
+/* OIDs of Template0 and Postgres database are fixed */
+#define Template0ObjectId		4
+#define PostgresObjectId		5
+
 /*
  * VariableCache is a data structure in shared memory that is used to track
  * OID and XID assignment state.  For largely historical reasons, there is

src/include/catalog/unused_oids

@@ -32,6 +32,15 @@ my @input_files = glob("pg_*.h");
 
 my $oids = Catalog::FindAllOidsFromHeaders(@input_files);
 
+# Push the template0 and postgres database OIDs.
+my $Template0ObjectId =
+  Catalog::FindDefinedSymbol('access/transam.h', '..', 'Template0ObjectId');
+push @{$oids}, $Template0ObjectId;
+
+my $PostgresObjectId =
+  Catalog::FindDefinedSymbol('access/transam.h', '..', 'PostgresObjectId');
+push @{$oids}, $PostgresObjectId;
+
 # Also push FirstGenbkiObjectId to serve as a terminator for the last gap.
 my $FirstGenbkiObjectId =
   Catalog::FindDefinedSymbol('access/transam.h', '..', 'FirstGenbkiObjectId');