Don't access catalogs to validate GUCs when not connected to a DB.

Vignesh found this bug in the check function for
default_table_access_method's check hook, but that was just copied
from older GUCs. Investigation by Michael and me then found the bug in
further places.

When not connected to a database (e.g. in a walsender connection), we
cannot perform (most) GUC checks that need database access. Even when
only shared tables are needed, unless they're
nailed (c.f. RelationCacheInitializePhase2()), they cannot be accessed
without pg_class etc. being present.

Fix by extending the existing IsTransactionState() checks to also
check for MyDatabaseOid.

Reported-By: Vignesh C, Michael Paquier, Andres Freund
Author: Vignesh C, Andres Freund
Discussion: https://postgr.es/m/CALDaNm1KXK9gbZfY-p_peRFm_XrBh1OwQO1Kk6Gig0c0fVZ2uw%40mail.gmail.com
Backpatch: 9.4-

1 files changed, 8 insertions, 6 deletions

diff --git a/src/backend/commands/tablespace.c b/src/backend/commands/tablespace.c
index 2b21edb8403..bf0a51cf7fd 100644
--- a/src/backend/commands/tablespace.c
+++ b/src/backend/commands/tablespace.c
@@ -1066,10 +1066,11 @@ bool
 check_default_tablespace(char **newval, void **extra, GucSource source)
 {
 	/*
-	 * If we aren't inside a transaction, we cannot do database access so
-	 * cannot verify the name.  Must accept the value on faith.
+	 * If we aren't inside a transaction, or connected to a database, we
+	 * cannot do the catalog accesses necessary to verify the name.  Must
+	 * accept the value on faith.
 	 */
-	if (IsTransactionState())
+	if (IsTransactionState() && MyDatabaseId != InvalidOid)
 	{
 		if (**newval != '\0' &&
 			!OidIsValid(get_tablespace_oid(*newval, true)))
@@ -1177,11 +1178,12 @@ check_temp_tablespaces(char **newval, void **extra, GucSource source)
 	}
 
 	/*
-	 * If we aren't inside a transaction, we cannot do database access so
-	 * cannot verify the individual names.  Must accept the list on faith.
+	 * If we aren't inside a transaction, or connected to a database, we
+	 * cannot do the catalog accesses necessary to verify the name.  Must
+	 * accept the value on faith.
 	 * Fortunately, there's then also no need to pass the data to fd.c.
 	 */
-	if (IsTransactionState())
+	if (IsTransactionState() && MyDatabaseId != InvalidOid)
 	{
 		temp_tablespaces_extra *myextra;
 		Oid		   *tblSpcs;