Fix failure with lock mode used for custom relation options

In-core relation options can use a custom lock mode since 47167b7, that
has lowered the lock available for some autovacuum parameters.  However
it forgot to consider custom relation options.  This causes failures
with ALTER TABLE SET when changing a custom relation option, as its lock
is not defined.  The existing APIs to define a custom reloption does not
allow to define a custom lock mode, so enforce its initialization to
AccessExclusiveMode which should be safe enough in all cases.  An
upcoming patch will extend the existing APIs to allow a custom lock mode
to be defined.

The problem can be reproduced with bloom indexes, so add a test there.

Reported-by: Nikolay Sharplov
Analyzed-by: Thomas Munro, Michael Paquier
Author: Michael Paquier
Reviewed-by: Kuntal Ghosh
Discussion: https://postgr.es/m/20190920013831.GD1844@paquier.xyz
Backpatch-through: 9.6

3 files changed, 9 insertions, 0 deletions

diff --git a/contrib/bloom/expected/bloom.out b/contrib/bloom/expected/bloom.out
index 5ab9e34f823..dae12a7d3e7 100644
--- a/contrib/bloom/expected/bloom.out
+++ b/contrib/bloom/expected/bloom.out
@@ -5,6 +5,7 @@ CREATE TABLE tst (
 );
 INSERT INTO tst SELECT i%10, substr(md5(i::text), 1, 1) FROM generate_series(1,2000) i;
 CREATE INDEX bloomidx ON tst USING bloom (i, t) WITH (col1 = 3);
+ALTER INDEX bloomidx SET (length=80);
 SET enable_seqscan=on;
 SET enable_bitmapscan=off;
 SET enable_indexscan=off;
diff --git a/contrib/bloom/sql/bloom.sql b/contrib/bloom/sql/bloom.sql
index 32755f2b1a5..4733e1e7050 100644
--- a/contrib/bloom/sql/bloom.sql
+++ b/contrib/bloom/sql/bloom.sql
@@ -7,6 +7,7 @@ CREATE TABLE tst (
 
 INSERT INTO tst SELECT i%10, substr(md5(i::text), 1, 1) FROM generate_series(1,2000) i;
 CREATE INDEX bloomidx ON tst USING bloom (i, t) WITH (col1 = 3);
+ALTER INDEX bloomidx SET (length=80);
 
 SET enable_seqscan=on;
 SET enable_bitmapscan=off;
diff --git a/src/backend/access/common/reloptions.c b/src/backend/access/common/reloptions.c
index 57730214993..2e1dcb93237 100644
--- a/src/backend/access/common/reloptions.c
+++ b/src/backend/access/common/reloptions.c
@@ -659,6 +659,13 @@ allocate_reloption(bits32 kinds, int type, const char *name, const char *desc)
 	newoption->namelen = strlen(name);
 	newoption->type = type;
 
+	/*
+	 * Set the default lock mode for this option.  There is no actual way
+	 * for a module to enforce it when declaring a custom relation option,
+	 * so just use the highest level, which is safe for all cases.
+	 */
+	newoption->lockmode = AccessExclusiveLock;
+
 	MemoryContextSwitchTo(oldcxt);
 
 	return newoption;