Check if ii_AmCache is NULL in aminsertcleanup

Fix a bug introduced by c1ec02b. It may happen that the executor
opens indexes on the result relation, but no rows end up being inserted.
Then the index_insert_cleanup still gets executed, but passes down NULL
to the AM callback. The AM callback may not expect this, as is the case
of brininsertcleanup, leading to a crash.

Fixed by only calling the cleanup callback if (ii_AmCache != NULL). This
way the AM can simply assume to only see a valid cache.

Reported-by: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4-w9qC-o9hQox9UHvdVZAYTp8OrPQOKtwbvzWaRejTT=Q@mail.gmail.com

Author: tvondra

src/backend/access/index/indexam.c

@@ -207,7 +207,7 @@ index_insert_cleanup(Relation indexRelation,
 	RELATION_CHECKS;
 	Assert(indexInfo);
 
-	if (indexRelation->rd_indam->aminsertcleanup)
+	if (indexRelation->rd_indam->aminsertcleanup && indexInfo->ii_AmCache)
 		indexRelation->rd_indam->aminsertcleanup(indexInfo);
 }
 

src/test/regress/expected/brin.out

@@ -572,3 +572,9 @@ CREATE UNLOGGED TABLE brintest_unlogged (n numrange);
 CREATE INDEX brinidx_unlogged ON brintest_unlogged USING brin (n);
 INSERT INTO brintest_unlogged VALUES (numrange(0, 2^1000::numeric));
 DROP TABLE brintest_unlogged;
+-- test that the insert optimization works if no rows end up inserted
+CREATE TABLE brin_insert_optimization (a int);
+INSERT INTO brin_insert_optimization VALUES (1);
+CREATE INDEX ON brin_insert_optimization USING brin (a);
+UPDATE brin_insert_optimization SET a = a;
+DROP TABLE brin_insert_optimization;

src/test/regress/sql/brin.sql

@@ -515,3 +515,10 @@ CREATE UNLOGGED TABLE brintest_unlogged (n numrange);
 CREATE INDEX brinidx_unlogged ON brintest_unlogged USING brin (n);
 INSERT INTO brintest_unlogged VALUES (numrange(0, 2^1000::numeric));
 DROP TABLE brintest_unlogged;
+
+-- test that the insert optimization works if no rows end up inserted
+CREATE TABLE brin_insert_optimization (a int);
+INSERT INTO brin_insert_optimization VALUES (1);
+CREATE INDEX ON brin_insert_optimization USING brin (a);
+UPDATE brin_insert_optimization SET a = a;
+DROP TABLE brin_insert_optimization;