Add a regression test for snapshot too old with hash indexes.

Amit Kapila, but I changed the comment not to be a copy-and-paste of
an existing one, and instead referred to it.

Discussion: http://postgr.es/m/CAA4eK1K0UJswCRf81WwJFO4H=+ZvbmKTNhAps-NkdmHRsq1GnQ@mail.gmail.com

src/test/modules/snapshot_too_old/Makefile

@@ -4,7 +4,7 @@
 # we have to clean those result files explicitly
 EXTRA_CLEAN = $(pg_regress_clean_files)
 
-ISOLATIONCHECKS=sto_using_cursor sto_using_select
+ISOLATIONCHECKS=sto_using_cursor sto_using_select sto_using_hash_index
 
 ifdef USE_PGXS
 PG_CONFIG = pg_config

src/test/modules/snapshot_too_old/expected/sto_using_hash_index.out

@@ -0,0 +1,15 @@
+Parsed test spec with 2 sessions
+
+starting permutation: noseq s1f1 s2sleep s2u s1f2
+step noseq: SET enable_seqscan = false;
+step s1f1: SELECT c FROM sto1 where c = 1000;
+c              
+
+1000           
+step s2sleep: SELECT setting, pg_sleep(6) FROM pg_settings WHERE name = 'old_snapshot_threshold';
+setting        pg_sleep       
+
+0                             
+step s2u: UPDATE sto1 SET c = 1001 WHERE c = 1000;
+step s1f2: SELECT c FROM sto1 where c = 1001;
+ERROR:  snapshot too old

src/test/modules/snapshot_too_old/specs/sto_using_hash_index.spec

@@ -0,0 +1,31 @@
+# This test is like sto_using_select, except that we test access via a
+# hash index.
+
+setup
+{
+    CREATE TABLE sto1 (c int NOT NULL);
+    INSERT INTO sto1 SELECT generate_series(1, 1000);
+    CREATE INDEX idx_sto1 ON sto1 USING HASH (c);
+}
+setup
+{
+    VACUUM ANALYZE sto1;
+}
+
+teardown
+{
+    DROP TABLE sto1;
+}
+
+session "s1"
+setup			{ BEGIN ISOLATION LEVEL REPEATABLE READ; }
+step "noseq"	{ SET enable_seqscan = false; }
+step "s1f1"		{ SELECT c FROM sto1 where c = 1000; }
+step "s1f2"		{ SELECT c FROM sto1 where c = 1001; }
+teardown		{ ROLLBACK; }
+
+session "s2"
+step "s2sleep"	{ SELECT setting, pg_sleep(6) FROM pg_settings WHERE name = 'old_snapshot_threshold'; }
+step "s2u"		{ UPDATE sto1 SET c = 1001 WHERE c = 1000; }
+
+permutation "noseq" "s1f1" "s2sleep" "s2u" "s1f2"