Add isolation test to reproduce dirty snapshot scan issue

This commit introduces an isolation test to reliably reproduce the issue where non-MVCC index scans using SnapshotDirty can miss tuples due to concurrent modifications. This situation can lead to incorrect results.

To facilitate this test, new injection point  added in the index_getnext_slot.

Changes include:

* Added injection point in src/backend/access/index/indexam.c
* Updated Makefile and meson.build to include the new dirty_index_scan isolation test.
* Created a new isolation spec dirty_index_scan.spec and its expected output to define and verify the test steps.
* This test complements the previous fix by demonstrating the issue and verifying that the fix effectively addresses it.

Author: nkey

src/backend/access/index/indexam.c

@@ -57,6 +57,7 @@
 #include "utils/ruleutils.h"
 #include "utils/snapmgr.h"
 #include "utils/syscache.h"
+#include "utils/injection_point.h"
 
 
 /* ----------------------------------------------------------------
@@ -741,6 +742,13 @@ index_getnext_slot(IndexScanDesc scan, ScanDirection direction, TupleTableSlot *
 		 * the index.
 		 */
 		Assert(ItemPointerIsValid(&scan->xs_heaptid));
+#ifdef USE_INJECTION_POINTS
+		if (!IsCatalogRelationOid(scan->indexRelation->rd_id))
+		{
+			INJECTION_POINT("index_getnext_slot_before_fetch");
+		}
+#endif
+
 		if (index_fetch_heap(scan, slot))
 			return true;
 	}

src/backend/executor/execIndexing.c

@@ -117,6 +117,7 @@
 #include "utils/multirangetypes.h"
 #include "utils/rangetypes.h"
 #include "utils/snapmgr.h"
+#include "utils/injection_point.h"
 
 /* waitMode argument to check_exclusion_or_unique_constraint() */
 typedef enum
@@ -943,6 +944,8 @@ check_exclusion_or_unique_constraint(Relation heap, Relation index,
 
 	ExecDropSingleTupleTableSlot(existing_slot);
 
+	if (!conflict)
+		INJECTION_POINT("check_exclusion_or_unique_constraint_no_conflict");
 	return !conflict;
 }
 

src/test/modules/injection_points/Makefile

@@ -14,7 +14,7 @@ PGFILEDESC = "injection_points - facility for injection points"
 REGRESS = injection_points hashagg reindex_conc
 REGRESS_OPTS = --dlpath=$(top_builddir)/src/test/regress
 
-ISOLATION = basic inplace syscache-update-pruned
+ISOLATION = basic inplace syscache-update-pruned dirty_index_scan
 
 TAP_TESTS = 1
 

src/test/modules/injection_points/expected/dirty_index_scan.out

@@ -0,0 +1,27 @@
+Parsed test spec with 3 sessions
+
+starting permutation: s1_s1 s2_s1 s3_s1
+injection_points_attach
+-----------------------
+                       
+(1 row)
+
+step s1_s1: INSERT INTO test.tbl VALUES(42, 1) on conflict(i) do update set n = EXCLUDED.n + 1; <waiting ...>
+step s2_s1: UPDATE test.tbl SET n = n + 1 WHERE i = 42; <waiting ...>
+step s3_s1: 
+	SELECT injection_points_detach('index_getnext_slot_before_fetch');
+	SELECT injection_points_wakeup('index_getnext_slot_before_fetch');
+ <waiting ...>
+step s1_s1: <... completed>
+step s2_s1: <... completed>
+step s3_s1: <... completed>
+injection_points_detach
+-----------------------
+                       
+(1 row)
+
+injection_points_wakeup
+-----------------------
+                       
+(1 row)
+

src/test/modules/injection_points/meson.build

@@ -47,6 +47,7 @@ tests += {
       'basic',
       'inplace',
       'syscache-update-pruned',
+      'dirty_index_scan',
     ],
     'runningcheck': false, # see syscache-update-pruned
   },

src/test/modules/injection_points/specs/dirty_index_scan.spec

@@ -0,0 +1,37 @@
+setup
+{
+	CREATE EXTENSION injection_points;
+	CREATE SCHEMA test;
+	CREATE UNLOGGED TABLE test.tbl(i int primary key, n int);
+	CREATE INDEX tbl_n_idx ON test.tbl(n);
+	INSERT INTO test.tbl VALUES(42,1);
+}
+
+teardown
+{
+	DROP SCHEMA test CASCADE;
+	DROP EXTENSION injection_points;
+}
+
+session s1
+setup	{
+	SELECT injection_points_set_local();
+	SELECT injection_points_attach('check_exclusion_or_unique_constraint_no_conflict', 'error');
+	SELECT injection_points_attach('index_getnext_slot_before_fetch', 'wait');
+}
+
+step s1_s1	{ INSERT INTO test.tbl VALUES(42, 1) on conflict(i) do update set n = EXCLUDED.n + 1; }
+
+session s2
+step s2_s1	{ UPDATE test.tbl SET n = n + 1 WHERE i = 42; }
+
+session s3
+step s3_s1		{
+	SELECT injection_points_detach('index_getnext_slot_before_fetch');
+	SELECT injection_points_wakeup('index_getnext_slot_before_fetch');
+}
+
+permutation
+	s1_s1
+	s2_s1(*)
+	s3_s1(s1_s1)