infoschema: fix a snapshot infoschema cache bug after v1 v2 switch

pkg/infoschema/cache.go

@@ -37,6 +37,10 @@ type InfoCache struct {
 
 	r    autoid.Requirement
 	Data *Data
+
+	// first known schema version records the first known schema version, all schemas between [firstKnownSchemaVersion, latest)
+	// are known as long as we keep the DDL history correctly.
+	firstKnownSchemaVersion int64
 }
 
 type schemaAndTimestamp struct {
@@ -97,6 +101,7 @@ func (h *InfoCache) Upsert(is InfoSchema, schemaTS uint64) func() {
 		infoschema: is,
 		timestamp:  int64(schemaTS),
 	})
+	h.firstKnownSchemaVersion = is.SchemaMetaVersion()
 
 	return func() {
 		// TODO: It's a bit tricky here, somewhere is holding the reference of the old infoschema.
@@ -223,8 +228,28 @@ func (h *InfoCache) getByVersionNoLock(version int64) InfoSchema {
 	//			return h.cache[i]
 	//		}
 	// ```
+	// upsert is a full reset of InfoCache, after upsert, the DDL history might lost and the assumption does not hold anymore.
+	// For example:
+	//     Before
+	//              infoschema 51
+	//              infoschema 52
+	//              infoschema 53
+	//              infoschema 54
+	//              infoschema 55
+	//              infoschema 56
+	//     After Upsert()
+	//              infoschema 56
+	//     Then load historial snapshot version 51
+	//              infoschema 51
+	//              infoschema 56
+	// Now, request for schema version 55, return infoschem 51 would be wrong!
+	//
+	if i == len(h.cache) {
+		return nil
+	}
 
-	if i < len(h.cache) && (i != 0 || h.cache[i].infoschema.SchemaMetaVersion() == version) {
+	if h.cache[i].infoschema.SchemaMetaVersion() == version ||
+		(i != 0 && h.cache[i].infoschema.SchemaMetaVersion() >= h.firstKnownSchemaVersion) {
 		infoschema_metrics.HitVersionCounter.Inc()
 		return h.cache[i].infoschema
 	}
@@ -289,6 +314,9 @@ func (h *InfoCache) Insert(is InfoSchema, schemaTS uint64) bool {
 			infoschema: is,
 			timestamp:  int64(schemaTS),
 		}
+		if len(h.cache) == 1 {
+			h.firstKnownSchemaVersion = is.SchemaMetaVersion()
+		}
 	} else if i < len(h.cache) {
 		// drop older schema
 		copy(h.cache[i+1:], h.cache[i:])

pkg/infoschema/test/infoschemav2test/BUILD.bazel

@@ -8,7 +8,7 @@ go_test(
         "v2_test.go",
     ],
     flaky = True,
-    shard_count = 7,
+    shard_count = 8,
     deps = [
         "//pkg/domain",
         "//pkg/domain/infosync",
@@ -21,6 +21,7 @@ go_test(
         "//pkg/testkit/testfailpoint",
         "//pkg/testkit/testsetup",
         "@com_github_stretchr_testify//require",
+        "@com_github_tikv_client_go_v2//oracle",
         "@org_uber_go_goleak//:goleak",
     ],
 )

pkg/infoschema/test/infoschemav2test/v2_test.go

@@ -33,6 +33,7 @@ import (
 	"github.com/pingcap/tidb/pkg/testkit"
 	"github.com/pingcap/tidb/pkg/testkit/testfailpoint"
 	"github.com/stretchr/testify/require"
+	"github.com/tikv/client-go/v2/oracle"
 )
 
 func TestSpecialSchemas(t *testing.T) {
@@ -375,3 +376,45 @@ func TestFullLoadAndSnapshot(t *testing.T) {
 	tk.MustExec("use db1")
 	tk.MustQuery("show tables").Check(testkit.Rows("t"))
 }
+
+func TestIssue54926(t *testing.T) {
+	store, dom := testkit.CreateMockStoreAndDomain(t)
+	tk := testkit.NewTestKit(t, store)
+	tk2 := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("set @@global.tidb_schema_cache_size = 0")
+	// For mocktikv, safe point is not initialized, we manually insert it for snapshot to use.
+	safePointName := "tikv_gc_safe_point"
+	safePointValue := "20160102-15:04:05 -0700"
+	safePointComment := "All versions after safe point can be accessed. (DO NOT EDIT)"
+	updateSafePoint := fmt.Sprintf(`INSERT INTO mysql.tidb VALUES ('%[1]s', '%[2]s', '%[3]s')
+	ON DUPLICATE KEY
+	UPDATE variable_value = '%[2]s', comment = '%[3]s'`, safePointName, safePointValue, safePointComment)
+	tk.MustExec(updateSafePoint)
+
+	time1 := time.Now()
+	time1TS := oracle.GoTimeToTS(time1)
+	schemaVer1 := tk.Session().GetInfoSchema().SchemaMetaVersion()
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t (id int primary key);")
+	tk.MustExec(`drop table if exists t`)
+	time.Sleep(50 * time.Millisecond)
+	time2 := time.Now()
+	time2TS := oracle.GoTimeToTS(time2)
+	schemaVer2 := tk.Session().GetInfoSchema().SchemaMetaVersion()
+
+	tk2.MustExec("create table test.t (id int primary key)")
+	tk.MustExec("set @@global.tidb_schema_cache_size = 1073741824")
+	dom.Reload()
+
+	// test set txn as of will flush/mutex tidb_snapshot
+	tk.MustExec(fmt.Sprintf(`set @@tidb_snapshot="%s"`, time1.Format("2006-1-2 15:04:05.000")))
+	require.Equal(t, time1TS, tk.Session().GetSessionVars().SnapshotTS)
+	require.NotNil(t, tk.Session().GetSessionVars().SnapshotInfoschema)
+	require.Equal(t, schemaVer1, tk.Session().GetInfoSchema().SchemaMetaVersion())
+	tk.MustExec(fmt.Sprintf(`SET TRANSACTION READ ONLY AS OF TIMESTAMP '%s'`, time2.Format("2006-1-2 15:04:05.000")))
+	require.Equal(t, uint64(0), tk.Session().GetSessionVars().SnapshotTS)
+	require.NotNil(t, tk.Session().GetSessionVars().SnapshotInfoschema)
+	require.Equal(t, time2TS, tk.Session().GetSessionVars().TxnReadTS.PeakTxnReadTS())
+	require.Equal(t, schemaVer2, tk.Session().GetInfoSchema().SchemaMetaVersion())
+}