sysvar: add validation check for tidb_max_dist_task_nodes (#59645)

ref #58944

Author: tangenta

pkg/disttask/framework/integrationtests/BUILD.bazel

@@ -17,7 +17,7 @@ go_test(
     ],
     flaky = True,
     race = "off",
-    shard_count = 22,
+    shard_count = 23,
     deps = [
         "//pkg/config",
         "//pkg/ddl",

pkg/disttask/framework/integrationtests/framework_scope_test.go

@@ -188,3 +188,19 @@ func TestTargetScope(t *testing.T) {
 		})
 	}
 }
+
+func TestTiDBMaxDistTaskNodesSettings(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+
+	tk.MustExec(`set global tidb_max_dist_task_nodes=-1`)
+	tk.MustQuery("select @@global.tidb_max_dist_task_nodes").Check(testkit.Rows("-1"))
+	tk.MustExec(`set global tidb_max_dist_task_nodes=1`)
+	tk.MustQuery("select @@global.tidb_max_dist_task_nodes").Check(testkit.Rows("1"))
+	tk.MustExec(`set global tidb_max_dist_task_nodes=128`)
+	tk.MustQuery("select @@global.tidb_max_dist_task_nodes").Check(testkit.Rows("128"))
+
+	tk.MustGetErrMsg(`set global tidb_max_dist_task_nodes=0`, "max_dist_task_nodes should be -1 or [1, 128]")
+	tk.MustExec(`set global tidb_max_dist_task_nodes=129`)
+	tk.MustQuery("select @@global.tidb_max_dist_task_nodes").Check(testkit.Rows("128"))
+}

pkg/sessionctx/variable/sysvar.go

@@ -3422,7 +3422,15 @@ var defaultSysVars = []*SysVar{
 		s.SharedLockPromotion = TiDBOptOn(val)
 		return nil
 	}},
-	{Scope: vardef.ScopeGlobal | vardef.ScopeSession, Name: vardef.TiDBMaxDistTaskNodes, Value: strconv.Itoa(vardef.DefTiDBMaxDistTaskNodes), Type: vardef.TypeInt, MinValue: -1, MaxValue: 128},
+	{Scope: vardef.ScopeGlobal | vardef.ScopeSession, Name: vardef.TiDBMaxDistTaskNodes, Value: strconv.Itoa(vardef.DefTiDBMaxDistTaskNodes), Type: vardef.TypeInt, MinValue: -1, MaxValue: 128,
+		Validation: func(s *SessionVars, normalizedValue string, originalValue string, scope vardef.ScopeFlag) (string, error) {
+			maxNodes := TidbOptInt(normalizedValue, vardef.DefTiDBMaxDistTaskNodes)
+			if maxNodes == 0 {
+				return normalizedValue, errors.New("max_dist_task_nodes should be -1 or [1, 128]")
+			}
+			return normalizedValue, nil
+		},
+	},
 	{Scope: vardef.ScopeGlobal, Name: vardef.TiDBTSOClientRPCMode, Value: vardef.DefTiDBTSOClientRPCMode, Type: vardef.TypeEnum, PossibleValues: []string{vardef.TSOClientRPCModeDefault, vardef.TSOClientRPCModeParallel, vardef.TSOClientRPCModeParallelFast},
 		SetGlobal: func(_ context.Context, s *SessionVars, val string) error {
 			return (*SetPDClientDynamicOption.Load())(vardef.TiDBTSOClientRPCMode, val)