Add lock_warnings system variable.

engine.go

@@ -268,6 +268,7 @@ func clearWarnings(ctx *sql.Context, node sql.Node) {
 	case *plan.Offset, *plan.Limit:
 		// `show warning limit x offset y` is valid, so we need to recurse
 		clearWarnings(ctx, n.Children()[0])
+	case *plan.Set:
 	case plan.ShowWarnings:
 		// ShowWarnings should not clear the warnings, but should still reset the warning count.
 		ctx.ClearWarningCount()

enginetest/queries/variable_queries.go

@@ -575,6 +575,66 @@ var VariableQueries = []ScriptTest{
 			},
 		},
 	},
+	{
+		Name: "locked warnings stay after query",
+		SetUpScript: []string{
+			"set @@lock_warnings = 1",
+			"select 1/0,1/0",
+			"select 1/1",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "show warnings",
+				Expected: []sql.Row{
+					{"Warning", 1365, "Division by 0"},
+					{"Warning", 1365, "Division by 0"}},
+			},
+			{
+				Query: "select 1/0",
+				Expected: []sql.Row{
+					{interface{}(nil)},
+				},
+			},
+			{
+				Query: "show warnings",
+				Expected: []sql.Row{
+					{"Warning", 1365, "Division by 0"},
+					{"Warning", 1365, "Division by 0"},
+					{"Warning", 1365, "Division by 0"},
+				},
+			},
+		},
+	},
+	{
+		Name: "unlocked warnings clear after query",
+		SetUpScript: []string{
+			"set @@lock_warnings = 0",
+			"select 1/0,1/0",
+			"select 1/1",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query:    "show warnings",
+				Expected: []sql.Row{},
+			},
+		},
+	},
+	{
+		Name: "warnings persist after locking between queries",
+		SetUpScript: []string{
+			"select 1/0",
+			"set @@lock_warnings = 1",
+			"select 1/1",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "show warnings",
+				Expected: []sql.Row{
+					{"Warning", 1365, "Division by 0"},
+				},
+			},
+		},
+	},
 	//TODO: do not override tables with user-var-like names...but why would you do this??
 	//{
 	//	Name: "user var table name no conflict",

sql/variables/system_variables.go

@@ -1253,6 +1253,23 @@ var systemVars = map[string]sql.SystemVariable{
 		Type:              types.NewSystemIntType("lock_wait_timeout", 1, 31536000, false),
 		Default:           int64(31536000),
 	},
+	"lock_warnings": &sql.MysqlSystemVariable{
+		Name:              "lock_warnings",
+		Scope:             sql.GetMysqlScope(sql.SystemVariableScope_Session),
+		Dynamic:           true,
+		SetVarHintApplies: false,
+		Type:              types.NewSystemBoolType("lock_warnings"),
+		Default:           int8(0),
+		NotifyChanged: func(ctx *sql.Context, _ sql.SystemVariableScope, value sql.SystemVarValue) error {
+			switch value.Val.(int8) {
+			case 0:
+				ctx.UnlockWarnings()
+			case 1:
+				ctx.LockWarnings()
+			}
+			return nil
+		},
+	},
 	"log_bin": &sql.MysqlSystemVariable{
 		Name:              "log_bin",
 		Scope:             sql.GetMysqlScope(sql.SystemVariableScope_Persist),