Add lock_warnings system variable.

engine.go

@@ -268,7 +268,8 @@ 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.ShowWarnings, *plan.Set:
+	case *plan.Set:
+	case plan.ShowWarnings:
 		// ShowWarnings should not clear the warnings, but should still reset the warning count.
 		ctx.ClearWarningCount()
 	default:
@@ -410,15 +411,6 @@ func (e *Engine) QueryWithBindings(ctx *sql.Context, query string, parsed sqlpar
 		return nil, nil, nil, err
 	}
 
-	shouldLock, err := ctx.GetSessionVariable(ctx, "lock_warnings")
-	if err != nil {
-		return nil, nil, nil, err
-	}
-	if shouldLock.(int8) == 1 {
-		ctx.LockWarnings()
-		defer ctx.UnlockWarnings()
-	}
-
 	// planbuilding can produce warnings, so we need to preserve them
 	numPrevWarnings := len(ctx.Session.Warnings())
 	bound, qFlags, err := e.bindQuery(ctx, query, parsed, bindings, binder, qFlags)

enginetest/queries/variable_queries.go

@@ -589,6 +589,20 @@ var VariableQueries = []ScriptTest{
 					{"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"},
+				},
+			},
 		},
 	},
 	{
@@ -605,6 +619,22 @@ var VariableQueries = []ScriptTest{
 			},
 		},
 	},
+	{
+		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

@@ -1255,11 +1255,20 @@ var systemVars = map[string]sql.SystemVariable{
 	},
 	"lock_warnings": &sql.MysqlSystemVariable{
 		Name:              "lock_warnings",
-		Scope:             sql.GetMysqlScope(sql.SystemVariableScope_Both),
+		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",