Add lock_warnings system variable.

engine.go

@@ -268,7 +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.ShowWarnings:
+	case plan.ShowWarnings, *plan.Set:
 		// ShowWarnings should not clear the warnings, but should still reset the warning count.
 		ctx.ClearWarningCount()
 	default:
@@ -410,6 +410,15 @@ 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

@@ -575,6 +575,36 @@ 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"}},
+			},
+		},
+	},
+	{
+		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{},
+			},
+		},
+	},
 	//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 +1253,14 @@ 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_Both),
+		Dynamic:           true,
+		SetVarHintApplies: false,
+		Type:              types.NewSystemBoolType("lock_warnings"),
+		Default:           int8(0),
+	},
 	"log_bin": &sql.MysqlSystemVariable{
 		Name:              "log_bin",
 		Scope:             sql.GetMysqlScope(sql.SystemVariableScope_Persist),