encoding/gob: avoid race on idToType

Fixes #23328

Change-Id: Ie4864d7f388d363860318fe41431d8a9719e9a75
Reviewed-on: https://go-review.googlesource.com/86075
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/test.go

@@ -1286,7 +1286,7 @@ func (t *tester) runFlag(rx string) string {
 func (t *tester) raceTest(dt *distTest) error {
 	t.addCmd(dt, "src", t.goTest(), "-race", "-i", "runtime/race", "flag", "os", "os/exec")
 	t.addCmd(dt, "src", t.goTest(), "-race", t.runFlag("Output"), "runtime/race")
-	t.addCmd(dt, "src", t.goTest(), "-race", t.runFlag("TestParse|TestEcho|TestStdinCloseRace|TestClosedPipeRace"), "flag", "os", "os/exec")
+	t.addCmd(dt, "src", t.goTest(), "-race", t.runFlag("TestParse|TestEcho|TestStdinCloseRace|TestClosedPipeRace|TestTypeRace"), "flag", "os", "os/exec", "encoding/gob")
 	// We don't want the following line, because it
 	// slows down all.bash (by 10 seconds on my laptop).
 	// The race builder should catch any error here, but doesn't.

src/encoding/gob/decode.go

@@ -1038,6 +1038,8 @@ func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId, inProgress map[re
 
 // typeString returns a human-readable description of the type identified by remoteId.
 func (dec *Decoder) typeString(remoteId typeId) string {
+	typeLock.Lock()
+	defer typeLock.Unlock()
 	if t := idToType[remoteId]; t != nil {
 		// globally known type.
 		return t.string()

src/encoding/gob/type_test.go

@@ -7,6 +7,7 @@ package gob
 import (
 	"bytes"
 	"reflect"
+	"sync"
 	"testing"
 )
 
@@ -218,3 +219,44 @@ func TestStressParallel(t *testing.T) {
 		<-c
 	}
 }
+
+// Issue 23328. Note that this test name is known to cmd/dist/test.go.
+func TestTypeRace(t *testing.T) {
+	c := make(chan bool)
+	var wg sync.WaitGroup
+	for i := 0; i < 2; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			var buf bytes.Buffer
+			enc := NewEncoder(&buf)
+			dec := NewDecoder(&buf)
+			var x interface{}
+			switch i {
+			case 0:
+				x = &N1{}
+			case 1:
+				x = &N2{}
+			default:
+				t.Errorf("bad i %d", i)
+				return
+			}
+			m := make(map[string]string)
+			<-c
+			if err := enc.Encode(x); err != nil {
+				t.Error(err)
+				return
+			}
+			if err := enc.Encode(x); err != nil {
+				t.Error(err)
+				return
+			}
+			if err := dec.Decode(&m); err == nil {
+				t.Error("decode unexpectedly succeeded")
+				return
+			}
+		}(i)
+	}
+	close(c)
+	wg.Wait()
+}