go/token: Fix race in FileSet.PositionFor.

Methods of FileSet are documented to be safe for concurrent use by
multiple goroutines, so FileSet is protected by a mutex and all its
methods use it to prevent concurrent mutations. All methods of File that
mutate the respective FileSet, including AddLine, do also lock its
mutex, but that does not help when PositionFor is invoked concurrently
and reads without synchronization what AddLine mutates.

The change adds acquiring a RLock around the racy call of File.position
and the respective test.

Fixes #16548

Change-Id: Iecaaa02630b2532cb29ab555376633ee862315dd
Reviewed-on: https://go-review.googlesource.com/25345
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/token/position.go

@@ -446,7 +446,9 @@ func (s *FileSet) File(p Pos) (f *File) {
 func (s *FileSet) PositionFor(p Pos, adjusted bool) (pos Position) {
 	if p != NoPos {
 		if f := s.file(p); f != nil {
+			s.mutex.RLock()
 			pos = f.position(p, adjusted)
+			s.mutex.RUnlock()
 		}
 	}
 	return

src/go/token/position_test.go

@@ -214,7 +214,7 @@ func TestFileSetCacheUnlikely(t *testing.T) {
 	}
 }
 
-// issue 4345. Test concurrent use of FileSet.Pos does not trigger a
+// issue 4345. Test that concurrent use of FileSet.Pos does not trigger a
 // race in the FileSet position cache.
 func TestFileSetRace(t *testing.T) {
 	fset := NewFileSet()
@@ -237,6 +237,35 @@ func TestFileSetRace(t *testing.T) {
 	stop.Wait()
 }
 
+// issue 16548. Test that concurrent use of File.AddLine and FileSet.PositionFor
+// does not trigger a race in the FileSet position cache.
+func TestFileSetRace2(t *testing.T) {
+	const N = 1e3
+	var (
+		fset = NewFileSet()
+		file = fset.AddFile("", -1, N)
+		ch   = make(chan int, 2)
+	)
+
+	go func() {
+		for i := 0; i < N; i++ {
+			file.AddLine(i)
+		}
+		ch <- 1
+	}()
+
+	go func() {
+		pos := file.Pos(0)
+		for i := 0; i < N; i++ {
+			fset.PositionFor(pos, false)
+		}
+		ch <- 1
+	}()
+
+	<-ch
+	<-ch
+}
+
 func TestPositionFor(t *testing.T) {
 	src := []byte(`
 foo