go/token: Fix race in FileSet.PositionFor.

Jan Mercl · bradfitz · commit 52fcff3ec147 · 2016-08-16T01:45:25.000Z
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 golang#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>
diff --git a/src/go/token/position.go b/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
diff --git a/src/go/token/position_test.go b/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

Original file line number	Diff line number	Diff line change
`@@ -446,7 +446,9 @@ func (s FileSet) File(p Pos) (f File) {`
`446`	`446`	`func (s *FileSet) PositionFor(p Pos, adjusted bool) (pos Position) {`
`447`	`447`	`if p != NoPos {`
`448`	`448`	`if f := s.file(p); f != nil {`
	`449`	`+ s.mutex.RLock()`
`449`	`450`	`pos = f.position(p, adjusted)`
	`451`	`+ s.mutex.RUnlock()`
`450`	`452`	`}`
`451`	`453`	`}`
`452`	`454`	`return`