token/position: provide files iterator

R=golang-dev, r2
CC=golang-dev
https://golang.org/cl/3541044

src/pkg/go/token/position.go

@@ -385,3 +385,25 @@ func (s *FileSet) AddFile(filename string, base, size int) *File {
 	s.files = append(s.files, f)
 	return f
 }
+
+
+// Files returns the files added to the file set.
+func (s *FileSet) Files() <-chan *File {
+	ch := make(chan *File)
+	go func() {
+		for i := 0; ; i++ {
+			var f *File
+			s.mutex.RLock()
+			if i < len(s.files) {
+				f = s.files[i]
+			}
+			s.mutex.RUnlock()
+			if f == nil {
+				break
+			}
+			ch <- f
+		}
+		close(ch)
+	}()
+	return ch
+}

src/pkg/go/token/position_test.go

@@ -138,3 +138,21 @@ func TestLineInfo(t *testing.T) {
 		checkPos(t, msg, fset.Position(p), Position{"bar", offs, 42, col})
 	}
 }
+
+
+func TestFiles(t *testing.T) {
+	fset := NewFileSet()
+	for i, test := range tests {
+		fset.AddFile(test.filename, fset.Base(), test.size)
+		j := 0
+		for g := range fset.Files() {
+			if g.Name() != tests[j].filename {
+				t.Errorf("expected filename = %s; got %s", tests[j].filename, g.Name())
+			}
+			j++
+		}
+		if j != i+1 {
+			t.Errorf("expected %d files; got %d", i+1, j)
+		}
+	}
+}