go/token: let FileSet.AddFile take a negative base

Negative base now means "automatic". Fixes a higher
level race.

Fixes #5418

R=gri
CC=golang-dev
https://golang.org/cl/9269043

src/pkg/go/parser/parser.go

@@ -64,7 +64,7 @@ type parser struct {
 }
 
 func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
-	p.file = fset.AddFile(filename, fset.Base(), len(src))
+	p.file = fset.AddFile(filename, -1, len(src))
 	var m scanner.Mode
 	if mode&ParseComments != 0 {
 		m = scanner.ScanComments

src/pkg/go/token/position.go

@@ -314,7 +314,8 @@ func (s *FileSet) Base() int {
 // AddFile adds a new file with a given filename, base offset, and file size
 // to the file set s and returns the file. Multiple files may have the same
 // name. The base offset must not be smaller than the FileSet's Base(), and
-// size must not be negative.
+// size must not be negative. As a special case, if a negative base is provided,
+// the current value of the FileSet's Base() is used instead.
 //
 // Adding the file will set the file set's Base() value to base + size + 1
 // as the minimum base value for the next file. The following relationship
@@ -329,6 +330,9 @@ func (s *FileSet) Base() int {
 func (s *FileSet) AddFile(filename string, base, size int) *File {
 	s.mutex.Lock()
 	defer s.mutex.Unlock()
+	if base < 0 {
+		base = s.base
+	}
 	if base < s.base || size < 0 {
 		panic("illegal base or size")
 	}

src/pkg/go/token/position_test.go

@@ -167,7 +167,13 @@ func TestLineInfo(t *testing.T) {
 func TestFiles(t *testing.T) {
 	fset := NewFileSet()
 	for i, test := range tests {
-		fset.AddFile(test.filename, fset.Base(), test.size)
+		base := fset.Base()
+		if i%2 == 1 {
+			// Setting a negative base is equivalent to
+			// fset.Base(), so test some of each.
+			base = -1
+		}
+		fset.AddFile(test.filename, base, test.size)
 		j := 0
 		fset.Iterate(func(f *File) bool {
 			if f.Name() != tests[j].filename {