os: make Readdir & Readdirnames return os.EOF at end

Fixes #678

R=rsc, r, alex.brainman, bsiegert, jdpoirier
CC=golang-dev
https://golang.org/cl/4536058

src/pkg/os/file_unix.go

@@ -70,19 +70,29 @@ func (file *File) Stat() (fi *FileInfo, err Error) {
 
 // Readdir reads the contents of the directory associated with file and
 // returns an array of up to count FileInfo structures, as would be returned
-// by Lstat, in directory order.  Subsequent calls on the same file will yield
+// by Lstat, in directory order. Subsequent calls on the same file will yield
 // further FileInfos.
-// A negative count means to read until EOF.
-// Readdir returns the array and an Error, if any.
-func (file *File) Readdir(count int) (fi []FileInfo, err Error) {
+//
+// If n > 0, Readdir returns at most n names. In this case, if
+// Readdirnames returns an empty slice, it will return a non-nil error
+// explaining why. At the end of a directory, the error is os.EOF.
+//
+// If n <= 0, Readdir returns all the FileInfo from the directory in
+// a single slice. In this case, if Readdir succeeds (reads all
+// the way to the end of the directory), it returns the slice and a
+// nil os.Error. If it encounters an error before the end of the
+// directory, Readdir returns the FileInfo read until that point
+// and a non-nil error.
+func (file *File) Readdir(n int) (fi []FileInfo, err Error) {
 	dirname := file.name
 	if dirname == "" {
 		dirname = "."
 	}
 	dirname += "/"
-	names, err1 := file.Readdirnames(count)
-	if err1 != nil {
-		return nil, err1
+	wantAll := n < 0
+	names, namesErr := file.Readdirnames(n)
+	if namesErr != nil && !wantAll {
+		return nil, namesErr
 	}
 	fi = make([]FileInfo, len(names))
 	for i, filename := range names {
@@ -93,6 +103,9 @@ func (file *File) Readdir(count int) (fi []FileInfo, err Error) {
 			fi[i] = *fip
 		}
 	}
+	if !wantAll && namesErr != EOF {
+		err = namesErr
+	}
 	return
 }