storer: change Open to return io.ReadCloser

In a future commit, we'll want to provide a storage interface that is
read-only.  If the Open method for storers returns a io.ReadWriteCloser,
then we'll be forced to implement a useless Write method.  Since we
don't actually need the ability to call Write, simply make the storer
interface return an io.ReadCloser instead, which lets us easily provide
the read-only interface we want.

Author: bk2204

file_storer.go

@@ -27,13 +27,13 @@ func newFileStorer(root, tmp string) *fileStorer {
 	}
 }
 
-// Open implements the storer.Open function, and returns a io.ReadWriteCloser
+// Open implements the storer.Open function, and returns a io.ReadCloser
 // for the given SHA. If the file does not exist, or if there was any other
 // error in opening the file, an error will be returned.
 //
 // It is the caller's responsibility to close the given file "f" after its use
 // is complete.
-func (fs *fileStorer) Open(sha []byte) (f io.ReadWriteCloser, err error) {
+func (fs *fileStorer) Open(sha []byte) (f io.ReadCloser, err error) {
 	return fs.open(fs.path(sha), os.O_RDONLY)
 }
 

memory_storer.go

@@ -51,7 +51,7 @@ func (ms *memoryStorer) Store(sha []byte, r io.Reader) (n int64, err error) {
 // Open implements the storer.Open function, and returns a io.ReadWriteCloser
 // for the given SHA. If a reader for the given SHA does not exist an error will
 // be returned.
-func (ms *memoryStorer) Open(sha []byte) (f io.ReadWriteCloser, err error) {
+func (ms *memoryStorer) Open(sha []byte) (f io.ReadCloser, err error) {
 	ms.mu.Lock()
 	defer ms.mu.Unlock()
 

storer.go

@@ -7,7 +7,7 @@ import "io"
 type storer interface {
 	// Open returns a handle on an existing object keyed by the given SHA.
 	// It returns an error if that file does not already exist.
-	Open(sha []byte) (f io.ReadWriteCloser, err error)
+	Open(sha []byte) (f io.ReadCloser, err error)
 
 	// Store copies the data given in "r" to the unique object path given by
 	// "sha". It returns an error if that file already exists (acting as if