mount: detect and deal with seeking beyond end of file - fixes #828

cmd/mount/fs.go

@@ -68,6 +68,7 @@ func mountOptions(device string) (options []fuse.MountOption) {
 // returns an error, and an error channel for the serve process to
 // report an error when fusermount is called.
 func mount(f fs.Fs, mountpoint string) (<-chan error, error) {
+	fs.Debug(f, "Mounting on %q", mountpoint)
 	c, err := fuse.Mount(mountpoint, mountOptions(f.Name()+":"+f.Root())...)
 	if err != nil {
 		return nil, err

cmd/mount/read.go

@@ -79,6 +79,14 @@ func (fh *ReadFileHandle) Read(ctx context.Context, req *fuse.ReadRequest, resp
 		return errClosedFileHandle
 	}
 	if req.Offset != fh.offset {
+		// Are we attempting to seek beyond the end of the
+		// file - if so just return EOF leaving the underlying
+		// file in an unchanged state.
+		if req.Offset >= fh.o.Size() {
+			fs.Debug(fh.o, "ReadFileHandle.Read attempt to read beyond end of file: %d > %d", req.Offset, fh.o.Size())
+			resp.Data = nil
+			return nil
+		}
 		err := fh.seek(req.Offset)
 		if err != nil {
 			return err

cmd/mount/read_test.go

@@ -90,6 +90,7 @@ func TestReadSeek(t *testing.T) {
 	fd, err := os.Open(run.path("testfile"))
 	assert.NoError(t, err)
 
+	// Seek to half way
 	_, err = fd.Seek(5, 0)
 	assert.NoError(t, err)
 
@@ -97,6 +98,23 @@ func TestReadSeek(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, buf, []byte("HELLO"))
 
+	// Test seeking to the end
+	_, err = fd.Seek(10, 0)
+	assert.NoError(t, err)
+
+	buf, err = ioutil.ReadAll(fd)
+	assert.NoError(t, err)
+	assert.Equal(t, buf, []byte(""))
+
+	// Test seeking beyond the end
+	_, err = fd.Seek(1000000, 0)
+	assert.NoError(t, err)
+
+	buf, err = ioutil.ReadAll(fd)
+	assert.NoError(t, err)
+	assert.Equal(t, buf, []byte(""))
+
+	// Now back to the start
 	_, err = fd.Seek(0, 0)
 	assert.NoError(t, err)