utils/fs: add ReadAt to memory file and tests.

utils/fs/memory/memory.go

@@ -189,6 +189,16 @@ func newFile(base, fullpath string, flag int) *file {
 }
 
 func (f *file) Read(b []byte) (int, error) {
+	n, err := f.ReadAt(b, f.position)
+	if err != nil {
+		return 0, err
+	}
+
+	f.position += int64(n)
+	return n, err
+}
+
+func (f *file) ReadAt(b []byte, off int64) (int, error) {
 	if f.IsClosed() {
 		return 0, fs.ErrClosed
 	}
@@ -197,7 +207,7 @@ func (f *file) Read(b []byte) (int, error) {
 		return 0, errors.New("read not supported")
 	}
 
-	n, err := f.content.ReadAt(b, f.position)
+	n, err := f.content.ReadAt(b, off)
 	f.position += int64(n)
 
 	return n, err

utils/fs/os/os.go

@@ -178,6 +178,6 @@ func (f *osFile) Close() error {
 	return f.file.Close()
 }
 
-func (f *osFile) ReadAt(p []byte, off int64) (n int, err error) {
+func (f *osFile) ReadAt(p []byte, off int64) (int, error) {
 	return f.file.ReadAt(p, off)
 }

utils/fs/test/fs_suite.go

@@ -379,3 +379,37 @@ func (s *FilesystemSuite) TestJoin(c *C) {
 func (s *FilesystemSuite) TestBase(c *C) {
 	c.Assert(s.Fs.Base(), Not(Equals), "")
 }
+
+func (s *FilesystemSuite) TestReadAtOnReadWrite(c *C) {
+	f, err := s.Fs.Create("foo")
+	c.Assert(err, IsNil)
+	_, err = f.Write([]byte("abcdefg"))
+	c.Assert(err, IsNil)
+	rf, ok := f.(io.ReaderAt)
+	c.Assert(ok, Equals, true)
+	b := make([]byte, 3)
+	n, err := rf.ReadAt(b, 2)
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, 3)
+	c.Assert(string(b), Equals, "cde")
+	c.Assert(f.Close(), IsNil)
+}
+
+func (s *FilesystemSuite) TestReadAtOnReadOnly(c *C) {
+	f, err := s.Fs.Create("foo")
+	c.Assert(err, IsNil)
+	_, err = f.Write([]byte("abcdefg"))
+	c.Assert(err, IsNil)
+	c.Assert(f.Close(), IsNil)
+
+	f, err = s.Fs.Open("foo")
+	c.Assert(err, IsNil)
+	rf, ok := f.(io.ReaderAt)
+	c.Assert(ok, Equals, true)
+	b := make([]byte, 3)
+	n, err := rf.ReadAt(b, 2)
+	c.Assert(err, IsNil)
+	c.Assert(n, Equals, 3)
+	c.Assert(string(b), Equals, "cde")
+	c.Assert(f.Close(), IsNil)
+}