plumbing: format/packfile, prevent large objects from being read into memory completely (#330)

This PR adds code to prevent large objects from being read into memory
from packfiles or the filesystem.

Objects greater than 1Mb are now no longer directly stored in the cache
or read completely into memory.

This PR differs and improves the previous broken #323 by fixing several
bugs in the reader and transparently wrapping ReaderAt as a Reader.

Signed-off-by: Andrew Thornton <art27@cantab.net>

1 files changed, 15 insertions, 0 deletions

diff --git a/plumbing/format/packfile/scanner.go b/plumbing/format/packfile/scanner.go
index 6e6a687..5d9e8fb 100644
--- a/plumbing/format/packfile/scanner.go
+++ b/plumbing/format/packfile/scanner.go
@@ -320,6 +320,21 @@ func (s *Scanner) NextObject(w io.Writer) (written int64, crc32 uint32, err erro
 	return
 }
 
+// ReadObject returns a reader for the object content and an error
+func (s *Scanner) ReadObject() (io.ReadCloser, error) {
+	s.pendingObject = nil
+	zr := zlibReaderPool.Get().(io.ReadCloser)
+
+	if err := zr.(zlib.Resetter).Reset(s.r, nil); err != nil {
+		return nil, fmt.Errorf("zlib reset error: %s", err)
+	}
+
+	return ioutil.NewReadCloserWithCloser(zr, func() error {
+		zlibReaderPool.Put(zr)
+		return nil
+	}), nil
+}
+
 // ReadRegularObject reads and write a non-deltified object
 // from it zlib stream in an object entry in the packfile.
 func (s *Scanner) copyObject(w io.Writer) (n int64, err error) {