Add fileReader

When calling Open on a File, the associated pool is used to find an
existing SizeReadSeekCloser for the underlying stream.

The Close method attempts to put the underlying SizeReadSeekCloser back in
the pool if it's not yet at the end of the stream.

reader.go

@@ -58,6 +58,42 @@ type File struct {
 	offset int64
 }
 
+type fileReader struct {
+	rc util.SizeReadSeekCloser
+	f  *File
+}
+
+func (fr *fileReader) Read(p []byte) (int, error) {
+	return fr.rc.Read(p)
+}
+
+func (fr *fileReader) Close() error {
+	if fr.rc == nil {
+		return nil
+	}
+
+	offset, err := fr.rc.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return err
+	}
+
+	f := fr.f
+
+	if offset == fr.rc.Size() { // EOF reached
+		if err := fr.rc.Close(); err != nil {
+			return err
+		}
+	} else {
+		if _, err := f.zip.pool[f.folder].Put(offset, fr.rc); err != nil {
+			return err
+		}
+	}
+
+	fr.rc = nil
+
+	return nil
+}
+
 // Open returns an io.ReadCloser that provides access to the File's contents.
 // Multiple files may be read concurrently.
 func (f *File) Open() (io.ReadCloser, error) {
@@ -66,16 +102,26 @@ func (f *File) Open() (io.ReadCloser, error) {
 		return ioutil.NopCloser(bytes.NewReader(nil)), nil
 	}
 
-	r, _, err := f.zip.folderReader(f.zip.si, f.folder)
-	if err != nil {
-		return nil, err
+	var err error
+
+	rc, _ := f.zip.pool[f.folder].Get(f.offset)
+	if rc == nil {
+		rc, _, err = f.zip.folderReader(f.zip.si, f.folder)
+		if err != nil {
+			return nil, err
+		}
 	}
 
-	if _, err := io.CopyN(ioutil.Discard, r, f.offset); err != nil {
+	if _, err = rc.Seek(f.offset, io.SeekStart); err != nil {
 		return nil, err
 	}
 
-	return plumbing.LimitReadCloser(r, int64(f.UncompressedSize)), nil
+	fr := &fileReader{
+		rc: rc,
+		f:  f,
+	}
+
+	return plumbing.LimitReadCloser(fr, int64(f.UncompressedSize)), nil
 }
 
 // OpenReaderWithPassword will open the 7-zip file specified by name using