Introduce Blob.DataAsyc (#94)

Signed-off-by: Duncan Ogilvie <mr.exodia.tpodt@gmail.com>

blob.go

@@ -6,7 +6,11 @@ package git
 
 import (
 	"bytes"
+	"fmt"
 	"io"
+	"io/ioutil"
+	"os"
+	"os/exec"
 )
 
 // Blob represents a Git object.
@@ -29,3 +33,36 @@ func (b *Blob) Data() (io.Reader, error) {
 func (b *Blob) DataPipeline(stdout, stderr io.Writer) error {
 	return NewCommand("show", b.ID.String()).RunInDirPipeline(b.repo.Path, stdout, stderr)
 }
+
+type cmdReadCloser struct {
+	cmd    *exec.Cmd
+	stdout io.Reader
+}
+
+func (c cmdReadCloser) Read(p []byte) (int, error) {
+	return c.stdout.Read(p)
+}
+
+func (c cmdReadCloser) Close() error {
+	io.Copy(ioutil.Discard, c.stdout)
+	return c.cmd.Wait()
+}
+
+// DataAsync gets a ReadCloser for the contents of a blob without reading it all.
+// Calling the Close function on the result will discard all unread output.
+func (b *Blob) DataAsync() (io.ReadCloser, error) {
+	cmd := exec.Command("git", "show", b.ID.String())
+	cmd.Dir = b.repo.Path
+	cmd.Stderr = os.Stderr
+
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, fmt.Errorf("StdoutPipe: %v", err)
+	}
+
+	if err = cmd.Start(); err != nil {
+		return nil, fmt.Errorf("Start: %v", err)
+	}
+
+	return cmdReadCloser{stdout: stdout, cmd: cmd}, nil
+}