Allow aborting the pack writing operation

In case of an error in the writer, the packbuilder will stay around
waiting for someone to read from its channel. The state associated
with a packbuilder is non-trivial and it will keep a reference to the
object, so the GC won't be able to free it.

Change the ForEach interface to also return a "stop" channel. Closing
the channel or writing into it will cause the first receive clause to
act, making the callback to return -1, aborting the operation and
ending the goroutine, freeing its hold on the packbuilder.

packbuilder.go

@@ -76,10 +76,11 @@ func (pb *Packbuilder) WriteToFile(name string) error {
 }
 
 func (pb *Packbuilder) Write(w io.Writer) error {
-	ch := pb.ForEach()
+	ch, stop := pb.ForEach()
 	for slice := range ch {
 		_, err := w.Write(slice)
 		if err != nil {
+			close(stop)
 			return err
 		}
 	}
@@ -90,22 +91,40 @@ func (pb *Packbuilder) Written() uint32 {
 	return uint32(C.git_packbuilder_written(pb.ptr))
 }
 
+type packbuilderCbData struct {
+	ch chan<- []byte
+	stop <-chan bool
+}
+
 //export packbuilderForEachCb
 func packbuilderForEachCb(buf unsafe.Pointer, size C.size_t, payload unsafe.Pointer) int {
-	ch := *(*chan []byte)(payload)
+	data := (*packbuilderCbData)(payload)
+	ch := data.ch
+	stop := data.stop
 
 	slice := C.GoBytes(buf, C.int(size))
-	ch <- slice
+	select {
+	case <- stop:
+		return -1
+	case ch <- slice:
+	}
+
 	return 0
 }
 
-func (pb *Packbuilder) forEachWrap(ch chan []byte) {
-	C._go_git_packbuilder_foreach(pb.ptr, unsafe.Pointer(&ch))
-	close(ch)
+func (pb *Packbuilder) forEachWrap(data *packbuilderCbData) {
+	C._go_git_packbuilder_foreach(pb.ptr, unsafe.Pointer(data))
+	close(data.ch)
 }
 
-func (pb *Packbuilder) ForEach() chan []byte {
-	ch := make(chan []byte, 0)
-	go pb.forEachWrap(ch)
-	return ch
+// Foreach sends the packfile as slices through the "data" channel. If
+// you want to stop the pack-building process (e.g. there's an error
+// writing to the output), close or write a value into the "stop"
+// channel.
+func (pb *Packbuilder) ForEach() (data <-chan []byte, stop chan<- bool) {
+	ch := make(chan []byte)
+	stop := make(chan bool)
+	data := packbuilderCbData{ch, stop}
+	go pb.forEachWrap(&data)
+	return ch, stop
 }