storage: reuse deltas from packfiles

* plumbing: add DeltaObject interface for EncodedObjects that
  are deltas and hold additional information about them, such
  as the hash of the base object.

* plumbing/storer: add DeltaObjectStorer interface for object
  storers that can return DeltaObject. Note that calls to
  EncodedObject will never return instances of DeltaObject.
  That requires explicit calls to DeltaObject.

* storage/filesystem: implement DeltaObjectStorer interface.

* plumbing/packfile: packfile encoder now supports reusing
  deltas that are already computed (e.g. from an existing
  packfile) if the storage implements DeltaObjectStorer.
  Reusing deltas boosts performance of packfile generation
  (e.g. on push).

1 files changed, 37 insertions, 0 deletions

diff --git a/storage/filesystem/deltaobject.go b/storage/filesystem/deltaobject.go
new file mode 100644
index 0000000..66cfb71
--- /dev/null
+++ b/storage/filesystem/deltaobject.go
@@ -0,0 +1,37 @@
+package filesystem
+
+import (
+	"gopkg.in/src-d/go-git.v4/plumbing"
+)
+
+type deltaObject struct {
+	plumbing.EncodedObject
+	base plumbing.Hash
+	hash plumbing.Hash
+	size int64
+}
+
+func newDeltaObject(
+	obj plumbing.EncodedObject,
+	hash plumbing.Hash,
+	base plumbing.Hash,
+	size int64) plumbing.DeltaObject {
+	return &deltaObject{
+		EncodedObject: obj,
+		hash:          hash,
+		base:          base,
+		size:          size,
+	}
+}
+
+func (o *deltaObject) BaseHash() plumbing.Hash {
+	return o.base
+}
+
+func (o *deltaObject) ActualSize() int64 {
+	return o.size
+}
+
+func (o *deltaObject) ActualHash() plumbing.Hash {
+	return o.hash
+}