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, 45 insertions, 1 deletions

diff --git a/plumbing/format/packfile/object_pack.go b/plumbing/format/packfile/object_pack.go
index a3e99c0..14337d1 100644
--- a/plumbing/format/packfile/object_pack.go
+++ b/plumbing/format/packfile/object_pack.go
@@ -1,6 +1,8 @@
 package packfile
 
-import "gopkg.in/src-d/go-git.v4/plumbing"
+import (
+	"gopkg.in/src-d/go-git.v4/plumbing"
+)
 
 // ObjectToPack is a representation of an object that is going to be into a
 // pack file.
@@ -39,6 +41,48 @@ func newDeltaObjectToPack(base *ObjectToPack, original, delta plumbing.EncodedOb
 	}
 }
 
+func (o *ObjectToPack) Type() plumbing.ObjectType {
+	if o.Original != nil {
+		return o.Original.Type()
+	}
+
+	if o.Base != nil {
+		return o.Base.Type()
+	}
+
+	if o.Object != nil {
+		return o.Object.Type()
+	}
+
+	panic("cannot get type")
+}
+
+func (o *ObjectToPack) Hash() plumbing.Hash {
+	if o.Original != nil {
+		return o.Original.Hash()
+	}
+
+	do, ok := o.Object.(plumbing.DeltaObject)
+	if ok {
+		return do.ActualHash()
+	}
+
+	panic("cannot get hash")
+}
+
+func (o *ObjectToPack) Size() int64 {
+	if o.Original != nil {
+		return o.Original.Size()
+	}
+
+	do, ok := o.Object.(plumbing.DeltaObject)
+	if ok {
+		return do.ActualSize()
+	}
+
+	panic("cannot get ObjectToPack size")
+}
+
 func (o *ObjectToPack) IsDelta() bool {
 	if o.Base != nil {
 		return true