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).

7 files changed, 312 insertions, 98 deletions

diff --git a/plumbing/format/packfile/delta_selector.go b/plumbing/format/packfile/delta_selector.go
index 20c8cea..4bee6d3 100644
--- a/plumbing/format/packfile/delta_selector.go
+++ b/plumbing/format/packfile/delta_selector.go
@@ -47,17 +47,127 @@ func (dw *deltaSelector) ObjectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack,
 func (dw *deltaSelector) objectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack, error) {
 	var objectsToPack []*ObjectToPack
 	for _, h := range hashes {
-		o, err := dw.storer.EncodedObject(plumbing.AnyObject, h)
+		o, err := dw.encodedDeltaObject(h)
 		if err != nil {
 			return nil, err
 		}
 
-		objectsToPack = append(objectsToPack, newObjectToPack(o))
+		otp := newObjectToPack(o)
+		if _, ok := o.(plumbing.DeltaObject); ok {
+			otp.Original = nil
+		}
+
+		objectsToPack = append(objectsToPack, otp)
+	}
+
+	if err := dw.fixAndBreakChains(objectsToPack); err != nil {
+		return nil, err
 	}
 
 	return objectsToPack, nil
 }
 
+func (dw *deltaSelector) encodedDeltaObject(h plumbing.Hash) (plumbing.EncodedObject, error) {
+	edos, ok := dw.storer.(storer.DeltaObjectStorer)
+	if !ok {
+		return dw.encodedObject(h)
+	}
+
+	return edos.DeltaObject(plumbing.AnyObject, h)
+}
+
+func (dw *deltaSelector) encodedObject(h plumbing.Hash) (plumbing.EncodedObject, error) {
+	return dw.storer.EncodedObject(plumbing.AnyObject, h)
+}
+
+func (dw *deltaSelector) fixAndBreakChains(objectsToPack []*ObjectToPack) error {
+	m := make(map[plumbing.Hash]*ObjectToPack, len(objectsToPack))
+	for _, otp := range objectsToPack {
+		m[otp.Hash()] = otp
+	}
+
+	for _, otp := range objectsToPack {
+		if err := dw.fixAndBreakChainsOne(m, otp); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (dw *deltaSelector) fixAndBreakChainsOne(objectsToPack map[plumbing.Hash]*ObjectToPack, otp *ObjectToPack) error {
+	isDelta := otp.Object.Type() == plumbing.OFSDeltaObject ||
+		otp.Object.Type() == plumbing.REFDeltaObject
+	if !isDelta {
+		return nil
+	}
+
+	// Initial ObjectToPack instances might have a delta assigned to Object
+	// but no actual base initially. Once Base is assigned to a delta, it means
+	// we already fixed it.
+	if otp.Base != nil {
+		return nil
+	}
+
+	do, ok := otp.Object.(plumbing.DeltaObject)
+	if !ok {
+		// if this is not a DeltaObject, then we cannot retrieve its base,
+		// so we have to break the delta chain here.
+		return dw.undeltify(otp)
+	}
+
+	base, ok := objectsToPack[do.BaseHash()]
+	if !ok {
+		// The base of the delta is not in our list of objects to pack, so
+		// we break the chain.
+		return dw.undeltify(otp)
+	}
+
+	if base.Size() <= otp.Size() {
+		// Bases should be bigger
+		return dw.undeltify(otp)
+	}
+
+	if err := dw.fixAndBreakChainsOne(objectsToPack, base); err != nil {
+		return err
+	}
+
+	otp.SetDelta(base, otp.Object)
+	return nil
+}
+
+func (dw *deltaSelector) restoreOriginal(otp *ObjectToPack) error {
+	if otp.Original != nil {
+		return nil
+	}
+
+	isDelta := otp.Object.Type() == plumbing.OFSDeltaObject ||
+		otp.Object.Type() == plumbing.REFDeltaObject
+	if !isDelta {
+		return nil
+	}
+
+	obj, err := dw.encodedObject(otp.Hash())
+	if err != nil {
+		return err
+	}
+
+	otp.Original = obj
+	return nil
+}
+
+// undeltify undeltifies an *ObjectToPack by retrieving the original object from
+// the storer and resetting it.
+func (dw *deltaSelector) undeltify(otp *ObjectToPack) error {
+	if err := dw.restoreOriginal(otp); err != nil {
+		return err
+	}
+
+	otp.Object = otp.Original
+	otp.Depth = 0
+	return nil
+}
+
 func (dw *deltaSelector) sort(objectsToPack []*ObjectToPack) {
 	sort.Sort(byTypeAndSize(objectsToPack))
 }
@@ -66,15 +176,24 @@ func (dw *deltaSelector) walk(objectsToPack []*ObjectToPack) error {
 	for i := 0; i < len(objectsToPack); i++ {
 		target := objectsToPack[i]
 
-		// We only want to create deltas from specific types
-		if !applyDelta[target.Original.Type()] {
+		// If we already have a delta, we don't try to find a new one for this
+		// object. This happens when a delta is set to be reused from an existing
+		// packfile.
+		if target.IsDelta() {
+			continue
+		}
+
+		// We only want to create deltas from specific types.
+		if !applyDelta[target.Type()] {
 			continue
 		}
 
 		for j := i - 1; j >= 0; j-- {
 			base := objectsToPack[j]
 			// Objects must use only the same type as their delta base.
-			if base.Original.Type() != target.Original.Type() {
+			// Since objectsToPack is sorted by type and size, once we find
+			// a different type, we know we won't find more of them.
+			if base.Type() != target.Type() {
 				break
 			}
 
@@ -89,7 +208,7 @@ func (dw *deltaSelector) walk(objectsToPack []*ObjectToPack) error {
 
 func (dw *deltaSelector) tryToDeltify(base, target *ObjectToPack) error {
 	// If the sizes are radically different, this is a bad pairing.
-	if target.Original.Size() < base.Original.Size()>>4 {
+	if target.Size() < base.Size()>>4 {
 		return nil
 	}
 
@@ -106,10 +225,20 @@ func (dw *deltaSelector) tryToDeltify(base, target *ObjectToPack) error {
 	}
 
 	// If we have to insert a lot to make this work, find another.
-	if base.Original.Size()-target.Object.Size() > msz {
+	if base.Size()-target.Size() > msz {
 		return nil
 	}
 
+	// Original object might not be present if we're reusing a delta, so we
+	// ensure it is restored.
+	if err := dw.restoreOriginal(target); err != nil {
+		return err
+	}
+
+	if err := dw.restoreOriginal(base); err != nil {
+		return err
+	}
+
 	// Now we can generate the delta using originals
 	delta, err := GetDelta(base.Original, target.Original)
 	if err != nil {
@@ -162,13 +291,13 @@ func (a byTypeAndSize) Len() int { return len(a) }
 func (a byTypeAndSize) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
 
 func (a byTypeAndSize) Less(i, j int) bool {
-	if a[i].Object.Type() < a[j].Object.Type() {
+	if a[i].Type() < a[j].Type() {
 		return false
 	}
 
-	if a[i].Object.Type() > a[j].Object.Type() {
+	if a[i].Type() > a[j].Type() {
 		return true
 	}
 
-	return a[i].Object.Size() > a[j].Object.Size()
+	return a[i].Size() > a[j].Size()
 }
diff --git a/plumbing/format/packfile/encoder.go b/plumbing/format/packfile/encoder.go
index ae83752..1426559 100644
--- a/plumbing/format/packfile/encoder.go
+++ b/plumbing/format/packfile/encoder.go
@@ -18,6 +18,9 @@ type Encoder struct {
 	w            *offsetWriter
 	zw           *zlib.Writer
 	hasher       plumbing.Hasher
+	// offsets is a map of object hashes to corresponding offsets in the packfile.
+	// It is used to determine offset of the base of a delta when a OFS_DELTA is
+	// used.
 	offsets      map[plumbing.Hash]int64
 	useRefDeltas bool
 }
@@ -78,25 +81,24 @@ func (e *Encoder) head(numEntries int) error {
 
 func (e *Encoder) entry(o *ObjectToPack) error {
 	offset := e.w.Offset()
+	e.offsets[o.Hash()] = offset
 
 	if o.IsDelta() {
 		if err := e.writeDeltaHeader(o, offset); err != nil {
 			return err
 		}
 	} else {
-		if err := e.entryHead(o.Object.Type(), o.Object.Size()); err != nil {
+		if err := e.entryHead(o.Type(), o.Size()); err != nil {
 			return err
 		}
 	}
 
-	// Save the position using the original hash, maybe a delta will need it
-	e.offsets[o.Original.Hash()] = offset
-
 	e.zw.Reset(e.w)
 	or, err := o.Object.Reader()
 	if err != nil {
 		return err
 	}
+
 	_, err = io.Copy(e.zw, or)
 	if err != nil {
 		return err
@@ -117,9 +119,9 @@ func (e *Encoder) writeDeltaHeader(o *ObjectToPack, offset int64) error {
 	}
 
 	if e.useRefDeltas {
-		return e.writeRefDeltaHeader(o.Base.Original.Hash())
+		return e.writeRefDeltaHeader(o.Base.Hash())
 	} else {
-		return e.writeOfsDeltaHeader(offset, o.Base.Original.Hash())
+		return e.writeOfsDeltaHeader(offset, o.Base.Hash())
 	}
 }
 
@@ -128,14 +130,19 @@ func (e *Encoder) writeRefDeltaHeader(base plumbing.Hash) error {
 }
 
 func (e *Encoder) writeOfsDeltaHeader(deltaOffset int64, base plumbing.Hash) error {
-	// because it is an offset delta, we need the base
-	// object position
-	offset, ok := e.offsets[base]
+	baseOffset, ok := e.offsets[base]
 	if !ok {
-		return fmt.Errorf("delta base not found. Hash: %v", base)
+		return fmt.Errorf("base for delta not found, base hash: %v", base)
+	}
+
+	// for OFS_DELTA, offset of the base is interpreted as negative offset
+	// relative to the type-byte of the header of the ofs-delta entry.
+	relativeOffset := deltaOffset-baseOffset
+	if relativeOffset <= 0 {
+		return fmt.Errorf("bad offset for OFS_DELTA entry: %d", relativeOffset)
 	}
 
-	return binary.WriteVariableWidthInt(e.w, deltaOffset-offset)
+	return binary.WriteVariableWidthInt(e.w, relativeOffset)
 }
 
 func (e *Encoder) entryHead(typeNum plumbing.ObjectType, size int64) error {
diff --git a/plumbing/format/packfile/encoder_advanced_test.go b/plumbing/format/packfile/encoder_advanced_test.go
new file mode 100644
index 0000000..d92e2c4
--- /dev/null
+++ b/plumbing/format/packfile/encoder_advanced_test.go
@@ -0,0 +1,91 @@
+package packfile_test
+
+import (
+	"bytes"
+	"math/rand"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	. "gopkg.in/src-d/go-git.v4/plumbing/format/packfile"
+	"gopkg.in/src-d/go-git.v4/plumbing/storer"
+	"gopkg.in/src-d/go-git.v4/storage/filesystem"
+	"gopkg.in/src-d/go-git.v4/storage/memory"
+
+	"github.com/src-d/go-git-fixtures"
+	. "gopkg.in/check.v1"
+)
+
+type EncoderAdvancedSuite struct {
+	fixtures.Suite
+}
+
+var _ = Suite(&EncoderAdvancedSuite{})
+
+func (s *EncoderAdvancedSuite) TestEncodeDecode(c *C) {
+	fixs := fixtures.Basic().ByTag("packfile").ByTag(".git")
+	fixs = append(fixs, fixtures.ByURL("https://github.com/src-d/go-git.git").
+		ByTag("packfile").ByTag(".git").One())
+	fixs.Test(c, func(f *fixtures.Fixture) {
+		storage, err := filesystem.NewStorage(f.DotGit())
+		c.Assert(err, IsNil)
+		s.testEncodeDecode(c, storage)
+	})
+
+}
+
+func (s *EncoderAdvancedSuite) testEncodeDecode(c *C, storage storer.Storer) {
+
+	objIter, err := storage.IterEncodedObjects(plumbing.AnyObject)
+	c.Assert(err, IsNil)
+
+	expectedObjects := map[plumbing.Hash]bool{}
+	var hashes []plumbing.Hash
+	err = objIter.ForEach(func(o plumbing.EncodedObject) error {
+		expectedObjects[o.Hash()] = true
+		hashes = append(hashes, o.Hash())
+		return err
+
+	})
+	c.Assert(err, IsNil)
+
+	// Shuffle hashes to avoid delta selector getting order right just because
+	// the initial order is correct.
+	auxHashes := make([]plumbing.Hash, len(hashes))
+	for i, j := range rand.Perm(len(hashes)) {
+		auxHashes[j] = hashes[i]
+	}
+	hashes = auxHashes
+
+	buf := bytes.NewBuffer(nil)
+	enc := NewEncoder(buf, storage, false)
+	_, err = enc.Encode(hashes)
+	c.Assert(err, IsNil)
+
+	scanner := NewScanner(buf)
+	storage = memory.NewStorage()
+	d, err := NewDecoder(scanner, storage)
+	c.Assert(err, IsNil)
+	_, err = d.Decode()
+	c.Assert(err, IsNil)
+
+	objIter, err = storage.IterEncodedObjects(plumbing.AnyObject)
+	c.Assert(err, IsNil)
+	obtainedObjects := map[plumbing.Hash]bool{}
+	err = objIter.ForEach(func(o plumbing.EncodedObject) error {
+		obtainedObjects[o.Hash()] = true
+		return nil
+	})
+	c.Assert(err, IsNil)
+	c.Assert(obtainedObjects, DeepEquals, expectedObjects)
+
+	for h := range obtainedObjects {
+		if !expectedObjects[h] {
+			c.Errorf("obtained unexpected object: %s", h)
+		}
+	}
+
+	for h := range expectedObjects {
+		if !obtainedObjects[h] {
+			c.Errorf("missing object: %s", h)
+		}
+	}
+}
diff --git a/plumbing/format/packfile/encoder_test.go b/plumbing/format/packfile/encoder_test.go
index 551d7ec..b5b0c42 100644
--- a/plumbing/format/packfile/encoder_test.go
+++ b/plumbing/format/packfile/encoder_test.go
@@ -2,11 +2,9 @@ package packfile
 
 import (
 	"bytes"
-	"io"
 
 	"github.com/src-d/go-git-fixtures"
 	"gopkg.in/src-d/go-git.v4/plumbing"
-	"gopkg.in/src-d/go-git.v4/plumbing/storer"
 	"gopkg.in/src-d/go-git.v4/storage/memory"
 
 	. "gopkg.in/check.v1"
@@ -88,80 +86,6 @@ func (s *EncoderSuite) TestHashNotFound(c *C) {
 	c.Assert(err, Equals, plumbing.ErrObjectNotFound)
 }
 
-func (s *EncoderSuite) TestDecodeEncodeDecode(c *C) {
-	fixtures.Basic().ByTag("packfile").Test(c, func(f *fixtures.Fixture) {
-		pf := f.Packfile()
-		ph := f.PackfileHash
-		storage := memory.NewStorage()
-		s.testDecodeEncodeDecode(c, pf, ph, storage)
-	})
-}
-
-func (s *EncoderSuite) testDecodeEncodeDecode(c *C,
-	pf io.ReadCloser,
-	ph plumbing.Hash,
-	storage storer.Storer) {
-
-	defer func() {
-		c.Assert(pf.Close(), IsNil)
-	}()
-
-	scanner := NewScanner(pf)
-
-	d, err := NewDecoder(scanner, storage)
-	c.Assert(err, IsNil)
-
-	ch, err := d.Decode()
-	c.Assert(err, IsNil)
-	c.Assert(ch, Equals, ph)
-
-	objIter, err := storage.IterEncodedObjects(plumbing.AnyObject)
-	c.Assert(err, IsNil)
-
-	expectedObjects := map[plumbing.Hash]bool{}
-	var hashes []plumbing.Hash
-	err = objIter.ForEach(func(o plumbing.EncodedObject) error {
-		expectedObjects[o.Hash()] = true
-		hashes = append(hashes, o.Hash())
-		return err
-
-	})
-	c.Assert(err, IsNil)
-
-	enc := NewEncoder(s.buf, storage, false)
-	_, err = enc.Encode(hashes)
-	c.Assert(err, IsNil)
-
-	scanner = NewScanner(s.buf)
-	storage = memory.NewStorage()
-	d, err = NewDecoder(scanner, storage)
-	c.Assert(err, IsNil)
-	_, err = d.Decode()
-	c.Assert(err, IsNil)
-
-	objIter, err = storage.IterEncodedObjects(plumbing.AnyObject)
-	c.Assert(err, IsNil)
-	obtainedObjects := map[plumbing.Hash]bool{}
-	err = objIter.ForEach(func(o plumbing.EncodedObject) error {
-		obtainedObjects[o.Hash()] = true
-		return nil
-	})
-	c.Assert(err, IsNil)
-	c.Assert(obtainedObjects, DeepEquals, expectedObjects)
-
-	for h := range obtainedObjects {
-		if !expectedObjects[h] {
-			c.Errorf("obtained unexpected object: %s", h)
-		}
-	}
-
-	for h := range expectedObjects {
-		if !obtainedObjects[h] {
-			c.Errorf("missing object: %s", h)
-		}
-	}
-}
-
 func (s *EncoderSuite) TestDecodeEncodeWithDeltaDecodeREF(c *C) {
 	s.enc = NewEncoder(s.buf, s.store, true)
 	s.simpleDeltaTest(c)
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
diff --git a/plumbing/object.go b/plumbing/object.go
index 3304da2..63396f0 100644
--- a/plumbing/object.go
+++ b/plumbing/object.go
@@ -23,6 +23,17 @@ type EncodedObject interface {
 	Writer() (io.WriteCloser, error)
 }
 
+// DeltaObject is an EncodedObject representing a delta.
+type DeltaObject interface {
+	EncodedObject
+	// BaseHash returns the hash of the object used as base for this delta.
+	BaseHash() Hash
+	// ActualHash returns the hash of the object after applying the delta.
+	ActualHash() Hash
+	// Size returns the size of the object after applying the delta.
+	ActualSize() int64
+}
+
 // ObjectType internal object type
 // Integer values from 0 to 7 map to those exposed by git.
 // AnyObject is used to represent any from 0 to 7.
diff --git a/plumbing/storer/object.go b/plumbing/storer/object.go
index a733ee6..3f41468 100644
--- a/plumbing/storer/object.go
+++ b/plumbing/storer/object.go
@@ -38,6 +38,14 @@ type EncodedObjectStorer interface {
 	IterEncodedObjects(plumbing.ObjectType) (EncodedObjectIter, error)
 }
 
+// DeltaObjectStorer is an EncodedObjectStorer that can return delta
+// objects.
+type DeltaObjectStorer interface {
+	// DeltaObject is the same as EncodedObject but without resolving deltas.
+	// Deltas will be returned as plumbing.DeltaObject instances.
+	DeltaObject(plumbing.ObjectType, plumbing.Hash) (plumbing.EncodedObject, error)
+}
+
 // Transactioner is a optional method for ObjectStorer, it enable transaction
 // base write and read operations in the storage
 type Transactioner interface {