3 files changed, 260 insertions, 50 deletions

diff --git a/plumbing/format/packfile/parser.go b/plumbing/format/packfile/parser.go
index edbc0e7..62f1d13 100644
--- a/plumbing/format/packfile/parser.go
+++ b/plumbing/format/packfile/parser.go
@@ -3,6 +3,7 @@ package packfile
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 
 	"github.com/go-git/go-git/v5/plumbing"
@@ -174,13 +175,25 @@ func (p *Parser) init() error {
 	return nil
 }
 
+type objectHeaderWriter func(typ plumbing.ObjectType, sz int64) error
+
+type lazyObjectWriter interface {
+	// LazyWriter enables an object to be lazily written.
+	// It returns:
+	// - w: a writer to receive the object's content.
+	// - lwh: a func to write the object header.
+	// - err: any error from the initial writer creation process.
+	//
+	// Note that if the object header is not written BEFORE the writer
+	// is used, this will result in an invalid object.
+	LazyWriter() (w io.WriteCloser, lwh objectHeaderWriter, err error)
+}
+
 func (p *Parser) indexObjects() error {
 	buf := sync.GetBytesBuffer()
 	defer sync.PutBytesBuffer(buf)
 
 	for i := uint32(0); i < p.count; i++ {
-		buf.Reset()
-
 		oh, err := p.scanner.NextObjectHeader()
 		if err != nil {
 			return err
@@ -220,21 +233,60 @@ func (p *Parser) indexObjects() error {
 			ota = newBaseObject(oh.Offset, oh.Length, t)
 		}
 
-		buf.Grow(int(oh.Length))
-		_, crc, err := p.scanner.NextObject(buf)
+		hasher := plumbing.NewHasher(oh.Type, oh.Length)
+		writers := []io.Writer{hasher}
+		var obj *plumbing.MemoryObject
+
+		// Lazy writing is only available for non-delta objects.
+		if p.storage != nil && !delta {
+			// When a storage is set and supports lazy writing,
+			// use that instead of creating a memory object.
+			if low, ok := p.storage.(lazyObjectWriter); ok {
+				ow, lwh, err := low.LazyWriter()
+				if err != nil {
+					return err
+				}
+
+				if err = lwh(oh.Type, oh.Length); err != nil {
+					return err
+				}
+
+				defer ow.Close()
+				writers = append(writers, ow)
+			} else {
+				obj = new(plumbing.MemoryObject)
+				obj.SetSize(oh.Length)
+				obj.SetType(oh.Type)
+
+				writers = append(writers, obj)
+			}
+		}
+		if delta && !p.scanner.IsSeekable {
+			buf.Reset()
+			buf.Grow(int(oh.Length))
+			writers = append(writers, buf)
+		}
+
+		mw := io.MultiWriter(writers...)
+
+		_, crc, err := p.scanner.NextObject(mw)
 		if err != nil {
 			return err
 		}
 
+		// Non delta objects needs to be added into the storage. This
+		// is only required when lazy writing is not supported.
+		if obj != nil {
+			if _, err := p.storage.SetEncodedObject(obj); err != nil {
+				return err
+			}
+		}
+
 		ota.Crc32 = crc
 		ota.Length = oh.Length
 
-		data := buf.Bytes()
 		if !delta {
-			sha1, err := getSHA1(ota.Type, data)
-			if err != nil {
-				return err
-			}
+			sha1 := hasher.Sum()
 
 			// Move children of placeholder parent into actual parent, in case this
 			// was a non-external delta reference.
@@ -249,20 +301,8 @@ func (p *Parser) indexObjects() error {
 			p.oiByHash[ota.SHA1] = ota
 		}
 
-		if p.storage != nil && !delta {
-			obj := new(plumbing.MemoryObject)
-			obj.SetSize(oh.Length)
-			obj.SetType(oh.Type)
-			if _, err := obj.Write(data); err != nil {
-				return err
-			}
-
-			if _, err := p.storage.SetEncodedObject(obj); err != nil {
-				return err
-			}
-		}
-
 		if delta && !p.scanner.IsSeekable {
+			data := buf.Bytes()
 			p.deltas[oh.Offset] = make([]byte, len(data))
 			copy(p.deltas[oh.Offset], data)
 		}
@@ -280,23 +320,29 @@ func (p *Parser) resolveDeltas() error {
 
 	for _, obj := range p.oi {
 		buf.Reset()
+		buf.Grow(int(obj.Length))
 		err := p.get(obj, buf)
 		if err != nil {
 			return err
 		}
-		content := buf.Bytes()
 
 		if err := p.onInflatedObjectHeader(obj.Type, obj.Length, obj.Offset); err != nil {
 			return err
 		}
 
-		if err := p.onInflatedObjectContent(obj.SHA1, obj.Offset, obj.Crc32, content); err != nil {
+		if err := p.onInflatedObjectContent(obj.SHA1, obj.Offset, obj.Crc32, nil); err != nil {
 			return err
 		}
 
 		if !obj.IsDelta() && len(obj.Children) > 0 {
+			// Dealing with an io.ReaderAt object, means we can
+			// create it once and reuse across all children.
+			r := bytes.NewReader(buf.Bytes())
 			for _, child := range obj.Children {
-				if err := p.resolveObject(io.Discard, child, content); err != nil {
+				// Even though we are discarding the output, we still need to read it to
+				// so that the scanner can advance to the next object, and the SHA1 can be
+				// calculated.
+				if err := p.resolveObject(io.Discard, child, r); err != nil {
 					return err
 				}
 				p.resolveExternalRef(child)
@@ -361,13 +407,13 @@ func (p *Parser) get(o *objectInfo, buf *bytes.Buffer) (err error) {
 	if o.DiskType.IsDelta() {
 		b := sync.GetBytesBuffer()
 		defer sync.PutBytesBuffer(b)
+		buf.Grow(int(o.Length))
 		err := p.get(o.Parent, b)
 		if err != nil {
 			return err
 		}
-		base := b.Bytes()
 
-		err = p.resolveObject(buf, o, base)
+		err = p.resolveObject(buf, o, bytes.NewReader(b.Bytes()))
 		if err != nil {
 			return err
 		}
@@ -378,6 +424,13 @@ func (p *Parser) get(o *objectInfo, buf *bytes.Buffer) (err error) {
 		}
 	}
 
+	// If the scanner is seekable, caching this data into
+	// memory by offset seems wasteful.
+	// There is a trade-off to be considered here in terms
+	// of execution time vs memory consumption.
+	//
+	// TODO: improve seekable execution time, so that we can
+	// skip this cache.
 	if len(o.Children) > 0 {
 		data := make([]byte, buf.Len())
 		copy(data, buf.Bytes())
@@ -386,10 +439,25 @@ func (p *Parser) get(o *objectInfo, buf *bytes.Buffer) (err error) {
 	return nil
 }
 
+// resolveObject resolves an object from base, using information
+// provided by o.
+//
+// This call has the side-effect of changing field values
+// from the object info o:
+//   - Type: OFSDeltaObject may become the target type (e.g. Blob).
+//   - Size: The size may be update with the target size.
+//   - Hash: Zero hashes will be calculated as part of the object
+//     resolution. Hence why this process can't be avoided even when w
+//     is an io.Discard.
+//
+// base must be an io.ReaderAt, which is a requirement from
+// patchDeltaStream. The main reason being that reversing an
+// delta object may lead to going backs and forths within base,
+// which is not supported by io.Reader.
 func (p *Parser) resolveObject(
 	w io.Writer,
 	o *objectInfo,
-	base []byte,
+	base io.ReaderAt,
 ) error {
 	if !o.DiskType.IsDelta() {
 		return nil
@@ -400,26 +468,46 @@ func (p *Parser) resolveObject(
 	if err != nil {
 		return err
 	}
-	data := buf.Bytes()
 
-	data, err = applyPatchBase(o, data, base)
+	writers := []io.Writer{w}
+	var obj *plumbing.MemoryObject
+	var lwh objectHeaderWriter
+
+	if p.storage != nil {
+		if low, ok := p.storage.(lazyObjectWriter); ok {
+			ow, wh, err := low.LazyWriter()
+			if err != nil {
+				return err
+			}
+			lwh = wh
+
+			defer ow.Close()
+			writers = append(writers, ow)
+		} else {
+			obj = new(plumbing.MemoryObject)
+			ow, err := obj.Writer()
+			if err != nil {
+				return err
+			}
+
+			writers = append(writers, ow)
+		}
+	}
+
+	mw := io.MultiWriter(writers...)
+
+	err = applyPatchBase(o, base, buf, mw, lwh)
 	if err != nil {
 		return err
 	}
 
-	if p.storage != nil {
-		obj := new(plumbing.MemoryObject)
-		obj.SetSize(o.Size())
+	if obj != nil {
 		obj.SetType(o.Type)
-		if _, err := obj.Write(data); err != nil {
-			return err
-		}
-
+		obj.SetSize(o.Size()) // Size here is correct as it was populated by applyPatchBase.
 		if _, err := p.storage.SetEncodedObject(obj); err != nil {
 			return err
 		}
 	}
-	_, err = w.Write(data)
 	return err
 }
 
@@ -443,24 +531,31 @@ func (p *Parser) readData(w io.Writer, o *objectInfo) error {
 	return nil
 }
 
-func applyPatchBase(ota *objectInfo, data, base []byte) ([]byte, error) {
-	patched, err := PatchDelta(base, data)
-	if err != nil {
-		return nil, err
+// applyPatchBase applies the patch to target.
+//
+// Note that ota will be updated based on the description in resolveObject.
+func applyPatchBase(ota *objectInfo, base io.ReaderAt, delta io.Reader, target io.Writer, wh objectHeaderWriter) error {
+	if target == nil {
+		return fmt.Errorf("cannot apply patch against nil target")
 	}
 
+	typ := ota.Type
 	if ota.SHA1 == plumbing.ZeroHash {
-		ota.Type = ota.Parent.Type
-		sha1, err := getSHA1(ota.Type, patched)
-		if err != nil {
-			return nil, err
-		}
+		typ = ota.Parent.Type
+	}
+
+	sz, h, err := patchDeltaWriter(target, base, delta, typ, wh)
+	if err != nil {
+		return err
+	}
 
-		ota.SHA1 = sha1
-		ota.Length = int64(len(patched))
+	if ota.SHA1 == plumbing.ZeroHash {
+		ota.Type = typ
+		ota.Length = int64(sz)
+		ota.SHA1 = h
 	}
 
-	return patched, nil
+	return nil
 }
 
 func getSHA1(t plumbing.ObjectType, data []byte) (plumbing.Hash, error) {
diff --git a/plumbing/format/packfile/patch_delta.go b/plumbing/format/packfile/patch_delta.go
index f00562d..67c20ff 100644
--- a/plumbing/format/packfile/patch_delta.go
+++ b/plumbing/format/packfile/patch_delta.go
@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"math"
 
@@ -265,6 +266,107 @@ func patchDelta(dst *bytes.Buffer, src, delta []byte) error {
 	return nil
 }
 
+func patchDeltaWriter(dst io.Writer, base io.ReaderAt, delta io.Reader,
+	typ plumbing.ObjectType, writeHeader objectHeaderWriter) (uint, plumbing.Hash, error) {
+	deltaBuf := bufio.NewReaderSize(delta, 1024)
+	srcSz, err := decodeLEB128ByteReader(deltaBuf)
+	if err != nil {
+		if err == io.EOF {
+			return 0, plumbing.ZeroHash, ErrInvalidDelta
+		}
+		return 0, plumbing.ZeroHash, err
+	}
+
+	if r, ok := base.(*bytes.Reader); ok && srcSz != uint(r.Size()) {
+		return 0, plumbing.ZeroHash, ErrInvalidDelta
+	}
+
+	targetSz, err := decodeLEB128ByteReader(deltaBuf)
+	if err != nil {
+		if err == io.EOF {
+			return 0, plumbing.ZeroHash, ErrInvalidDelta
+		}
+		return 0, plumbing.ZeroHash, err
+	}
+
+	// If header still needs to be written, caller will provide
+	// a LazyObjectWriterHeader. This seems to be the case when
+	// dealing with thin-packs.
+	if writeHeader != nil {
+		err = writeHeader(typ, int64(targetSz))
+		if err != nil {
+			return 0, plumbing.ZeroHash, fmt.Errorf("could not lazy write header: %w", err)
+		}
+	}
+
+	remainingTargetSz := targetSz
+
+	hasher := plumbing.NewHasher(typ, int64(targetSz))
+	mw := io.MultiWriter(dst, hasher)
+
+	bufp := sync.GetByteSlice()
+	defer sync.PutByteSlice(bufp)
+
+	sr := io.NewSectionReader(base, int64(0), int64(srcSz))
+	// Keep both the io.LimitedReader types, so we can reset N.
+	baselr := io.LimitReader(sr, 0).(*io.LimitedReader)
+	deltalr := io.LimitReader(deltaBuf, 0).(*io.LimitedReader)
+
+	for {
+		buf := *bufp
+		cmd, err := deltaBuf.ReadByte()
+		if err == io.EOF {
+			return 0, plumbing.ZeroHash, ErrInvalidDelta
+		}
+		if err != nil {
+			return 0, plumbing.ZeroHash, err
+		}
+
+		if isCopyFromSrc(cmd) {
+			offset, err := decodeOffsetByteReader(cmd, deltaBuf)
+			if err != nil {
+				return 0, plumbing.ZeroHash, err
+			}
+			sz, err := decodeSizeByteReader(cmd, deltaBuf)
+			if err != nil {
+				return 0, plumbing.ZeroHash, err
+			}
+
+			if invalidSize(sz, targetSz) ||
+				invalidOffsetSize(offset, sz, srcSz) {
+				return 0, plumbing.ZeroHash, err
+			}
+
+			if _, err := sr.Seek(int64(offset), io.SeekStart); err != nil {
+				return 0, plumbing.ZeroHash, err
+			}
+			baselr.N = int64(sz)
+			if _, err := io.CopyBuffer(mw, baselr, buf); err != nil {
+				return 0, plumbing.ZeroHash, err
+			}
+			remainingTargetSz -= sz
+		} else if isCopyFromDelta(cmd) {
+			sz := uint(cmd) // cmd is the size itself
+			if invalidSize(sz, targetSz) {
+				return 0, plumbing.ZeroHash, ErrInvalidDelta
+			}
+			deltalr.N = int64(sz)
+			if _, err := io.CopyBuffer(mw, deltalr, buf); err != nil {
+				return 0, plumbing.ZeroHash, err
+			}
+
+			remainingTargetSz -= sz
+		} else {
+			return 0, plumbing.ZeroHash, err
+		}
+		if remainingTargetSz <= 0 {
+			break
+		}
+	}
+
+	return targetSz, hasher.Sum(), nil
+}
+
 // Decodes a number encoded as an unsigned LEB128 at the start of some
 // binary data and returns the decoded number and the rest of the
 // stream.
diff --git a/storage/filesystem/object.go b/storage/filesystem/object.go
index 846a7b8..e812fe9 100644
--- a/storage/filesystem/object.go
+++ b/storage/filesystem/object.go
@@ -146,6 +146,19 @@ func (s *ObjectStorage) SetEncodedObject(o plumbing.EncodedObject) (h plumbing.H
 	return o.Hash(), err
 }
 
+// LazyWriter returns a lazy ObjectWriter that is bound to a DotGit file.
+// It first write the header passing on the object type and size, so
+// that the object contents can be written later, without the need to
+// create a MemoryObject and buffering its entire contents into memory.
+func (s *ObjectStorage) LazyWriter() (w io.WriteCloser, wh func(typ plumbing.ObjectType, sz int64) error, err error) {
+	ow, err := s.dir.NewObject()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return ow, ow.WriteHeader, nil
+}
+
 // HasEncodedObject returns nil if the object exists, without actually
 // reading the object data from storage.
 func (s *ObjectStorage) HasEncodedObject(h plumbing.Hash) (err error) {