plumbing: packfile/scanner, readability/performance improvements, zlib pooling

Signed-off-by: Arran Walker <arran.walker@fiveturns.org>

3 files changed, 143 insertions, 105 deletions

diff --git a/plumbing/format/packfile/common.go b/plumbing/format/packfile/common.go
index 0d9ed54..f82c1ab 100644
--- a/plumbing/format/packfile/common.go
+++ b/plumbing/format/packfile/common.go
@@ -2,6 +2,7 @@ package packfile
 
 import (
 	"bytes"
+	"compress/zlib"
 	"io"
 	"sync"
 
@@ -66,3 +67,12 @@ var bufPool = sync.Pool{
 		return bytes.NewBuffer(nil)
 	},
 }
+
+var zlibInitBytes = []byte{0x78, 0x9c, 0x01, 0x00, 0x00, 0xff, 0xff, 0x00, 0x00, 0x00, 0x01}
+
+var zlibReaderPool = sync.Pool{
+	New: func() interface{} {
+		r, _ := zlib.NewReader(bytes.NewReader(zlibInitBytes))
+		return r
+	},
+}
diff --git a/plumbing/format/packfile/scanner.go b/plumbing/format/packfile/scanner.go
index 614b0d1..7b44192 100644
--- a/plumbing/format/packfile/scanner.go
+++ b/plumbing/format/packfile/scanner.go
@@ -39,8 +39,7 @@ type ObjectHeader struct {
 }
 
 type Scanner struct {
-	r   reader
-	zr  readerResetter
+	r   *scannerReader
 	crc hash.Hash32
 
 	// pendingObject is used to detect if an object has been read, or still
@@ -56,19 +55,27 @@ type Scanner struct {
 // NewScanner returns a new Scanner based on a reader, if the given reader
 // implements io.ReadSeeker the Scanner will be also Seekable
 func NewScanner(r io.Reader) *Scanner {
-	seeker, ok := r.(io.ReadSeeker)
-	if !ok {
-		seeker = &trackableReader{Reader: r}
-	}
+	_, ok := r.(io.ReadSeeker)
 
 	crc := crc32.NewIEEE()
 	return &Scanner{
-		r:          newTeeReader(newByteReadSeeker(seeker), crc),
+		r:          newScannerReader(r, crc),
 		crc:        crc,
 		IsSeekable: ok,
 	}
 }
 
+func (s *Scanner) Reset(r io.Reader) {
+	_, ok := r.(io.ReadSeeker)
+
+	s.r.Reset(r)
+	s.crc.Reset()
+	s.IsSeekable = ok
+	s.pendingObject = nil
+	s.version = 0
+	s.objects = 0
+}
+
 // Header reads the whole packfile header (signature, version and object count).
 // It returns the version and the object count and performs checks on the
 // validity of the signature and the version fields.
@@ -182,8 +189,7 @@ func (s *Scanner) NextObjectHeader() (*ObjectHeader, error) {
 // nextObjectHeader returns the ObjectHeader for the next object in the reader
 // without the Offset field
 func (s *Scanner) nextObjectHeader() (*ObjectHeader, error) {
-	defer s.Flush()
-
+	s.r.Flush()
 	s.crc.Reset()
 
 	h := &ObjectHeader{}
@@ -304,35 +310,29 @@ func (s *Scanner) readLength(first byte) (int64, error) {
 // NextObject writes the content of the next object into the reader, returns
 // the number of bytes written, the CRC32 of the content and an error, if any
 func (s *Scanner) NextObject(w io.Writer) (written int64, crc32 uint32, err error) {
-	defer s.crc.Reset()
-
 	s.pendingObject = nil
 	written, err = s.copyObject(w)
-	s.Flush()
+
+	s.r.Flush()
 	crc32 = s.crc.Sum32()
+	s.crc.Reset()
+
 	return
 }
 
 // ReadRegularObject reads and write a non-deltified object
 // from it zlib stream in an object entry in the packfile.
 func (s *Scanner) copyObject(w io.Writer) (n int64, err error) {
-	if s.zr == nil {
-		var zr io.ReadCloser
-		zr, err = zlib.NewReader(s.r)
-		if err != nil {
-			return 0, fmt.Errorf("zlib initialization error: %s", err)
-		}
+	zr := zlibReaderPool.Get().(io.ReadCloser)
+	defer zlibReaderPool.Put(zr)
 
-		s.zr = zr.(readerResetter)
-	} else {
-		if err = s.zr.Reset(s.r, nil); err != nil {
-			return 0, fmt.Errorf("zlib reset error: %s", err)
-		}
+	if err = zr.(zlib.Resetter).Reset(s.r, nil); err != nil {
+		return 0, fmt.Errorf("zlib reset error: %s", err)
 	}
 
-	defer ioutil.CheckClose(s.zr, &err)
+	defer ioutil.CheckClose(zr, &err)
 	buf := byteSlicePool.Get().([]byte)
-	n, err = io.CopyBuffer(w, s.zr, buf)
+	n, err = io.CopyBuffer(w, zr, buf)
 	byteSlicePool.Put(buf)
 	return
 }
@@ -378,110 +378,89 @@ func (s *Scanner) Close() error {
 	return err
 }
 
-// Flush finishes writing the buffer to crc hasher in case we are using
-// a teeReader. Otherwise it is a no-op.
+// Flush is a no-op (deprecated)
 func (s *Scanner) Flush() error {
-	tee, ok := s.r.(*teeReader)
-	if ok {
-		return tee.Flush()
-	}
 	return nil
 }
 
-type trackableReader struct {
-	count int64
-	io.Reader
+// scannerReader has the following characteristics:
+// - Provides an io.SeekReader impl for bufio.Reader, when the underlying
+//   reader supports it.
+// - Keeps track of the current read position, for when the underlying reader
+//   isn't an io.SeekReader, but we still want to know the current offset.
+// - Writes to the hash writer what it reads, with the aid of a smaller buffer.
+//   The buffer helps avoid a performance penality for performing small writes
+//   to the crc32 hash writer.
+type scannerReader struct {
+	reader io.Reader
+	crc    io.Writer
+	rbuf   *bufio.Reader
+	wbuf   *bufio.Writer
+	offset int64
 }
 
-// Read reads up to len(p) bytes into p.
-func (r *trackableReader) Read(p []byte) (n int, err error) {
-	n, err = r.Reader.Read(p)
-	r.count += int64(n)
-
-	return
-}
-
-// Seek only supports io.SeekCurrent, any other operation fails
-func (r *trackableReader) Seek(offset int64, whence int) (int64, error) {
-	if whence != io.SeekCurrent {
-		return -1, ErrSeekNotSupported
+func newScannerReader(r io.Reader, h io.Writer) *scannerReader {
+	sr := &scannerReader{
+		rbuf: bufio.NewReader(nil),
+		wbuf: bufio.NewWriterSize(nil, 64),
+		crc:  h,
 	}
+	sr.Reset(r)
 
-	return r.count, nil
+	return sr
 }
 
-func newByteReadSeeker(r io.ReadSeeker) *bufferedSeeker {
-	return &bufferedSeeker{
-		r:      r,
-		Reader: *bufio.NewReader(r),
-	}
-}
+func (r *scannerReader) Reset(reader io.Reader) {
+	r.reader = reader
+	r.rbuf.Reset(r.reader)
+	r.wbuf.Reset(r.crc)
 
-type bufferedSeeker struct {
-	r io.ReadSeeker
-	bufio.Reader
-}
-
-func (r *bufferedSeeker) Seek(offset int64, whence int) (int64, error) {
-	if whence == io.SeekCurrent && offset == 0 {
-		current, err := r.r.Seek(offset, whence)
-		if err != nil {
-			return current, err
-		}
-
-		return current - int64(r.Buffered()), nil
+	r.offset = 0
+	if seeker, ok := r.reader.(io.ReadSeeker); ok {
+		r.offset, _ = seeker.Seek(0, io.SeekCurrent)
 	}
-
-	defer r.Reader.Reset(r.r)
-	return r.r.Seek(offset, whence)
 }
 
-type readerResetter interface {
-	io.ReadCloser
-	zlib.Resetter
-}
+func (r *scannerReader) Read(p []byte) (n int, err error) {
+	n, err = r.rbuf.Read(p)
 
-type reader interface {
-	io.Reader
-	io.ByteReader
-	io.Seeker
+	r.offset += int64(n)
+	if _, err := r.wbuf.Write(p[:n]); err != nil {
+		return n, err
+	}
+	return
 }
 
-type teeReader struct {
-	reader
-	w         hash.Hash32
-	bufWriter *bufio.Writer
+func (r *scannerReader) ReadByte() (b byte, err error) {
+	b, err = r.rbuf.ReadByte()
+	if err == nil {
+		r.offset++
+		return b, r.wbuf.WriteByte(b)
+	}
+	return
 }
 
-func newTeeReader(r reader, h hash.Hash32) *teeReader {
-	return &teeReader{
-		reader:    r,
-		w:         h,
-		bufWriter: bufio.NewWriter(h),
-	}
+func (r *scannerReader) Flush() error {
+	return r.wbuf.Flush()
 }
 
-func (r *teeReader) Read(p []byte) (n int, err error) {
-	r.Flush()
+// Seek seeks to a location. If the underlying reader is not an io.ReadSeeker,
+// then only whence=io.SeekCurrent is supported, any other operation fails.
+func (r *scannerReader) Seek(offset int64, whence int) (int64, error) {
+	var err error
 
-	n, err = r.reader.Read(p)
-	if n > 0 {
-		if n, err := r.w.Write(p[:n]); err != nil {
-			return n, err
+	if seeker, ok := r.reader.(io.ReadSeeker); !ok {
+		if whence != io.SeekCurrent || offset != 0 {
+			return -1, ErrSeekNotSupported
+		}
+	} else {
+		if whence == io.SeekCurrent && offset == 0 {
+			return r.offset, nil
 		}
-	}
-	return
-}
 
-func (r *teeReader) ReadByte() (b byte, err error) {
-	b, err = r.reader.ReadByte()
-	if err == nil {
-		return b, r.bufWriter.WriteByte(b)
+		r.offset, err = seeker.Seek(offset, whence)
+		r.rbuf.Reset(r.reader)
 	}
 
-	return
-}
-
-func (r *teeReader) Flush() (err error) {
-	return r.bufWriter.Flush()
+	return r.offset, err
 }
diff --git a/plumbing/format/packfile/scanner_test.go b/plumbing/format/packfile/scanner_test.go
index 091b457..a401d6d 100644
--- a/plumbing/format/packfile/scanner_test.go
+++ b/plumbing/format/packfile/scanner_test.go
@@ -135,6 +135,55 @@ func (s *ScannerSuite) TestSeekObjectHeaderNonSeekable(c *C) {
 	c.Assert(err, Equals, ErrSeekNotSupported)
 }
 
+func (s *ScannerSuite) TestReaderReset(c *C) {
+	r := fixtures.Basic().One().Packfile()
+	p := NewScanner(r)
+
+	version, objects, err := p.Header()
+	c.Assert(version, Equals, VersionSupported)
+	c.Assert(objects, Equals, uint32(31))
+
+	h, err := p.SeekObjectHeader(expectedHeadersOFS[0].Offset)
+	c.Assert(err, IsNil)
+	c.Assert(h, DeepEquals, &expectedHeadersOFS[0])
+
+	p.Reset(r)
+	c.Assert(p.pendingObject, IsNil)
+	c.Assert(p.version, Equals, uint32(0))
+	c.Assert(p.objects, Equals, uint32(0))
+	c.Assert(p.r.reader, Equals, r)
+	c.Assert(p.r.offset > expectedHeadersOFS[0].Offset, Equals, true)
+
+	p.Reset(bytes.NewReader(nil))
+	c.Assert(p.r.offset, Equals, int64(0))
+}
+
+func (s *ScannerSuite) TestReaderResetSeeks(c *C) {
+	r := fixtures.Basic().One().Packfile()
+
+	// seekable
+	p := NewScanner(r)
+	c.Assert(p.IsSeekable, Equals, true)
+	h, err := p.SeekObjectHeader(expectedHeadersOFS[0].Offset)
+	c.Assert(err, IsNil)
+	c.Assert(h, DeepEquals, &expectedHeadersOFS[0])
+
+	// reset with seekable
+	p.Reset(r)
+	c.Assert(p.IsSeekable, Equals, true)
+	h, err = p.SeekObjectHeader(expectedHeadersOFS[1].Offset)
+	c.Assert(err, IsNil)
+	c.Assert(h, DeepEquals, &expectedHeadersOFS[1])
+
+	// reset with non-seekable
+	f := fixtures.Basic().ByTag("ref-delta").One()
+	p.Reset(io.MultiReader(f.Packfile()))
+	c.Assert(p.IsSeekable, Equals, false)
+
+	_, err = p.SeekObjectHeader(expectedHeadersOFS[4].Offset)
+	c.Assert(err, Equals, ErrSeekNotSupported)
+}
+
 var expectedHeadersOFS = []ObjectHeader{
 	{Type: plumbing.CommitObject, Offset: 12, Length: 254},
 	{Type: plumbing.OFSDeltaObject, Offset: 186, Length: 93, OffsetReference: 12},