compress/gzip: allow stopping at end of first stream

Allows parsing some file formats that assign special
meaning to which stream data is found in.

Will do the same for compress/bzip2 once this is
reviewed and submitted.

Fixes #6486.

LGTM=nigeltao
R=nigeltao, dan.kortschak
CC=adg, bradfitz, golang-codereviews, r
https://golang.org/cl/159120044

1 files changed, 27 insertions, 0 deletions

diff --git a/src/compress/gzip/gunzip.go b/src/compress/gzip/gunzip.go
index df1d5aa2be..72ee55c4fa 100644
--- a/src/compress/gzip/gunzip.go
+++ b/src/compress/gzip/gunzip.go
@@ -74,6 +74,7 @@ type Reader struct {
 	flg          byte
 	buf          [512]byte
 	err          error
+	multistream  bool
 }
 
 // NewReader creates a new Reader reading the given reader.
@@ -83,6 +84,7 @@ type Reader struct {
 func NewReader(r io.Reader) (*Reader, error) {
 	z := new(Reader)
 	z.r = makeReader(r)
+	z.multistream = true
 	z.digest = crc32.NewIEEE()
 	if err := z.readHeader(true); err != nil {
 		return nil, err
@@ -102,9 +104,30 @@ func (z *Reader) Reset(r io.Reader) error {
 	}
 	z.size = 0
 	z.err = nil
+	z.multistream = true
 	return z.readHeader(true)
 }
 
+// Multistream controls whether the reader supports multistream files.
+//
+// If enabled (the default), the Reader expects the input to be a sequence
+// of individually gzipped data streams, each with its own header and
+// trailer, ending at EOF. The effect is that the concatenation of a sequence
+// of gzipped files is treated as equivalent to the gzip of the concatenation
+// of the sequence. This is standard behavior for gzip readers.
+//
+// Calling Multistream(false) disables this behavior; disabling the behavior
+// can be useful when reading file formats that distinguish individual gzip
+// data streams or mix gzip data streams with other data streams.
+// In this mode, when the Reader reaches the end of the data stream,
+// Read returns io.EOF. If the underlying reader implements io.ByteReader,
+// it will be left positioned just after the gzip stream.
+// To start the next stream, call z.Reset(r) followed by z.Multistream(false).
+// If there is no next stream, z.Reset(r) will return io.EOF.
+func (z *Reader) Multistream(ok bool) {
+	z.multistream = ok
+}
+
 // GZIP (RFC 1952) is little-endian, unlike ZLIB (RFC 1950).
 func get4(p []byte) uint32 {
 	return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
@@ -245,6 +268,10 @@ func (z *Reader) Read(p []byte) (n int, err error) {
 	}
 
 	// File is ok; is there another?
+	if !z.multistream {
+		return 0, io.EOF
+	}
+
 	if err = z.readHeader(false); err != nil {
 		z.err = err
 		return