archive/zip: enable overriding (de)compressors per file

Implement setting the compression level for a zip archive by registering
a per-Writer compressor through Writer.RegisterCompressor.  If no
compressors are registered, fall back to the ones registered at the
package level.  Also implements per-Reader decompressors.

Fixes #8359

Change-Id: I93b27c81947b0f817b42e0067aa610ff267fdb21
Reviewed-on: https://go-review.googlesource.com/16669Reviewed-by: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Klaus Post <klauspost@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/zip/example_test.go

@@ -7,6 +7,7 @@ package zip_test
 import (
 	"archive/zip"
 	"bytes"
+	"compress/flate"
 	"fmt"
 	"io"
 	"log"
@@ -73,3 +74,31 @@ func ExampleReader() {
 	// Contents of README:
 	// This is the source code repository for the Go programming language.
 }
+
+func ExampleWriter_RegisterCompressor() {
+	// Override the default Deflate compressor with a higher compression
+	// level.
+
+	// Create a buffer to write our archive to.
+	buf := new(bytes.Buffer)
+
+	// Create a new zip archive.
+	w := zip.NewWriter(buf)
+
+	var fw *flate.Writer
+
+	// Register the deflator.
+	w.RegisterCompressor(zip.Deflate, func(out io.Writer) (io.WriteCloser, error) {
+		var err error
+		if fw == nil {
+			// Creating a flate compressor for every file is
+			// expensive, create one and reuse it.
+			fw, err = flate.NewWriter(out, flate.BestCompression)
+		} else {
+			fw.Reset(out)
+		}
+		return fw, err
+	})
+
+	// Proceed to add files to w.
+}

src/archive/zip/reader.go

@@ -25,6 +25,7 @@ type Reader struct {
 	r             io.ReaderAt
 	File          []*File
 	Comment       string
+	decompressors map[uint16]Decompressor
 }
 
 type ReadCloser struct {
@@ -34,6 +35,7 @@ type ReadCloser struct {
 
 type File struct {
 	FileHeader
+	zip          *Reader
 	zipr         io.ReaderAt
 	zipsize      int64
 	headerOffset int64
@@ -95,7 +97,7 @@ func (z *Reader) init(r io.ReaderAt, size int64) error {
 	// a bad one, and then only report a ErrFormat or UnexpectedEOF if
 	// the file count modulo 65536 is incorrect.
 	for {
-		f := &File{zipr: r, zipsize: size}
+		f := &File{zip: z, zipr: r, zipsize: size}
 		err = readDirectoryHeader(f, buf)
 		if err == ErrFormat || err == io.ErrUnexpectedEOF {
 			break
@@ -113,6 +115,26 @@ func (z *Reader) init(r io.ReaderAt, size int64) error {
 	return nil
 }
 
+// RegisterDecompressor registers or overrides a custom decompressor for a
+// specific method ID. If a decompressor for a given method is not found,
+// Reader will default to looking up the decompressor at the package level.
+//
+// Must not be called concurrently with Open on any Files in the Reader.
+func (z *Reader) RegisterDecompressor(method uint16, dcomp Decompressor) {
+	if z.decompressors == nil {
+		z.decompressors = make(map[uint16]Decompressor)
+	}
+	z.decompressors[method] = dcomp
+}
+
+func (z *Reader) decompressor(method uint16) Decompressor {
+	dcomp := z.decompressors[method]
+	if dcomp == nil {
+		dcomp = decompressor(method)
+	}
+	return dcomp
+}
+
 // Close closes the Zip file, rendering it unusable for I/O.
 func (rc *ReadCloser) Close() error {
 	return rc.f.Close()
@@ -140,7 +162,7 @@ func (f *File) Open() (rc io.ReadCloser, err error) {
 	}
 	size := int64(f.CompressedSize64)
 	r := io.NewSectionReader(f.zipr, f.headerOffset+bodyOffset, size)
-	dcomp := decompressor(f.Method)
+	dcomp := f.zip.decompressor(f.Method)
 	if dcomp == nil {
 		err = ErrAlgorithm
 		return

src/archive/zip/writer.go

@@ -14,7 +14,6 @@ import (
 )
 
 // TODO(adg): support zip file comments
-// TODO(adg): support specifying deflate level
 
 // Writer implements a zip file writer.
 type Writer struct {
@@ -22,6 +21,7 @@ type Writer struct {
 	dir         []*header
 	last        *fileWriter
 	closed      bool
+	compressors map[uint16]Compressor
 }
 
 type header struct {
@@ -220,7 +220,7 @@ func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
 		compCount: &countWriter{w: w.cw},
 		crc32:     crc32.NewIEEE(),
 	}
-	comp := compressor(fh.Method)
+	comp := w.compressor(fh.Method)
 	if comp == nil {
 		return nil, ErrAlgorithm
 	}
@@ -270,6 +270,24 @@ func writeHeader(w io.Writer, h *FileHeader) error {
 	return err
 }
 
+// RegisterCompressor registers or overrides a custom compressor for a specific
+// method ID. If a compressor for a given method is not found, Writer will
+// default to looking up the compressor at the package level.
+func (w *Writer) RegisterCompressor(method uint16, comp Compressor) {
+	if w.compressors == nil {
+		w.compressors = make(map[uint16]Compressor)
+	}
+	w.compressors[method] = comp
+}
+
+func (w *Writer) compressor(method uint16) Compressor {
+	comp := w.compressors[method]
+	if comp == nil {
+		comp = compressor(method)
+	}
+	return comp
+}
+
 type fileWriter struct {
 	*header
 	zipw      io.Writer