archive/zip: clarify expectations of RegisterCompressor and RegisterDecompressor

Clarify that Compressor and Decompressor callbacks must support being invoked
concurrently, but that the writer or reader returned need not be.

Updates #8359

Change-Id: Ia407b581dd124185f165c25f5701018a8ce4357a
Reviewed-on: https://go-review.googlesource.com/18627
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/zip/example_test.go

@@ -76,8 +76,7 @@ func ExampleReader() {
 }
 
 func ExampleWriter_RegisterCompressor() {
-	// Override the default Deflate compressor with a higher compression
-	// level.
+	// Override the default Deflate compressor with a higher compression level.
 
 	// Create a buffer to write our archive to.
 	buf := new(bytes.Buffer)
@@ -85,19 +84,9 @@ func ExampleWriter_RegisterCompressor() {
 	// Create a new zip archive.
 	w := zip.NewWriter(buf)
 
-	var fw *flate.Writer
-
-	// Register the deflator.
+	// Register a custom Deflate compressor.
 	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
+		return flate.NewWriter(out, flate.BestCompression)
 	})
 
 	// Proceed to add files to w.

src/archive/zip/reader.go

@@ -118,8 +118,6 @@ func (z *Reader) init(r io.ReaderAt, size int64) error {
 // 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)

src/archive/zip/register.go

@@ -12,15 +12,19 @@ import (
 	"sync"
 )
 
-// A Compressor returns a compressing writer, writing to the
-// provided writer. On Close, any pending data should be flushed.
-type Compressor func(io.Writer) (io.WriteCloser, error)
+// A Compressor returns a new compressing writer, writing to w.
+// The WriteCloser's Close method must be used to flush pending data to w.
+// The Compressor itself must be safe to invoke from multiple goroutines
+// simultaneously, but each returned writer will be used only by
+// one goroutine at a time.
+type Compressor func(w io.Writer) (io.WriteCloser, error)
 
-// Decompressor is a function that wraps a Reader with a decompressing Reader.
-// The decompressed ReadCloser is returned to callers who open files from
-// within the archive.  These callers are responsible for closing this reader
-// when they're finished reading.
-type Decompressor func(io.Reader) io.ReadCloser
+// A Decompressor returns a new decompressing reader, reading from r.
+// The ReadCloser's Close method must be used to release associated resources.
+// The Decompressor itself must be safe to invoke from multiple goroutines
+// simultaneously, but each returned reader will be used only by
+// one goroutine at a time.
+type Decompressor func(r io.Reader) io.ReadCloser
 
 var flateWriterPool sync.Pool
 
@@ -75,14 +79,15 @@ var (
 )
 
 // RegisterDecompressor allows custom decompressors for a specified method ID.
-func RegisterDecompressor(method uint16, d Decompressor) {
+// The common methods Store and Deflate are built in.
+func RegisterDecompressor(method uint16, dcomp Decompressor) {
 	mu.Lock()
 	defer mu.Unlock()
 
 	if _, ok := decompressors[method]; ok {
 		panic("decompressor already registered")
 	}
-	decompressors[method] = d
+	decompressors[method] = dcomp
 }
 
 // RegisterCompressor registers custom compressors for a specified method ID.