compress/gzip: add Writer.Flush to call flate.Writer's Flush

From a discussion on golang-nuts.

R=golang-dev, dsymonds, nigeltao, coocood, adg
CC=golang-dev
https://golang.org/cl/8251043

doc/go1.1.html

@@ -631,6 +631,14 @@ so it implements the
 <a href="/pkg/io/#WriterTo"><code>io.WriterTo</code></a> interface.
 </li>
 
+<li>
+The <a href="/pkg/compress/gzip/"><code>compress/gzip</code></a> package has
+a new <a href="/pkg/compress/gzip/#Writer.Flush"><code>Flush</code></a>
+method for its
+<a href="/pkg/compress/gzip/#Writer"><code>Writer</code></a>
+type that flushes its underlying <code>flate.Writer</code>.
+</li>
+
 <li>
 The <a href="/pkg/crypto/hmac/"><code>crypto/hmac</code></a> package has a new function,
 <a href="/pkg/crypto/hmac/#Equal"><code>Equal</code></a>, to compare two MACs.

src/pkg/compress/gzip/gzip.go

@@ -28,7 +28,7 @@ type Writer struct {
 	Header
 	w          io.Writer
 	level      int
-	compressor io.WriteCloser
+	compressor *flate.Writer
 	digest     hash.Hash32
 	size       uint32
 	closed     bool
@@ -191,6 +191,28 @@ func (z *Writer) Write(p []byte) (int, error) {
 	return n, z.err
 }
 
+// Flush flushes any pending compressed data to the underlying writer.
+//
+// It is useful mainly in compressed network protocols, to ensure that
+// a remote reader has enough data to reconstruct a packet. Flush does
+// not return until the data has been written. If the underlying
+// writer returns an error, Flush returns that error.
+//
+// In the terminology of the zlib library, Flush is equivalent to Z_SYNC_FLUSH.
+func (z *Writer) Flush() error {
+	if z.err != nil {
+		return z.err
+	}
+	if z.closed {
+		return nil
+	}
+	if z.compressor == nil {
+		z.Write(nil)
+	}
+	z.err = z.compressor.Flush()
+	return z.err
+}
+
 // Close closes the Writer. It does not close the underlying io.Writer.
 func (z *Writer) Close() error {
 	if z.err != nil {

src/pkg/compress/gzip/gzip_test.go

@@ -157,3 +157,43 @@ func TestLatin1RoundTrip(t *testing.T) {
 		}
 	}
 }
+
+func TestWriterFlush(t *testing.T) {
+	buf := new(bytes.Buffer)
+
+	w := NewWriter(buf)
+	w.Comment = "comment"
+	w.Extra = []byte("extra")
+	w.ModTime = time.Unix(1e8, 0)
+	w.Name = "name"
+
+	n0 := buf.Len()
+	if n0 != 0 {
+		t.Fatalf("buffer size = %d before writes; want 0", n0)
+	}
+
+	if err := w.Flush(); err != nil {
+		t.Fatal(err)
+	}
+
+	n1 := buf.Len()
+	if n1 == 0 {
+		t.Fatal("no data after first flush")
+	}
+
+	w.Write([]byte("x"))
+
+	n2 := buf.Len()
+	if n1 != n2 {
+		t.Fatalf("after writing a single byte, size changed from %d to %d; want no change", n1, n2)
+	}
+
+	if err := w.Flush(); err != nil {
+		t.Fatal(err)
+	}
+
+	n3 := buf.Len()
+	if n2 == n3 {
+		t.Fatal("Flush didn't flush any data")
+	}
+}