net/http: fix requests failing on short gzip body

Fixes #7750.

LGTM=bradfitz
R=golang-codereviews, ibilicc, bradfitz
CC=golang-codereviews
https://golang.org/cl/84850043

src/pkg/net/http/transport.go

@@ -812,13 +812,7 @@ func (pc *persistConn) readLoop() {
 				resp.Header.Del("Content-Encoding")
 				resp.Header.Del("Content-Length")
 				resp.ContentLength = -1
-				gzReader, zerr := gzip.NewReader(resp.Body)
-				if zerr != nil {
-					pc.close()
-					err = zerr
-				} else {
-					resp.Body = &readerAndCloser{gzReader, resp.Body}
-				}
+				resp.Body = &gzipReader{body: resp.Body}
 			}
 			resp.Body = &bodyEOFSignal{body: resp.Body}
 		}
@@ -1156,6 +1150,27 @@ func (es *bodyEOFSignal) condfn(err error) {
 	es.fn = nil
 }
 
+// gzipReader wraps a response body so it can lazily
+// call gzip.NewReader on the first call to Read
+type gzipReader struct {
+	body io.ReadCloser // underlying Response.Body
+	zr   io.Reader     // lazily-initialized gzip reader
+}
+
+func (gz *gzipReader) Read(p []byte) (n int, err error) {
+	if gz.zr == nil {
+		gz.zr, err = gzip.NewReader(gz.body)
+		if err != nil {
+			return 0, err
+		}
+	}
+	return gz.zr.Read(p)
+}
+
+func (gz *gzipReader) Close() error {
+	return gz.body.Close()
+}
+
 type readerAndCloser struct {
 	io.Reader
 	io.Closer

src/pkg/net/http/transport_test.go

@@ -803,6 +803,33 @@ func TestTransportGzipRecursive(t *testing.T) {
 	}
 }
 
+// golang.org/issue/7750: request fails when server replies with
+// a short gzip body
+func TestTransportGzipShort(t *testing.T) {
+	defer afterTest(t)
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header().Set("Content-Encoding", "gzip")
+		w.Write([]byte{0x1f, 0x8b})
+	}))
+	defer ts.Close()
+
+	tr := &Transport{}
+	defer tr.CloseIdleConnections()
+	c := &Client{Transport: tr}
+	res, err := c.Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	_, err = ioutil.ReadAll(res.Body)
+	if err == nil {
+		t.Fatal("Expect an error from reading a body.")
+	}
+	if err != io.ErrUnexpectedEOF {
+		t.Errorf("ReadAll error = %v; want io.ErrUnexpectedEOF", err)
+	}
+}
+
 // tests that persistent goroutine connections shut down when no longer desired.
 func TestTransportPersistConnLeak(t *testing.T) {
 	if runtime.GOOS == "plan9" {