fix: send the 101 status line before the connection is handed over

WriteHeader only committed when a Content-Type was already set, so the plain
upgrade sequence, 101 with no content type followed by Hijack, recorded the
status and never forwarded it: Hijack set hijacked and close returned at that
guard, leaving the client with the raw protocol bytes and no status line where
net/http would have written one.

101 now commits immediately, which it can do because it carries no body to sniff
and is already excluded from compression. The regression test omits Content-Type
and reads the raw socket, so it fails without the fix.

Reported by umputun on #51.

Author: paskal

gzip.go

@@ -61,8 +61,9 @@ func (w *gzipResponseWriter) WriteHeader(status int) {
 	w.status = status
 
 	// with a content type in hand the decision can be made right away, otherwise it waits for the
-	// first Write so the body can be sniffed
-	if ctype := w.Header().Get("Content-Type"); ctype != "" {
+	// first Write so the body can be sniffed. 101 cannot wait: the upgrade sequence carries no body
+	// and usually no content type, and the Hijack that follows would leave the status unsent
+	if ctype := w.Header().Get("Content-Type"); ctype != "" || status == http.StatusSwitchingProtocols {
 		w.decide(ctype)
 		w.commit()
 	}

gzip_test.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/http/httptest"
 	"strconv"
@@ -791,3 +792,42 @@ func TestGzipWriterCapabilitiesMatchUnderlying(t *testing.T) {
 		})
 	}
 }
+
+func TestGzipSwitchingProtocolsWithoutContentType(t *testing.T) {
+	// the plain upgrade sequence: 101 with no content type, then the handler takes the connection.
+	// the status has to be on the wire before Hijack, or it is never sent at all
+	handler := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Upgrade", "foo")
+		w.Header().Set("Connection", "Upgrade")
+		w.WriteHeader(http.StatusSwitchingProtocols)
+
+		hj, ok := w.(http.Hijacker)
+		require.True(t, ok, "upgrade needs a Hijacker")
+		conn, buf, err := hj.Hijack()
+		require.NoError(t, err)
+		defer conn.Close()
+
+		_, err = buf.WriteString("raw-protocol-bytes")
+		require.NoError(t, err)
+		require.NoError(t, buf.Flush())
+	})
+
+	ts := httptest.NewServer(Gzip()(handler))
+	defer ts.Close()
+
+	conn, err := net.Dial("tcp", strings.TrimPrefix(ts.URL, "http://"))
+	require.NoError(t, err)
+	defer conn.Close()
+
+	_, err = fmt.Fprint(conn, "GET /upgrade HTTP/1.1\r\nHost: example.com\r\n"+
+		"Accept-Encoding: gzip\r\nUpgrade: foo\r\nConnection: Upgrade\r\n\r\n")
+	require.NoError(t, err)
+	require.NoError(t, conn.SetReadDeadline(time.Now().Add(5*time.Second)))
+
+	got, err := io.ReadAll(conn)
+	require.NoError(t, err)
+
+	assert.Contains(t, string(got), "101 Switching Protocols", "the 101 status line must reach the client")
+	assert.Contains(t, string(got), "raw-protocol-bytes")
+	assert.NotContains(t, string(got), "Content-Encoding: gzip", "an upgraded connection must not be gzipped")
+}