fix: preserve -pr http11 across retryablehttp-go fallback (#2240)

common/httpx/httpx.go

@@ -153,7 +153,7 @@ func New(options *Options) (*HTTPX, error) {
 		DisableKeepAlives: true,
 	}
 
-	if httpx.Options.Protocol == "http11" {
+	if httpx.Options.Protocol == HTTP11 {
 		// disable http2
 		_ = os.Setenv("GODEBUG", "http2client=0")
 		transport.TLSNextProto = map[string]func(string, *tls.Conn) http.RoundTripper{}
@@ -183,6 +183,15 @@ func New(options *Options) (*HTTPX, error) {
 		CheckRedirect: redirectFunc,
 	}, retryablehttpOptions)
 
+	// When HTTP/1.1-only mode is enforced via -pr http11, prevent retryablehttp-go
+	// from silently upgrading to HTTP/2 on retry. retryablehttp-go falls back to
+	// HTTPClient2 (an HTTP/2-capable client) when it encounters "malformed HTTP
+	// version" errors from servers that speak HTTP/2. Pointing HTTPClient2 at the
+	// same HTTP/1.1-only client neutralises the fallback. See: #2240
+	if httpx.Options.Protocol == HTTP11 {
+		httpx.client.HTTPClient2 = httpx.client.HTTPClient
+	}
+
 	transport2 := &http2.Transport{
 		TLSClientConfig: &tls.Config{
 			InsecureSkipVerify: true,

common/httpx/httpx_test.go

@@ -28,3 +28,29 @@ func TestDo(t *testing.T) {
 		require.Greater(t, len(resp.Raw), 800)
 	})
 }
+
+// TestHTTP11ProtocolEnforcement verifies that -pr http11 prevents retryablehttp-go's
+// HTTP/2 fallback from bypassing the protocol restriction (#2240).
+func TestHTTP11ProtocolEnforcement(t *testing.T) {
+	t.Run("http11 mode neutralises HTTPClient2 fallback", func(t *testing.T) {
+		opts := DefaultOptions
+		opts.Protocol = HTTP11
+		ht, err := New(&opts)
+		require.Nil(t, err)
+
+		// HTTPClient2 must be the same as HTTPClient so the fallback
+		// path still uses HTTP/1.1-only transport.
+		require.Same(t, ht.client.HTTPClient, ht.client.HTTPClient2,
+			"HTTPClient2 must equal HTTPClient in http11 mode to prevent HTTP/2 fallback")
+	})
+
+	t.Run("default mode keeps separate HTTPClient2 for HTTP/2", func(t *testing.T) {
+		ht, err := New(&DefaultOptions)
+		require.Nil(t, err)
+
+		// In default mode the two clients must be distinct — HTTPClient2
+		// is the HTTP/2-capable client used for protocol detection/fallback.
+		require.NotSame(t, ht.client.HTTPClient, ht.client.HTTPClient2,
+			"HTTPClient2 must differ from HTTPClient in default mode")
+	})
+}