fix(http11): prevent retryablehttp HTTP/2 fallback from bypassing -pr http11 (#2240)

common/httpx/httpx.go

@@ -183,6 +183,15 @@ func New(options *Options) (*HTTPX, error) {
 		CheckRedirect: redirectFunc,
 	}, retryablehttpOptions)
 
+	// When HTTP/1.1-only mode is enforced, prevent retryablehttp-go's HTTP/2 fallback
+	// from bypassing the protocol restriction. retryablehttp-go falls back to HTTPClient2
+	// (an HTTP/2-capable client) when it encounters a "malformed HTTP version" error from
+	// a server that speaks HTTP/2. Replacing HTTPClient2 with the HTTP/1.1-only HTTPClient
+	// ensures the -pr http11 flag is honoured end-to-end. 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

@@ -8,6 +8,32 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// TestHTTP11ProtocolEnforcement verifies that -pr http11 prevents the retryablehttp-go
+// HTTP/2 fallback from bypassing the HTTP/1.1-only restriction (#2240).
+func TestHTTP11ProtocolEnforcement(t *testing.T) {
+	t.Run("http11 mode disables HTTPClient2 fallback", func(t *testing.T) {
+		opts := DefaultOptions
+		opts.Protocol = "http11"
+		ht, err := New(&opts)
+		require.Nil(t, err)
+
+		// When Protocol == "http11", HTTPClient2 must point to the same underlying
+		// client as HTTPClient so the retryablehttp-go fallback is neutralised.
+		require.Equal(t, ht.client.HTTPClient, ht.client.HTTPClient2,
+			"HTTPClient2 must equal HTTPClient in http11 mode to prevent HTTP/2 fallback")
+	})
+
+	t.Run("default mode keeps distinct HTTPClient2", func(t *testing.T) {
+		ht, err := New(&DefaultOptions)
+		require.Nil(t, err)
+
+		// Without an explicit http11 restriction the two clients must differ
+		// (HTTPClient2 is the native HTTP/2 client used for protocol detection).
+		require.NotEqual(t, ht.client.HTTPClient, ht.client.HTTPClient2,
+			"HTTPClient2 must be distinct from HTTPClient in default mode")
+	})
+}
+
 func TestDo(t *testing.T) {
 	ht, err := New(&DefaultOptions)
 	require.Nil(t, err)