fix: retry on GOAWAY errors when using cached HTTP/2 connections

RoundTripOnlyCachedConn bypasses the retry loop in RoundTripOpt, so
GOAWAY and other retryable errors were returned directly to callers.

Now we check CanRetryError before returning, allowing retryable errors
to fall through and create a new connection.

Changes:
- Export canRetryError as CanRetryError
- Check CanRetryError in roundTrip when RoundTripOnlyCachedConn fails

Author: 0xLyndon

internal/http2/transport.go

@@ -527,7 +527,7 @@ var (
 // It returns either a request to retry (either the same request, or a
 // modified clone), or an error if the request can't be replayed.
 func shouldRetryRequest(req *http.Request, err error) (*http.Request, error) {
-	if !canRetryError(err) {
+	if !CanRetryError(err) {
 		return nil, err
 	}
 	// If the Body is nil (or http.NoBody), it's safe to reuse
@@ -558,7 +558,7 @@ func shouldRetryRequest(req *http.Request, err error) (*http.Request, error) {
 	return nil, fmt.Errorf("http2: Transport: cannot retry err [%v] after Request.Body was written; define Request.GetBody to avoid this error", err)
 }
 
-func canRetryError(err error) bool {
+func CanRetryError(err error) bool {
 	if err == errClientConnUnusable || err == errClientConnGotGoAway {
 		return true
 	}

transport.go

@@ -944,7 +944,7 @@ func (t *Transport) roundTrip(req *http.Request) (resp *http.Response, err error
 
 	if scheme == "https" && t.forceHttpVersion != h1 {
 		resp, err := t.t2.RoundTripOnlyCachedConn(req)
-		if err != h2internal.ErrNoCachedConn {
+		if err != h2internal.ErrNoCachedConn && !h2internal.CanRetryError(err) {
 			return resp, err
 		}
 		req, err = rewindBody(req)