fix: return error immediately on roundtrip error

[cih9088]

Rationale

From https://pkg.go.dev/net/http#RoundTripper

In particular, RoundTrip must return err == nil if it obtained a response, regardless of the response's HTTP status code. A non-nil err should be reserved for failure to obtain a response

So if err != nil, resp is nil which causes nil pointer dereference on line 94 of the following snippet.

httpcache/internal/validationresponsehandler.go

Lines 93 to 104 in 60e8c9f

	if (err != nil || isStaleErrorAllowed(resp.StatusCode)) && req.Method == http.MethodGet {
	ccResp = ParseCCResponseDirectives(resp.Header)
	ccRespOnce = true
	if r.siep.CanStaleOnError(ctx.Freshness, ccResp) {
	// RFC 9111 §4.2.4 Serving Stale Responses
	// RFC 9111 §4.3.3 Handling Validation Responses (5xx errors)
	SetAgeHeader(ctx.Stored.Data, r.clock, ctx.Freshness.Age)
	CacheStatusStale.ApplyTo(ctx.Stored.Data.Header)
	r.l.LogCacheStaleIfError(req, ctx.URLKey, ctx.ToMisc(ccResp))
	return ctx.Stored.Data, nil
	}
	}

Changed

• return err immediately on upstream roundtripper error
• HandleValidationResponse does not accept error anymore.

---

Note that on cache miss and background revalidation returns error immediately.

On cache miss,

httpcache/roundtripper.go

Lines 271 to 274 in 60e8c9f

	resp, start, end, err := r.roundTripTimed(req)
	if err != nil {
	return nil, err
	}

On background revalidation,

httpcache/roundtripper.go

Lines 424 to 428 in 60e8c9f

	resp, start, end, err := r.roundTripTimed(req)
	if err != nil {
	errc <- err
	return
	}