channels for sequences

[johakoch]

use channels to pass (error) results to and receive results from proxies/requests

---

Reviewer checklist

• Read PR description: a summary about the changes is required
• Changelog updated
• Documentation: docs/{Reference, Cli, ...}, Docker and cli help/usage
• Pulled branch, manually tested
• Verified requirements are met
• Reviewed the code
• Reviewed the related tests

[johakoch]

I don't think, we need a changelog entry here.
@malud please, have a look.

[malud]

LGTM, just one more func. Even client cancels seems to work as expected.

[malud]

Hm unstable tests.

=== RUN   TestJWKsMaxStale
http_integration_test.go:3621: expected status 200, got: 401 ()
RUN   TestOpenAPIValidator_RelativeServerURL
openapi_test.go:138: backend error: status is not supported

Also the openApi test failure I have already seen on this branch.

[johakoch]

Hm unstable tests.

=== RUN   TestJWKsMaxStale
http_integration_test.go:3621: expected status 200, got: 401 ()
RUN   TestOpenAPIValidator_RelativeServerURL
openapi_test.go:138: backend error: status is not supported

I think, both test failures are unrelated to the changes of this PR.

If I remember correctly, I saw TestJWKsMaxStale failures somewhere else, too.

level=error msg="access control error: stale: received no valid JWKs data: <nil>, status code 500"

Maybe the resource request is sent before the AC has its JWKS information.

TestOpenAPIValidator_RelativeServerURL creates a request to the test backend and passes it to an OpenAPI validating backend. No producers involved.

However, TestOpenAPIValidator_RelativeServerURL could be improved to actually log the entries before the test is stopped by helper.Must(err):

	_, err = backend.RoundTrip(req)
	if err != nil {
		for _, entry := range hook.AllEntries() {
			t.Log(entry.String())
		}
		helper.Must(err)
	}

instead of

	_, err = backend.RoundTrip(req)
	if err != nil {
		helper.Must(err)
	}

	if t.Failed() {
		for _, entry := range hook.AllEntries() {
			t.Log(entry.String())
		}
	}