#766 fix proxy stream

[malud]

closes #766

Reviewer checklist

• Read PR description: a summary about the changes is required
• Changelog updated
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• Pulled branch, manually tested
• Verified requirements are met
• Reviewed the code
• Reviewed the related tests

[johakoch]

This solves the problem described in #766.
One observation: I tried the config from the issue and got cancels sometimes, e.g.

ERRO[0333] internal server error: body copy failed: context canceled  build=dev handler=endpoint type=couper_daemon uid=clfkru9s6vh37hl8dhs0 version=0

due to the backend timeout,

or

ERRO[0118] internal server error: body copy failed: read error during body copy: unexpected EOF  build=dev handler=endpoint type=couper_daemon uid=clfl6u1s6vhc5ia5ji2g version=0

. In both cases, the access log had status=200, which is, at least, misleading.

---

TestBackend_MaxConnections_BodyClose fails quite regularly:

$ go test -timeout 300s -race github.com/avenga/couper/server -run TestBackend_MaxConnections_BodyClose -count=1
INTEGRATION: create test backend...
--- FAIL: TestBackend_MaxConnections_BodyClose (71.03s)
    --- FAIL: TestBackend_MaxConnections_BodyClose/parallel (0.00s)
        --- FAIL: TestBackend_MaxConnections_BodyClose/parallel/_/proxy-seq (70.00s)
            http_backend_test.go:130: Get "http://couper.dev:8080/proxy-seq": context deadline exceeded
        --- FAIL: TestBackend_MaxConnections_BodyClose/parallel/_/proxy-seq-ref (70.01s)
            http_backend_test.go:130: Get "http://couper.dev:8080/proxy-seq-ref": context deadline exceeded
        --- FAIL: TestBackend_MaxConnections_BodyClose/parallel/_/ws (70.01s)
            http_backend_test.go:130: Get "http://couper.dev:8080/ws": context deadline exceeded
        --- FAIL: TestBackend_MaxConnections_BodyClose/parallel/_/default2 (70.01s)
            http_backend_test.go:130: Get "http://couper.dev:8080/default2": context deadline exceeded
        --- FAIL: TestBackend_MaxConnections_BodyClose/parallel/_/ (70.01s)
            http_backend_test.go:130: Get "http://couper.dev:8080/": context deadline exceeded
        --- FAIL: TestBackend_MaxConnections_BodyClose/parallel/_/default (70.01s)
            http_backend_test.go:130: Get "http://couper.dev:8080/default": context deadline exceeded
        --- FAIL: TestBackend_MaxConnections_BodyClose/parallel/_/named (70.01s)
            http_backend_test.go:130: Get "http://couper.dev:8080/named": context deadline exceeded

TestBackend_MaxConnections_BodyClose fails too often. This test (even with the same go code ported to release.1.12) passes 10/10.

[johakoch]

If I change the paths to

	paths := []string{
		"/named",
		"/named",
		"/named",
		"/named",
	}

(which doesn't have default request/proxy), TestBackend_MaxConnections_BodyClose passes 10/10.

If I change it to

	paths := []string{
		"/",
		"/",
		"/",
		"/",
	}

(which has default request), TestBackend_MaxConnections_BodyClose fails 8/10.

So I guess, this is related to the changes to the handling of default requests/proxies in this PR.

In Backend.innerRoundTrip(), all backend requests are passed to

	beresp, err := b.transport.RoundTrip(req)

but at least some get a "context canceled" error.

[johakoch]

The test started to fail with commit "Fix buffer condition and related body parse in combination with syncedVariables provided by each backend instance", then passed with commit "Fix random order related to the default block while preparing roundtrips" and failed again with commit "reduce produce wait-time; TODO".

Apparently, the

		time.Sleep(time.Nanosecond * 10) // TODO: refactor to ch pipe

is not stable enought. The previous

		time.Sleep(time.Millisecond * 5) // TODO: refactor to ch pipe

was more stable.