Offset should not be applied for explicitly set timeout

[tdabasinskas]

As per the following code, blackbox_exporter always applies --timeout-offset (defaulting to 0.5s) to the timeout:

blackbox_exporter/main.go

Lines 75 to 92 in dc53582

	var timeoutSeconds float64
	if v := r.Header.Get("X-Prometheus-Scrape-Timeout-Seconds"); v != "" {
	var err error
	timeoutSeconds, err = strconv.ParseFloat(v, 64)
	if err != nil {
	http.Error(w, fmt.Sprintf("Failed to parse timeout from Prometheus header: %s", err), http.StatusInternalServerError)
	return
	}
	}
	if timeoutSeconds == 0 {
	timeoutSeconds = 10
	}
	if module.Timeout.Seconds() < timeoutSeconds && module.Timeout.Seconds() > 0 {
	timeoutSeconds = module.Timeout.Seconds()
	}
	timeoutSeconds -= *timeoutOffset
	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(timeoutSeconds*float64(time.Second)))

There's a problem with this setting in the following cases:

• --timeout-offset=0: Prometheus might fail to scrape blackbox_exporter as the timeout (when combined with the network latency) might exceed Prometheus scrape timeout (that's the reason it's not 0 by default).
• --timeout-offset=0.5: it's not possible to have HTTP modules that expect response faster than 0.5 (as timeout: 500ms effectively becomes timeout: 0ms).

Would it be possible to apply the offset (timeoutSeconds -= *timeoutOffset ) only when there's no explicitly defined timeout for a module? Meaning, that if the user explicitly set timeout field (which, as per the conditions, must be more than 0 and less than Prometheus scrape timeout) for some specific module, we can assume the user knows what he's doing and we use this timeout as is (without applying any offset).

An alternative can also be having per-module timeout-offset property, which allows the module to override not only the default time but default offset as well.

I can do a PR if agreed.

[brian-brazil]

only when there's no explicitly defined timeout for a module?

We can do that, presuming the header-offset is still a limit.

[tdabasinskas]

By header-offset you mean X-Prometheus-Scrape-Timeout-Seconds? If so, yes, that would still be the upper limit.

[brian-brazil]

Yes