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fix(inst-fetch,inst-xhr) Ignore network events with zero-timing
Background: 1. For historical reasons, the perf/resource timing spec uses 0 as a special value to denote when some timing information is either not applicable (e.g. no redirect occured) or not available (when producing an opaque resource timing object for CORS resources). 2. However, in some limited cases, 0 can also be a legitimate value for these timing events. Note that this is rare in real life – these fields are high-res performance timestamp relative to the performance time-origin, which is typically the navigation event for the initial page load. For a _resource_ timing to have a 0 timestamp, it would have to be initiated simultaneously with page load, it is unclear if this can actually happen IRL. Reportedly (#1769), at one point this was observed in some browsers during automated tests where things happen very fast and the browser artifically supress the timing resolution. It was unclear if the report was about the navigation timing entry or resource timing entries. It is also unclear if these utilities are intended for anything other than the internal fetch/XHR instrumentation, but they are public API, so if someone uses these functions on the initial page navigation event, then it is possible for the function to see legitimate 0-value inputs. 3. When creating span events, we do not use the timeOrigin-relative perf timestamps as-is. Rather, we convert them back to the UNIX epoch timestamps. When this conversion is applied to those 0 timestamps, it creates nonsensical/misleading events that are quite difficult to distinguish for downstream consumers. It would be nice if the W3C specs would have left the N/A values as `undefined`, but that's not the world we live in and so we have to work with what we've got. History: 1. Initially, the code ignored 0-value timestamps. 2. #1769 identified cases of valid 0-value timestamps and removed the check. 3. This causes the other category of bugs where we created the nonsensical events (#2457, #3848, #4478) 4. #3897 added a special-case for `secureConnectionStart` by way of tapping into auxiliary metadata 5. That appoach cannot be generalized for the other cases, so #4486 added some rather convoluted checks. 6. As part of refactoring the tests to use service workers, a new bug #5314 was identified. Presumably, the patch in #4486 was writen that way (as opposed to just checking for 0) to avoid breaking #1769, but I suspect it ends up breaking some of those use cases anyway. Options: 1. Keep the patch from #4486 largely as-is but change the reference time from `fetchStart` to `startTime`. 2. Revert to the very original intent of the code and drop everything with 0-values. 3. This commit attempts a compromise position, when `startTime === 0` (which probably means we are called with the initial navigation event), 0-values are plausibly valid so we include them by default, but in any other cases (`startTime > 0`), legitimate 0-values should not be possible so we drop them by default. Part of the issue here is that it's unclear how these utilities are being used since they are public APIs. In core, these functions are only called by the fetch/xhr instrumentation with resource timing entries, where legitimate 0-value seems extremely unlikely. In my opinion, the rarity of legitimate 0-value timing values in the real world (recall that it doesn't just mean something happened very fast, but something happened very fast _at the same instant that the page loaded_) should yield to the conflicting interst (to avoid non-sensical events that are difficult to process downstream) here, especially when it appears that the only observed cases are from automated testsing. Personally I would be in favor of the stronger position (2), but attempted to strike a balance here to keep things useful the other cases. In #2457, @johnbley argued: > I think that for this piece of tech, our telemetry should report > whatever the browser says with minimal processing, and let > downstream/backend/easier-to-modify components deal with > normalization/truncation/processing if desired (e.g., throwing > away absurd times like "that page took 6 years or -5 hours to > load", or declaring that "no redirects happened so the redirect > timing is not 0 but non-existent"). Does that not work in your > world for some reason? I could see this perspective if the "minimal processing" means that we sends the 0-values to the backends as 0s. It's still unfortunate but at least it would be easy to write a query/filter to process them away. However, given we are actively normalizes the 0-values into the absolute epoch timestamp of `performance.timeOrigin`, I think this becomes actively hostile to backend/downstream processing, as it turns a relatively clear signal (value=0) into something that would require stateful machinary to reverse engineer (infer the timeOrigin from a trace), or imperfect huristics (ignore things that seems to take way too long). It also takes up bytesize/bandwidth to transmit in the vast majority of cases where it would be actively unhelpful. Instead, I'd argue it creates way less harm to drop the 0-values. When a 0-value is expected but not sent, it may skew the distrubtion on some aggregrate metrics slightly (e.g. making "average time for SSL handshake" appears large), but probably nothing would break. If we are expecting backends to do work to normalize data anyway, then IMO it would also be way easier for them to notice the missing items and insert it back than the other way around. Fixes #3199 Fixes #5314 See also #1769, #2457, #3848, #3879, #4478, #4486
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