Extract TTFB metric to getLoadingDurations util

[oandregal]

What?

Refactors the TimeToFirstByte performance metric to the getLoadingDurations utility.

Why?

This makes it work like any other metric.

How?

By moving the calculation to the getLoadingDurations utility and use it directly in the test.

Testing Instructions

Go to the performance CI job of this PR and verify that the front-end metrics are still reported and work as expected.

[github-actions]

Size Change: 0 B

Total Size: 1.31 MB

ℹ️ View Unchanged

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_{compressed-size-action}

[kevin940726]

LGTM 👍

[oandregal]

I'm a bit confused about the existing error:

● Front End Performance › Time To First Byte (TTFB)

Evaluation failed: TypeError: Cannot read properties of undefined (reading 'startTime')

 at ../../__puppeteer_evaluation_script__:17:36
 at ExecutionContext._evaluateInternal (../../node_modules/puppeteer-core/src/common/ExecutionContext.ts:273:13)
 at ExecutionContext.evaluate (../../node_modules/puppeteer-core/src/common/ExecutionContext.ts:140:12)
 at getLoadingDurations (specs/performance/utils.js:86:9)
 at Object.<anonymous> (specs/performance/front-end-classic-theme.test.js:41:32)

There's no startTime imported by utils.js now, though there was one in my first commit. I wonder what I'm missing here 🤔 @kevin940726 @mikachan I'd benefit from a second pair of eyes!

[mikachan]

There's no startTime imported by utils.js now, though there was one in my first commit.

Hmm, I think this error is referring to the startTime on line 104. Although I'm not sure why it's undefined here, as I assume this has been working fine previously.

Reading this, perhaps we should be wrapping paintTimings in JSON.stringify? Something like this: JSON.stringify( performance.getEntriesByName( 'first-paint' ) )[ 0 ].startTime and JSON.stringify( performance.getEntriesByName( 'first-contentful-paint' ) )[ 0 ].startTime.

[kevin940726]

I believe this is because we run getLoadingDurations() as soon as the page is loaded, which doesn't necessarily mean it's painted. While we only need the TTFB metrics, the util collects all kinds of different metrics in the same function. I think it's not an issue in other tests because we usually delay running this function until the pages are fully painted.

Maybe it's time to refactor this util to make it more resilient. Using PerformanceObserver with buffered: true might be a good solution for this issue?

[oandregal]

Closing this for now, it doesn't seem the right approach.