1943130 - Breakable lines are fetch twice in the debugger frontend

Status: RESOLVED FIXED

(DevTools :: Debugger, enhancement)

Description

[Alexandre Poirot [:ochameau]]

While investigating bug 1942774, I saw duplicated actions for retrieving breakable lines of each source.
We were also unconditionaly fetching breakable lines of all HTML sources, regardless of them being selected or not.
This may have led to performance troubles on websites having many iframes or iframe with large html page.

Comment 1

[Alexandre Poirot [:ochameau]]

Attachment: Bug 1943130 - [devtools] Avoid fetching breakable lines more than once per source.

We can't use either memoizableAction, nor the promise middleware.

• memoizableAction helps memoize on the long run,
  but doesn't handle mutliple calls made for the same ressource
  before the async computation of the value.
• promise middleware explicitely prevents exposing the actual
  Promise to the reducer, which prevents exposing it to the selector.

Ultimately, we have to expose that async promise done to compute the reducer data,
that's the one the frontend should wait for on any subsequent run for the same ressource.

These two existing APIs are weak and should probably be removed in favor
of something similar to the current patch.
Last, but not least, this approach doesn't leak all data indefinitely
until DevTools closes (memoizableAction does)

Also tweak the new-source action in order to force fetching breakable lines
of HTML sources, only if that source is currently selected.
We were computing breakable lines for all HTML sources, for example when
having iframes.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/cb56e068ecea
[devtools] Avoid fetching breakable lines more than once per source. r=devtools-reviewers,bomsy

Comment 3

[Cristina Horotan [:chorotan]]

https://hg.mozilla.org/mozilla-central/rev/cb56e068ecea

Comment 4

[Florin Bilt]

(In reply to Pulsebot from comment #2)

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/cb56e068ecea
[devtools] Avoid fetching breakable lines more than once per source.
r=devtools-reviewers,bomsy

Perfherder has detected a devtools performance change from push cb56e068ecea83724815f25fc65bdc652c412d42.

Improvements:

Ratio	Test	Platform	Options	Absolute values (old vs new)
8%	damp complicated.jsdebugger.close.DAMP	windows11-64-shippable-qr	e10s fission stylo webrender	11.54 -> 10.61

Details of the alert can be found in the alert summary, including links to graphs and comparisons for each of the affected tests.

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