[Webkit-unassigned] [Bug 235744] [XCBuild] WTF's headers are copied via a script and are invisible to the build system

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=235744

--- Comment #20 from Elliott Williams <emw at apple.com> ---
(In reply to Alexey Proskuryakov from comment #19)
> > - Failing the build when the xcfilelists _do_ change, so it's not possible
> > for the files to silently get mutated on a CI machine. 
> 
> I do not understand why mutating in CI is impossible. Don't our scripts
> mutate the source xcfilelists before failing the build?

Yes, they do. I didn't mean to suggest that we somehow avoid mutations altogether, only that we avoid silently-successful mutations.


> > - Checking (and potentially regenerating) xcfilelists in every build, so
> > that regenerations are caught early.
> 
> What are the configurations where we check xcfilelists? I thought that this
> was disabled in many places.
> […]
> This explanation seems unlikely to me. People who work on non-Mac platforms
> would not be building with Xcode at all before EWS. And those who do
> wouldn't build for all platforms, which can well have different inputs and
> outputs due to different features being enabled.

It turns out the check is skipped if Xcode is making a production build or using a public SDK: https://github.com/WebKit/WebKit/blob/f370fc201e04319d45a75961233022e1f46d4f5b/Source/JavaScriptCore/Scripts/check-xcfilelists.sh#L4

This is probably a big part of why we don't see infra instability due to the generator, but doesn't really explain how changes from non-Mac contributors eventually make it into xcfilelists.

> Perhaps subtle reasons like these are why we aren't hitting problems all the
> time, but this means that we are walking a very thin line.
>
> […]
> 
> I still don't feel like I understand the situation enough to confidently
> suggest what's right. The first patch feels safer at this point FWIW.

Agreed. I'm going to move forward with the first patch, and plan to revisit xcfilelist generation later. Thank you very much for talking this through!

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