[webkit-dev] Deployment of new EWS Non-Unified builder

[Emilio Cobos Álvarez]

If it's useful as a data point, in Gecko we used not to care about 
non-unified builds. This worked kind of ok, because the file 
arrangements were mostly deterministic by directory. However, folks 
running with weird build configurations always ended up hitting these 
issues (and they might not know that they're not their fault).

Furthermore, various IDEs and tooling rely on dependencies being right / 
files building in non-unified mode to provide useful diagnostics while 
editing the files (e.g, we have a code review bot that runs clang-tidy 
on the files you touched, and if the dependencies are not right it just 
fails).

To that end, we have a bot that builds a "hybrid" build configuration, 
and runs on CI. It's "hybrid" in the sense there's a per-directory opt 
out; there are some directories that still haven't been fixed to build 
without unified builds, but the goal is that eventually everything 
should build in non-unified builds.

See [1] for the details. In my experience, the occasional fix-up for 
non-unified builds is better / simpler than having to untangle the mess 
after the fact (see [2] for a recent example).

Cheers,

  -- Emilio

[1]: 
https://firefox-source-docs.mozilla.org/build/buildsystem/unified-builds.html#building-outside-of-the-unified-environment
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1772513

On 5/21/22 06:17, dpino via webkit-dev wrote:
> Hi,
> 
> Last year we started a thread to discuss the possibility of deploying a
> new EWS bot that builds WebKit with Non-Unified sources [1]. This thread
> explains the technical reasons why a non-unified build bot is desirable.
> If you're aware of the problems introduced by unified sources
> compilation, skip the next paragraph.
> 
> Unified sources compilation consists of merging several source files
> together into one big file to improve building times. Usually the same
> sources files are grouped together, but compilation flags or downstream
> changes can create different source file groups. When that happens, you
> may hit a build error. Since unified sources group files together,
> missing headers in one file can be satisfied by another file in the same
> group. What's happening in WebKit development currently is that source
> code that breaks non-unified compilation frequently lands without even
> being noticed. The breaks are usually related to missing headers.
> Embedders that need to support WebKit builds with different flags or
> maintain downstream changes are more likely to hit these compilation
> errors.
> 
> Last year's attempt to deploy an EWS Non-Unified builder ended up in a
> deployment of a post-commit bot plus two workers running in the UAT. It
> was actually hard to get the Non-Unified builder working [2], something
> that we achieved at the beginning of this year (kudos to Adrián and
> Lauro). Since then we have been maintaining the post-commit bot very
> closely. There are periods of time where the bot has remained green for
> a long period of time.
> 
> But lately maintaining the bot green has become harder and harder,
> specially due to the big refactorizations that have happened in WebKit
> source code lately. The bot very rarely stays green longer than 2 or 3
> days without close maintenance. We believe that we all should share the
> responsibility of keeping the non-unified build working, and therefore
> we want to proceed with the deployment of a EWS Non-Unified builder.
> 
> Initially, we'll provide two workers on a modern host with 8 cores
> assigned each. We have found this setup faster for most patches than
> many of the existing EWS queues, as it only runs a build in non-unified
> mode, without test or upload steps. If this were not fast enough, we
> would consider increasing the number of workers in the queue. We set up
> a page[3] with a rough comparison to the regular (unified) bot and the
> build times are pretty similar.
> 
> Diego
> 
> [1]
> https://www.mail-archive.com/webkit-dev@lists.webkit.org/msg30077.html
> [2] https://bugs.webkit.org/show_bug.cgi?id=226088
> [3] https://people.igalia.com/lmoura/webkit-bots-dashboard/unified.html
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