[webkit-dev] New EWS Non-Unified builder

[dpino]

Hi everyone,

In Igalia we have been discussing the need of deploying a new builder
which builds WebKit using non-unified sources, and we know that at least
the folks at Sony are also in favor.

One side effect of Unified Source building is that it hides compilation
errors. The kinds of errors that usually get hidden by unified builds
are missing headers inclusions and missing definitions of functions
declared inline; the latter being tricky to debug because it results in
mysterious linker errors. This is caused by unified builds stashing
several .cpp files together for compilation, so the definitions and
header inclusions done in one “leak” into the others. As for missing
header inclusion errors, a source file might include a header definition
as a co-lateral effect of being stashed together with another file that
indeed includes the missing header.

These hidden compilation errors may arise later at some point if unified
source files are stashed together in a different manner.

The current situation is requiring periodical maintenance. You can check
build fixes commits due to unified source compilation with:

    $ git log --pretty=short --grep "Non-unified"

Here are some examples:
    https://bugs.webkit.org/show_bug.cgi?id=222652
    https://bugs.webkit.org/show_bug.cgi?id=222755
    https://bugs.webkit.org/show_bug.cgi?id=221701

A new builder which builds WebKit with non-unified Source will highly
help to improve this situation. Compilation errors will be detected as
soon as possible and will save a lot of time not only for the developers
who are currently doing this manual maintenance but for anyone who would
like to build WebKit, and may stumble on compilation errors accidentally
introduced due to unified sources.

While correct compilation of the codebase can only be guaranteed with
non-unified source builds, we do not propose switching the current EWS
compilation builders to non-unified because it's slower and the EWS
LayoutTests and API test bots use the products built by the EWS builders
— we do not want to delay getting results from those. That's why we are
proposing a new builder: it will run on parallel, resulting in no
slowdown for the other EWS builders, which will keep using unified
builds.

How this new builder will impact developers? The EWS LayoutTest bots
take at least 1 hour to complete a build. We think that as long as this
new EWS Non-Unified builder is within that time budget, this new EWS
wont' slow down development speed.

Thoughts?

Best regards,

Diego