Plugin artifacts for UE 5.4 are huge (too many objects files) about cesium-unreal

[csciguy8]

Downloaded UE 5.4.1 locally and it seems to fix this issue.

I built UnrealGame, Development, and the extra (non-unity) object files weren't being produced.

Fix could be as easy as just updating our UE_5.4.zip on S3 @kring

[csciguy8]

@j9liu tried 5.4.0, built locally, and didn't see the extra object files either. This hints it could be a CI build problem?

[kring]

The problem is that 5.4 changed how UBT decides which files are in the "working set". Working set files are excluded from the adaptive unity build, and so they balloon the size of our plugin.

Specifically, the 5.4 bug (that's what I'd call it) was introduced on this line:
https://github.com/EpicGames/UnrealEngine/blob/5.4.1-release/Engine/Source/Programs/UnrealBuildTool/System/SourceFileWorkingSet.cs#L395

As a result of that line, any time we're using an "installed" engine (which we are, because we didn't build it from source), we get a PerforceSourceFileWorkingSet. That working set implementation deems any file that is not read-only to be part of the working set.

As for how to fix it, I'm less sure. Setting all our source files to read-only before doing the build on CI would probably work.

[kring]

What I wrote above is probably not the full story, though, because as far as I can tell UE 5.3 would have used the PerforceSourceFileWorkingSet too, and it would have behaved the same way. So I'm not sure why this is a new problem.

[csciguy8]

I just realized that the slow builds observed in 5.4 vs 5.3 can probably be attributed to this bug too. Non-unity builds are slower.