[bootstrap] Use separate module cache for builds

[edymtt]

This will ensure that multiple build jobs running in parallel on the
same machine using different Swift compilers (like it could be the case in CI) do not interfere with
each other.

Addresses rdar://71487295

[abertelrud]

Thanks! This looks good to me, but it would be good to hear what @neonichu thinks as well.

[abertelrud]

@swift-ci please smoke test

[abertelrud]

Should the module cache directory perhaps also be a parameter to SwiftPM so that explicit flags like this don't need to be used?

[neonichu]

Is this a broader concern or specific to CI with bootstrap? What if I run two invocations of swift test locally, can I run into similar issues?

[edymtt]

@abertelrud I cannot offer a good opinion on your question -- I don't have a good idea on how common is to compile applications in parallel on the same machine using different Swift compilers which relies on different formats of the module cache (I can only think of this being needed in CI environments)

Also I ignore if there are situations in which we use a different compiler to load the manifest (in which case the parameter could be confusing)

[edymtt]

@neonichu my PR is specific to Swift CI usage of bootstrap -- this can happen in general, but I believe it's up to the user to configure the module cache path if necessary

Likely the same will happen with swift test, which is a scenario I haven't tested yet -- although from a quick peek at logs from swiftlang/swift#34782, I can see that -module-cache-path is passed already (since get_swiftpm_flags is called in that code path as well)

env SWIFTCI_USE_LOCAL_DEPS=1 <snip>/x86_64-unknown-linux-gnu/release/swift-test --enable-test-discovery --parallel --build-path /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-main/buildbot_linux/swiftpm-linux-x86_64 --configuration release -Xlinker -rpath -Xlinker $ORIGIN/../lib/swift/linux -Xswiftc -module-cache-path -Xswiftc /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-main/buildbot_linux/swiftpm-linux-x86_64/module-cache -Xmanifest -module-cache-path -Xmanifest /home/buildnode/jenkins/workspace/swift-PR-Linux/branch-main/buildbot_linux/swiftpm-linux-x86_64/module-cache

[edymtt]

For the sake of completeness, I've tried to run the tests for swiftpm with this PR -- while the compilation of the tests does not write to the default module cache, executing the tests themselves do.

I would prefer to address this in a separate PR.

[neonichu]

Oh, I see, the issue is specific to using different compilers in parallel? That doesn't seem as if that would be something that's commonly done by users.

[edymtt]

Correct -- expanded the PR description to reflect this