build: partially repair cross-repository testing

[compnerd]

When doing cross-repository testing, we need to ensure that we use the proper set of dependencies. With a change to IndexStoreDB relying on swift-lmdb, we uncovered this latent issue which filtered out the dependency and resulted in undefined symbol references.

[ahoppen]

I assume that you’re seeing missing symbols from swift-lmdb? If so, I don’t think this is the right approach because it re-builds modules that have already been built using CMake. You should add the appropriate search paths for swift-lmdb to https://github.com/swiftlang/swift/blob/8bf940614627a58ca708ce2639220c5e59172a69/utils/build.ps1#L2536

[compnerd]

However, that wouldn't allow you the necessary control per se. You need to manually manage every single dependency per target as the C/C++ dependencies will not flow (unlikely Swift where the autolinking would allow you do that).

[ahoppen]

Yes, that is the downside of the current design. As I see it we have two ways to go:

1. Re-use the targets that are already built using CMake during testing.
2. Re-build all SourceKit-LSP dependencies using SwiftPM (and work around the maximum symbol issue by building swift-syntax as a a dynamic library)

I don’t think that we should do some mixed in-between way of re-using some dependencies from the CMake build and re-building other dependencies. I believe that (1) is the better approach for now because it avoids building code twice, reducing CI times and tests the object files that are going to be used in the toolchain, aligning the tests more closely with what we really ship. I agree that having to manage the list of C/C++ targets that we link against is annoying but I think it’s the best option we have right now.