[SR-7724] libdispatch gets built twice on Linux when sourcekit is enabled

[kevints]

Previous ID	SR-7724
Radar	None
Original Reporter	@kevints
Type	Bug
Status	In Progress
Resolution

Additional Detail from JIRA

Votes	0
Component/s	libdispatch, Source Tooling
Labels	Bug
Assignee	@kevints
Priority	Medium

md5: be0f60374adcf80ee5d8090c026d8f95

Issue Description:

This appears to be a bug in the build system, on Linux sourcekit will cause libdispatch to be built a second time, using the CMake ExternalProject feature.

This unfortunately doesn't preserve all of the options that were passed to build-script (which builds libdispatch separately here, but a seemingly-arbitrary subset of them.

We should ideally refactor the build so that dispatch is only built once, and with the same collection of arguments.

[kevints]

Note that the swift-cmake-built libdispatch gets deleted by build-script here: https://github.com/apple/swift/blob/3156180c082d469fdeaa003b7d4c7b7fe0f4ecfb/utils/build-script-impl#L2511-L2512

[belkadan]

I think this is deliberate. SourceKit uses the C parts of Dispatch, but not the Swift parts. The alternative would be to build the Swift repo in two stages. cc @akyrtzi, @compnerd

[kevints]

Hmm, I'm not sure I follow. At the end of the build only one libdispatch.so is installed on the system and both build invocations pass -DENABLE_SWIFT=YES. If they do need to be different they could at least use different build dirs to avoid the need for the

call rm -fr "${LIBDISPATCH_BUILD_DIR}"

which has the effect of making the libdispatch build nonincremental on Linux

[compnerd]

The ordering at this point is tracked well enough to allow building libdispatch with the SDK overlay in a single go. I think that there was some issue with the build at one point, not sure if is still there (and I don't remember the details 🙁).

[kevints]

PR: swiftlang/swift#19640