build: build target libdispatch as necessary

[compnerd]

This should not be predicated on the host libdispatch being built, only
if concurrency is being built in multithreaded mode.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @drexin

[compnerd]

@swift-ci please test

[drexin]

Thanks!

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 3b1f01d

[drexin]

Linux test failed because of a flaky test. The relevant part succeeded.

[drexin]

@swift-ci smoke test Linux

[drexin]

@swift-ci smoke test linux

[davezarzycki]

Hi @compnerd – Just FYI, this broke my unified build setup (which doesn't use libdispatch). I'm trying to figure out a workaround but I suspect I'll create a pull request soon. Here is the error:

CMake Error at /usr/share/cmake/Modules/ExternalProject.cmake:2555 (message):
  No download info given for 'libdispatch-linux-x86_64' and its source
  directory:

   /home/dave/b/u/t/tools/swift/libdispatch-linux-x86_64-prefix/src/libdispatch-linux-x86_64

  is not an existing non-empty directory.  Please specify one of:

   * SOURCE_DIR with an existing non-empty directory
   * DOWNLOAD_COMMAND
   * URL
   * GIT_REPOSITORY
   * SVN_REPOSITORY
   * HG_REPOSITORY
   * CVS_REPOSITORY and CVS_MODULE
Call Stack (most recent call first):
  /usr/share/cmake/Modules/ExternalProject.cmake:3206 (_ep_add_download_command)
  /home/dave/ro_s/u/swift/cmake/modules/Libdispatch.cmake:76 (ExternalProject_Add)
  /home/dave/ro_s/u/swift/CMakeLists.txt:964 (include)

[davezarzycki]

I don't see an obvious workaround and I don't fully appreciate what this pull request is trying to solve. What does seem to be happening now is that libdispatch is now mandatory when it didn't used to be. Was this intentional? Where was the discussion?

[davezarzycki]

Digging around some more, it seems like the relationship with libdispatch has never been formalized as a CMake option, so what we end up with is a mess of code trying to infer what to do. In practice, we have a tri-state:

1. Use host libdispatch (default for Apple platforms)
2. Build libdispatch (default for non-Apple platforms)
3. Don't use libdispatch (for reasons unknown)

I think I have a solution: #35939

[kubamracek]

@compnerd I think this may also be the reason why the "minimal" bot is now red? Could you take a look? https://ci.swift.org/job/oss-swift-test-stdlib-with-toolchain-minimal/157/

[edymtt]

@kubamracek @compnerd I would have a PR to address that leveraging the flag introduced in #35939

#35976