Add swiftpm-xctest-helper rpath on macOS.

[dan-zheng]

Add an extra rpath for swiftpm-xctest-helper on macOS:
@executable_path/../../../lib/swift/macosx.

This should fix SR-12599 and SR-12600, not yet verified:

$ swift test --filter foo
error: signalled(6): /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2020-04-14-a.xctoolchain/usr/libexec/swift/pm/swiftpm-xctest-helper /Users/danielzheng/swift-tf/Package/.build/x86_64-apple-macosx/debug/PackagePackageTests.xctest /var/folders/m_/6f7q8zfs3n9fr0c_4gy8840m00hc_q/T/TemporaryFile.B8gUFG output:
    dyld: Library not loaded: @rpath/XCTest.framework/Versions/A/XCTest
      Referenced from: /Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2020-04-14-a.xctoolchain/usr/libexec/swift/pm/swiftpm-xctest-helper
      Reason: image not found

Building Swift toolchains now to verify the fix.
Edit: this patch doesn't fix the issues in its current form.

---

This idea was suggested by @abertelrud in #2692 (comment) as a more focused fix for SR-12599 and SR-12600.

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

This PR did not fix SR-12599 or SR-12600, so I'm converting it back to a draft.

$ otool -l swiftpm-xctest-helper | grep LC_RPATH -A2
          cmd LC_RPATH
      cmdsize 168
         path swift/swift-nightly-install/Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2020-04-17-a.xctoolchain/usr/lib/swift/macosx (offset 12)
--
          cmd LC_RPATH
      cmdsize 128
         path /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/usr/lib/swift (offset 12)
--
          cmd LC_RPATH
      cmdsize 32
         path @loader_path (offset 12)
--
          cmd LC_RPATH
      cmdsize 168
         path swift/swift-nightly-install/Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2020-04-17-a.xctoolchain/usr/lib/swift/macosx (offset 12)
--
          cmd LC_RPATH
      cmdsize 56
         path @executable_path/../lib/swift/macosx (offset 12)

@abertelrud: I wonder if you have any ideas why this PR didn't work?
#2692 remains a functional fix.

[abertelrud]

Thanks for taking the time to try this fix, and apologies for not replying until now. I don't know offhand why your diff doesn't work, but I'll take a look and see what I can find. Otherwise, we could take the fix that adds the rpath to all of them, but as I'm also looking into a similar issue, I'd like to take a stab at figuring out how to do more targeted rpaths. BTW, I am just now learning CMake myself, so I will need to explore a bit.

[dan-zheng]

Hi @abertelrud,

I wonder if you have any updates regarding this PR or #2692, as a fix for the swiftpm-xctest-helper rpath issue? We've been manually cherry-picking it as part of every macOS toolchain build, and our toolchain-building users are confused why they encounter it too.

Thanks!

[abertelrud]

Hi, apologies for the delay. I've taken a look at this now, and am seeing the same as you. In fact, it doesn't look as if CMake is being used for building swiftpm-xctest-heper (at least not when using Utilities/bootstrap): because even if I deliberately introduce a syntax error into the CMakeLists.txt file for swiftpm-xctest-helper, I get no error. @aciidb0mb3r or @neonichu, do you know whether it's expected that the CMakeLists.txt file for swiftpm-xctest-helper is unused when bootstrapping, and if so, how best to add linker options to it?

[abertelrud]

Looking closer at the log, it looks as if the swiftpm-xctest-helper is being built using SwiftPM even in the bootstrap case, so the CMakeLists.txt might be there only to for the Utilities/build-using-cmake script.

If so, the right place for these flags would be in the SwiftPM package manifest itself.

[abertelrud]

Something like this seems to work:

diff --git a/Package.swift b/Package.swift
index 5244b1fb..0f4ce338 100644
--- a/Package.swift
+++ b/Package.swift
@@ -167,7 +167,9 @@ let package = Package(
         .target(
             /** Shim tool to find test names on OS X */
             name: "swiftpm-xctest-helper",
-            dependencies: []),
+            linkerSettings: [
+                .unsafeFlags(["-Xlinker", "-rpath", "-Xlinker", "@executable_path/../../../lib/swift/macosx"], .when(platforms: [.macOS])),
+            ]),
 
         // MARK: Additional Test Dependencies

I have confirmed that this results in the right rpath in the produced binary, but I am not sure whether that is the right way to do it, and whether there would be any unwanted side effects. @aciidb0mb3r or @neonichu, what do you think?

[aciidgh]

This should be ok for now since we don't export swiftpm-xctest-helper as a product so should have no effect on packages that depend on libSwiftPM.

[dan-zheng]

I confirmed that #2694 (comment) works as a fix for SR-12600. I made it into a PR: #2785.
This PR doesn't work as a fix and is obsolete.