Change symlink error to warning

[stevapple]

If linking oldBuildPath fails, SwiftPM will emit a warning instead of an error.

Motivation:

Symlinking oldBuildPath is not a vital step of swift build, but emitting an error will depress the run and test commands. This largely troubled Windows users, since symlinking requires elevated privileges on Windows.

Modifications:

Codes of linking oldBuildPath is now wrapped with do-catch to capture the error and change it into a warning.

Result:

If linking oldBuildPath fails, SwiftPM will emit a warning instead of an error.

[tomerd]

I think this is good change. @abertelrud @neonichu wdyt?

[tomerd]

@swift-ci please smoke test

[stevapple]

@abertelrud How do you think of this patch?

[abertelrud]

@stevapple Apologies for my delayed response here. This looks good to me.

[abertelrud]

Looks great to me. Thanks!

[abertelrud]

@swift-ci please smoke test

[abertelrud]

@swift-ci please smoke test linux

[tomerd]

with this change any error in attempting to delete the oldBuildPath is ignored. intentional?

[stevapple]

Yes. In most cases deletion error will also lead to symlink error (if not, there is actually nothing wrong), so I ignored it here.

[neonichu]

Since this is already in a do catch block, we should not need to ignore the error.

Suggested change

	try? localFileSystem.removeFileTree(oldBuildPath)
	try localFileSystem.removeFileTree(oldBuildPath)

[stevapple]

This may lead to confusion because the warning here indicates an error in symlinking instead of deleting.

If we want to handle it, there should be another nested do-catch, which I don’t think necessary🤔

[tomerd]

we can change the warning to say unable to delete \(oldBuildPath) or create symbolic link at \(oldBuildPath): \(error)

[tomerd]

@swift-ci please smoke test

[abertelrud]

The failure looks unrelated to the change, and looks like the same one I saw in a different PR:

Undefined symbols for architecture x86_64:
  "swift::swift51override_conformsToSwiftProtocol(swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*, __swift::__runtime::llvm::StringRef, swift::TargetProtocolConformanceDescriptor<swift::InProcess> const* (*)(swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolDescriptor<swift::InProcess> const*, __swift::__runtime::llvm::StringRef))", referenced from:
      _Swift50Overrides in libswiftCompatibility50.a(Overrides.cpp.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Rerunning just for macOS to see if it was a one-off.

[abertelrud]

@swift-ci please smoke test macos

[abertelrud]

@tomerd All ok to merge this?