[Merge Modules] Omit merge-modules job when only one input is given.

[artemcm]

And said input matches the expected output path.

Otherwise both the .compile and the .mergeModule jobs have an identical output:
.temporary(<module_name>.swiftmodule) which will trigger a fatal error in the MultiJobExecutor because it may cause LLBuild cycles downstream.

[nkcsgexi]

Are we certain that we will use the correct output file name Foo.swiftmodule instead of filename~particial.swiftmodule in the frontend command?

[artemcm]

Are we certain that we will use the correct output file name Foo.swiftmodule instead of filename~particial.swiftmodule in the frontend command?

We ensure with this check:

soleInput.fileHandle == mergeOutput

That this will only omit merge-modules if the final module output matches that from the compile job.
In a plain compile with one source file and with -emit-module, we will have the situation you describe where we have one module Foo~partial.swiftmodule in a temporary directory somewhere and the output of merge-modules is Foo.swiftmodule where the user wants it. In that case, the job will not be skipped.

This is motivated by the -g case where the final module output is in a temporary directory and matches exactly the module output of the single-source-file.

[nkcsgexi]

ah, that makes sense. Thank you for the explanation! @artemcm

[artemcm]

@swift-ci please test

[artemcm]

@swift-ci please test

[artemcm]

@swift-ci please test