x/sys/unix: mkmerge_test.go is not tested on the Go builders

[bcmills]

unix/mkmerge.go was updated in CL 296889 to add //go:build lines (#41184).

That caused the test in unix/mkmerge_test.go to start failing, but nobody noticed because mkmerge_test.go itself has the ignore constraint — it must be run explicitly to run it at all, and the Go builders do not know to do that.

Probably the simplest solution is to factor mkmerge.go out into a proper package (perhaps x/sys/unix/internal/mkmerge?), in which case its test will be run automatically by the builders.

Another option would be to add a Test function in x/sys/unix itself that runs go test mkmerge.go mkmerge_test.go as a subprocess.

CC @tklauser @jupj @ianlancetaylor

[jupj]

Factoring mkmerge.go out into a proper package sounds like a good and simple solution, testing is transparent in that way IMHO.

Do we need to verify that the Go builders catch the failing tests in mkmerge_test.go, before fixing the tests?

In this case the process could be:

1. First patch set: factor mkmerge.go and mkmerge_test.go into x/sys/unix/internal/mkmerge
2. See that the Go builders fail for mkmerge_test.go
3. Follow-up patch set (or separate CL?): fix the tests in mkmerge_test.go and see that Go builders do not fail anymore.

[bcmills]

I already fixed the failing test, so we don't need to observe it on the builder. Just factoring out a package ought to suffice.

If you want to see it in action, I would suggest publishing a patchset to Gerrit with an intentional regression in the test, running TryBots on that Gerrit change to confirm that the regression is caught, and then publishing a second patchset that removes the regression.

[gopherbot]

Change https://golang.org/cl/363334 mentions this issue: x/sys/unix: factor out mkmerge into a package

[jupj]

I did an attempt at package x/sys/unix/internal/merge in CL 363334. When running in Linux, x/sys/unix/mkall.sh prints the following error for mkmerge:

----- GENERATING: merging generated files -----
go: go.mod file not found in current directory or any parent directory; see 'go help modules'
could not merge zerrors files: exit status 1
***** FAILURE:    merging generated files *****

The Docker-based build system for Linux uses a volume mounted at x/sys/unix, which doesn't include x/sys/go.mod, hence package internal/mkmerge is not part of any module when used within the Docker-based build system.

Thoughts on how to address this?

[bcmills]

Oh, neato! We should probably change the build system to mount x/sys instead of x/sys/unix, since x/sys/go.mod is what declares the version of the language against which the package is written.

[gopherbot]

Change https://golang.org/cl/363594 mentions this issue: unix: mount Docker-based builder at x/sys

[jupj]

Thanks for the input!

I sent CL 363594 to change the build system to mount x/sys. Package internal/mkmerge seems to work as intended now.