cmd/go: builds fail when GOPATH=/dev/null

[josharian]

When run with GOPATH=/dev/null, go build fails:

$ GOPATH=/dev/null go test -run=TestLldbPython -v -count=1 runtime
=== RUN   TestLldbPython
--- FAIL: TestLldbPython (0.21s)
    runtime-lldb_test.go:169: building source exit status 1
        go: failed to create cache directory /dev/null/pkg/mod/cache: mkdir /dev/null: not a directory
FAIL
FAIL	runtime	0.220s

That seems fine. Who would run with GOPATH=/dev/null‽ It's not even a directory.

But it's not fine, because run.bash sets GOPATH=/dev/null. Why? The comment says: The $GOPATH value doesn't need to point to an actual directory, it just needs to pass the semantic checks performed by Go.

I don't know who is wrong here, but I do want all.bash to pass. I have a workaround I can mail just for TestLldbPython (set GOPATH=). Or we can fix this mess...somewhere else.

Opinions?

@bcmills @jayconrod @bradfitz

[cuonglm]

@josharian IMHO, set GOPATH=/dev/null in run.bash prevent it from conflict with GOROOT. If the test required a valid GOPATH, it should set explicitly.

[bcmills]

GOPATH/pkg/mod contains the module cache, which includes a lockfile that guards edits to the user's go.mod and go.sum files (see cmd/go/internal/modfetch.SideLock).

If those files do not need to be updated and no modules need to be downloaded, then the lock does not need to be acquired, and GOPATH does not need to be set. I'm guessing that the failure you're observing is an interaction with #30790, which perhaps introduces more edits to the go.mod file than strictly necessary.

Regardless of the cause, we could proceed without locking if GOPATH does not exist and cannot be created, but I think an explicit failure in that case is preferable to an undetected race.

[gopherbot]

Change https://golang.org/cl/169877 mentions this issue: cmd/go/internal/modload: do not implicitly add a 'go' directive to an existing go.mod file

[gopherbot]

Change https://golang.org/cl/169918 mentions this issue: runtime: fix TestLldbPython in module mode (again)

[josharian]

Thanks. I went ahead and mailed my simple workaround for you to take a look at.