x/build/cmd/coordinator: switch to "./..." import path pattern for testing packages in a module

[dmitshur]

Back when GOPATH mode was default, assuming one GOPATH workspace, running "go test ./..." in $GOPATH/src/root had the same effect as running "go test {root}/...". The latter was more explicit and did the stated thing even if it was accidentally run from the wrong directory. Maybe that's why it was chosen, or maybe it was chosen arbitrarily, since both worked and did the same thing.

That invocation persisted to today, when module mode is used, and for each module to be tested the coordinator runs "go test {module-root}/..." invocations at the module root to test packages contained inside that module:

// A goTestRun represents a single invocation of the 'go test' command.
type goTestRun struct {
	Dir      string   // Directory where 'go test' should be executed.
	Patterns []string // Import path patterns to provide to 'go test'.
}
// The default behavior is to test the pattern "golang.org/x/{repo}/..."
// in the repository root.
repoPath := importPathOfRepo(st.SubName)
testRuns := []goTestRun{{
	Dir:      "gopath/src/" + repoPath,
	Patterns: []string{repoPath + "/..."},
}}

(source)

From #51283 I've learned there's a difference in that "{module-root}/..." may select a package with import path "{module-root}" even if that package is in another module, but "./..." won't do that.

Since there's no need for the pattern to match any packages outside of the very module it's meant to cover, we could consider switching. It would not fail if a go.sum entry that go mod tidy doesn't currently insert isn't added in some cases (see #51283), remove possibility of unintentional duplicate coverage of packages in nested modules, and least importantly it is shorter visually.

CC @golang/release, @bcmills, @matloob.

[bcmills]

In my opinion this is the appropriate fix for the builders for the semantics of cmd/go as it exists today.

[dmitshur]

Moved to NeedsFix after bringing this up and discussing with the team.

There's not a lot of code that needs to change to implement this, but doing what's needed to validate that it's working as expected (and follow up as needed) may end up being the bulk of the work. (Issue #22437 is also relevant.)

[gopherbot]

Change https://go.dev/cl/418543 mentions this issue: cmd/coordinator: use "./..." pattern when testing golang.org/x repos