cmd/go: go list fails with "no non-test Go files" but succeeds upon re-running

[findleyr]

While working on an unrelated gopls bug, I stumbled into a scenario where go list fails non-deterministically depending on whether GOCACHE is populated.

Test setup: (from x/tools/gopls/internal/regtest/misc.TestImportTestVariant)

-- go.mod --
module mod.com

go 1.12
-- client/test/role.go --
package test

import _ "mod.com/client"

type RoleSetup struct{}
-- client/client_role_test.go --
package client_test

import (
	"testing"
	_ "mod.com/client"
	ctest "mod.com/client/test"
)

func TestClient(t *testing.T) {
	_ = ctest.RoleSetup{}
}
-- client/client_test.go --
package client

import "testing"

func TestClient(t *testing.T) {}

Notably client/test/role.go imports mod.com/client, which has no go files (an unintentional bug in the test setup). However, I observed that the entire workspace load fails for gopls in this scenario, due to the following go command bug:

$ go clean -cache
$ go list -compiled=true -- mod.com/...
go build mod.com/client: no non-test Go files in /tmp/gopls-regtest-3664541497/TestImportTestVariant/default/work/client
mod.com/client
mod.com/client/test
$ go list -compiled=true -- mod.com/...
mod.com/client
mod.com/client/test

The first time the go command runs, it returns a non-zero exit code with the "no non-test Go files" error printed to stderr. The second time, it succeeds with no error.

I see no reason for the error upon the first invocation, but even so the error should of course not depend on the state of the cache.

In my testing, this bug exists at least back to Go 1.12, so it's hard to make an argument for urgency (other than the fact that it just cost me two hours of debugging! :) )

CC @bcmills @adonovan