statemgr: add a NewUnlockErrorFull state manager for tests

[mildwonkey]

I've frequently needed to coerce Unlock() errors for tests and it's been
awkward and fraught every time, so I decided to add a full state manger
that returns mostly errors. I intend to use this in conjunction with
the clistate.Locker interface, which first calls statemgr.Lock() (to block if the
mutex is in use) at the start of clistate.Unlock(), so statemgr.Lock() rather awkwardly needed to succeed.

[codecov]

Codecov Report

Merging #25823 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Impacted Files	Coverage Δ
states/statemgr/statemgr_fake.go	44.68% <0.00%> (-18.96%)	⬇️
dag/marshal.go	53.33% <0.00%> (-1.12%)	⬇️

[alisdair]

This looks good to me in isolation! 👍 I'm not familiar with tests that would use this, so I can't quite picture how it'd be used. If you have one to hand, could you point at a test where you'd use it?

[alisdair]

I don't see any of the struct members other than t being used in the methods below. Would it make sense to remove them?

[mildwonkey]

Honestly I don't think we need any of the struct members, since all the state functions return errors. I'll remove them all!

[mildwonkey]

Absolutely @alisdair , I should have included this from the start. The community PR that pushed me over the edge is this one: #25729

The Unlock function first locks the state, which is why I needed a statemgr that would return success (and a non-empty lockId) for lock and an error for unlock:

terraform/command/clistate/state.go

Lines 132 to 138 in b5f4f9a

	func (l *locker) Unlock(parentErr error) error {
	l.mu.Lock()
	defer l.mu.Unlock()
	if l.lockID == "" {
	return parentErr
	}

And here's the very simple test I have ready to add to that PR:

func TestUnlock(t *testing.T) {
	ui := new(cli.MockUi)

	l := NewLocker(context.Background(), 0, ui, &colorstring.Colorize{Disable: true})
	l.Lock(statemgr.NewUnlockErrorFull(nil, nil), "test-lock")

	err := l.Unlock(nil)
	if err == nil {
		t.Error("expected unlock error")
	}
}

[ghost]

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