Why does the following test produce a data race?

[Gtsiat]

Hi, the below test:

func TestEventAndCanInGoroutines(t *testing.T) {
	fsm := NewFSM(
		"closed",
		Events{
			{Name: "open", Src: []string{"closed"}, Dst: "open"},
			{Name: "close", Src: []string{"open"}, Dst: "closed"},
		},
		Callbacks{},
	)

	wg := sync.WaitGroup{}
	for i := 0; i < 10; i++ {
		wg.Add(2)
		go func(n int) {
			defer wg.Done()
			var err error
			// different event per loop
			if n%2 == 0 {
				err = fsm.Event(context.Background(), "open")
			} else {
				err = fsm.Event(context.Background(), "close")

			}
			fmt.Println(fsm.Current())
			if err != nil {
				_ = err
			}
		}(i)

		go func() {
			defer wg.Done()
			t.Logf("Can: %t", fsm.Can("close"))
		}()
	}

	wg.Wait()
}

produces the following output when ran with -race enabled:

==================
WARNING: DATA RACE
Write at 0x00c0001280a0 by goroutine 13:
  github.com/looplab/fsm.(*FSM).Event()
      /Users/gtsiat/repos/fsm/fsm.go:372 +0x618
  github.com/looplab/fsm.TestEventAndCanInGoroutines.func1()
      /Users/gtsiat/repos/fsm/fsm_test.go:1062 +0xe0
  github.com/looplab/fsm.TestEventAndCanInGoroutines.gowrap1()
      /Users/gtsiat/repos/fsm/fsm_test.go:1069 +0x44

Previous read at 0x00c0001280a0 by goroutine 18:
  github.com/looplab/fsm.(*FSM).Can()
      /Users/gtsiat/repos/fsm/fsm.go:234 +0x10c
  github.com/looplab/fsm.TestEventAndCanInGoroutines.func2()
      /Users/gtsiat/repos/fsm/fsm_test.go:1073 +0x8c

Goroutine 13 (running) created at:
  github.com/looplab/fsm.TestEventAndCanInGoroutines()
      /Users/gtsiat/repos/fsm/fsm_test.go:1055 +0x348
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1689 +0x180
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1742 +0x40

Goroutine 18 (running) created at:
  github.com/looplab/fsm.TestEventAndCanInGoroutines()
      /Users/gtsiat/repos/fsm/fsm_test.go:1071 +0x214
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1689 +0x180
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.22.1/libexec/src/testing/testing.go:1742 +0x40
==================

The exact command used to run the test was:

go clean -testcache && go test -v -race -run TestEventAndCanInGoroutines

By replacing the stateMu.RLock and stateMu.RUnlock with stateMu.Lock and stateMu.Unlock in the func (f *FSM) Can(event string) bool method, the issue seems to get fixed, but I would lie if I knew that I understand why. Can someone that has experience with this repo explain why it happens?

[denisvaloha]

The problem is that Event() tries to assign the f.transition to transitionFunc() while Can() is using it. Locking the event mutex should fix the issue.

[maxekman]

Fixed by #107.