fix: make variant metrics concurrency safe

[sighphyre]

This adds a mutex lock to the metrics for GetVariant, which fixes a crash bug which occurs when the `GetVariant`` method is called in a tight loop concurrently.

The toggle data that's modified in the countVariants is bound to the bucket, which is locked by the mutex, so I want to say this safe.

I haven't added a test because concurrency reasons but this can be arbitrarily reproduced with the following hacky code snippet from version 3.2.x to 3.7.2 (doesn't occur with equivalent IsEnabled calls).

timer := time.NewTimer(1 * time.Second)

for {
	<-timer.C

	go func() {
		for i := 0; i < 10000; i++ {
			variant := unleash.GetVariant("test")
			fmt.Printf("'%s' enabled? %v\n", "test", variant)
			timer.Reset(1 * time.Second)
		}
	}()

	go func() {
		for i := 0; i < 10000; i++ {
			variant := unleash.GetVariant("test")
			fmt.Printf("'%s' enabled? %v\n", "test", variant)
			timer.Reset(1 * time.Second)
		}
	}()
}

[gastonfournier]

How does this actually work? I'm used to having the unlock after the guarded portion of the code, but maybe it's the defer what makes the difference...

[FredrikOseberg]

Defer runs this function after everything has been executed and as the function exits.

…

On Tue, Jan 24, 2023 at 4:56 PM Gastón Fournier ***@***.***> wrote: ***@***.**** commented on this pull request. ------------------------------ In metrics.go <#129 (comment)> : > + + m.bucketMu.Lock() + defer m.bucketMu.Unlock() + How does this actually work? I'm used to having the unlock after the guarded portion of the code, but maybe it's the defer what makes the difference... — Reply to this email directly, view it on GitHub <#129 (review)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AD2WIPSEINWLQP5ODZ3N6EDWT73RTANCNFSM6AAAAAAUE2I3KM> . You are receiving this because your review was requested.Message ID: ***@***.***>