add deadlock check with duplicate taking a lock in a goroutine

[smileusd]

fix #5

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This change is 

[sasha-s]

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Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[a-robinson]

Was it intended that this also fires for calling RLock() twice in the same goroutine? Based on the README update and the test I'd say it looks unintended, but want to double check before changing it to not fire for such situations.

[sasha-s]

This was unintentional, but it might be a good thing.
As far as I remember, doing calling RLock() twice from the same goroutine is dangerous:
it could deadlock if another goroutine tries to grab a regular (write) lock, while the first one is holding an RLock.
There was some discussion here:
#4 (comment)
To be more specific,
this code occasionally deadlocks:

var a sync.RWMutex
    var wg sync.WaitGroup

    sleep := func() {
      time.Sleep(time.Duration((rand.Intn(20))) * time.Millisecond)
    }

    rlockTwice := func() {
      defer wg.Done()
      sleep()
      a.RLock()
      sleep()
      a.RLock()
      sleep()
      a.RUnlock()
      sleep()
      a.RUnlock()
    }

    lock := func() {
      defer wg.Done()
      sleep()
      a.Lock()
      sleep()
      a.Unlock()
    }

    for i := 0; i < 10; i++ {
      wg.Add(2)
      go rlockTwice()
      go lock()
    }

-- with go version go1.9.3 darwin/amd64

What do you think?

[a-robinson]

Looks good, thanks @sasha-s!

[sasha-s]

I updated the readme, thanks for bringing it up.