Deadlock between Watcher and Debounce threads (fix #187)

[bpasero]

fix https://github.com/parcel-bundler/watcher/issues/187

There are two threads involved in Watcher.cc and Debounce.cc each calling into respective methods of the other and thus each potentially holding onto locks of the other. This can lead to a deadlock in the following scenario:

While the Debounce.cc thread is processing callbacks in the Debounce::notify() method, it holds its own lock. The method loops over callbacks to process in Watcher.cc which itself requires a lock in Watcher.cc. If an event gets reported while the debouncer is in Watcher.triggerCallbacks(), a deadlock is present, because:

• assume the event thread is thread A
• assume the debouncer thread is thread B
• A holds its own lock in Watcher::notify() and calls into Debounce.trigger() which requires the debouncer lock
• B at this time is inside Debounce::notify() holding its own lock processing callbacks and about to call into Watcher.triggerCallbacks()
• A deadlocks waiting for B to release the debouncer lock
• B deadlocks waiting for A to release the watcher lock

[devongovett]

Thanks for the deep investigation here! This looks good to me.