Race condition between close/read leads to deadlock

[bherw]

Because the close method does not lock and the read method does not check the closed value after locking, it's possible for a deadlock to occur under the following series of events:

Thread 1 (reader): read last element from the channel
Thread 1: call operator>>
Thread 1: check closed(), which is false
Thread 1: lock mutex
Thread 1: check the predicate in wait()
Thread 2 (writer): call close()
Thread 2: set closed=true
Thread 2: call cnd_.notify_all()
Thread 1: calls wait() without predicate (in the implementation of the wait with predicate method)

At this point, Thread 1 waits forever because the notify_all occurred before the actual call to wait().

[bherw]

I don't have a good minimal test case for this, but it shows up often enough in our integration tests--maybe one in 500-600 usages of the channel.