Queue can result in nested monitor deadlock

[Jonathan Amsterdam]

This is a reply to Alan Morgan, Paul McGuire and Duncan Booth.

I need mutex M because I have other fields in my class that need to be
thread-safe.

The reason I want to use a Queue and not a list is that a Queue has
additional synchronization besides the mutex. For instance, Queue.get()
will block the caller until the queue is non-empty. Of course I could
build this behavior on top of a list, but then I'm reinventing the
wheel: Queue.Queue is the perfect thing, except for the problem I
describe.

I can't release my mutex M and then call Queue.get().  That could be a
race condition: between the time M is released and Queue's mutex is
acquired, another thread could get into my object and mess things up.
(We'd need a specific example to see this, and there may be many cases
where I could safely release M before calling Queue.get(), but in
general it won't work.)