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I would like to try to clarify the problem a little and
suggest some possible solution approaches. 

While this patch solves a particular threading.settrace()
problem (and possibly a potential threading.setprofile
problem), the more I think about this, I'm not sure it will
solve all of them or is necessary in all cases.

To reiterate the problem: 

It was noticed that having tracing (Threading.settrace) or
profiling turned on while inside threading.py can cause a
thread hang when _active_limbo_lock.aquire() is called
recursively: once while code uses a method in threading.py
like _delete(), and another time when tracing or profiling
routine is called by settrace from within a Threading method
and the tracing/profiling code calls one of the Threading
methods like enumerate() to get information for its own
purposes. (The patch addresses this for _delete but I'm not
sure it would address it if the first call were say enumerate).

One possibility and clearly the most reliable one because it
relies least on code using Threading, would be for
threading.py to check for this kind of recursive invocation
 (at the module level, not the method level) which might be
done by scanning a call stack. More later. 

Another possibility might be to document this behavior and
put the burden on the profiler/debugger/tracer or anything
that might cause some set of threading routines to be called
recursively. To address the problem outside of Threading
code, what might be done is call
_active_limbo_lock.aquire(blocking=0) before calling a
Threading routine like enumerate(), and use the Threading
routine only only if the lock is acquired.

This will work, but it may get the "cannot acquire lock"
status too often, namely in situations where there isn't a
recursive call. Better than this would again be to somehow
indicate that "a call to a Threading routine which does
locking" is in progress. 

A simple and reliable way to do this would be to share the
responsibility: the Threading methods would set a boolean
variable set to indicate this condition. Code using
Threading could test this before making calls which would
cause recursive invocation.