avoid cancel_handle close race with weakref.finalize

[richardsheridan]

Fixes #1271. This is essentially the reference counting scheme from that issue, but python is doing the refcounting. I've also made sure the "nonblocking" case is a full checkpoint, win32 errors don't pass silently, and the logic is protected from KeyboardInterrupt.

I wrote this in response to #1834 (comment) where @honglei seemed more interested in the bugfix than the features of #1834. (Hopefully you can test this branch in your application?)

Ideally, if someone reviews this they would also have a look at #1834 so I can either close or merge that one.

[codecov]

Codecov Report

Merging #2374 (2e9d220) into master (b24672a) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2374      +/-   ##
==========================================
- Coverage   92.45%   92.42%   -0.04%     
==========================================
  Files         118      118              
  Lines       16342    16348       +6     
  Branches     3155     3156       +1     
==========================================
  Hits        15109    15109              
- Misses       1104     1110       +6     
  Partials      129      129              

Impacted Files	Coverage Δ
trio/_core/_io_windows.py	0.00% <ø> (ø)
trio/_core/_windows_cffi.py	0.00% <0.00%> (ø)
trio/_wait_for_object.py	0.00% <0.00%> (ø)
trio/tests/test_wait_for_object.py	10.29% <0.00%> (-0.08%)	⬇️

[honglei]

at the beginning I wanna make pynorm https://github.com/USNavalResearchLaboratory/norm work with asyncio and fastapi, so I need an async version of WaitForSingleObject , then I found trio.
But trio is not asyncio, #892 ,
so I need some time to learn/understand how the trio makes things work.

import pynorm
instance:pynorm.Instance = pynorm.Instance()
async def watch_norm_events():
    '''
    NormGetNextEvent docs : non-blocking operation may be achieved by using the NormGetDescriptor() function
    to obtain a descriptor(int for Unix or HANDLE for WIN32) suitable for 
    asynchronous input/output (I/O) notification
    using such system calls the Unix select() or Win32 WaitForMultipleObjects() calls. 
    The descriptor is signaled when a notification event is pending and a call to 
    NormGetNextEvent() will not block
    '''
    global instance
    #instance.setDebugLevel(level=12)

    handle = instance.getDescriptor() #handler opened by norm lib
    while True:
        await WaitForSingleObject(handle)  # wait for event 
        event: pynorm.event.Event = instance.getNextEvent( )
        print(event) 

[richardsheridan]

This PR has 100% diff branch coverage on my machine.

[njsmith]

Any reason for this change? The two versions should act the same, given that we're creating a new CapacityLimiter on every call.

[richardsheridan]

Nothing super important, but it avoids an import and an attribute lookup. Do you remember why the default thread limiter is not used?

[richardsheridan]

Just answering my own question here: #4 (comment)