Raise deferred KeyboardInterrupt out of run() after cancelling all tasks

[oremanj]

Fixes #151, first step of #733.

[codecov]

Codecov Report

Merging #1537 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1537      +/-   ##
==========================================
- Coverage   99.67%   99.67%   -0.01%     
==========================================
  Files         108      108              
  Lines       13358    13345      -13     
  Branches     1012     1008       -4     
==========================================
- Hits        13315    13302      -13     
  Misses         28       28              
  Partials       15       15              

Impacted Files	Coverage Δ
trio/_core/_traps.py	100.00% <ø> (ø)
trio/_core/_exceptions.py	100.00% <100.00%> (ø)
trio/_core/_io_kqueue.py	83.01% <100.00%> (ø)
trio/_core/_io_windows.py	98.54% <100.00%> (ø)
trio/_core/_run.py	99.72% <100.00%> (-0.01%)	⬇️
trio/_core/tests/test_ki.py	100.00% <100.00%> (ø)
trio/_core/tests/test_run.py	100.00% <100.00%> (ø)

[oremanj]

The coverage "failure" is because trio._core._run now contains fewer statements, so the same set of uncovered statements results in a lower coverage percentage.

[pquentin]

Sorry for the merge conflict, I just merged the Black pull request

[njsmith]

I'm not sure cancelling the system nursery is a great idea... until now, we had an invariant that system tasks would only be cancelled (a) after the main task exited, or (b) after a system task crashed. So they basically had the "service task" semantics discussed in #1521.

I feel like KeyboardInterrupt should immediately cancel the main task, but not immediately cancel system tasks.

[oremanj]

That makes sense to me, but I think it will be easiest to implement by just doing #1521, because I don't see any way to wrap the main task in a dedicated cancel scope without adding another frame to every Trio traceback.

[njsmith]

Maybe possibly we could do something like this?

    async def init(self, async_fn, args):
        async with open_nursery() as system_nursery:
            self.system_nursery = system_nursery
            async with open_nursery() as main_task_nursery:   ##### <-----
                try:
                    self.main_task = self.spawn_impl(async_fn, args, main_task_nursery, None)
                except BaseException as exc:
                    self.main_task_outcome = Error(exc)
                    system_nursery.cancel_scope.cancel()
                self.entry_queue.spawn()

but yeah, I see your point