Issue 34563: invalid assert on big output of multiprocessing.Process

the bug is reproduced on big multiprocessing.Process output
when the size of the output gets bigger than signed int the value becomes negative, thus
```            
assert left > 0
```
in multiprocessing/connection.py:337 raises an exception like the following

```
Traceback (most recent call last):
  File "D:\GitHub\cpython\lib\threading.py", line 917, in _bootstrap_inner
    self.run()
  File "D:\GitHub\cpython\lib\threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "D:\GitHub\cpython\lib\multiprocessing\pool.py", line 470, in _handle_results
    task = get()
  File "D:\GitHub\cpython\lib\multiprocessing\connection.py", line 250, in recv
    buf = self._recv_bytes()
  File "D:\GitHub\cpython\lib\multiprocessing\connection.py", line 318, in _recv_bytes
    return self._get_more_data(ov, maxsize)
  File "D:\GitHub\cpython\lib\multiprocessing\connection.py", line 337, in _get_more_data
    assert left > 0
AssertionError
```


this assert looks invalid in this case because in C code the left value is DWORD (unsigned long), which cannot be negative by definition.

By bigger than signed int i meant bigger than positive signed int

New changeset 266f4904a222a784080e29aad0916849e507515d by Victor Stinner (Alexander Buchkovsky) in branch 'master':
bpo-34563: Fix for invalid assert on big output of multiprocessing.Process (GH-9027)
https://github.com/python/cpython/commit/266f4904a222a784080e29aad0916849e507515d

New changeset e8ca8806a9f47b3bac4ae1c6b8a54c47d1aad8f3 by Victor Stinner (Miss Islington (bot)) in branch '3.7':
bpo-34563: Fix for invalid assert on big output of multiprocessing.Process (GH-9027) (GH-9064)
https://github.com/python/cpython/commit/e8ca8806a9f47b3bac4ae1c6b8a54c47d1aad8f3

New changeset 23f427a0fdb888212136cf8745a9f5f832a3f374 by Victor Stinner (Alexander Buchkovsky) in branch '3.6':
[3.6] bpo-34563: Fix for invalid assert on big output of multiprocessing.Process (GH-9027) (GH-9069)
https://github.com/python/cpython/commit/23f427a0fdb888212136cf8745a9f5f832a3f374

Thanks Alexander Buchkovsky for the fix: it has been merged into 3.6, 3.7 and master branches. Thanks Oleksandr Buchkovskyi for the bug report!

--

I looked at the Python 2.7 code and it doesn't look to be impacted by this bug.

Modules/_multiprocessing/pipe_connection.c uses ReadFile() and PeekNamedPipe(), but the result type of the two C functions using it is not a PyObject* and conn_recv_string() looks good to me. Moreover, the type of the buflength parameter of conn_recv_string() is size_t, so there is no signed issue.

thank you for all the help in merging this fix!

resolution: fixed
stage: patch review -> resolved