While working on the PEP 475, I noticed that the faulthandler module doesn't handle well reentrant calls to its signal handlers. If a signal is received while faulthandler is displaying the traceback, faulthandler restarts to display a new traceback (and then finish to display the previous traceback).

Moreover, faulthandler uses the C function write() to display the traceback and it ignores write() errors. It should retry write() if write() is interrupted by a signal (fails with EINTR).

Attached py_write_noraise.patch:

- Add a new _Py_write_noraise() function
- Document functions releasing the GIL

Attached traceback_eintr.patch:

- Modify functions displaying traceback to use _Py_write_noraise() instead of write()

Attached faulthandler_eintr.patch:

- Add a reentrant flag to the two signal handlers of faulthandler
- Modify faulthandler.dump_traceback() to call PyErr_CheckSignals(), to call the Python signal handler if a signal was received while faulthandler was displaying the traceback of all Python threads

New changeset 60e001ec141d by Victor Stinner in branch 'default':
Issue #23836: Add _Py_write_noraise() function
https://hg.python.org/cpython/rev/60e001ec141d

New changeset d9b9e2a68e7c by Victor Stinner in branch 'default':
Issue #23836: Document functions releasing the GIL in fileutils.c
https://hg.python.org/cpython/rev/d9b9e2a68e7c

New changeset 12e065efc821 by Victor Stinner in branch 'default':
Issue #23836: Use _Py_write_noraise() to retry on EINTR in trip_signal() of
https://hg.python.org/cpython/rev/12e065efc821

New changeset e3c97621d8b4 by Victor Stinner in branch 'default':
Issue #23836: Use _Py_write_noraise() to retry on EINTR in child_exec() of
https://hg.python.org/cpython/rev/e3c97621d8b4

New changeset 29ab05c5e9dc by Victor Stinner in branch 'default':
Issue #23836: Use _Py_write_noraise() to retry on EINTR in _Py_DumpTraceback()
https://hg.python.org/cpython/rev/29ab05c5e9dc

New changeset f07b855afbb2 by Victor Stinner in branch 'default':
Issue #23836: Fix the faulthandler module to handle reentrant calls
https://hg.python.org/cpython/rev/f07b855afbb2