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To factorize code handling EINTR, I propose add 2 new functions to fileutils.h: _Py_read() and _Py_write().
Attached patch adds these functions and use them in the os and _io modules.
The code of the functions is based on the code from the os and _io modules. The main change is that the functions now truncate code if greater than PY_SSIZE_T_MAX. Other changes are just refactoring.

select_write.patch: use _Py_write() in the select module.

I created the issue #23709 for the ossaudiodev module: "Refactor ossaudiodev: use _Py_read and _Py_write with the Py_buffer".

With py_read_write.patch in debug mode, test_threading crash with an assertion error because of the issue #15751. Currently, it's not possible to call assert(PyGILState_Check()); inside Py_NewInterpreter(), while creating a subinterpreter (ex: _testcapi.run_in_subinterp()).
The workaround is to comment the assertion in _Py_read() and Py_write() until the issue #15751 is fixed.

Oops, there was a bug in FileIO.readall(), fixed in the new patch py_read_write-2.patch.
I also commented assert(PyGILState_Check()); to workaround the issue #15751.

New changeset c3c47ea32f72 by Victor Stinner in branch 'default':
Issue #23708: Add _Py_read() and _Py_write() functions to factorize code handle
https://hg.python.org/cpython/rev/c3c47ea32f72 

New changeset e232b57ee784 by Victor Stinner in branch 'default':
Issue #23708: select.devpoll now retries its internal write() when interrupted
https://hg.python.org/cpython/rev/e232b57ee784 

I commited both patches. Thanks Antoine for the review of py_read_write-2.patch, I removed the commented assertion.

New changeset 116e4c40115f by Victor Stinner in branch 'default':
Issue #23708: Fix _Py_read() compilation error on Windows
https://hg.python.org/cpython/rev/116e4c40115f 

An assertion failed in _Py_read() while running test_signal on AMD64 Snow Leop 3.x:
http://buildbot.python.org/all/builders/AMD64%20Snow%20Leop%203.x/builds/2779/steps/test/logs/stdio
[321/393/1] test_signal
Assertion failed: (errno == EINTR && PyErr_Occurred()), function _Py_read, file Python/fileutils.c, line 1181.
Fatal Python error: Aborted
Current thread 0x00007fff71296cc0 (most recent call first):
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/subprocess.py", line 1407 in _execute_child
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/subprocess.py", line 855 in __init__
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/test_signal.py", line 126 in run_test
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/test_signal.py", line 180 in test_main
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/case.py", line 577 in run
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/case.py", line 625 in __call__
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 122 in run
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 84 in __call__
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 122 in run
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 84 in __call__
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/runner.py", line 176 in run
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/support/__init__.py", line 1772 in _run_suite
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/support/__init__.py", line 1806 in run_unittest
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/test_signal.py", line 1123 in test_main
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1284 in runtest_inner
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 967 in runtest
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 532 in main
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1568 in main_in_temp_cwd
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1593 in <module>
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/runpy.py", line 85 in _run_code
 File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/runpy.py", line 170 in _run_module_as_main

New changeset 6dd201b6bb4f by Victor Stinner in branch 'default':
Issue #23708: Split assertion expression in two assertions in _Py_read() and
https://hg.python.org/cpython/rev/6dd201b6bb4f 

Same error on "x86 Tiger 3.x" buildbot:
http://buildbot.python.org/all/builders/x86%20Tiger%203.x/builds/9381/steps/test/logs/stdio
[345/393] test_signal
Python/fileutils.c:1181: failed assertion `errno == EINTR && PyErr_Occurred()'
Fatal Python error: Aborted
Current thread 0xa000d000 (most recent call first):
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/subprocess.py", line 1407 in _execute_child
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/subprocess.py", line 855 in __init__
 File "/Users/db3l/buildarea/3.x.bolen-tiger/build/Lib/test/test_signal.py", line 126 in run_test
 ...

New changeset 07fd54208434 by Victor Stinner in branch 'default':
Issue #23708: Save/restore errno in _Py_read() and _Py_write()
https://hg.python.org/cpython/rev/07fd54208434 

> New changeset 07fd54208434 by Victor Stinner in branch 'default':
> Issue #23708: Save/restore errno in _Py_read() and _Py_write()
When I wrote _Py_read()/_Py_write(), I added assertions on errno to ensure that errno is not modified. I chose to use assertions instead of save/restore errno because on Linux errno is not modified. Since they *are* platforms where errno is modified, it's safer to always save/restore errno.
_Py_read()/_Py_write() *must* set set errno because some callers uses it. It's more convinient to use errno than having to get the errno attribute of the raised OSError exception.
test_signal pass again on "AMD64 Snow Leop 3.x" buildbot, I close the issue.