gh-138013: Move I/O test infrastructre to test_io.utils

[cmaloney]

Moves the I/O test support code to test.test_io.utils leaving all test cases as they are. The code was moved via copy/paste then adjusted imports as needed (remove unneded + add all required).

---

I plan to do a code update + two more direct code movement to split tests from test_general to increase parallelism and logical layout. Times are runtime on my 64 bit linux debug build output by slow tests when running

./python -m test test_io -uall,walltime,largefile,extralargefile -M16G -o -j0

1. Move buffered tests into test_bufferedio
2. Remove custom load_tests w/ test list from test_general gh-138013: Remove load_tests in test_io.test_general #138771 (see: gh-127647: Fix and enable I/O protocol tests #138369)
3. Move SignalsTest to test_signals (21.4s)
4. Move TextIOWrapperTest and IncrementalDecoder tests to test_textio (1.1s)

The longest remaining test in test_general after that for me is test_daemon_threads_shutdown_*_deadlock which takes ~7s (and is marked with requires_resource('walltime')). Runtime of test_io reduces from ~34.1s -> 21.4s (SignalsTest is now longest) via increased parallelism.

• Issue: Move Buffered tests from test_io to test_bufio #138013

[cmaloney]

Windows bot failure looks real and a weird intermittent issue I've been running into locally some. Investigating:

test_uninitialized (test.test_io.test_bufferedio.CBufferedRWPairTest.test_uninitialized) ... Warning -- Unraisable exception
Exception ignored while finalizing file <_io.BufferedRWPair object at 0x000001AC6C4654F0>:
Traceback (most recent call last):
  File "D:\a\cpython\cpython\Lib\re\_parser.py", line 176, in append
    def append(self, code):
    
ValueError: flush of closed file
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.BufferedWriter>:
Traceback (most recent call last):
  File "D:\a\cpython\cpython\Lib\re\_parser.py", line 176, in append
    def append(self, code):
    
ValueError: flush of closed file

[cmaloney]

I ran all the test_io tests in a loop for two days without reproducing... Reading through the BufferedRWPair code I think there are some tear-down race conditions but this PR doesn't change them, just effects test timing (which may make them a little more likely). That the stack trace points to a distinct module is really weird for me / I don't understand (likely a distinct bug). I plan to make a PR for some of the things I found in manual review in BufferedRWPair, but it is cases I've been able to figure out a good test case for; just hunches.

tl; dr: I think this PR splitting the tests is sound. It might make an existing flaky / race case more visible

[cmaloney]

Ubuntu Github Action got a better backtrace:

Exception ignored while finalizing file <_io.BufferedWriter>:
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_io/_support.py", line 41, in writable
    def writable(self):
    
ValueError: flush of closed file
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.BufferedRWPair object at 0x200017dfb50>:
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_io/_support.py", line 41, in writable
    def writable(self):
    
ValueError: flush of closed file

[cmaloney]

Found a reproducer and filed an issue for the BufferedRWPair GC: gh-138720

[vstinner]

What do you think of test_io.utils name instead? test_asyncio, test_ast and test_interpreters packages use this name.

[cmaloney]

Works for me; This PR needs GH-138724 (or another fix for gh-138720) as that modifies a buffered test which is moved int his PR.

I see sort of three options

1. close this PR and make the "split" of utils a PR of its own (no test moving) with these two requested changes; then make the test moving a distinct PR
2. close this PR and re-make it with the new name (utils) doing the pure copy-paste after a fix for GC of unclosed io.BuffererdRWPair after .readinto results in unraisable exception #138720 is landed
3. hand-tweak inside of the copy part of this PR; I had been avoiding that to try and keep this pure-"copy paste" + "add imports"

Happy to do any of the above, my leaning is 2, but 1 would make it so more of the work of splitting test_general can be landed sooner potentially

[cmaloney]

thought of a 4:

4. Remove the test_buffered moving from this, make it just the moving to test_io.utils

[cmaloney]

Decided to implement 4

[vstinner]

Merged, thanks.