Revert "gh-129005: _pyio.BufferedIO remove copy on readall (#129454)"

[cmaloney]

This reverts commit e1c4ba9.

This seems to be the commit where bots started failing. Not sure why.

cc: @vstinner

• Issue: Reduce copies when reading files in pyio, match behavior of _io #129005

[vstinner]

Ok, let's revert until the issue is understood and fixed.

[vstinner]

Example of failure, PPC64LE Fedora Stable 3.x: https://buildbot.python.org/#/builders/90/builds/5505

ERROR: test_nonblock_pipe_write_bigbuf (test.test_io.PyMiscIOTest.test_nonblock_pipe_write_bigbuf)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1250, in write
    self._flush_unlocked()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1282, in _flush_unlocked
    raise BlockingIOError(
        errno.EAGAIN,
        "write could not complete without blocking", 0)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4453, in _test_nonblock_pipe_write
    wf.write(msg)
    ~~~~~~~~^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1258, in write
    raise BlockingIOError(e.errno, e.strerror, written)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1250, in write
    self._flush_unlocked()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1282, in _flush_unlocked
    raise BlockingIOError(
        errno.EAGAIN,
        "write could not complete without blocking", 0)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4462, in _test_nonblock_pipe_write
    wf.write(msg)
    ~~~~~~~~^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1258, in write
    raise BlockingIOError(e.errno, e.strerror, written)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4421, in test_nonblock_pipe_write_bigbuf
    self._test_nonblock_pipe_write(16*1024)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4446, in _test_nonblock_pipe_write
    with rf, wf:
             ^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 482, in __exit__
    self.close()
    ~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1309, in close
    self.flush()
    ~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1270, in flush
    self._flush_unlocked()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1282, in _flush_unlocked
    raise BlockingIOError(
        errno.EAGAIN,
        "write could not complete without blocking", 0)
BlockingIOError: [Errno 11] write could not complete without blocking

======================================================================
ERROR: test_nonblock_pipe_write_smallbuf (test.test_io.PyMiscIOTest.test_nonblock_pipe_write_smallbuf)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1250, in write
    self._flush_unlocked()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1282, in _flush_unlocked
    raise BlockingIOError(
        errno.EAGAIN,
        "write could not complete without blocking", 0)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4453, in _test_nonblock_pipe_write
    wf.write(msg)
    ~~~~~~~~^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1258, in write
    raise BlockingIOError(e.errno, e.strerror, written)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1250, in write
    self._flush_unlocked()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1282, in _flush_unlocked
    raise BlockingIOError(
        errno.EAGAIN,
        "write could not complete without blocking", 0)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4462, in _test_nonblock_pipe_write
    wf.write(msg)
    ~~~~~~~~^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1258, in write
    raise BlockingIOError(e.errno, e.strerror, written)
BlockingIOError: [Errno 11] write could not complete without blocking

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4427, in test_nonblock_pipe_write_smallbuf
    self._test_nonblock_pipe_write(1024)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/test/test_io.py", line 4446, in _test_nonblock_pipe_write
    with rf, wf:
             ^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 482, in __exit__
    self.close()
    ~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1309, in close
    self.flush()
    ~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1270, in flush
    self._flush_unlocked()
    ~~~~~~~~~~~~~~~~~~~~^^
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-ppc64le/build/Lib/_pyio.py", line 1282, in _flush_unlocked
    raise BlockingIOError(
        errno.EAGAIN,
        "write could not complete without blocking", 0)
BlockingIOError: [Errno 11] write could not complete without blocking

[cmaloney]

The test does a lot of writes until it blocks, catches that exception, then does a "readall" from the pipe (to get it so a write shouldn't block) followed by a small write of "BLOCKED" which is expected to never block. That write blocks, which raises an exception and gets the pile of stack traces. As the test exits, close / flush happens on each of the objects which tries to flush the buffers, resulting in more writes, which also block (and get to the full set of exceptions).

Makes it feel to me like either the .read() / .readall() isn't emptying the pipe, or the fact the pipe is no longer full isn't updating before the next write() occurs. Nothing concrete though. The shortcut made a bit faster, but I'm pretty sure _io.BufferedIO is still faster than _pyio.BufferedIO, and _io.BufferedIO is also run through this test...)