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