bpo-43260: io: Prevent large data remains in textio buffer.

[methane]

When very large data remains in TextIOWrapper, it can not flush the internal
buffer forever because of MemoryError.

bpo-43260

[eryksun]

This needs a test in a subprocess. In POSIX the maximum available address space or heap size can be easily controlled. For example:

>>> import resource
>>> hard_limit = resource.getrlimit(resource.RLIMIT_AS)[1]
>>> resource.setrlimit(resource.RLIMIT_AS, (128 * (1<<20), hard_limit))
>>> f = open('test.txt', 'w')
>>> f.write('a' * (64 * (1<<20)))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
MemoryError
>>> f.write('a')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
MemoryError

It's possible in Windows with kernel Job objects, but Python's standard library doesn't support them, not without ctypes.

[methane]

@eryksun I don't like such tests. Such tests are fragile.

[methane]

Having buffer in Pure Python TextIOWrapper doesn't make sense. It is just an overhead.

[eryksun]

Having buffer in Pure Python TextIOWrapper doesn't make sense. It is just an overhead.

That's fine. I just wanted to know if there was more work to do in order to align this behavior with _pyio.

_pyio.TextIOWrapper is intentionally designed to behave as if write_through is always enabled. I do not understand this decision, but it is what it is. To me it seems that the optimization to internally buffer small writes is fundamental to the behavior of io.TextIOWrapper, i.e. it is not an internal implementation detail since it fundamentally alters the behavior. So it should be emulated by _pyio regardless of the effect on performance at this level.

[methane]

To me it seems that the optimization to internally buffer small writes is fundamental to the behavior of io.TextIOWrapper, i.e. it is not an internal implementation detail since it fundamentally alters the behavior. So it should be emulated by _pyio regardless of the effect on performance at this level.

I don't think so. There is a BufferedIO under the TextIOWrapper. No need to add one additonal buffer layer.

C implementation of TextIOWrapper is fast. Since it is very fast, calling BufferedIO.write() is slower than appending internal buffer.

On the other hand, Python implementation is not so fast. Calling overhead is not so large. So adding one more buffering layer isn't worth enough.

[eryksun]

On the other hand, Python implementation is not so fast. Calling overhead is not so large. So adding one more buffering layer isn't worth enough.

Again, it's nothing to do with performance, but instead matching the observable behavior of io.TextIOWrapper with regard to when and how much it writes to the underlying buffer, which I expected _pyio.TextIOWrapper to do for the sake of valid experimentation. When I asked the question, I wasn't aware that it's intentionally designed to deviate from the observable behavior of io.TextIOWrapper in this case.

[miss-islington]

Thanks @methane for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

[miss-islington]

Sorry @methane, I had trouble checking out the 3.9 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 01806d5beba3d208bb56adba6829097d803bf54f 3.9

[miss-islington]

Sorry, @methane, I could not cleanly backport this to 3.8 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 01806d5beba3d208bb56adba6829097d803bf54f 3.8

[bedevere-bot]

GH-24617 is a backport of this pull request to the 3.9 branch.

[miss-islington]

Thanks @methane for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[miss-islington]

Sorry @methane, I had trouble checking out the 3.8 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 01806d5beba3d208bb56adba6829097d803bf54f 3.8

[bedevere-bot]

GH-24618 is a backport of this pull request to the 3.8 branch.