The WASI and x86 Windows failures are around large readall on a large zip file, working on figuring out what I changed / broke for those cases.

See also#90102.

@serhiy-storchaka
Cool! Hadn't realized there were a couple attempts at that previously. This doesn't change the behavior ofisatty call at all, and the isatty check is only optimized during__init__ so I think is safe in terms of what didn't quite work with#112495 (changes to the underlying file/fd after open; the stat + don't call isatty is inside the same single library call in this PR)

I don't have a strong preference around my "is regular" check vs. S_ISCHR (#112495) vs. Size. If there's one which is best to use or a combo happy to implement that.

The test_zipimport highlighted a couple issues to me which I think should be solved, but separately from this (which keeps the behavior from before this PR):

1. If the size is large, then this code with slight hard to notice/read changes (which does break buildbots) allocates larger than the max byte string size

   cpython/Objects/bytesobject.c

   Lines 86 to 90 in690b935

   	if ((size_t)size> (size_t)PY_SSIZE_T_MAX-PyBytesObject_SIZE) {
   	PyErr_SetString(PyExc_OverflowError,
   	"byte string is too large");
   	returnNULL;
   	}

   vs.

   cpython/Include/internal/pycore_fileutils.h

   Lines 65 to 76 in690b935

   	#if defined(MS_WINDOWS)|| defined(__APPLE__)
   	/* On Windows, the count parameter of read() is an int (bpo-9015, bpo-9611).
   	On macOS 10.13, read() and write() with more than INT_MAX bytes
   	fail with EINVAL (bpo-24658). */
   	# define_PY_READ_MAX INT_MAX
   	# define_PY_WRITE_MAX INT_MAX
   	#else
   	/* write() should truncate the input to PY_SSIZE_T_MAX bytes,
   	but it's safer to do it ourself to have a portable behaviour */
   	# define_PY_READ_MAX PY_SSIZE_T_MAX
   	# define_PY_WRITE_MAX PY_SSIZE_T_MAX
   	#endif

   
   This is worked around currently by "if the size is large, always use a dynamically resized buffer" and the end reads end up being small (ex. 1 byte fortest_largefile). It feels like that's pointing out two things which I would like to solve with a separate PRs (This keeps the same behavior around PY_READ_MAX + estimated size as before this PR)
   1. It would be good to have a test intest_fileio around this behavior, rather than relying on other modules. In specific, on a very large / max size file a seek to near the end of the file then do a.read(). Also truncate the file then do a read.
   2. The zipfile module relies on read without a size after seeking to a certain distance from the end of the file, and discards if size doesn't match what it expects exactly. That would work more efficiently by doing a fixed-size read

2. In the CPython codebase doing a seek after open then doing a readall / read without size occurs multiple times, which likely means it is common other places as well

   1. Should improve behavior for that (Likely either seek and truncate should invalidate the cached size or some form of general "if large readall must call fstat to get up to date info")
   2. Should add a test that we don't over-allocate which can lead to an OOM on smaller memory boxes in these cases (is what broke the bot ingh-120754: Update estimated_size in C truncate #121357)

3. The dynamic buffer resizing doesn't pay attention to the max PyBytes size and should be improved. Currently it tries allocating _PY_READ_MAX which is bigger than the biggest possible bytes resulting in a bytes too large failure).

Created PRs to improve

1. zipfile to not do unbounded reads --gh-113977, gh-120754: Remove unbounded reads from zipfile #122101
2. explicit I/O tests around seek + readall on a large file (and comment on the existing truncate + readall) --GH-120754: Add more tests around seek + readall #122103

@vstinner I have opened a new PRGH-123412 which contains just the refactor tostat_atopen member, memory allocation management of that new member. It has two commits, one for_io and a second for_pyio, trying to keep those in sync but allowing the changes to hopefully be looked at side to side. I'll work on a new PR to skip checkingisatty for common regular files on top of that.