gh-90102: Remove isatty call during regular open

[cmaloney]

TTYs are always character devices. If the interpreter knows a file is not a character device when it would call isatty, skip the isatty call. Inside open() in the same python library call there is a fresh stat result that contains that information. Use the stat result to skip a system call.

This shortcut was suggested by @eryksun in 2021. isatty is not necessarily constant over time, but inside the python library call which opened the fd and has yet to return the fd, it seems reasonable to rely on the fd not changing. The fd may be visible to caller code which passes in an opener or intercepts specific calls.

For gh-120754, with this change reading text with open('README.md').read() is down to 6 system calls:

openat(AT_FDCWD, "README.rst", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=8921, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "This is Python version 3.14.0 al"..., 8922) = 8921
read(3, "", 1)                          = 0
close(3)                                = 0

When reading bytes with buffering=0 (disabling BufferedIO which remoevs the lseek), reading a regular file is down to 5 system calls (strace python -c "from pathlib import Path; Path('README.rst').read_bytes()")

Benchmark

Run on a 2024 MacBook Air running Squoia 15.0

Benchmark code

import pyperf
from pathlib import Path


def read_all(all_paths):
    for p in all_paths:
        # note: Using open rather than pathlib since 
        # pathlib.read_bytes() skips BufferedIO and that makes a
        # significant performance delta around the reduced isatty 
        # requirement. Reading bytes rather than text since at 
        # least one `.py` file in tree results in UnicodeDecodeError
        open(p, 'rb').read()


def read_file(path_obj):
    path_obj.read_text()


all_rst = list(Path("Doc").glob("**/*.rst"))
all_py = list(Path(".").glob("**/*.py"))
assert all_rst, "Should have found rst files"
assert all_py, "Should have found python source files"


runner = pyperf.Runner()
runner.bench_func("read_file_small", read_file, Path("Doc/howto/clinic.rst"))
runner.bench_func("read_file_large", read_file, Path("Doc/c-api/typeobj.rst"))

runner.bench_func("read_all_rst", read_all, all_rst)
runner.bench_func("read_all_py", read_all, all_py)

Before:

.....................
read_file_small: Mean +- std dev: 8.30 us +- 0.09 us
.....................
read_file_large: Mean +- std dev: 21.1 us +- 0.2 us
.....................
read_all_rst: Mean +- std dev: 4.42 ms +- 0.05 ms
.....................
read_all_py: Mean +- std dev: 19.5 ms +- 0.2 ms

After:

.....................
read_file_small: Mean +- std dev: 7.78 us +- 0.07 us
.....................
read_file_large: Mean +- std dev: 20.8 us +- 0.4 us
.....................
read_all_rst: Mean +- std dev: 4.16 ms +- 0.05 ms
.....................
read_all_py: Mean +- std dev: 18.5 ms +- 0.3 ms

---

For files which are recently read / in OS filecache (or on fast devices), there is a ~15% performance overhead with BufferedIO (see: #122111 where I updated Path.read_bytes() and measured the change). Unfortunately multiple attempts I've made to do small reworks to make the lseek unnecessary in BufferedIO have resulted in no performance change and a lot of complexity.

I have been developing a more broad refactoring that could reduce the BufferedIO overhead as well as several other I/O overheads while meeting current API expectations (ex. each layer of the stack re-figures out readable and writeable, every call must re-validate with many branches the fd state and arguments, _pyio needs to copy at least once in user-space because os.read can't be passed a buffer / always allocates a new one, etc.).

I'm planning to put together a talk on "Journey to the center of open().read()" and submit it to present at a San Francisco bay area python meetup since as I've been working on this I've found it's a very intricate set of operations which didn't match my mental image in some interesting ways. Hope is to then do a second talk with sample working implementation which shows how could rework internals while keeping the existing Python I/O API to reduce overheads, increase readability, solve some longstanding bugs, and possibly enable usage of io_uring for more performance improvement. Overarching goal would be to get down to one largely python native I/O implementation with improved performance from the optimizations as well as opening new performance improvement avenues.

@vstinner This replaces #121593 on top of #123412

• Issue: Avoid calling isatty() for most open() calls #90102

[vstinner]

LGTM

[vstinner]

Merged, thank you. At the first read, I didn't understand the relationship between S_ISCHR() and isatty(). But your comment makes sense and explains it correctly.

[ngnpope]

I think this should be:

Suggested change

	return True
	return False

[cmaloney]

Yep, Py_RETURN_FALSE; I got right in the C but not the pyio. Making a new PR to update....

[cmaloney]

#125089