gh-129205: Add os.readinto API for reading data into a caller provided buffer

[cmaloney]

Adds os.readinto API as well as associated tests. Most functionality is provided by _Py_read which is fairly well exercised by os.read tests.

For testing I have been running:

make buildbottest python -bb -E -Wd -m test -M32g -uall test_os _test_eintr -vvv

I skipped adding to test_largefile, that path should get exercised in part by test_os.FileTests.test_large_readinto, and will get exercised reading 2GB+ of bytes as move _pyio to use os.readinto (test_largefile.PyLargeFileTest).

Not sure how to make a negative length buffer for testing that case getting handled as expected (_Py_read takes a size_t and Py_buffer.len is a Py_ssize_t)

Should probably have buildbots run once reviewed. This should work across platforms, but I've only tested on Linux and things like large file tests aren't run by all bots.

• Issue: Add os.readinto API for reading data into a caller provided buffer #129205

[cmaloney]

@vstinner ready for you to take a look

[picnixz]

Some test nits

[cmaloney]

note: Clinic wrapping code takes -1 return + PyErr_Occurred to indicate error and will return a NULL PyObject* for the function. So should never get back a PyLong object which is negative.

[vstinner]

LGTM. Thanks for the update, the implementation now looks better :-)

[picnixz]

Just some wording comments and LGTM.

[picnixz]

Suggested change

	datas= [b"hello", b"world", b"spam"]
	bufs= [bytearray(5), bytearray(5), bytearray(4)]
	code='\n'.join((
	'import os, sys, time',
	'',
	'wr = int(sys.argv[1])',
	'datas = %r'%datas,
	'sleep_time = %r'%self.sleep_time,
	'',
	'for data in datas:',
	' # let the parent block on read()',
	' time.sleep(sleep_time)',
	' os.write(wr, data)',
	))
	data= [b"hello", b"world", b"spam"]
	bufs= [bytearray(5), bytearray(5), bytearray(4)]
	code='\n'.join((
	'import os, sys, time',
	'',
	'wr = int(sys.argv[1])',
	f'data = {data!r}',
	f'sleep_time = {self.sleep_time!r}',
	'',
	'for datum in data:',
	' # let the parent block on read()',
	' time.sleep(sleep_time)',
	' os.write(wr, datum)',
	))

Strictly speaking, data is already the plural form of datum.

[cmaloney]

For data -> datum the code is copied from test_read just before with minor tweaks; I think the two being very close in code is useful for maintenance more than naming. I think I can get rid of bufs and zip though which will remove the ambiguities / make the code read better

[vstinner]

Merged, thank you!

[vstinner]

As the initial author of test_eintr, I'm likely the one who wrote datas typo. So I wrote PR gh-129316 to fix grammar and modernize the code base (use f-strings) 😁