gh-154324: Fix os.sendfile() error reporting on illumos

[serhiy-storchaka]

illumos sendfile(3EXT) returns the number of transferred bytes by adding it to *off without initializing it, so the offset cannot be used to detect a partial write when the call fails before writing any data, and errors were reported as successful transfers.

Use sendfilev(3EXT) instead, which reports the number of transferred bytes in an explicit out parameter.

• Issue: os.sendfile() can return a bogus number of transferred bytes on illumos #154324

[serhiy-storchaka]

@kulikjak, could you please look at this and verify if it works on real Solaris?

[kulikjak]

This one is Illumos specific - sendfile correctly returns zero in this case on Solaris.

$> LD_LIBRARY_PATH=./build/sparcv9/ ./build/sparcv9/python
Python 3.16.0a0 (main, Jul 22 2026, 10:55:24) [GCC 15.2.0] on sunos5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> fd = os.open("file", os.O_RDONLY)   # not writable, sendfile() fails with EBADF
>>> os.sendfile(fd, fd, 0, 65536)
0

sendfile on Illumos does work differently than on Oracle Solaris - the "sendfile raises EINVAL rather than returning 0" part applies to both, but the "On illumos specifically sendfile() may perform a partial write ..." part (added with #23154) does not apply to us. (We considered hiding it behind Illumos specific #ifdef, but gcc does not differentiate between these two platforms, so we would need some detection in configure, and since it works on both (just does nothing on Oracle Solaris), we kept it like this).

I again tested it with your change on our end, and all tests still pass.

I never tried using sendfilev - maybe it won't need the first part either? I will test that.

[serhiy-storchaka]

I tested sendfilev() at and past EOF on illumos, and it still needs the first part:

file size 10
offset=0    count=4096   sendfilev ret=-1 EINVAL xferred=10
offset=10   count=4096   sendfilev ret=-1 EINVAL xferred=0
offset=110  count=4096   sendfilev ret=-1 EINVAL xferred=0

At offset >= size it fails with EINVAL and reports nothing transferred, the same as sendfile().

The two parts are complementary: the fstat() check turns EOF into 0, and xferred makes the first line report 10 instead of an error.