Error on a 32-bit buildbot worker where ssize_t maximum = 2,147,483,647 (2**31-1) bytes = ~2.0 GiB.

test_largefile uses:

# size of file to create (>2 GiB; 2 GiB == 2,147,483,648 bytes)
size = 2_500_000_000

x86 Gentoo Installed with X 3.x:
https://buildbot.python.org/all/#/builders/103/builds/3162

======================================================================
ERROR: test_it (test.test_largefile.TestCopyfile)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "/buildbot/buildarea/cpython/3.x.ware-gentoo-x86.installed/build/target/lib/python3.9/test/test_largefile.py", line 160, in test_it
    shutil.copyfile(TESTFN, TESTFN2)
  File "/buildbot/buildarea/cpython/3.x.ware-gentoo-x86.installed/build/target/lib/python3.9/shutil.py", line 266, in copyfile
    _fastcopy_sendfile(fsrc, fdst)
  File "/buildbot/buildarea/cpython/3.x.ware-gentoo-x86.installed/build/target/lib/python3.9/shutil.py", line 145, in _fastcopy_sendfile
    sent = os.sendfile(outfd, infd, offset, blocksize)
OverflowError: Python int too large to convert to C ssize_t

On Linux (Fedora 30), man sendfile shows me the prototype:

   ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);

Extract of Lib/shutil.py:

# Hopefully the whole file will be copied in a single call.
# sendfile() is called in a loop 'till EOF is reached (0 return)
# so a bufsize smaller or bigger than the actual file size
# should not make any difference, also in case the file content
# changes while being copied.
try:
    blocksize = max(os.fstat(infd).st_size, 2 ** 23)  # min 8MB
except Exception:
    blocksize = 2 ** 27  # 128MB

    offset = 0
    while True:
        try:
            sent = os.sendfile(outfd, infd, offset, blocksize)
        except OSError as err:
            ...
        else:
            if sent == 0:
                break  # EOF
            offset += sent

Extract of the Linux implementation of os.sendfile():

    int in, out;
    Py_ssize_t ret;
    off_t offset;
    ...
    Py_ssize_t count;
    PyObject *offobj;
    static char *keywords[] = {"out", "in",
                                "offset", "count", NULL};
    if (!PyArg_ParseTupleAndKeywords(args, kwdict, "iiOn:sendfile",
            keywords, &out, &in, &offobj, &count))
        return NULL;
    ...
    if (!Py_off_t_converter(offobj, &offset))
        return NULL;

do {
    Py_BEGIN_ALLOW_THREADS
    ret = sendfile(out, in, &offset, count);
    Py_END_ALLOW_THREADS
} while (ret < 0 && errno == EINTR && !(async_err = PyErr_CheckSignals()));

with:

static int
Py_off_t_converter(PyObject *arg, void *addr)
{
#ifdef HAVE_LARGEFILE_SUPPORT
    *((Py_off_t *)addr) = PyLong_AsLongLong(arg);
#else
    *((Py_off_t *)addr) = PyLong_AsLong(arg);
#endif
    if (PyErr_Occurred())
        return 0;
    return 1;
}

I understand that the error comes from the 4th sendfile() parameter: "count". The C code (of the Linux implementation) uses the "n" format for Py_ssize_t: Python/getargs.c calls PyLong_AsSsize_t().

On a 64-bit system, it's less likely to reach Py_ssize_t maximum value (max = 2**63-1), but it's easy to reach on a 32-bit system (max = 2**31-1).

Oh, it's likely a regression caused by:

commit 5bcc6d8
Author: Giampaolo Rodola <g.rodola@gmail.com>
Date: Mon Sep 30 12:51:55 2019 +0800

bpo-37096: Add large-file tests for modules using sendfile(2) (GH-13676)

https://buildbot.python.org/all/#/builders/103/builds/3162

configure:

checking for sendfile... yes
checking whether to enable large file support... yes

pythoninfo:

platform.libc_ver: glibc 2.29
platform.platform: Linux-4.19.72-gentoo-i686-AMD_Athlon-tm-_64_X2_Dual_Core_Processor_5000+-with-glibc2.29

Similar failure on ARMv7 Debian buster 3.x:
https://buildbot.python.org/all/#/builders/176/builds/1372

pythoninfo:
sys.maxsize: 2147483647

New changeset 94e1650 by Giampaolo Rodola in branch 'master':
bpo-38319: Fix shutil._fastcopy_sendfile(): set sendfile() max block size (GH-16491)
94e1650

Looks like it worked:
https://buildbot.python.org/all/#/builders/176/builds/1383

New changeset 938c00c by Łukasz Langa (Miss Islington (bot)) in branch '3.8':
bpo-38319: Fix shutil._fastcopy_sendfile(): set sendfile() max block size (GH-16491) (bpo-16506)
938c00c

32-bit buildbots are back to green. Thanks for the fix!