[PATCH 1 of 6] util: add iterfile to workaround a fileobj.__iter__ issue with EINTR

[Pierre-Yves David]

On 11/14/2016 11:49 PM, Gregory Szorc wrote:
> On Mon, Nov 14, 2016 at 3:35 PM, Jun Wu <quark at fb.com
> <mailto:quark at fb.com>> wrote:
>
>     # HG changeset patch
>     # User Jun Wu <quark at fb.com <mailto:quark at fb.com>>
>     # Date 1479166374 0
>     #      Mon Nov 14 23:32:54 2016 +0000
>     # Node ID 7e3bb7754d338399dee3ee41770b7d6624b81fc1
>     # Parent  038547a14d850f14ecd2671852093dc07848a134
>     # Available At https://bitbucket.org/quark-zju/hg-draft
>     <https://bitbucket.org/quark-zju/hg-draft>
>     #              hg pull https://bitbucket.org/quark-zju/hg-draft
>     <https://bitbucket.org/quark-zju/hg-draft> -r 7e3bb7754d33
>     util: add iterfile to workaround a fileobj.__iter__ issue with EINTR
>
>     The fileobj.__iter__ implementation in Python 2.7.12 (hg changeset
>     45d4cea97b04) is buggy: it cannot handle EINTR correctly.
>
>     In Objects/fileobject.c:
>
>         size_t Py_UniversalNewlineFread(....) {
>             ....
>             if (!f->f_univ_newline)
>                 return fread(buf, 1, n, stream);
>             ....
>         }
>
>     According to the "fread" man page:
>
>         If an error occurs, or the end of the file is reached, the
>     return value
>         is a short item count (or zero).
>
>     Therefore it's possible for "fread" (and "Py_UniversalNewlineFread") to
>     return a positive value while errno is set to EINTR and ferror(stream)
>     changes from zero to non-zero.
>
>     There are multiple "Py_UniversalNewlineFread": "file_read",
>     "file_readinto",
>     "file_readlines", "readahead". While the first 3 have code to handle the
>     EINTR case, the last one "readahead" doesn't:
>
>         static int readahead(PyFileObject *f, Py_ssize_t bufsize) {
>             ....
>             chunksize = Py_UniversalNewlineFread(
>                 f->f_buf, bufsize, f->f_fp, (PyObject *)f);
>             ....
>             if (chunksize == 0) {
>                 if (ferror(f->f_fp)) {
>                     PyErr_SetFromErrno(PyExc_IOError);
>                     ....
>                 }
>             }
>             ....
>         }
>
>     It means "readahead" could ignore EINTR, if "Py_UniversalNewlineFread"
>     returns a non-zero value. And at the next time "readahead" got
>     executed, if
>     "Py_UniversalNewlineFread" returns 0, "readahead" would raise a
>     Python error
>     without a incorrect errno - could be 0 - thus "IOError: [Errno 0]
>     Error".
>
>     The only user of "readahead" is "readahead_get_line_skip".
>     The only user of "readahead_get_line_skip" is "file_iternext", aka.
>     "fileobj.__iter__", which should be avoided.
>
>     There are multiple places where the pattern "for x in fp" is used. This
>     patch adds a "iterfile" method in "util.py" so we can migrate our
>     code from
>     "for x in fp" to "fox x in util.iterfile(fp)".
>
>
> Bleh. Is this bug reported upstream? (Not that it helps us much.)
>
> This seems like a pretty trivial workaround. So LGTM.

This seems okay and I trust Greg review here. This is pushed.

-- 
Pierre-Yves David