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

Gregory Szorc gregory.szorc at gmail.com
Mon Nov 14 18:49:06 EST 2016


On Mon, Nov 14, 2016 at 3:35 PM, Jun Wu <quark at fb.com> wrote:

> # HG changeset patch
> # User Jun Wu <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
> #              hg pull 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.


>
> diff --git a/mercurial/util.py b/mercurial/util.py
> --- a/mercurial/util.py
> +++ b/mercurial/util.py
> @@ -2191,4 +2191,9 @@ def wrap(line, width, initindent='', han
>      return wrapper.fill(line).encode(encoding.encoding)
>
> +def iterfile(fp):
> +    """like fp.__iter__ but does not have issues with EINTR. Python
> 2.7.12 is
> +    known to have such issues."""
> +    return iter(fp.readline, '')
> +
>  def iterlines(iterator):
>      for chunk in iterator:
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