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Subject: |
df hangs on fifos/named pipes |
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Date: |
Sat, 28 Oct 2017 08:18:11 +0100 |
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User-agent: |
Mutt/1.5.24 (2015-08-30) |
test case:
mkfifo p
df p
That hangs, unless you make "p" non-readable or some other process
has the fifo open in write mode.
The reason is that df tries to open the fifo in read-only mode,
according to comments in the source code so as to trigger a
potential automout.
That goes back to this commit:
> commit dbd17157d7e693b8de9737f802db0e235ff5a3e6
> Author: Tomas Smetana <address@hidden>
> Date: Tue Apr 28 11:21:49 2009 +0200
>
> df: use open(2), not stat, to trigger automounting
>
> * src/df.c (main): When iterating over command-line arguments,
> attempting to ensure each backing file system is mounted, use
> open, not stat. stat is no longer sufficient to trigger
> automounting, in some cases. Based on a suggestion from Ian Kent.
> More details in http://bugzilla.redhat.com/497830
More info at the bugzilla link.
It's arguable whether df, a reporting tool, should have such a
side effect as automounting a file system.
The fifo issue though is a bug IMO, especially considering that
POSIX explicitely says that df should work on fifos.
Here, it may be enough to add the O_DIRECTORY flag to open()
where available if we only care about automounting files of type
directory (or portably use opendir()).
Or use O_PATH on Linux 3.6+ followed by openat() on non-fifos if
open(O_PATH) is not enough to trigger the automount in the
unlikely event we care about automounting non-directory files
(and report their disk usage).
Or not open() at all, and not automount file systems.
Note that busybox, heirloom or ast-open df implementations on
Linux don't have the problem (and presumably don't automount
file systems). Nor does FreeBSD.
Reproduced with:
$ df --version
df (GNU coreutils) 8.25
and:
$ df --version
df (GNU coreutils) 8.27.46-e13fe
That was discovered by Martijn Dekker, CCed, when looking for a
portable way to identify the file system of an arbitrary file.
--
Stephane
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--- Begin Message ---
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Subject: |
Re: bug#29038: df hangs on fifos/named pipes |
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Date: |
Mon, 30 Oct 2017 09:53:01 -0700 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 |
On 29/10/17 23:16, Paul Eggert wrote:
> Pádraig Brady wrote:
>
>> I suppose we could stat() and if that succeeds && !fifo, only then call
>> open() ?
>> Patch to do that is attached.
>
> Better is to use open with O_NONBLOCK, as this avoids interpreting the file
> name
> twice in the usual case. Better yet is to use O_PATH if available, as this
> avoids interpreting the file name twice even when the file is unreadable or
> is a
> FIFO that would block. Also, the comment just before the code should be
> changed
> to match the altered code. Proposed followup patch attached.
>
> It is puzzling that df calls fstat or stat, when it should just be calling
> fstatfs or statfs. But that is a different matter.
I pushed my change with the comments fixed up.
I thought about O_NONBLOCK and O_PATH but thought these might not
induce or wait for the auto mount to occur.
thanks for the review,
Pádraig
marking this as done...
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