linux-ext4 - [Bug 18372] New: A filled up named pipe always has a size of zero!

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Message-ID: <bug-18372-13602@https.bugzilla.kernel.org/>
Date:	Sun, 12 Sep 2010 21:47:04 GMT
From:	bugzilla-daemon@...zilla.kernel.org
To:	linux-ext4@...r.kernel.org
Subject: [Bug 18372] New: A filled up named pipe always has a size of zero!

https://bugzilla.kernel.org/show_bug.cgi?id=18372

           Summary: A filled up named pipe always has a size of zero!
           Product: File System
           Version: 2.5
    Kernel Version: 2.6.32-22-generic (also older and newer ones)
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: ext4
        AssignedTo: fs_ext4@...nel-bugs.osdl.org
        ReportedBy: tom.bzliknl83947@...ine.de
                CC: tom.bzliknl83947@...ine.de
        Regression: No

Hi, there!

The size of a filled named pipe is always zero. This is wrong! A read operation
is going to hang for try to read from an empty pipe. To go around this trap it
is necessary to do a conditional test on the filesize, [ -s pipe ]. Problem:
The test doesn't work, because filesize is always zero.

Try:
mknod pipe &
sleep 3600  <>pipe &
echo 'date' >pipe
[ -s pipe ] && echo "filled up"
ls -l pipe; stat pipe    # if size zero, it is wrong
[ -s pipe ] && (read val <pipe; echo $val)

This is a critical error. While parallel processing a script would be blocked.
In worst case it causes a deadlock situation. Every changing of a condition of
a file should be reported whithin the i-node. 

I found it in several disto: Ubuntu KarmicKoala 9.1, LucidLynx 10.04,
Linux Mint Debian Edition 5, Slackware 13.1 x64

Best regards
TOM

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