[Nepomuk] The zombie processes bug ( 302143 )

[Vishesh Handa]

Hey David.

Do you think you could please look at this?


On Wed, Dec 12, 2012 at 7:58 AM, Simeon Bird <bladud at gmail.com> wrote:

> ( the report is https://bugs.kde.org/show_bug.cgi?id=302143 )
>
> The last couple of days (not sure why: I think triggered somehow by the
> virtuoso deadlocks Vishesh posted a patch for recently)
> I started hitting the nepomuk zombie processes bug, and so I figured this
> was a good opportunity to debug it.
>
> Turns out the root cause is a (quite silly) QProcess bug. I found the
> source here:
> http://qt.gitorious.org/qt/qt/blobs/4.8/src/corelib/io/qprocess_unix.cpp
> The short version is: QProcess doesn't check errors properly.
>
> The longer version:
>
> When QProcess->start() is called, Qt creates a pipe to the process to get
> its exit value and output.
> It does this with qt_create_pipe, which calls qt_safe_pipe. qt_safe_pipe,
> on failure, returns 1.
> If this happens, qt_create_pipe fails, sets errno, prints a warning:
> [/usr/bin/nepomukservicestub] QProcessPrivate::createPipe: Cannot create
> pipe 0x1987228: Too many open files
> and returns void, carefully ignoring the error.
>
> The calling function, QProcessPrivate::startProcess, does not check errno,
> and thus continues on its merry
> way assuming the pipe has been created successfully, and creates a
> QSocketNotifier with it.
> Since the pipe is not valid, this fails and prints a warning:
> [/usr/bin/nepomukservicestub] QSocketNotifier: Invalid socket specified
>
> The calling process again does not check for an error, continues on its
> merry way and
> forks off the child process (incidentally obliterating the value of errno
> from qt_create_pipe).
> Note that since the child process is actually created correctly, no
> QProcess error is set,
> so we can't fix it by checking for error().
>
> The child process then has no way to pass its exit value to the calling
> process, since the
> communication pipes it would normally use do not exist, and thus when it
> exits it becomes
> a zombie.
>
> As a bonus, once the first timeout timer for a broken process happens,
> waitForFinished is called,
> which crashes, because it is trying to wait on a pipe which does not exist.
> (This was reported with a patch a year ago, but not fixed:
> https://bugreports.qt-project.org/browse/QTBUG-18934 )
>
> There is another KDE bug which seems to have the same root cause:
> https://bugs.kde.org/show_bug.cgi?id=252602
>
> So far as I can see, this really needs to be fixed in QProcess.
> The fix would, I guess, make qt_create_pipe return an integer, and
> then have startProcess check the return value,
> set processError and abort.
>
> Can this be done in a reasonable timeframe? Does anyone know how to submit
> Qt patches?
>
> Simeon
>
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>
>


-- 
Vishesh Handa
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