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Date: Tue, 18 Feb 2014 10:00:36 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20140218090036.GA32585@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 2/3] qtest: make QEMU our direct child
 process
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To: Markus Armbruster <armbru@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Andreas Faerber <afaerber@suse.de>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, Anthony Liguori <aliguori@amazon.com>

On Mon, Feb 17, 2014 at 05:44:55PM +0100, Markus Armbruster wrote:
> Stefan Hajnoczi <stefanha@redhat.com> writes:
> 
> > qtest_init() cannot use exec*p() to launch QEMU since the exec*p()
> > functions take an argument array while qtest_init() takes char
> > *extra_args.  Therefore we execute /bin/sh -c <command-line> and let the
> > shell parse the argument string.
> >
> > This left /bin/sh as our child process and our child's child was QEMU.
> > We still want QEMU's pid so the -pidfile option was used to let QEMU
> > report its pid.
> >
> > The pidfile needs to be unlinked when the test case exits or fails.  In
> > other words, the pidfile creates a new problem for us!
> >
> > Simplify all this using the shell 'exec' command.  It allows us to
> > replace the /bin/sh process with QEMU.  Then we no longer need to use
> > -pidfile because we already know our fork child's pid.
> >
> > Note: Yes, it seems silly to exec /bin/sh when we could just exec QEMU
> > directly.  But remember qtest_init() takes a single char *extra_args
> > command-line fragment instead of a real argv[] array, so we need
> > /bin/sh's argument parsing behavior.
> 
> Sounds like a design mistake to me.

I wouldn't call char *extra_args a mistake because strings are still
simpler to manipulate in C than char *argv[] arrays.  So we write less
code in test cases and libqtest.c at the expense of a roundabout way to
spawn the process.

Stefan