Re: [Qemu-devel] [PATCH 2/3] qtest: make QEMU our direct child process

[Stefan Hajnoczi <stefanha@gmail.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