Re: [PATCH 1/3] qsd: Add pre-init argument parsing pass

[Markus Armbruster <armbru@redhat.com>]

Hanna Reitz <hreitz@redhat.com> writes:

> We want to add a --daemonize argument to QSD's command line.

Why?

>                                                               This will
> require forking the process before we do any complex initialization
> steps, like setting up the block layer or QMP.  Therefore, we must scan
> the command line for it long before our current process_options() call.

Can you explain in a bit more detail why early forking is required?

I have a strong dislike for parsing more than once...

> Instead of adding custom new code to do so, just reuse process_options()
> and give it a @pre_init_pass argument to distinguish the two passes.  I
> believe there are some other switches but --daemonize that deserve
> parsing in the first pass:
>
> - --help and --version are supposed to only print some text and then
>   immediately exit (so any initialization we do would be for naught).
>   This changes behavior, because now "--blockdev inv-drv --help" will
>   print a help text instead of complaining about the --blockdev
>   argument.
>   Note that this is similar in behavior to other tools, though: "--help"
>   is generally immediately acted upon when finding it in the argument
>   list, potentially before other arguments (even ones before it) are
>   acted on.  For example, "ls /does-not-exist --help" prints a help text
>   and does not complain about ENOENT.
>
> - --pidfile does not need initialization, and is already exempted from
>   the sequential order that process_options() claims to strictly follow
>   (the PID file is only created after all arguments are processed, not
>   at the time the --pidfile argument appears), so it makes sense to
>   include it in the same category as --daemonize.
>
> - Invalid arguments should always be reported as soon as possible.  (The
>   same caveat with --help applies: That means that "--blockdev inv-drv
>   --inv-arg" will now complain about --inv-arg, not inv-drv.)
>
> Note that we could decide to check only for --daemonize in the first
> pass, and defer --help, --version, and checking for invalid arguments to
> the second one, thus largely keeping our current behavior.  However,
> this would break "--help --daemonize": The child would print the help
> text to stdout, which is redirected to /dev/null, and so the text would
> disappear.  We would need to have the text be printed to stderr instead,
> and this would then make the parent process exit with EXIT_FAILURE,
> which is probably not what we want for --help.
>
> This patch does make some references to --daemonize without having
> implemented it yet, but that will happen in the next patch.
>
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>