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

[Markus Armbruster]

Hanna Reitz <hreitz@redhat.com> writes:

> On 21.01.22 15:26, Markus Armbruster wrote:
>> Hanna Reitz <hreitz@redhat.com> writes:
>>
>>> On 21.01.22 11:27, Markus Armbruster wrote:
>>>> Hanna Reitz <hreitz@redhat.com> writes:
>>>>> The problem I face is that currently there is no ergonomic way to wait
>>>>> until the QSD is up and running (besides looping until the PID file
>>>>> exists), and I don’t think a utility program that doesn’t know the QSD
>>>>> could provide this.  (For example, it looks like daemonize(1) will
>>>>> have the parent exit immediately, regardless of whether the child is
>>>>> set up or not.)
>>>>
>>>> Why do you need to wait for QSD to be ready?
>>>>
>>>> I'm asking because with common daemons, I don't wait, I just connect to
>>>> their socket and start talking.  They'll reply only when ready.
>>>>
>>> That only applies when you want to talk to a socket, which I often
>>> don’t do.  Most of the time I use the storage daemon, I pass all
>>> --blockdev and --export options through the command line and don’t
>>>   create any socket at all.  When I use the QSD just to export some
>>> block device, I generally don’t need QMP.
>>
>> If you export via NBD, why can't you just connect to NBD socket?
>
> I’m not sure what exactly you mean by this, because the socket doesn’t
> exist before the QSD is launched.  If I launch the QSD in the
> background and have it create an NBD server on a Unix socket, then
> this socket will not exist until the respective --nbd-server option is
> parsed.  Trying to connect to it immediately after the QSD has been
> launched may work (if the QSD was quicker to parse the option and
> create the server than me trying to connect) or may yield ECONNREFUSED
> or ENOENT, depending on whether the socket file existed before or not.

This is similar to "with common daemons, [...] I just connect to their
socket and start talking."

> Also, outside of the iotests, I personally generally usually use FUSE
> exports instead of NBD exports.

You could wait for the mount to appear with stat -f.