Re: [Qemu-devel] [libvirt] [PATCH v6 2/2] vl: fix use of --daemonize with --preconfig

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Jun 12, 2018 at 01:50:33PM +0100, Daniel P. Berrangé wrote:
> On Tue, Jun 12, 2018 at 02:42:05PM +0200, Igor Mammedov wrote:
> > We can keep daemonizing flow in QEMU as it's now.
> > But Eduardo's idea about libvirt created socked + letting QEMU connect to it
> > has a merit. It should fix current deadlock issue with as monitor
> > won't be depending on lead exit event.
> 
> NB, libvirt only ever uses --daemonize when probing capabilities, never
> when launching QEMU for a real VM. In the latter case, we now use FD
> passing, so libvirt opens the UNIX domain socket listener, and passes
> this into QEMU. So libvirt knows it can connect to the listener
> immediately and will only ever get a failure if QEMU has exited.

So, what I'm really missing here is: do we have a good reason to
support --daemonize + --preconfig today?

The options I see are:

1) complete daemonization before preconfig main loop
----------------------------------------------------

By "complete daemonization" I mean doing chdir("/"),
stderr/stdout cleanup, chroot, and UID magic before calling
exit(0) on the main QEMU process.

Pros:
* More intuitive

Cons:
* Can break existing initialization code that don't expect
  this to happen.
  (can this be fixed?)
* Can break any preconfig-time QMP commands that rely on opening
  files
  (is it a real problem?)
* Any initialization error conditions that currently rely on
  error_report()+exit() will be very inconvenient to handle
  properly
  (this can be fixed eventually, but it's not trivial)


2) incomplete daemonization before preconfig main loop
------------------------------------------------------

This means calling exit(0) on the main process before doing the
chdir/stderr/etc magic.

Pros:
* Less likely to break initialization code and other QMP commands

Cons:
* Not what's expected from a well-behaved daemon.
  (If we're not daemonizing properly, what's the point of using
  -daemonize at all?)
* More difficult to change behavior later.


3) daemonize only after leaving preconfig state
-----------------------------------------------

AFAICS, this is the current behavior.

Pros:
* Less likely to break init code
* Keeps existing code as is

Cons:
* Less intuitive
* -daemonize becomes useless as synchronization point for monitor
  availability
* Would this be useful for anybody, at all?
* We won't be able to change this behavior later


4) Not supporting -preconfig + -daemonize
-----------------------------------------

Pros:
* Simple to implement.
* Avoids unexpected bugs.
* Saves our time.
* We can change this behavior later.

Cons:
* People might want us to support it eventually.



I believe the only reasonable options are (1) and (4).

-- 
Eduardo