On Mon, Mar 01, 2021 at 04:24:09PM +0000, Richard W.M. Jones wrote: > On Mon, Mar 01, 2021 at 04:15:47PM +0000, Daniel P. Berrangé wrote: > > On Mon, Mar 01, 2021 at 04:08:57PM +0000, Stefan Hajnoczi wrote: > > > Daemons often have a --pidfile option where the pid is written to a file > > > so that scripts can stop the daemon by sending a signal. > > > > > > The pid file also acts as a lock to prevent multiple instances of the > > > daemon from launching for a given pid file. > > > > > > QEMU, qemu-nbd, qemu-ga, virtiofsd, and qemu-pr-helper all support the > > > --pidfile option. Add it to qemu-storage-daemon too. > > > > > > Reported-by: Richard W.M. Jones <rjo...@redhat.com> > > > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > > > --- > > > docs/tools/qemu-storage-daemon.rst | 10 ++++++++++ > > > storage-daemon/qemu-storage-daemon.c | 29 ++++++++++++++++++++++++++++ > > > 2 files changed, 39 insertions(+) > > > > > > diff --git a/docs/tools/qemu-storage-daemon.rst > > > b/docs/tools/qemu-storage-daemon.rst > > > index f63627eaf6..8f4ab16ffc 100644 > > > --- a/docs/tools/qemu-storage-daemon.rst > > > +++ b/docs/tools/qemu-storage-daemon.rst > > > @@ -118,6 +118,16 @@ Standard options: > > > List object properties with ``<type>,help``. See the :manpage:`qemu(1)` > > > manual page for a description of the object properties. > > > > > > +.. option:: --pidfile PATH > > > + > > > + is the path to a file where the daemon writes its pid. This allows > > > scripts to > > > + stop the daemon by sending a signal:: > > > + > > > + $ kill -SIGTERM $(<path/to/qsd.pid) > > > + > > > + A file lock is applied to the file so only one instance of the daemon > > > can run > > > + with a given pid file path. The daemon unlinks its pid file when > > > terminating. > > > > Usually a pidfile wants to have some explicit synchronization rules > > defined. AFAICS, qsd doesn't have a --daemonize option to sync against. > > If we're using the FD passing trick for the monitor, however, we want > > a guarantee that the pidfile is written before the monitor accepts the > > first client. > > > > IOW, the parent process needs to know that once it has done the QMP > > handshake, there is guaranteed tobe a pidfile present, or if the > > QMP handshake fails, then the app is guaranteed to have quit. > > > > IIUC, this impl should be ok in this respect, because we won't process > > the QMP handdshake until the main loop runs, at which point the pidfile > > is present. So we just need to document that the pidfile is guaranteed > > to be written by the time QMP is active. > > I'm not sure if I follow this exactly, but from my point of view I'd > like to know that: > > (1) If we're using --nbd-server addr.type=inet|unix then the PID file > must not be created until the socket has been created and is > listening. Here I mean the NBD socket, but the same would apply to > the QMP socket or any other listening socket. This allows you to do > scripting sanely (qemu-storage-daemon ... &) without arbitrary sleeps.
Okay. This is guaranteed by the code (--chardev creates the character device and listens before the pid file is written). > (2) If we're using the FD passing trick instead, we don't care and > would probably not use the --pidfile option anyway (since we have the > PID from calling fork). Yep. I will document the things that you and Dan mentioned. Stefan
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