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. (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). Rich. > > > + > > Examples > > -------- > > Launch the daemon with QMP monitor socket ``qmp.sock`` so clients can > > execute > > diff --git a/storage-daemon/qemu-storage-daemon.c > > b/storage-daemon/qemu-storage-daemon.c > > index 9021a46b3a..011ae49ac3 100644 > > --- a/storage-daemon/qemu-storage-daemon.c > > +++ b/storage-daemon/qemu-storage-daemon.c > > @@ -59,6 +59,7 @@ > > #include "sysemu/runstate.h" > > #include "trace/control.h" > > > > +static const char *pid_file; > > static volatile bool exit_requested = false; > > > > void qemu_system_killed(int signal, pid_t pid) > > @@ -126,6 +127,7 @@ enum { > > OPTION_MONITOR, > > OPTION_NBD_SERVER, > > OPTION_OBJECT, > > + OPTION_PIDFILE, > > }; > > > > extern QemuOptsList qemu_chardev_opts; > > @@ -164,6 +166,7 @@ static void process_options(int argc, char *argv[]) > > {"monitor", required_argument, NULL, OPTION_MONITOR}, > > {"nbd-server", required_argument, NULL, OPTION_NBD_SERVER}, > > {"object", required_argument, NULL, OPTION_OBJECT}, > > + {"pidfile", required_argument, NULL, OPTION_PIDFILE}, > > {"trace", required_argument, NULL, 'T'}, > > {"version", no_argument, NULL, 'V'}, > > {0, 0, 0, 0} > > @@ -275,6 +278,9 @@ static void process_options(int argc, char *argv[]) > > qobject_unref(args); > > break; > > } > > + case OPTION_PIDFILE: > > + pid_file = optarg; > > + break; > > default: > > g_assert_not_reached(); > > } > > @@ -285,6 +291,27 @@ static void process_options(int argc, char *argv[]) > > } > > } > > > > +static void pid_file_cleanup(void) > > +{ > > + unlink(pid_file); > > +} > > + > > +static void pid_file_init(void) > > +{ > > + Error *err = NULL; > > + > > + if (!pid_file) { > > + return; > > + } > > + > > + if (!qemu_write_pidfile(pid_file, &err)) { > > + error_reportf_err(err, "cannot create PID file: "); > > + exit(EXIT_FAILURE); > > + } > > + > > + atexit(pid_file_cleanup); > > +} > > + > > int main(int argc, char *argv[]) > > { > > #ifdef CONFIG_POSIX > > @@ -312,6 +339,8 @@ int main(int argc, char *argv[]) > > qemu_init_main_loop(&error_fatal); > > process_options(argc, argv); > > > > + pid_file_init(); > > + > > while (!exit_requested) { > > main_loop_wait(false); > > } > > -- > > 2.29.2 > > > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 100 libraries supported. http://fedoraproject.org/wiki/MinGW