Re: [libvirt PATCH] qemu: allow passt to self-daemonize

["Michal Prívozník" <mprivozn@redhat.com>]

On 2/9/23 00:13, Laine Stump wrote:
> I initially had the passt process being started in an identical
> fashion to the slirp-helper - libvirt was daemonizing the new process
> and recording its pid in a pidfile. The problem with this is that,
> since it is daemonized immediately, any startup error in passt happens
> after the daemonization, and thus isn't seen by libvirt - libvirt
> believes that the process has started successfully and continues on
> its merry way. The result was that sometimes a guest would be started,
> but there would be no passt process for qemu to use for network
> traffic.
> 
> Instead, we should be starting passt in the same manner we start
> dnsmasq - we just exec it as normal (along with a request that passt
> create the pidfile, which is just another option on the passt
> commandline) and wait for the child process to exit; passt then has a
> chance to parse its commandline and complete all the setup prior to
> daemonizing itself; if it encounters an error and exits with a non-0
> code, libvirt will see the code and know about the failure. We can
> then grab the output from stderr, log that so the "user" has some idea
> of what went wrong, and then fail the guest startup.
> 
> Signed-off-by: Laine Stump <laine@redhat.com>
> ---
>  src/qemu/qemu_passt.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/src/qemu/qemu_passt.c b/src/qemu/qemu_passt.c
> index 0f09bf3db8..f640a69c00 100644
> --- a/src/qemu/qemu_passt.c
> +++ b/src/qemu/qemu_passt.c
> @@ -141,24 +141,23 @@ qemuPasstStart(virDomainObj *vm,
>      g_autofree char *passtSocketName = qemuPasstCreateSocketPath(vm, net);
>      g_autoptr(virCommand) cmd = NULL;
>      g_autofree char *pidfile = qemuPasstCreatePidFilename(vm, net);
> +    g_autofree char *errbuf = NULL;
>      char macaddr[VIR_MAC_STRING_BUFLEN];
>      size_t i;
>      pid_t pid = (pid_t) -1;
>      int exitstatus = 0;
>      int cmdret = 0;
> -    VIR_AUTOCLOSE errfd = -1;
>  
>      cmd = virCommandNew(PASST);
>  
>      virCommandClearCaps(cmd);
> -    virCommandSetPidFile(cmd, pidfile);
> -    virCommandSetErrorFD(cmd, &errfd);
> -    virCommandDaemonize(cmd);
> +    virCommandSetErrorBuffer(cmd, &errbuf);
>  
>      virCommandAddArgList(cmd,
>                           "--one-off",
>                           "--socket", passtSocketName,
>                           "--mac-addr", virMacAddrFormat(&net->mac, macaddr),
> +                         "--pid", pidfile,

The only problem with this approach is that our virPidFile*() functions
rely on locking the very first byte. And when reading the pidfile, we
try to lock the file and if we succeeded it means the file wasn't locked
which means the process holding the lock died and thus the pid in the
pidfile is stale.

Now, I don't see passt locking the pidfile at all. So effectively, after
this patch qemuPasstStop() would do nothing (well, okay, it'll remove
the pidfile), qemuPasstSetupCgroup() does nothing, etc.

What we usually do in this case, is: we let our code write the pidfile
(just like the current code does), but then have a loop that waits a bit
for socket to show up. If it doesn't in say 5 seconds we kill the child
process (which we know the PID of). You can take inspiration from:
qemuDBusStart() or qemuProcessStartManagedPRDaemon().

Michal