Re: [Qemu-devel] [PATCH] log: Redirect stderr to logfile if deamonized

[Daniel P. Berrange]

On Tue, Mar 01, 2016 at 12:15:21PM +0100, Gerd Hoffmann wrote:
> On Do, 2016-02-18 at 13:38 +0200, Dimitris Aragiorgis wrote:
> > In case of daemonize, use the logfile passed with the -D option in
> > order to redirect stderr to it instead of /dev/null.
> > 
> > Also remove some unused code in log.h.
> 
> Patch breaks interaction with libvirt.  libvirt hangs on startup, while
> probing qemu capabilities.  qemu runs in probing mode (command line is
> "/home/kraxel/projects/qemu/build-default/x86_64-softmmu/qemu-system-x86_64 
> -S -no-user-config -nodefaults -nographic -M none -qmp 
> unix:/var/lib/libvirt/qemu/capabilities.monitor.sock,server,nowait -pidfile 
> /var/lib/libvirt/qemu/capabilities.pidfile -daemonize" according to 
> "systemctl status libvirtd -l"), apparently both qemu and libvirt wait for 
> each other.

When libvirt is probing capabilities it passes a pipe file descriptor
for stderr. It reads from this pipe to detect any errors printed by
QEMU before daemonizing. When QEMU daemonizes, it closes this FD and
sets stderr to /dev/null. Libvirt knows qemu has successfully started
up at this point.

With this patch though, this code:

@@ -275,7 +276,10 @@ void os_setup_post(void)
 
         dup2(fd, 0);
         dup2(fd, 1);
-        dup2(fd, 2);
+        /* In case -D is given do not redirect stderr to /dev/null */
+        if (!qemu_logfile) {
+            dup2(fd, 2);
+        }
 
         close(fd);
 

means that QEMU will never close stderr anymore, so libvirt things
QEMU is still starting up....forever.

Given current libvirt behaviour / expectations, I think the only
option is to revert this change.

IMHO if applications want qemu logs to go to stderr, they should
explicitly ask for that to happen via a CLI arg.

Regards,
Daniel
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