Re: [Xen-changelog] Detach Xend from terminal, courtesy of Horms <horms@verge.net.au>.

[Anthony Liguori <aliguori@us.ibm.com>]

Sorry I missed this on Xen devel, but could someone explain this patch a 
little more.  The old behavior purposefully delayed the terminal 
detaching until Xend was ready to accept connections.  This is necessary 
to avoid a race condition with executing the first xm command after a 
xend start.

I don't quite understand why two fork()s would be required to properly 
daemonize.  I don't know of any other daemons that do that (certainly, 
xenstored and xenconsoled don't).

I could be missing something so if someone could clarify I'd greatly 
appreciate it.

Thanks,

Anthony Liguori



Xen patchbot -unstable wrote:

># HG changeset patch
># User emellor@leeni.uk.xensource.com
># Node ID efc71a3e9f6f7487ad349677369d6221a06ead3f
># Parent  52f214d983fb3edf2e265984ed9fe103dd7d2440
>Detach Xend from terminal, courtesy of Horms <horms@verge.net.au>.
>
>* For setsid to effectively detach a process from the terminal,
>  the following needs to occur:
>
>    fork ()   Return control to the shell
>    setsid () New session with no controlling terminal
>    fork ()   The session leader (parent) exits and thus
>              the resulting child process can never regain the terminal
>
>  This patch adds the second fork
>
>* The call to self.daemonize(), which now forks, is moved to
>  run before self.tracing(), as now that it actually disconnects
>  from the terminal, and thus the prevailing process, the trace
>  loses the processes created in self.run().
>
>Signed-off-by: Ewan Mellor <ewan@xensource.com>
>
>diff -r 52f214d983fb -r efc71a3e9f6f tools/python/xen/xend/server/SrvDaemon.py
>--- a/tools/python/xen/xend/server/SrvDaemon.py	Thu Dec  8 16:11:48 2005
>+++ b/tools/python/xen/xend/server/SrvDaemon.py	Thu Dec  8 16:17:53 2005
>@@ -123,9 +123,18 @@
> 
>     def daemonize(self):
>         if not XEND_DAEMONIZE: return
>+ 
>         # Detach from TTY.
>+
>+        # Become the group leader (already a child process)
>         os.setsid()
> 
>+        # Fork, this allows the group leader to exit,
>+        # which means the child can never again regain control of the
>+        # terminal
>+        if self.fork_pid(XEND_PID_FILE):
>+            self.exit()
>+ 
>         # Detach from standard file descriptors, and redirect them to
>         # /dev/null or the log as appropriate.
>         os.close(0)
>@@ -164,7 +173,7 @@
>         # we can avoid a race condition during startup
>         
>         r,w = os.pipe()
>-        if self.fork_pid(XEND_PID_FILE):
>+        if os.fork():
>             os.close(w)
>             r = os.fdopen(r, 'r')
>             try:
>@@ -178,6 +187,7 @@
>         else:
>             os.close(r)
>             # Child
>+            self.daemonize()
>             self.tracing(trace)
>             self.run(os.fdopen(w, 'w'))
> 
>@@ -274,7 +284,6 @@
> 
>             relocate.listenRelocation()
>             servers = SrvServer.create()
>-            self.daemonize()
>             servers.start(status)
>         except Exception, ex:
>             print >>sys.stderr, 'Exception starting xend:', ex
>
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