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>

tools/python/xen/xend/server/SrvDaemon.py

@@ -123,9 +123,18 @@ class Daemon:
 
     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 @@ class Daemon:
         # 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 @@ class Daemon:
         else:
             os.close(r)
             # Child
+            self.daemonize()
             self.tracing(trace)
             self.run(os.fdopen(w, 'w'))
 
@@ -274,7 +284,6 @@ class Daemon:
 
             relocate.listenRelocation()
             servers = SrvServer.create()
-            self.daemonize()
             servers.start(status)
         except Exception, ex:
             print >>sys.stderr, 'Exception starting xend:', ex