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

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Ewan Mellor wrote:

> > 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).
> >    
> It's a standard technique on Unixen.  You need to fork before calling setsid
> to ensure that you are not a process group leader, then afterwards to ensure
> that children cannot regain the terminal.  See
>  
I see, I've only ever needed this behavior once and did something 
embarrassingly more complex to achieve it :-)  While not necessary for 
xenconsoled and xenstored, it certainly doesn't hurt.
> http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16
>
> (If xenstored and xenconsoled aren't doing this, then they ought to.)
>  
Patch is attached.

> We have preserved the behaviour whereby the Xend process does not exit until it
> is ready to accept connections -- it's just one fork further on (and there's
> a patch with another fork on its way, for your amusement, but that's a different
> story ;-)
>  
So that Xend can restart itself?  Yeah, I saw that one :-)  I'll reserve 
a commen there.   Thanks for the explaination!
Regards,

Anthony Liguori

> Ewan. 
>
>  

# HG changeset patch
# User Anthony Liguori <anthony@xxxxxxxxxxxxx>
# Node ID e55633c669d11b48cf16d0ddaebbb836d7b3f5f6
# Parent  53cff3f88e45cb5230da39f86d84b6606da0cdbb
Make sure to fork again after setsid() so that child cannot regain CTTY.

Signed-off-by: Anthony Liguori <aliguori@xxxxxxxxxx>

diff -r 53cff3f88e45 -r e55633c669d1 tools/console/daemon/utils.c
--- a/tools/console/daemon/utils.c      Fri Dec  9 11:05:06 2005 +0000
+++ b/tools/console/daemon/utils.c      Fri Dec  9 16:33:01 2005 -0500
@@ -90,6 +90,12 @@
 
        setsid();
 
+       if ((pid = fork()) > 0) {
+               exit(0);
+       } else if (pid == -1) {
+               err(errno, "fork() failed");
+       }
+
        /* redirect fd 0,1,2 to /dev/null */
        if ((fd = open("/dev/null",O_RDWR)) == -1) {
                exit(1);
diff -r 53cff3f88e45 -r e55633c669d1 tools/xenstore/xenstored_core.c
--- a/tools/xenstore/xenstored_core.c   Fri Dec  9 11:05:06 2005 +0000
+++ b/tools/xenstore/xenstored_core.c   Fri Dec  9 16:33:01 2005 -0500
@@ -1491,6 +1491,13 @@
 
        /* Session leader so ^C doesn't whack us. */
        setsid();
+
+       /* Let session leader exit so child cannot regain CTTY */
+       if ((pid = fork()) < 0)
+               barf_perror("Failed to fork daemon");
+       if (pid != 0)
+               exit(0);
+
 #ifndef TESTING        /* Relative paths for socket names */
        /* Move off any mount points we might be in. */
        chdir("/");

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