[PATCH] properly daemonize vncviewer

[PATCH] properly daemonize vncviewer

[Charles Coffing]

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Hi,

Currently, vncviewer is spawned from xm, but it doesn't properly get
daemonized.  The attached patch makes vncviewer run completely separate
from xm.

There are various reasons it should be daemonized, but the particular
problem we hit was that YaST called "xm create" and waited on output on
stdout/stderr; xm then spawned vncviewer (which never closed its
inherited stdout and stderr); xm then would exit, but YaST still had
open file descriptors, and therefore waited forever.  It would be
possible to work around in YaST, but it seemed cleaner to daemonize
vncviewer.

We've been running with a variant of this patch (a variant because we
use tightvnc, which requires different arguments) for many months, and
it works well.

Please consider applying to xen-unstable.

Thanks.

Signed-off-by:  Charles Coffing <ccoffing@novell.com>


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Index: xen-unstable/tools/python/xen/xm/create.py
===================================================================
--- xen-unstable.orig/tools/python/xen/xm/create.py
+++ xen-unstable/tools/python/xen/xm/create.py
@@ -857,11 +857,49 @@ def choose_vnc_display():
 vncpid = None
 
 def spawn_vnc(display):
+    """Spawns a vncviewer that listens on the specified display.  On success,
+    returns the port that the vncviewer is listening on and sets the global
+    vncpid.  On failure, returns 0.  Note that vncviewer is daemonized.
+    """
     vncargs = (["vncviewer", "-log", "*:stdout:0",
             "-listen", "%d" % (VNC_BASE_PORT + display) ])
-    global vncpid    
-    vncpid = os.spawnvp(os.P_NOWAIT, "vncviewer", vncargs)
 
+    r, w = os.pipe()
+    pid = os.fork()
+
+    if pid == 0:
+        os.close(r)
+        w = os.fdopen(w, 'w')
+        os.setsid()
+        try:
+            pid2 = os.fork()
+        except:
+            pid2 = None
+        if pid2 == 0:
+            os.chdir("/")
+            for fd in range(0, 256):
+                try:
+                    os.close(fd)
+                except:
+                    pass
+            os.open("/dev/null", os.O_RDWR)
+            os.dup2(0, 1)
+            os.dup2(0, 2)
+            os.execvp("vncviewer", vncargs)
+            os._exit(1)
+        else:
+            w.write(str(pid2 or 0))
+            w.close()
+            os._exit(0)
+
+    global vncpid
+    os.close(w)
+    r = os.fdopen(r)
+    vncpid = int(r.read())
+    r.close()
+    os.waitpid(pid, 0)
+    if vncpid == 0:
+        return 0
     return VNC_BASE_PORT + display
     
 def preprocess_vnc(vals):

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Re: [PATCH] properly daemonize vncviewer

[Keir Fraser]

On 1 Aug 2006, at 20:11, Charles Coffing wrote:

> We've been running with a variant of this patch (a variant because we
> use tightvnc, which requires different arguments) for many months, and
> it works well.
>
> Please consider applying to xen-unstable.

Is there no Python library function that can do this (I couldn't find 
one)?

If not, can you at least put the new code in a function with a 
spawn-like interface (i.e., takes two parameters -- program name and 
program argument list). It's okay to put the new function somewhere 
nearby in create.py.

  -- Keir

Re: [PATCH] properly daemonize vncviewer

[Charles Coffing]

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On Wed, Aug 2, 2006 at  2:32 AM, Keir Fraser <Keir.Fraser@cl.cam.ac.uk>
wrote: 
> On 1 Aug 2006, at 20:11, Charles Coffing wrote:
> 
>> We've been running with a variant of this patch (a variant because
we
>> use tightvnc, which requires different arguments) for many months,
and
>> it works well.
>>
>> Please consider applying to xen- unstable.
> 
> Is there no Python library function that can do this (I couldn't find

> one)?

No.  And as an added twist, I wanted to get the PID of the daemonized
program back (without the mess or risk of writing to a temporary file)
and I didn't see any such thing out there already.

> If not, can you at least put the new code in a function with a 
> spawn- like interface (i.e., takes two parameters --  program name
and 
> program argument list). It's okay to put the new function somewhere 
> nearby in create.py.

I've attached the updated patch with the suggested changes.



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Index: xen-3.0-testing/tools/python/xen/xm/create.py
===================================================================
--- xen-3.0-testing.orig/tools/python/xen/xm/create.py
+++ xen-3.0-testing/tools/python/xen/xm/create.py
@@ -765,14 +765,58 @@ def choose_vnc_display():
 
 vncpid = None
 
+def daemonize(prog, args):
+    """Runs a program as a daemon with the list of arguments.  Returns the PID
+    of the daemonized program, or returns 0 on error.
+    """
+    r, w = os.pipe()
+    pid = os.fork()
+
+    if pid == 0:
+        os.close(r)
+        w = os.fdopen(w, 'w')
+        os.setsid()
+        try:
+            pid2 = os.fork()
+        except:
+            pid2 = None
+        if pid2 == 0:
+            os.chdir("/")
+            for fd in range(0, 256):
+                try:
+                    os.close(fd)
+                except:
+                    pass
+            os.open("/dev/null", os.O_RDWR)
+            os.dup2(0, 1)
+            os.dup2(0, 2)
+            os.execvp(prog, args)
+            os._exit(1)
+        else:
+            w.write(str(pid2 or 0))
+            w.close()
+            os._exit(0)
+
+    os.close(w)
+    r = os.fdopen(r)
+    daemon_pid = int(r.read())
+    r.close()
+    os.waitpid(pid, 0)
+    return daemon_pid
+
 def spawn_vnc(display):
+    """Spawns a vncviewer that listens on the specified display.  On success,
+    returns the port that the vncviewer is listening on and sets the global
+    vncpid.  On failure, returns 0.  Note that vncviewer is daemonized.
+    """
     vncargs = (["vncviewer", "-log", "*:stdout:0",
             "-listen", "%d" % (VNC_BASE_PORT + display) ])
-    global vncpid    
-    vncpid = os.spawnvp(os.P_NOWAIT, "vncviewer", vncargs)
-
+    global vncpid
+    vncpid = daemonize("vncviewer", vncargs)
+    if vncpid == 0:
+        return 0
     return VNC_BASE_PORT + display
-    
+
 def preprocess_vnc(vals):
     """If vnc was specified, spawn a vncviewer in listen mode
     and pass its address to the domain on the kernel command line.

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