[PATCH] Fix xend xenstore handling

[john.levon@sun.com]

# HG changeset patch
# User john.levon@sun.com
# Date 1198248324 28800
# Node ID b29e155a85ef663170b0651798c84db79bafcdd8
# Parent  d7974e8d8f5131356e7c8fc87f880d737044e91b
Fix xend xenstore handling.

xend can get into a situation where two processes are attempting to
interact with the xenstore socket, with disastrous results. Fix the two
bad users of xstransact, add a big warning, and fix the destructor so
future mistakes will be detected earlier.

Signed-off-by: John Levon <john.levon@sun.com>

diff --git a/tools/python/xen/xend/XendDomainInfo.py b/tools/python/xen/xend/XendDomainInfo.py
--- a/tools/python/xen/xend/XendDomainInfo.py
+++ b/tools/python/xen/xend/XendDomainInfo.py
@@ -1533,10 +1533,9 @@ class XendDomainInfo:
                 except:
                     # Log and swallow any exceptions in removal --
                     # there's nothing more we can do.
-                        log.exception("Device release failed: %s; %s; %s",
-                                      self.info['name_label'], devclass, dev)
-
-            
+                    log.exception("Device release failed: %s; %s; %s",
+                                  self.info['name_label'], devclass, dev)
+        t.abort()
 
     def getDeviceController(self, name):
         """Get the device controller for this domain, and if it
@@ -1848,7 +1847,6 @@ class XendDomainInfo:
         # build list of phantom devices to be removed after normal devices
         plist = []
         if self.domid is not None:
-            from xen.xend.xenstore.xstransact import xstransact
             t = xstransact("%s/device/vbd" % GetDomainPath(self.domid))
             for dev in t.list():
                 backend_phantom_vbd = xstransact.Read("%s/device/vbd/%s/phantom_vbd" \
@@ -1858,6 +1856,7 @@ class XendDomainInfo:
                                       % backend_phantom_vbd)
                     plist.append(backend_phantom_vbd)
                     plist.append(frontend_phantom_vbd)
+            t.abort()
         return plist
 
     def _cleanup_phantom_devs(self, plist):
diff --git a/tools/python/xen/xend/server/pciif.py b/tools/python/xen/xend/server/pciif.py
--- a/tools/python/xen/xend/server/pciif.py
+++ b/tools/python/xen/xend/server/pciif.py
@@ -22,8 +22,6 @@ from xen.xend import sxp
 from xen.xend import sxp
 from xen.xend.XendError import VmError
 from xen.xend.XendLogging import log
-
-from xen.xend.xenstore.xstransact import xstransact
 
 from xen.xend.server.DevController import DevController
 
diff --git a/tools/python/xen/xend/xenstore/xstransact.py b/tools/python/xen/xend/xenstore/xstransact.py
--- a/tools/python/xen/xend/xenstore/xstransact.py
+++ b/tools/python/xen/xend/xenstore/xstransact.py
@@ -7,8 +7,16 @@
 
 from xen.xend.xenstore.xsutil import xshandle
 
+class xstransact:
+    """WARNING: Be very careful if you're instantiating an xstransact object
+       yourself (i.e. not using the capitalized static helpers like .Read().
+       It is essential that you clean up the object in place via
+       t.commit/abort(): GC can happen at any time, including contexts where
+       it's not safe to to use the shared xenstore socket fd. In particular,
+       if xend forks, and GC occurs, we can have two processes trying to
+       use the same xenstore fd, and all hell breaks loose.
+       """
 
-class xstransact:
 
     def __init__(self, path = ""):
         
@@ -22,8 +30,9 @@ class xstransact:
         self.in_transaction = True
 
     def __del__(self):
+        # see above.
         if self.in_transaction:
-            xshandle().transaction_end(self.transaction, True)
+            raise RuntimeError("ERROR: GC of live transaction")
 
     def commit(self):
         if not self.in_transaction: