[Kimchi-devel] [PATCH 1/4] Use locks to prevent concurrent updates to VMs

[Crístian Deives]

If several users try to update the same VM at the same time, an error
may happen due to concurrent access.

Define a dictionary containing one lock per each VM, and use it when
updating the VM: the lock is acquired before the update operation begins
and it is released when the operation finishes (whether by success or
failure).

Signed-off-by: Crístian Deives <cristiandeives at gmail.com>
---
 src/kimchi/model/vms.py | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/kimchi/model/vms.py b/src/kimchi/model/vms.py
index 106e9bc..e3819dc 100644
--- a/src/kimchi/model/vms.py
+++ b/src/kimchi/model/vms.py
@@ -23,6 +23,7 @@ from lxml import etree, objectify
 import os
 import random
 import string
+import threading
 import time
 import uuid
 from xml.etree import ElementTree
@@ -75,6 +76,9 @@ XPATH_DOMAIN_UUID = '/domain/uuid'
 
 XPATH_NUMA_CELL = './cpu/numa/cell'
 
+# key: VM name; value: lock object
+vm_locks = {}
+
 
 class VMsModel(object):
     def __init__(self, **kargs):
@@ -196,10 +200,16 @@ class VMModel(object):
         self.stats = {}
 
     def update(self, name, params):
-        dom = self.get_vm(name, self.conn)
-        dom = self._static_vm_update(dom, params)
-        self._live_vm_update(dom, params)
-        return dom.name().decode('utf-8')
+        lock = vm_locks.get(name)
+        if lock is None:
+            lock = threading.Lock()
+            vm_locks[name] = lock
+
+        with lock:
+            dom = self.get_vm(name, self.conn)
+            dom = self._static_vm_update(dom, params)
+            self._live_vm_update(dom, params)
+            return dom.name().decode('utf-8')
 
     def clone(self, name):
         """Clone a virtual machine based on an existing one.
-- 
2.4.3