Handle pidfile exception for dnsmasq

Capture the exception in dnsmasq_pid_for method. If the pidfile cannot be read
for some reason, it should be treated as if it does not exist. This prevents
issues where the filesystem write delay leaves the file created but empty.
Fixes bug 865399.

(cherry picked from commit a25f106)

Change-Id: Ifd79c7143060702bfe359f0e0a35867c685e27df

Authors

@@ -120,6 +120,7 @@ Vladimir Popovski <vladimir@zadarastorage.com>
 William Wolf <throughnothing@gmail.com>
 Yoshiaki Tamura <yoshi@midokura.jp>
 Youcef Laribi <Youcef.Laribi@eu.citrix.com>
+Yun Shen <Yun.Shen@hp.com>
 Yuriy Taraday <yorik.sar@gmail.com>
 Zhixue Wu <Zhixue.Wu@citrix.com>
 Zed Shaw <zedshaw@zedshaw.com>

nova/network/linux_net.py

@@ -793,8 +793,11 @@ def _dnsmasq_pid_for(dev):
     pid_file = _dhcp_file(dev, 'pid')
 
     if os.path.exists(pid_file):
-        with open(pid_file, 'r') as f:
-            return int(f.read())
+        try:
+            with open(pid_file, 'r') as f:
+                return int(f.read())
+        except (ValueError, IOError):
+            return None
 
 
 def _ra_pid_for(dev):