Allow using lockfile per jenkins master

When a jjb run is thrown when another jjb is already running,
it can cause corruption of cache. Start using a lockfile to
ensure this won't be happening and run securely on automated
systems.

Change-Id: I3ac37e738b3bb87c04a47afb8adb3e25f8fb4ea8

jenkins_jobs/builder.py

@@ -16,6 +16,7 @@
 # Manage jobs in Jenkins server
 
 import errno
+import fcntl
 import hashlib
 import io
 import logging
@@ -47,6 +48,7 @@ class CacheStorage(object):
     # removed global module references during teardown.
     _yaml = yaml
     _logger = logger
+    _fcntl = fcntl
 
     def __init__(self, jenkins_url, flush=False):
         cache_dir = self.get_cache_dir()
@@ -54,6 +56,11 @@ class CacheStorage(object):
         host_vary = re.sub('[^A-Za-z0-9\-\~]', '_', jenkins_url)
         self.cachefilename = os.path.join(
             cache_dir, 'cache-host-jobs-' + host_vary + '.yml')
+
+        # generate named lockfile if not exists, and lock it
+        while not self._lock(cache_dir, host_vary):
+            time.sleep(1)
+
         if flush or not os.path.isfile(self.cachefilename):
             self.data = {}
         else:
@@ -61,6 +68,25 @@ class CacheStorage(object):
                 self.data = yaml.load(yfile)
         logger.debug("Using cache: '{0}'".format(self.cachefilename))
 
+    def _lock(self, cache_dir, jenkins_master):
+        path = os.path.join(cache_dir, "lock-jjb.%s" % jenkins_master)
+        self.lockfile = open(path, 'w')
+
+        try:
+            self._fcntl.lockf(self.lockfile,
+                              self._fcntl.LOCK_EX | self._fcntl.LOCK_NB)
+        except IOError:
+            return False
+        return True
+
+    def _unlock(self):
+        if getattr(self, 'lockfile', None) is not None:
+            try:
+                self._fcntl.lockf(self.lockfile, self._fcntl.LOCK_UN)
+                self.lockfile.close()
+            except IOError:
+                pass
+
     @staticmethod
     def get_cache_dir():
         home = os.path.expanduser('~')
@@ -111,6 +137,7 @@ class CacheStorage(object):
                 self._logger.info("Cache saved")
                 self._logger.debug("Cache written out to '%s'" %
                                    self.cachefilename)
+        self._unlock()
 
     def __del__(self):
         self.save()

tests/cachestorage/test_cachestorage.py

@@ -32,7 +32,8 @@ class TestCaseCacheStorage(LoggingFixture, testtools.TestCase):
 
         with mock.patch('jenkins_jobs.builder.CacheStorage.save') as save_mock:
             with mock.patch('os.path.isfile', return_value=False):
-                jenkins_jobs.builder.CacheStorage("dummy")
+                with mock.patch('jenkins_jobs.builder.CacheStorage._lock'):
+                    jenkins_jobs.builder.CacheStorage("dummy")
             save_mock.assert_called_with()
 
     @mock.patch('jenkins_jobs.builder.CacheStorage.get_cache_dir',