Set keepalives for gerrit connections

Some gerrit clients connect through stateful firewalls,
which are occasionally "flushed", which leads to hanging of the connection.
This could be avoided by tcp keepalives.
To turn keepalives on, set "keepalive" in gerrit connection attributes to non-zero
(0 means no keepalives sent)
The default is turn it on with timeout of 60 seconds.

Change-Id: Ic3d11e0ac2108f02832e2114ba8bc1adcc87cb4f

doc/source/connections.rst

@@ -38,6 +38,9 @@ Create a connection with gerrit.
   Path to SSH key to use when logging into above server.
   ``sshkey=/home/zuul/.ssh/id_rsa``
 
+**keepalive**
+  Optional: Keepalive timeout, 0 means no keepalive.
+  ``keepalive=60``
 
 Gerrit Configuration
 ~~~~~~~~~~~~~~~~~~~~

etc/zuul.conf-sample

@@ -32,6 +32,7 @@ server=review.example.com
 ;baseurl=https://review.example.com/r
 user=jenkins
 sshkey=/home/jenkins/.ssh/id_rsa
+;keepalive=60
 
 [connection smtp]
 driver=smtp

zuul/connection/gerrit.py

@@ -135,13 +135,14 @@ class GerritWatcher(threading.Thread):
     poll_timeout = 500
 
     def __init__(self, gerrit_connection, username, hostname, port=29418,
-                 keyfile=None):
+                 keyfile=None, keepalive=60):
         threading.Thread.__init__(self)
         self.username = username
         self.keyfile = keyfile
         self.hostname = hostname
         self.port = port
         self.gerrit_connection = gerrit_connection
+        self.keepalive = keepalive
         self._stopped = False
 
     def _read(self, fd):
@@ -172,6 +173,8 @@ class GerritWatcher(threading.Thread):
                            username=self.username,
                            port=self.port,
                            key_filename=self.keyfile)
+            transport = client.get_transport()
+            transport.set_keepalive(self.keepalive)
 
             stdin, stdout, stderr = client.exec_command("gerrit stream-events")
 
@@ -224,6 +227,7 @@ class GerritConnection(BaseConnection):
         self.server = self.connection_config.get('server')
         self.port = int(self.connection_config.get('port', 29418))
         self.keyfile = self.connection_config.get('sshkey', None)
+        self.keepalive = int(self.connection_config.get('keepalive', 60))
         self.watcher_thread = None
         self.event_queue = None
         self.client = None
@@ -356,6 +360,8 @@ class GerritConnection(BaseConnection):
                        username=self.user,
                        port=self.port,
                        key_filename=self.keyfile)
+        transport = client.get_transport()
+        transport.set_keepalive(self.keepalive)
         self.client = client
 
     def _ssh(self, command, stdin_data=None):
@@ -461,7 +467,8 @@ class GerritConnection(BaseConnection):
             self.user,
             self.server,
             self.port,
-            keyfile=self.keyfile)
+            keyfile=self.keyfile,
+            keepalive=self.keepalive)
         self.watcher_thread.start()
 
     def _stop_event_connector(self):