Handle empty reads as closed connections

When the new Opendev 3.2 Gerrit closes the SSH connection, gerritlib
goes into a hard loop.  Either this has never really worked, or our
new version of gerrit closes the SSH connection differently and
triggers this.

Currently the _read() function in this code interprets a 0 read as
data and continues the event loop.  This means the _listen() function
never exits.  The poll() keeps returning that there is a POLLIN event
and the _read() function keeps reading the empty FD.

Modify this to set the state to DISCONNECTED for a blank read.
Additionally improve the error message if we see invalid data (which
shouldn't happen).  Insert a short-circuit in the listen to check if
the state has switched to DISCONNECTED and exit the loop in that case.

Change-Id: I46849bcbfc3a0d409baa654867018994ce717481

gerritlib/gerrit.py

@@ -136,15 +136,25 @@ class GerritWatcher(threading.Thread):
 
     def _read(self, fd):
         line = fd.readline()
-        data = json.loads(line)
-        self.log.debug("Received data from Gerrit event stream: \n%s" %
-                       pprint.pformat(data))
-        self.gerrit.addEvent(data)
+        if line:
+            try:
+                data = json.loads(line)
+                self.log.debug("Received data from Gerrit event stream: \n%s" %
+                               pprint.pformat(data))
+                self.gerrit.addEvent(data)
+            except json.decoder.JSONDecodeError:
+                self.log.debug("Can not parse data from Gerrit event stream: \n%s" %
+                               pprint.pformat(line))
+        else:
+            # blank read indicates EOF and the connection closed
+            self.state = DISCONNECTED
 
     def _listen(self, stdout, stderr):
         poll = select.poll()
         poll.register(stdout.channel)
         while True:
+            if self.state == DISCONNECTED:
+                return
             ret = poll.poll()
             for (fd, event) in ret:
                 if fd == stdout.channel.fileno():