[2790] Short-term workaround for the blocking read loop in zonemgr

In the end we'll need to address the threading model in zonemgr, but this should be a good short-term workaround for the problem.

src/bin/zonemgr/zonemgr.py.in

@@ -690,7 +690,19 @@ class Zonemgr:
         self.running = True
         try:
             while not self._shutdown_event.is_set():
-                self._module_cc.check_command(False)
+                fileno = self._module_cc.get_socket().fileno()
+                # Wait with select() until there is something to read,
+                # and then read it using a non-blocking read
+                # This may or may not be relevant data for this loop,
+                # but due to the way the zonemgr does threading, we
+                # can't have a blocking read loop here.
+                try:
+                    (reads, _, _) = select.select([fileno], [], [])
+                except select.error as se:
+                    if se.args[0] != errno.EINTR:
+                        raise
+                if fileno in reads:
+                    self._module_cc.check_command(True)
         finally:
             self._module_cc.send_stopping()