python: Add cooperative_yield() API method to Idl.

When using eventlet monkey_patch()'d code, greenthreads can be
blocked on connection for several seconds while the database
contents are parsed. Eventlet recommends adding a sleep(0) call
to cooperatively yield in cpu-bound code. asyncio code has
asyncio.sleep(0). This patch adds an API  method that defaults to
doing nothing, but can be overridden to yield as needed.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

python/ovs/db/idl.py

@@ -481,6 +481,15 @@ class Idl(object):
         :type updates:  Row
         """
 
+    def cooperative_yield(self):
+        """Hook for cooperatively yielding to eventlet/gevent/asyncio/etc.
+
+        When a block of code is going to spend a lot of time cpu-bound without
+        doing any I/O, it can cause greenthread/coroutine libraries to block.
+        This call should be added to code where this can happen, but defaults
+        to doing nothing to avoid overhead where it is not needed.
+        """
+
     def __send_cond_change(self, table, cond):
         monitor_cond_change = {table.name: [{"where": cond}]}
         old_uuid = str(self.uuid)
@@ -646,6 +655,8 @@ class Idl(object):
                                       'is not an object'
                                       % (table_name, uuid_string))
 
+                self.cooperative_yield()
+
                 if version == OVSDB_UPDATE2:
                     changes = self.__process_update2(table, uuid, row_update)
                     if changes: