Add API endpoints to ConfigDBConnector to support pre-loading data wi…

…thout blackout (sonic-net#587)

Currently if someone wishes to use ConfigDBConnector to operate on updates coming from CONFIG_DB AND they wish to pull all the initial data from the table to initialize their only option is to pull the data and then run the `listen()` method on ConfigDBConnector.

This creates a "blackout" period where between the time the tables are downloaded and the `listen()` method is called, any new updates to CONFIG_DB will not be handled.

To resolve this I have changed two things in ConfigDBConnector. All of which are 100% backwards compatible...
1. I split `listen()` into `listen()` and `process()` where `listen()` may now be called with `start=False` as an argument which does not start immediately processing updates from CONFIG_DB and calling handlers. This allows you to call `listen()` *first* and then download table data and then call `process()` to start processing updates. This way no updates will be missed as they will be queued by redis while the table data is being downloaded by the consumer. 
2. I added a `cache` argument to `process` in which you pass any initial table data your system is operating on. When a table update is processed the system checks if that keys value is different than what is stored in the cache (if it is present) and only fires the callback handler in the case that the data has changes (added / deleted / updated). This prevents the same data from being processed multiple times.

common/configdb.h

@@ -57,6 +57,7 @@ class ConfigDBConnector_Native : public SonicV2Connector_Native
 
             ## Note: callback is difficult to implement by SWIG C++, so keep in python
             self.handlers = {}
+            self.fire_init_data = {}
 
         @property
         def KEY_SEPARATOR(self):
@@ -71,10 +72,28 @@ class ConfigDBConnector_Native : public SonicV2Connector_Native
             return self.getDbName()
 
         ## Note: callback is difficult to implement by SWIG C++, so keep in python
-        def listen(self):
-            ## Start listen Redis keyspace events and will trigger corresponding handlers when content of a table changes.
+        def listen(self, init_data_handler=None):
+            ## Start listen Redis keyspace event. Pass a callback function to `init` to handle initial table data.
             self.pubsub = self.get_redis_client(self.db_name).pubsub()
             self.pubsub.psubscribe("__keyspace@{}__:*".format(self.get_dbid(self.db_name)))
+
+            # Build a cache of data for all subscribed tables that will recieve the initial table data so we dont send duplicate event notifications
+            init_data = {tbl: self.get_table(tbl) for tbl in self.handlers if init_data_handler or self.fire_init_data[tbl]}
+
+            # Function to send initial data as series of updates through individual table callback handlers
+            def load_data(tbl, data):
+                if self.fire_init_data[tbl]:
+                    for row, x in data.items():
+                        self.__fire(tbl, row, x)
+                    return False
+                return True
+
+            init_callback_data = {tbl: data for tbl, data in init_data.items() if load_data(tbl, data)}	
+
+            # Pass all initial data that we DID NOT send as updates to handlers through the init callback if provided by caller
+            if init_data_handler:
+                init_data_handler(init_callback_data)
+
             while True:
                 item = self.pubsub.listen_message()
                 if item['type'] == 'pmessage':
@@ -84,6 +103,12 @@ class ConfigDBConnector_Native : public SonicV2Connector_Native
                         if table in self.handlers:
                             client = self.get_redis_client(self.db_name)
                             data = self.raw_to_typed(client.hgetall(key))
+                            if table in init_data and row in init_data[table]:
+                                cache_hit = init_data[table][row] == data
+                                del init_data[table][row]
+                                if not init_data[table]:
+                                    del init_data[table]
+                                if cache_hit: continue
                             self.__fire(table, row, data)
                     except ValueError:
                         pass    #Ignore non table-formated redis entries
@@ -153,8 +178,9 @@ class ConfigDBConnector_Native : public SonicV2Connector_Native
                 handler = self.handlers[table]
                 handler(table, key, data)
 
-        def subscribe(self, table, handler):
+        def subscribe(self, table, handler, fire_init_data=False):
             self.handlers[table] = handler
+            self.fire_init_data[table] = fire_init_data
 
         def unsubscribe(self, table):
             if table in self.handlers:

tests/test_redis_ut.py

@@ -1,6 +1,7 @@
 import os
 import time
 import pytest
+import multiprocessing
 from threading import Thread
 from pympler.tracker import SummaryTracker
 from swsscommon import swsscommon
@@ -621,6 +622,50 @@ def thread_coming_entry():
     config_db.unsubscribe(table_name)
     assert table_name not in config_db.handlers
 
+def test_ConfigDBInit():
+    table_name_1 = 'TEST_TABLE_1'
+    table_name_2 = 'TEST_TABLE_2'
+    test_key = 'key1'
+    test_data = {'field1': 'value1'}
+    test_data_update = {'field1': 'value2'}
+
+    manager = multiprocessing.Manager()
+    ret_data = manager.dict()
+
+    def test_handler(table, key, data, ret):
+        ret[table] = {key: data}
+
+    def test_init_handler(data, ret):
+        ret.update(data)
+
+    def thread_listen(ret):
+        config_db = ConfigDBConnector()
+        config_db.connect(wait_for_init=False)
+
+        config_db.subscribe(table_name_1, lambda table, key, data: test_handler(table, key, data, ret),
+                            fire_init_data=False)
+        config_db.subscribe(table_name_2, lambda table, key, data: test_handler(table, key, data, ret),
+                            fire_init_data=True)
+
+        config_db.listen(init_data_handler=lambda data: test_init_handler(data, ret))
+
+    config_db = ConfigDBConnector()
+    config_db.connect(wait_for_init=False)
+    client = config_db.get_redis_client(config_db.CONFIG_DB)
+    client.flushdb()
+
+    # Init table data
+    config_db.set_entry(table_name_1, test_key, test_data)
+    config_db.set_entry(table_name_2, test_key, test_data)
+
+    thread = multiprocessing.Process(target=thread_listen, args=(ret_data,))
+    thread.start()
+    time.sleep(5)
+    thread.terminate()
+
+    assert ret_data[table_name_1] == {test_key: test_data}
+    assert ret_data[table_name_2] == {test_key: test_data}
+
 
 def test_DBConnectFailure():
     """ Verify that a DB connection failure will not cause a process abort