broker.py: Validate subscriptions.

Author: peterhinch

v3/docs/DRIVERS.md

@@ -1173,14 +1173,10 @@ The `topic` arg is typically a string but may be any hashable object. A
 
 #### Agent types
 
-An `agent` may be any of the following:
-
-* `Queue` When a message is received it receives 2-tuple `(topic, message)`. If
-extra args were passed on subscription the queue receives a 3-tuple.
-`(topic, message, (args...))`.
-* `RingbufQueue` When a message is received it receives 2-tuple `(topic, message)`.
-If extra args were passed on subscription it receives a 3-tuple,
-`(topic, message, (args...))`.
+An `agent` may be an instance of any of the following:
+
+* `RingbufQueue` Received messages are queued as a 2-tuple `(topic, message)`.
+* `Queue` Received messages are queued as a 2-tuple `(topic, message)`.
 * `function` Called when a message is received. Args: topic, message plus any
 further args.
 * `bound method` Called when a message is received. Args: topic, message plus any
@@ -1193,7 +1189,8 @@ message plus any further args.
 * `Event` Set when a message is received.
 
 Note that synchronous `agent` instances must run to completion quickly otherwise
-the `publish` method will be slowed.
+the `publish` method will be slowed. See [Notes](./DRIVERS.md#93-notes) for
+further details on queue behaviour.
 
 #### Broker class variable
 
@@ -1202,24 +1199,25 @@ the `publish` method will be slowed.
 #### example
 ```py
 import asyncio
-from primitives import Broker, Queue
+from primitives import Broker, RingbufQueue
 
 broker = Broker()
-queue = Queue()  # Or (e.g. RingbufQueue(20))
+queue = RingbufQueue(20)
 async def sender(t):
     for x in range(t):
         await asyncio.sleep(1)
         broker.publish("foo_topic", f"test {x}")
 
+async def receiver():
+    async for topic, message in queue:
+        print(topic, message)
+
 async def main():
     broker.subscribe("foo_topic", queue)
-    n = 10
-    asyncio.create_task(sender(n))
-    print("Letting queue part-fill")
-    await asyncio.sleep(5)
-    for _ in range(n):
-        topic, message = await queue.get()
-        print(topic, message)
+    rx = asyncio.create_task(receiver())
+    await sender(10)
+    await asyncio.sleep(2)
+    rx.cancel()
 
 asyncio.run(main())
 ```
@@ -1236,7 +1234,8 @@ async def messages(client):
 Assuming the MQTT client is subscribed to multiple topics, message strings are
 directed to individual tasks each supporting one topic.
 
-The following illustrates a use case for `agent` args.
+The following illustrates a use case for passing args to an `agent` (pin nos.
+are for Pyoard 1.1).
 ```py
 import asyncio
 from primitives import Broker
@@ -1319,17 +1318,23 @@ If a message causes a queue to fill, a message will silently be lost. It is the
 responsibility of the subscriber to avoid this. In the case of a `Queue`
 instance the lost message is the one causing the overflow. In the case of
 `RingbufQueue` the oldest message in the queue is discarded. In some
-applications this behaviour is preferable.
+applications this behaviour is preferable. In general `RingbufQueue` is
+preferred as it is optimised for microcontroller use and supports retrieval by
+an asynchronous iterator.
+
+If either queue type is subscribed with args, publications will queued as a
+3-tuple `(topic, message, (args...))`. There is no obvious use case for this.
 
 #### exceptions
 
 An instance of an `agent` objects is owned by a subscribing tasks but is
 executed by a publishing task. If a function used as an `agent` throws an
 exception, the traceback will point to a `Broker.publish` call.
 
-The `Broker` class does not throw exceptions. There are a number of non-fatal
-conditions which can occur such as a queue overflow or an attempt to unsubscribe
-an `agent` twice. The `Broker` will report these if `Broker.Verboase=True`.
+The `Broker` class throws a `ValueError` if `.subscribe` is called with an
+invalid `agent` type. There are a number of non-fatal conditions which can occur
+such as a queue overflow or an attempt to unsubscribe an `agent` twice. The
+`Broker` will report these if `Broker.Verbose=True`.
 
 ###### [Contents](./DRIVERS.md#0-contents)
 

v3/primitives/broker.py

@@ -14,10 +14,22 @@ class Agent:
     pass
 
 
+def _validate(a):
+    return (
+        isinstance(a, asyncio.Event)
+        or isinstance(a, Queue)
+        or isinstance(a, RingbufQueue)
+        or isinstance(a, Agent)
+        or callable(a)
+    )
+
+
 class Broker(dict):
     Verbose = True
 
     def subscribe(self, topic, agent, *args):
+        if not _validate(agent):
+            raise ValueError("Invalid agent:", agent)
         aa = (agent, args)
         if not (t := self.get(topic, False)):
             self[topic] = {aa}
@@ -51,7 +63,7 @@ def publish(self, topic, message):
                 try:
                     agent.put_nowait(t if args else t[:2])
                 except Exception:  # Queue discards current message. RingbufQueue discards oldest
-                    Broker.verbose and print(f"Message lost topic {topic} message {message}")
+                    Broker.Verbose and print(f"Message lost topic {topic} message {message}")
                 continue
             # agent is function, method, coroutine or bound coroutine
             res = agent(topic, message, *args)

v3/primitives/tests/broker_test.py

@@ -59,6 +59,7 @@ async def print_ringbuf_q(q):
 
 
 async def main():
+    Broker.Verbose = False  # Suppress q full messages
     tc = TestClass()
     q = Queue(10)
     rq = RingbufQueue(10)
@@ -96,8 +97,6 @@ async def main():
     print()
     print("Unsubscribing method")
     broker.unsubscribe("foo_topic", tc.get_data)  # Async method
-    # print("Pause 5s")
-    # await asyncio.sleep(5)
     print("Retrieving foo_topic messages from Queue")
     try:
         await asyncio.wait_for(print_queue(q), 5)
@@ -108,9 +107,19 @@ async def main():
         await asyncio.wait_for(print_ringbuf_q(rq), 5)
     except asyncio.TimeoutError:
         print("Timeout")
-    print("Check error on invalid unsubscribe")
+    print()
+    print("*** Testing error reports and exception ***")
+    print()
+    Broker.Verbose = True
+    print("*** Check error on invalid unsubscribe ***")
     broker.unsubscribe("rats", "more rats")  # Invalid topic
     broker.unsubscribe("foo_topic", "rats")  # Invalid agent
+    print("*** Check exception on invalid subscribe ***")
+    try:
+        broker.subscribe("foo_topic", "rubbish_agent")
+        print("Test FAIL")
+    except ValueError:
+        print("Test PASS")
 
 
 asyncio.run(main())