Fix RuntimeError in ClusterPubSub sharded message generator (#3889)

* Fix RuntimeError in ClusterPubSub sharded message generator

* Applying review comments

Author: petyaslavova

redis/cluster.py

@@ -2208,7 +2208,8 @@ def _sharded_message_generator(self):
 
     def _pubsubs_generator(self):
         while True:
-            yield from self.node_pubsub_mapping.values()
+            current_nodes = list(self.node_pubsub_mapping.values())
+            yield from current_nodes
 
     def get_sharded_message(
         self, ignore_subscribe_messages=False, timeout=0.0, target_node=None

tests/test_pubsub.py

@@ -871,6 +871,120 @@ def test_pubsub_shardnumsub(self, r):
         channels = [(b"foo", 1), (b"bar", 2), (b"baz", 3)]
         assert r.pubsub_shardnumsub("foo", "bar", "baz", target_nodes="all") == channels
 
+    @pytest.mark.onlycluster
+    @skip_if_server_version_lt("7.0.0")
+    def test_ssubscribe_multiple_channels_different_nodes(self, r):
+        """
+        Test subscribing to multiple sharded channels on different nodes.
+        Validates that the generator properly handles multiple node_pubsub_mapping entries.
+        """
+        pubsub = r.pubsub()
+        channel1 = "test-channel:{0}"
+        channel2 = "test-channel:{6}"
+
+        # Subscribe to first channel
+        pubsub.ssubscribe(channel1)
+        msg = wait_for_message(pubsub, timeout=1.0, func=pubsub.get_sharded_message)
+        assert msg is not None
+        assert msg["type"] == "ssubscribe"
+
+        # Subscribe to second channel (likely different node)
+        pubsub.ssubscribe(channel2)
+        msg = wait_for_message(pubsub, timeout=1.0, func=pubsub.get_sharded_message)
+        assert msg is not None
+        assert msg["type"] == "ssubscribe"
+
+        # Verify both channels are in shard_channels
+        assert channel1.encode() in pubsub.shard_channels
+        assert channel2.encode() in pubsub.shard_channels
+
+        pubsub.close()
+
+    @pytest.mark.onlycluster
+    @skip_if_server_version_lt("7.0.0")
+    def test_ssubscribe_multiple_channels_publish_and_read(self, r):
+        """
+        Test publishing to multiple sharded channels and reading messages.
+        Validates that _sharded_message_generator properly cycles through
+        multiple node_pubsub_mapping entries.
+        """
+        pubsub = r.pubsub()
+        channel1 = "test-channel:{0}"
+        channel2 = "test-channel:{6}"
+        msg1_data = "message-1"
+        msg2_data = "message-2"
+
+        # Subscribe to both channels
+        pubsub.ssubscribe(channel1, channel2)
+
+        # Read subscription confirmations
+        for _ in range(2):
+            msg = wait_for_message(pubsub, timeout=1.0, func=pubsub.get_sharded_message)
+            assert msg is not None
+            assert msg["type"] == "ssubscribe"
+
+        # Publish messages to both channels
+        r.spublish(channel1, msg1_data)
+        r.spublish(channel2, msg2_data)
+
+        # Read messages - should get both messages
+        messages = []
+        for _ in range(2):
+            msg = wait_for_message(pubsub, timeout=1.0, func=pubsub.get_sharded_message)
+            assert msg is not None
+            assert msg["type"] == "smessage"
+            messages.append(msg)
+
+        # Verify we got messages from both channels
+        channels_received = {msg["channel"] for msg in messages}
+        assert channel1.encode() in channels_received
+        assert channel2.encode() in channels_received
+
+        pubsub.close()
+
+    @pytest.mark.onlycluster
+    @skip_if_server_version_lt("7.0.0")
+    def test_generator_handles_concurrent_mapping_changes(self, r):
+        """
+        Test that the generator properly handles mapping changes during iteration.
+        This validates the fix for the RuntimeError: dictionary changed size during iteration.
+        """
+        pubsub = r.pubsub()
+        channel1 = "test-channel:{0}"
+        channel2 = "test-channel:{6}"
+
+        # Subscribe to first channel
+        pubsub.ssubscribe(channel1)
+        msg = wait_for_message(pubsub, timeout=1.0, func=pubsub.get_sharded_message)
+        assert msg is not None
+        assert msg["type"] == "ssubscribe"
+
+        # Get initial mapping size (cluster pubsub only)
+        assert hasattr(pubsub, "node_pubsub_mapping"), "Test requires ClusterPubSub"
+        initial_size = len(pubsub.node_pubsub_mapping)
+
+        # Subscribe to second channel (modifies mapping during potential iteration)
+        pubsub.ssubscribe(channel2)
+        msg = wait_for_message(pubsub, timeout=1.0, func=pubsub.get_sharded_message)
+        assert msg is not None
+        assert msg["type"] == "ssubscribe"
+
+        # Verify mapping was updated
+        assert len(pubsub.node_pubsub_mapping) >= initial_size
+
+        # Publish and read messages - should not raise RuntimeError
+        r.spublish(channel1, "msg1")
+        r.spublish(channel2, "msg2")
+
+        messages_received = 0
+        for _ in range(2):
+            msg = wait_for_message(pubsub, timeout=1.0, func=pubsub.get_sharded_message)
+            if msg and msg["type"] == "smessage":
+                messages_received += 1
+
+        assert messages_received == 2
+        pubsub.close()
+
 
 class TestPubSubPings:
     @skip_if_server_version_lt("3.0.0")