fix: built-in MQTT bridge handling of QoS 0 messages published to the cloud topic

crates/extensions/tedge_mqtt_bridge/src/lib.rs

@@ -377,7 +377,23 @@ async fn half_bridge(
 
             // Keep track of packet IDs so we can acknowledge messages
             Event::Outgoing(Outgoing::Publish(pkid)) => {
-                if let hash_map::Entry::Vacant(e) = forward_pkid_to_received_msg.entry(pkid) {
+                if pkid == 0 {
+                    // Messages with pkid 0 (meaning QoS=0) should not be added to the hashmap
+                    // as multiple messages with the pkid=0 can be received
+                    match companion_bridge_half.recv().await {
+                        // A message was forwarded by the other bridge half, note the packet id
+                        Some(Some((topic, msg))) => {
+                            loop_breaker.forward_on_topic(topic, &msg);
+                        }
+
+                        // A healthcheck message was published, ignore this packet id
+                        Some(None) => {}
+
+                        // The other bridge half has disconnected, break the loop and shut down the bridge
+                        None => break,
+                    }
+                } else if let hash_map::Entry::Vacant(e) = forward_pkid_to_received_msg.entry(pkid)
+                {
                     match companion_bridge_half.recv().await {
                         // A message was forwarded by the other bridge half, note the packet id
                         Some(Some((topic, msg))) => {

tests/RobotFramework/tests/cumulocity/bridge_config/builtin_bridge.robot

@@ -0,0 +1,37 @@
+*** Settings ***
+Resource    ../../../resources/common.resource
+Library    Cumulocity
+Library    ThinEdgeIO
+
+Test Setup    Custom Setup
+Test Teardown    Get Logs
+
+*** Test Cases ***
+
+Connection test
+    [Documentation]    Repeatedly test the cloud connection
+    FOR    ${attempt}    IN RANGE    0    10    1
+        ${output}=    Execute Command    tedge connect c8y --test    timeout=10
+        Should Not Contain  ${output}    connection check failed
+    END
+
+
+Support publishing QoS 0 messages to c8y topic #2960
+    [Documentation]    Verify the publishing of multiple QoS 0 message directly to the cloud connection
+    ...                Note 1: Since QoS 0 aren't guarenteed to be delivered, use a non-strict assertion on the exact event count in the cloud.
+    ...                        During testing the test would reliably fail if the expected count is less than 10 messages.
+    ...                Note 2: The bridge will automatically change the QoS from 0 when translating messages from te/# to c8y/#,
+    ...                we can't use the te/# topics in the test.
+    FOR    ${attempt}    IN RANGE    0    20    1
+        Execute Command    tedge mqtt pub -q 0 c8y/s/us '400,test_q0,Test event with qos 0 attempt ${attempt}'    timeout=10
+    END
+    Cumulocity.Device Should Have Event/s    expected_text=Test event with qos 0.*    type=test_q0    minimum=10
+
+
+*** Keywords ***
+
+Custom Setup
+    ${DEVICE_SN}=    Setup
+    Set Suite Variable    ${DEVICE_SN}
+    Execute Command    tedge config set mqtt.bridge.built_in true
+    Execute Command    tedge reconnect c8y