Fix MQTTv5 QoS 2 CONNACK interop and add WOLFMQTT_MAX_QOS build cap by dgarske · Pull Request #537 · wolfSSL/wolfMQTT

dgarske · 2026-05-13T16:52:35Z

Fix MQTTv5 QoS 2 interop: stop emitting Maximum QoS = 2 in CONNACK ([MQTT-3.2.2.3.4] Protocol Error; mosquitto disconnects).
Add WOLFMQTT_MAX_QOS (0/1/2, default 2) wired through configure.ac (--enable-max-qos=N) and CMake (-DWOLFMQTT_MAX_QOS=N).
Capped builds compile out the broker QoS-2 state machine (PUBREC/PUBREL/PUBCOMP, inbound dedup), cap subscribe grants and will QoS, and DISCONNECT QoS > cap publishes with reason 0x9B QoS Not Supported. Client clamps max_qos against the build cap. ~2.6 KB .text savings at MAX_QOS=1.
CI: broker-check.yml adds MAX_QOS=0/1/2 entries (capped = build-only since broker.test has QoS-2 cases); cmake-build.yml now a matrix over the same values.
Minor: CMake broker+TLS include-order fix.

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Pull request overview

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Removes an invalid MQTT v5 CONNACK property that violated the spec and caused strict clients to disconnect.

Changes:

Drop emission of MQTT_PROP_MAX_QOS = 2 in CONNACK, which is a protocol error per [MQTT-3.2.2.3.4].
Add an explanatory comment documenting why the property is intentionally omitted.

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embhorn · 2026-05-13T18:35:33Z

Should this be a compile-time configuration option for the broker? Then if QoS-2 is max, don't send the property.

…pped

Builds with -DWOLFMQTT_MAX_QOS=1 (or 0) now compile out the broker's inbound QoS-2 dedup state, BrokerHandle_PublishRel / PublishRec, and the PUBLISH_REC/REL/COMP dispatch cases; subscribe grants are capped at the build's max; PUBLISH at QoS > cap is rejected with a v5 DISCONNECT reason 0x9B (QoS Not Supported); will QoS is clamped on CONNECT. Client side clamps initial max_qos and post-CONNACK property value against the same cap so MqttPublishMsg rejects out-of-range publishes locally. Default build (WOLFMQTT_MAX_QOS=2) is unchanged: broker.test passes in full, no behavior change. Measured code-size delta on src/.libs/mqtt_broker (.text): default 48178 -> capped 45587 (-2591 bytes / -5.4%). Verified against mosquitto_pub -q 2: capped broker disconnects with "Message QoS not supported on broker, try a lower QoS."

…akeLists

… mqtt_types.h and reorder includes in mqtt_broker.c

Copilot

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 13 comments.

+    if (pub.qos > WOLFMQTT_MAX_QOS) {
+        WBLOG_ERR(broker,
+            "broker: PUBLISH QoS %d exceeds WOLFMQTT_MAX_QOS=%d sock=%d",
+            pub.qos, WOLFMQTT_MAX_QOS, (int)bc->sock);
+    #ifdef WOLFMQTT_V5
+        if (bc->protocol_level >= MQTT_CONNECT_PROTOCOL_LEVEL_5) {
+            (void)BrokerSend_Disconnect(bc, MQTT_REASON_QOS_NOT_SUPPORTED);
+        }
+    #endif
+        rc = MQTT_CODE_ERROR_MALFORMED_DATA;
+        goto publish_cleanup;
+    }


+         * sent Will QoS > advertised Max QoS would already be in
+         * Protocol Error territory, but for v3.1.1 (no advertisement)
+         * we silently downgrade rather than rejecting CONNECT. */


+ * Define in user_settings.h to compile a QoS-capped client/broker. Wired
+ * into broker CONNACK Maximum QoS property emission today; broader code
+ * gating lives on the mqtt_qos_max branch. */
+#ifndef WOLFMQTT_MAX_QOS
+    #define WOLFMQTT_MAX_QOS 2
+#endif
+#if (WOLFMQTT_MAX_QOS < 0) || (WOLFMQTT_MAX_QOS > 2)
+    #error "WOLFMQTT_MAX_QOS must be 0, 1, or 2"


+/* Maximum QoS supported by this build. Legal values: 0, 1, 2. Default 2.
+ * Define in user_settings.h to compile a QoS-capped client/broker. Wired
+ * into broker CONNACK Maximum QoS property emission today; broader code
+ * gating lives on the mqtt_qos_max branch. */


+          - name: "Broker MAX_QOS=1 (build only)"
+            cflags: ""
+            wolfmqtt_opts: "--enable-v5 --enable-broker --enable-max-qos=1"
+            skip_broker_test: "yes"


    T12_PROPS=yes
    grep -q "Property CB: Type 37" "${TMP_DIR}/t12.log" 2>/dev/null || T12_PROPS=no
-    grep -q "Property CB: Type 36" "${TMP_DIR}/t12.log" 2>/dev/null || T12_PROPS=no
+    grep -q "Property CB: Type 40" "${TMP_DIR}/t12.log" 2>/dev/null || T12_PROPS=no


+        # Smoke-build the CMake project under each WOLFMQTT_MAX_QOS value
+        # to keep the build-option plumbing exercised. "" means leave the
+        # cache value at its 2 default.


 #include "wolfmqtt/mqtt_broker.h"
+#include "wolfmqtt/mqtt_types.h"


+#if !defined(WOLFMQTT_USER_SETTINGS) && \
+    !defined(_WIN32) && !defined(USE_WINDOWS_API)
+    /* If options.h is missing use the "./configure" script. Otherwise, copy
+     * the template "wolfmqtt/options.h.in" into "wolfmqtt/options.h" */
+    #include <wolfmqtt/options.h>
+#endif


+        /* [MQTT-3.2.2.3.4] Maximum QoS property MUST be 0 or 1. Absence
+         * of the property signals server supports Maximum QoS 2. Emitting
+         * Maximum QoS = 2 is a Protocol Error and strict v5 clients (e.g.
+         * mosquitto) will disconnect on receipt. Emit the property only
+         * when this build caps below QoS 2 via WOLFMQTT_MAX_QOS. */
+    #if WOLFMQTT_MAX_QOS < 2
        prop = MqttProps_Add(&ack.props);
        if (prop != NULL) {
            prop->type = MQTT_PROP_MAX_QOS;
-            prop->data_byte = MQTT_QOS_2;
+            prop->data_byte = (byte)WOLFMQTT_MAX_QOS;
        }
+    #endif


dgarske self-assigned this May 13, 2026

Copilot AI review requested due to automatic review settings May 13, 2026 16:52

Copilot AI reviewed May 13, 2026

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Fix for MQTTv5 QoS 2 interop

13f42e6

dgarske force-pushed the broker_fixes branch from 8017b09 to 13f42e6 Compare May 13, 2026 18:21

Copilot started reviewing on behalf of dgarske May 13, 2026 19:16 View session

dgarske added 5 commits May 13, 2026 15:23

Add WOLFMQTT_MAX_QOS knob; broker emits Max QoS property only when ca…

13bf635

…pped

Add --enable-max-qos / WOLFMQTT_MAX_QOS option to configure.ac and CM…

01fe400

…akeLists

CI: exercise --enable-max-qos=0|1|2 in broker-check and cmake-build

40644b1

Fix CMake broker+TLS build: include options.h in mqtt_broker.h before…

3502df2

… mqtt_types.h and reorder includes in mqtt_broker.c

dgarske changed the title ~~Fix for MQTTv5 QoS 2 interop~~ Fix MQTTv5 QoS 2 CONNACK interop and add WOLFMQTT_MAX_QOS build cap May 14, 2026

dgarske assigned embhorn and unassigned dgarske May 14, 2026

dgarske requested review from Copilot and embhorn May 14, 2026 20:29

Copilot AI reviewed May 14, 2026

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Copilot started reviewing on behalf of dgarske May 14, 2026 20:42 View session

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Fix MQTTv5 QoS 2 CONNACK interop and add WOLFMQTT_MAX_QOS build cap#537

Fix MQTTv5 QoS 2 CONNACK interop and add WOLFMQTT_MAX_QOS build cap#537
dgarske wants to merge 6 commits into
wolfSSL:masterfrom
dgarske:broker_fixes

dgarske commented May 13, 2026 •

edited

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		#include "wolfmqtt/mqtt_broker.h"
		#include "wolfmqtt/mqtt_types.h"

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dgarske commented May 13, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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embhorn commented May 13, 2026

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dgarske commented May 13, 2026 •

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