By default keep mutex locked if we tried to write. The wolfSSL TLS en…

…gine requires an SSL_Write that returns WANT_WRITE to be called with the same buffer/sz, not a different one, even if no data was sent. If user wants to enable the feature anyways they can use `WOLFMQTT_ALLOW_NODATA_UNLOCK`. Only the write that has this logic as the issue doesn't exist for an SSL_Read.

src/mqtt_client.c

@@ -26,6 +26,32 @@
 
 #include "wolfmqtt/mqtt_client.h"
 
+/* DOCUMENTED BUILD OPTIONS:
+ *
+ * WOLFMQTT_MULTITHREAD: Enables multi-thread support with mutex protection on
+ *  client struct, write and read. When a pending response is needed its added
+ *  to a linked list and if another thread reads the expected response it is
+ *  flagged, so the other thread knows it completed.
+ *
+ * WOLFMQTT_NONBLOCK: Enabled transport support for returning WANT READ/WRITE,
+ *  which becomes WOLFMQTT_CODE_CONTINUE. This prevents blocking if the
+ *  transport (socket) has no data.
+ *
+ * WOLFMQTT_V5: Enables MQTT v5.0 support
+ *
+ * WOLFMQTT_ALLOW_NODATA_UNLOCK: Used with multi-threading and non-blocking to
+ *   allow unlock if no data was sent/received. Note the TLS stack typically
+ *   requires an attempt to write to continue with same write, not different.
+ *   By default if we attempt a write we keep the mutex locked and return
+ *   MQTT_CODE_CONTINUE
+ *
+ * WOLFMQTT_USER_THREADING: Allows custom mutex functions to be defined by the
+ *  user. Example: wm_SemInit
+ *
+ * WOLFMQTT_DEBUG_CLIENT: Enables verbose PRINTF for the client code.
+ */
+
+
 /* Private functions */
 
 /* forward declarations */
@@ -221,7 +247,7 @@ static int MqttReadStart(MqttClient* client, MqttMsgStat* stat)
         MQTT_TRACE_MSG("Warning, recv already locked!");
         rc = MQTT_CODE_ERROR_SYSTEM;
     }
-    /* detect if a write is already in progress */
+    /* detect if a read is already in progress */
     if (wm_SemLock(&client->lockClient) == 0) {
         if (client->read.total > 0) {
             MQTT_TRACE_MSG("Partial read in progress!");
@@ -692,7 +718,8 @@ static int MqttClient_DecodePacket(MqttClient* client, byte* rx_buf,
             #ifdef WOLFMQTT_DISCONNECT_CB
             /* Call disconnect callback with reason code */
             if ((packet_obj != NULL) && client->disconnect_cb) {
-                client->disconnect_cb(client, p_disc->reason_code, client->disconnect_ctx);
+                client->disconnect_cb(client, p_disc->reason_code,
+                    client->disconnect_ctx);
             }
             #endif
         #else
@@ -1087,8 +1114,9 @@ static int MqttClient_WaitType(MqttClient *client, void *packet_obj,
             if (rc <= 0) {
             #ifdef WOLFMQTT_NONBLOCK
                 if (rc == MQTT_CODE_CONTINUE &&
-                        (client->packet.stat > MQTT_PK_BEGIN ||
-                         client->read.total > 0)) {
+                    (client->packet.stat > MQTT_PK_BEGIN ||
+                     client->read.total > 0)
+                ) {
                     /* advance state, since we received some data */
                     mms_stat->read = MQTT_MSG_HEADER;
                 }
@@ -1580,7 +1608,11 @@ int MqttClient_Connect(MqttClient *client, MqttConnect *mc_connect)
         /* Send connect packet */
         rc = MqttPacket_Write(client, client->tx_buf, xfer);
     #ifdef WOLFMQTT_NONBLOCK
-        if (rc == MQTT_CODE_CONTINUE && client->write.total > 0) {
+        if (rc == MQTT_CODE_CONTINUE
+        #ifdef WOLFMQTT_ALLOW_NODATA_UNLOCK
+            && client->write.total > 0
+        #endif
+        ) {
             /* keep send locked and return early */
             return rc;
         }
@@ -1986,9 +2018,14 @@ static int MqttPublishMsg(MqttClient *client, MqttPublish *publish,
             /* Send publish packet */
             rc = MqttPacket_Write(client, client->tx_buf, xfer);
         #ifdef WOLFMQTT_NONBLOCK
-            if (rc == MQTT_CODE_CONTINUE && client->write.total > 0)
+            if (rc == MQTT_CODE_CONTINUE
+            #ifdef WOLFMQTT_ALLOW_NODATA_UNLOCK
+                && client->write.total > 0
+            #endif
+            ) {
                 /* keep send locked and return early */
                 return rc;
+            }
         #endif
             client->write.len = 0; /* reset len, so publish chunk resets */
 
@@ -2175,7 +2212,11 @@ int MqttClient_Subscribe(MqttClient *client, MqttSubscribe *subscribe)
         /* Send subscribe packet */
         rc = MqttPacket_Write(client, client->tx_buf, xfer);
     #ifdef WOLFMQTT_NONBLOCK
-        if (rc == MQTT_CODE_CONTINUE && client->write.total > 0) {
+        if (rc == MQTT_CODE_CONTINUE
+        #ifdef WOLFMQTT_ALLOW_NODATA_UNLOCK
+            && client->write.total > 0
+        #endif
+        ) {
             /* keep send locked and return early */
             return rc;
         }
@@ -2276,7 +2317,11 @@ int MqttClient_Unsubscribe(MqttClient *client, MqttUnsubscribe *unsubscribe)
         /* Send unsubscribe packet */
         rc = MqttPacket_Write(client, client->tx_buf, xfer);
     #ifdef WOLFMQTT_NONBLOCK
-        if (rc == MQTT_CODE_CONTINUE && client->write.total > 0) {
+        if (rc == MQTT_CODE_CONTINUE
+        #ifdef WOLFMQTT_ALLOW_NODATA_UNLOCK
+            && client->write.total > 0
+        #endif
+        ) {
             /* keep send locked and return early */
             return rc;
         }
@@ -2369,7 +2414,11 @@ int MqttClient_Ping_ex(MqttClient *client, MqttPing* ping)
         /* Send ping req packet */
         rc = MqttPacket_Write(client, client->tx_buf, xfer);
     #ifdef WOLFMQTT_NONBLOCK
-        if (rc == MQTT_CODE_CONTINUE && client->write.total > 0) {
+        if (rc == MQTT_CODE_CONTINUE
+        #ifdef WOLFMQTT_ALLOW_NODATA_UNLOCK
+            && client->write.total > 0
+        #endif
+        ) {
             /* keep send locked and return early */
             return rc;
         }
@@ -2461,8 +2510,11 @@ int MqttClient_Disconnect_ex(MqttClient *client, MqttDisconnect *p_disconnect)
     rc = MqttPacket_Write(client, client->tx_buf, xfer);
 #ifdef WOLFMQTT_NONBLOCK
     /* if disconnect context avail allow partial write in non-blocking mode */
-    if (p_disconnect != NULL &&
-            rc == MQTT_CODE_CONTINUE && client->write.total > 0) {
+    if (p_disconnect != NULL && rc == MQTT_CODE_CONTINUE
+    #ifdef WOLFMQTT_ALLOW_NODATA_UNLOCK
+        && client->write.total > 0
+    #endif
+    ) {
         /* keep send locked and return early */
         return rc;
     }
@@ -2544,7 +2596,11 @@ int MqttClient_Auth(MqttClient *client, MqttAuth* auth)
         /* Send authentication packet */
         rc = MqttPacket_Write(client, client->tx_buf, xfer);
     #ifdef WOLFMQTT_NONBLOCK
-        if (rc == MQTT_CODE_CONTINUE && client->write.total > 0) {
+        if (rc == MQTT_CODE_CONTINUE
+        #ifdef WOLFMQTT_ALLOW_NODATA_UNLOCK
+            && client->write.total > 0
+        #endif
+        ) {
             /* keep send locked and return early */
             return rc;
         }