feat: Apply MQTT duplex client as a frame for TA

Apply a `mosquitto` based client as the frame of the MQTT protocol interface of
tangled-accelerator.

accelerator/BUILD

@@ -26,6 +26,17 @@ cc_binary(
     ],
 )
 
+cc_binary(
+    name = "accelerator_mqtt",
+    srcs = ["mqtt_main.c"],
+    deps = [
+        ":ta_config",
+        ":ta_errors",
+        "//mqtt_utils",
+        "@entangled//utils/handles:signal",
+    ],
+)
+
 cc_image(
     name = "ta_image",
     binary = ":accelerator",

accelerator/mqtt_main.c

@@ -0,0 +1,52 @@
+#include "mqtt_utils/client_common.h"
+#include "mqtt_utils/duplex_callback.h"
+#include "mqtt_utils/duplex_utils.h"
+
+int main(int argc, char *argv[]) {
+  rc_mosq_retcode_t ret;
+  mosq_config_t cfg;
+  struct mosquitto *mosq = NULL;
+
+  // Initialize `mosq` and `cfg`
+  // if we want to opertate this program under multi-threading, see https://github.com/eclipse/mosquitto/issues/450
+  duplex_config_init(&mosq, &cfg);
+
+  // Set callback functions
+  ret = duplex_callback_func_set(mosq, &cfg);
+  if (ret) {
+    fprintf(stderr, "Error: %s\n", mosquitto_strerror(ret));
+    goto done;
+  }
+
+  // Set the configures and message for testing
+  ret = gossip_channel_set(&cfg, HOST, TOPIC, TOPIC_RES);
+  if (ret) {
+    fprintf(stderr, "Error: %s\n", mosquitto_strerror(ret));
+    goto done;
+  }
+
+  // Set the message that is going to be sent. This function could be used in the function `duplex_loop`
+  // We just put it here for demostration.
+  ret = gossip_message_set(&cfg, MESSAGE);
+  if (ret) {
+    fprintf(stderr, "Error: %s\n", mosquitto_strerror(ret));
+    goto done;
+  }
+
+  // Set cfg as `userdata` field of `mosq` which allows the callback functions to use `cfg`.
+  mosquitto_user_data_set(mosq, &cfg);
+
+  // Start listening subscribing topics, once we received a message from the listening topics, we can send corresponding
+  // message.
+  // if we need to take the above task forever, just put it in a infinit loop.
+  ret = duplex_loop(mosq, &cfg);
+  if (ret) {
+    fprintf(stderr, "Error: %s\n", mosquitto_strerror(ret));
+  }
+
+done:
+  mosquitto_destroy(mosq);
+  mosquitto_lib_cleanup();
+  mosq_config_cleanup(&cfg);
+  return ret;
+}

mqtt_utils/BUILD

@@ -0,0 +1,28 @@
+cc_library(
+    name = "mqtt_utils",
+    srcs = [
+        "duplex_callback.c",
+        "duplex_utils.c",
+    ],
+    hdrs = [
+        "duplex_callback.h",
+        "duplex_utils.h",
+    ],
+    visibility = ["//visibility:public"],
+    deps = [":mqtt_common"],
+)
+
+cc_library(
+    name = "mqtt_common",
+    srcs = [
+        "client_common.c",
+        "pub_utils.c",
+        "sub_utils.c",
+    ],
+    hdrs = [
+        "client_common.h",
+        "pub_utils.h",
+        "sub_utils.h",
+    ],
+    deps = ["//third_party:mosquitto"],
+)