[C] az_mqtt adapter: static clients, no heap (AZ_IOT_AZ_MQTT_STATIC_CLIENTS)

c/CHANGELOG.md

@@ -11,7 +11,8 @@
   or mbedTLS (`AZ_IOT_AZ_MQTT_TLS`), Schannel on Windows; WebSockets, HTTP CONNECT proxy, and
   key references through an OpenSSL 3 provider. Per-client sizes are set when compiled, with or
   without CMake: a footprint (`AZ_IOT_AZ_MQTT_FOOTPRINT`), separate send and receive sizes, or a
-  config header. See [docs/client-configuration.md](docs/client-configuration.md#az_mqtt-adapter-sizes).
+  config header. With `AZ_IOT_AZ_MQTT_STATIC_CLIENTS` > 0 (default 0) the adapter allocates nothing: a fixed number
+  of clients in static storage. See [docs/client-configuration.md](docs/client-configuration.md#az_mqtt-adapter-sizes).
 - Logging for support diagnostics: rotating file sink (`az_iot_log_file_sink_open()`), lines
   `<UTC ISO 8601 time> [LEVEL] [component] [thread] [file:line] message` from the built-in sinks, connection configuration,
   state changes and retries at `INFO`, and a `...` marker on truncated messages.

c/adapters/az_mqtt/CMakeLists.txt

@@ -21,6 +21,14 @@ set(AZ_IOT_AZ_MQTT_INFLIGHT_MAX "" CACHE STRING
     "az_mqtt adapter: requests in flight at once, 1-32767; MQTT 5: also the Receive Maximum advertised (empty: footprint's)")
 set(AZ_IOT_AZ_MQTT_USER_PROPERTIES_MAX "" CACHE STRING
     "az_mqtt adapter: MQTT 5 user properties per packet (empty: footprint's)")
+set(AZ_IOT_AZ_MQTT_STATIC_CLIENTS "" CACHE STRING
+    "az_mqtt adapter: clients of each MQTT version in static storage, no heap; 0 or empty: allocated when created")
+set(AZ_IOT_AZ_MQTT_TRANSPORT_SIZE "" CACHE STRING
+    "az_mqtt adapter, static clients: bytes for the transport (empty: 200 KiB on Windows, 8 KiB elsewhere)")
+set(AZ_IOT_AZ_MQTT_CONNECT_STRINGS_SIZE "" CACHE STRING
+    "az_mqtt adapter, static clients: bytes for a connect's strings (empty: 8 KiB)")
+set(AZ_IOT_AZ_MQTT_MESSAGE_STORAGE_SIZE "" CACHE STRING
+    "az_mqtt adapter: store of unacknowledged QoS 1/2 PUBLISH; 0: refuse clean_start false (MQTT 5: with session expiry > 0) (empty: send size + 18)")
 set(AZ_IOT_AZ_MQTT_CONFIG_FILE "" CACHE STRING
     "az_mqtt adapter: header with size definitions, as written in a quoted #include (empty: none)")
 
@@ -69,6 +77,8 @@ add_library(azure::iot::adapter_az_mqtt ALIAS az_iot_adapter_az_mqtt)
 target_include_directories(az_iot_adapter_az_mqtt
     PUBLIC
         $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/inc>
+    PRIVATE
+        ${PROJECT_SOURCE_DIR}/src/core # internal/crypto.h: az_iot_crypto__wipe()
 )
 target_link_libraries(az_iot_adapter_az_mqtt
     PUBLIC
@@ -86,7 +96,8 @@ elseif(NOT _az_mqtt_footprint STREQUAL "")
         "AZ_IOT_AZ_MQTT_FOOTPRINT must be DEFAULT or CONSTRAINED (got '${AZ_IOT_AZ_MQTT_FOOTPRINT}')")
 endif()
 unset(_az_mqtt_footprint)
-foreach(_v SEND_BUFFER_SIZE RECEIVE_BUFFER_SIZE INFLIGHT_MAX USER_PROPERTIES_MAX)
+foreach(_v SEND_BUFFER_SIZE RECEIVE_BUFFER_SIZE INFLIGHT_MAX USER_PROPERTIES_MAX STATIC_CLIENTS
+        TRANSPORT_SIZE CONNECT_STRINGS_SIZE MESSAGE_STORAGE_SIZE)
     if(NOT AZ_IOT_AZ_MQTT_${_v} STREQUAL "")
         target_compile_definitions(az_iot_adapter_az_mqtt PRIVATE
             AZ_IOT_AZ_MQTT_${_v}=${AZ_IOT_AZ_MQTT_${_v}})

c/adapters/az_mqtt/az_iot_az_mqtt_config.h

@@ -83,6 +83,30 @@
 #define AZ_IOT_AZ_MQTT_USER_PROPERTIES_MAX 16
 #endif
 
+/** @brief Clients of each MQTT version kept in static storage, so the adapter allocates nothing;
+ * create() returns NULL when all are in use. 0: each client is allocated when created. */
+#ifndef AZ_IOT_AZ_MQTT_STATIC_CLIENTS
+#define AZ_IOT_AZ_MQTT_STATIC_CLIENTS 0
+#endif
+
+/** @brief Static clients: bytes reserved for the transport. create() returns NULL when
+ * az_mqtt_transport_sizeof() is larger (OpenSSL: under 1 KiB; mbedTLS: about 5 KiB; Schannel:
+ * about 197 KiB). */
+#ifndef AZ_IOT_AZ_MQTT_TRANSPORT_SIZE
+#ifdef _WIN32
+#define AZ_IOT_AZ_MQTT_TRANSPORT_SIZE (200 * 1024)
+#else
+#define AZ_IOT_AZ_MQTT_TRANSPORT_SIZE (8 * 1024)
+#endif
+#endif
+
+/** @brief Static clients: bytes for the copies of a connect's strings (host, credentials, TLS
+ * paths or PEM, proxy, will, user properties), each with a NUL. A connect whose copies do not
+ * fit returns AZ_IOT_ERR_NOT_ENOUGH_SPACE. */
+#ifndef AZ_IOT_AZ_MQTT_CONNECT_STRINGS_SIZE
+#define AZ_IOT_AZ_MQTT_CONNECT_STRINGS_SIZE (8 * 1024)
+#endif
+
 #if AZ_IOT_AZ_MQTT_SEND_BUFFER_SIZE < 1 || AZ_IOT_AZ_MQTT_SEND_BUFFER_SIZE > 268435455
 #error "AZ_IOT_AZ_MQTT_SEND_BUFFER_SIZE must be 1 to 268435455"
 #endif
@@ -93,6 +117,12 @@
 #if AZ_IOT_AZ_MQTT_INFLIGHT_MAX < 1 || AZ_IOT_AZ_MQTT_INFLIGHT_MAX > 32767
 #error "AZ_IOT_AZ_MQTT_INFLIGHT_MAX must be 1 to 32767"
 #endif
+#if AZ_IOT_AZ_MQTT_STATIC_CLIENTS < 0
+#error "AZ_IOT_AZ_MQTT_STATIC_CLIENTS must be 0 or more"
+#endif
+#if AZ_IOT_AZ_MQTT_TRANSPORT_SIZE < 1 || AZ_IOT_AZ_MQTT_CONNECT_STRINGS_SIZE < 1
+#error "AZ_IOT_AZ_MQTT_TRANSPORT_SIZE and AZ_IOT_AZ_MQTT_CONNECT_STRINGS_SIZE must be 1 or more"
+#endif
 #if AZ_IOT_AZ_MQTT_USER_PROPERTIES_MAX < 1
 #error "AZ_IOT_AZ_MQTT_USER_PROPERTIES_MAX must be at least 1"
 #endif

c/adapters/az_mqtt/az_iot_mqtt_az_mqtt.c

@@ -22,20 +22,10 @@
 #include <stdlib.h>
 #include <string.h>
 
-char* az_iot_az_mqtt_strdup(const char* s)
-{
-  if (s == NULL)
-  {
-    return NULL;
-  }
-  size_t const n = strlen(s) + 1;
-  char* copy = (char*)malloc(n);
-  if (copy != NULL)
-  {
-    memcpy(copy, s, n);
-  }
-  return copy;
-}
+#if AZ_IOT_AZ_MQTT_STATIC_CLIENTS > 0 && defined(__GNUC__)
+// Static clients allocate nothing: a heap call below fails to compile.
+#pragma GCC poison malloc calloc realloc free
+#endif
 
 bool az_iot_az_mqtt_has_text(const char* s) { return s != NULL && s[0] != '\0'; }
 
@@ -141,7 +131,9 @@ const char* az_iot_az_mqtt_string_writer_add(az_iot_az_mqtt_string_writer* w, az
   return out;
 }
 
+#if AZ_IOT_AZ_MQTT_STATIC_CLIENTS == 0
 void az_iot_az_mqtt_factory_free(void* factory_ctx) { free(factory_ctx); }
+#endif
 
 void az_iot_az_mqtt_factory_destroy(az_iot_mqtt_factory* factory)
 {