semphr: implement the FreeRTOS recursive mutex API

sim/semphr.cpp

@@ -1,5 +1,6 @@
 #include "semphr.h"
 #include <SDL.h>
+#include <mutex>
 #include <stdexcept>
 
 QueueHandle_t xSemaphoreCreateMutex() {
@@ -40,3 +41,18 @@ BaseType_t xSemaphoreGive(SemaphoreHandle_t xSemaphore) {
   pxQueue->queue.pop_back();
   return true;
 }
+
+SemaphoreHandle_t xSemaphoreCreateRecursiveMutex() {
+  // Only ever locked with portMAX_DELAY, so std::recursive_mutex maps directly.
+  return reinterpret_cast<SemaphoreHandle_t>(new std::recursive_mutex());
+}
+
+BaseType_t xSemaphoreTakeRecursive(SemaphoreHandle_t xSemaphore, TickType_t /*xTicksToWait*/) {
+  reinterpret_cast<std::recursive_mutex*>(xSemaphore)->lock();
+  return true;
+}
+
+BaseType_t xSemaphoreGiveRecursive(SemaphoreHandle_t xSemaphore) {
+  reinterpret_cast<std::recursive_mutex*>(xSemaphore)->unlock();
+  return true;
+}

sim/semphr.h

@@ -9,3 +9,10 @@ QueueHandle_t xSemaphoreCreateMutex();
 
 BaseType_t xSemaphoreTake(SemaphoreHandle_t xSemaphore, TickType_t xTicksToWait);
 BaseType_t xSemaphoreGive(SemaphoreHandle_t xSemaphore);
+
+// Recursive mutex, as used by FS. Handles from xSemaphoreCreateRecursiveMutex
+// must only be passed to the *Recursive functions and vice versa (same rule as
+// real FreeRTOS).
+SemaphoreHandle_t xSemaphoreCreateRecursiveMutex();
+BaseType_t xSemaphoreTakeRecursive(SemaphoreHandle_t xSemaphore, TickType_t xTicksToWait);
+BaseType_t xSemaphoreGiveRecursive(SemaphoreHandle_t xSemaphore);