pbio/task: don't run completed task on cancel

This adds a check to avoid canceling an already completed task.
Otherwise, it was possible that running one iteration of the
protothread could actually restart the task.

Author: dlech

lib/pbio/src/task.c

@@ -30,8 +30,12 @@ void pbio_task_init(pbio_task_t *task, pbio_task_thread_t thread, void *context)
  * Runs the task protothread until the next yield.
  * @param [in]  task    The task.
  * @returns             True if the protothread has completed, otherwise false.
+ *
+ * Do not call this on a completed task.
  */
 bool pbio_task_run_once(pbio_task_t *task) {
+    assert(task->status == PBIO_ERROR_AGAIN);
+
     if (PT_SCHEDULE(task->thread(&task->pt, task))) {
         return false;
     }
@@ -43,10 +47,14 @@ bool pbio_task_run_once(pbio_task_t *task) {
 }
 
 /**
- * Cancels @p task and runs one iteration.
+ * Cancels @p task and runs one iteration unless the task is already complete.
  * @param [in]  task The task.
  */
 void pbio_task_cancel(pbio_task_t *task) {
+    if (task->status != PBIO_ERROR_AGAIN) {
+        return;
+    }
+
     task->cancel = true;
     pbio_task_run_once(task);
 }