eina: add helper to return already known to fail future.

src/lib/eina/eina_promise.c

@@ -814,6 +814,27 @@ eina_future_resolved(Eina_Future_Scheduler *scheduler, Eina_Value value)
    return NULL;
 }
 
+EAPI Eina_Future *
+eina_future_rejected(Eina_Future_Scheduler *scheduler, Eina_Error err)
+{
+   Eina_Promise *p;
+   Eina_Future *f;
+
+   EINA_SAFETY_ON_NULL_GOTO(scheduler, error);
+
+   p = eina_promise_new(scheduler, _dummy_cancel, NULL);
+   EINA_SAFETY_ON_NULL_GOTO(p, error);
+
+   f = eina_future_new(p);
+   EINA_SAFETY_ON_NULL_GOTO(f, error);
+
+   eina_promise_reject(p, err);
+   return f;
+
+ error:
+   return NULL;
+}
+
 EAPI Eina_Future *
 eina_future_then_from_desc(Eina_Future *prev, const Eina_Future_Desc desc)
 {

src/lib/eina/eina_promise.h

@@ -785,6 +785,33 @@ EAPI Eina_Future *eina_future_new(Eina_Promise *p) EINA_ARG_NONNULL(1) EINA_WARN
  */
 EAPI Eina_Future *eina_future_resolved(Eina_Future_Scheduler *scheduler, Eina_Value value) EINA_ARG_NONNULL(1);
 
+/**
+ * Creates a new future that is already rejected to a specified error.
+ *
+ * This function creates a new future with an already known error,
+ * that will be resolved and dispatched by the given @a scheduler as
+ * usual.
+ *
+ * This is a helper that behaves the same as eina_promise_new()
+ * followed by eina_future_new() and then eina_promise_rejected().
+ *
+ * Futures can also be canceled using eina_future_cancel(), which will
+ * cause the whole chain to be cancelled alongside with any pending
+ * promise.
+ *
+ * @param scheduler The scheduler to use.
+ * @param err An Eina_Error value
+ *
+ * @return The future or @c NULL on error.
+ *
+ * @see eina_promise_new()
+ * @see eina_future_new()
+ * @see eina_promise_reject()
+ * @see eina_promise_resolve()
+ * @see eina_future_cancel()
+ */
+EAPI Eina_Future *eina_future_rejected(Eina_Future_Scheduler *scheduler, Eina_Error err);
+
 /**
  * Register an Eina_Future_Desc to be used when the future is resolve/rejected.
  *