LibCore+Userland: Allow canceling promises

To make EventLoop cancel its managed Promises, we need the ability to
cancel them in the first place.

Userland/Libraries/LibCore/Promise.h

@@ -10,6 +10,7 @@
 #include <LibCore/Object.h>
 
 namespace Core {
+
 template<typename Result>
 class Promise : public Object {
     C_OBJECT(Promise);
@@ -19,22 +20,33 @@ public:
 
     void resolve(Result&& result)
     {
-        m_pending = move(result);
+        m_pending_or_error = move(result);
+
         if (on_resolved)
-            on_resolved(m_pending.value());
+            on_resolved(m_pending_or_error.value());
     }
 
-    bool is_resolved()
+    void cancel(Error error)
     {
-        return m_pending.has_value();
-    };
+        m_pending_or_error = move(error);
+    }
 
-    Result await()
+    bool is_canceled()
     {
-        while (!is_resolved()) {
+        return m_pending_or_error.has_value() && m_pending_or_error->is_error();
+    }
+
+    bool is_resolved() const
+    {
+        return m_pending_or_error.has_value() && !m_pending_or_error->is_error();
+    }
+
+    ErrorOr<Result> await()
+    {
+        while (!m_pending_or_error.has_value())
             Core::EventLoop::current().pump();
-        }
-        return m_pending.release_value();
+
+        return m_pending_or_error.release_value();
     }
 
     // Converts a Promise<A> to a Promise<B> using a function func: A -> B
@@ -52,6 +64,7 @@ public:
 private:
     Promise() = default;
 
-    Optional<Result> m_pending;
+    Optional<ErrorOr<Result>> m_pending_or_error;
 };
+
 }

Userland/Libraries/LibFileSystemAccessClient/Client.cpp

@@ -170,7 +170,7 @@ int Client::get_new_id()
 
 Result Client::handle_promise(int id)
 {
-    auto result = m_promises.get(id)->promise->await();
+    auto result = TRY(m_promises.get(id)->promise->await());
     m_promises.remove(id);
     return result;
 }