LibCore: Slightly rework the Core::Promise API

The previous iteration of this API was somewhat odd and rough in random
places, which degraded usability and made less than perfect sense.
This commit reworks the API to be a little closer to more
conventional promise APIs (a la javascript promises).

Also adds a test to ensure the class even works.

Userland/Libraries/LibCore/Forward.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/Forward.h>
+
 namespace Core {
 
 class AnonymousBuffer;
@@ -32,7 +34,7 @@ class Object;
 class ObjectClassRegistration;
 class ProcessStatisticsReader;
 class Socket;
-template<typename Result>
+template<typename Result, typename TError = AK::Error>
 class Promise;
 class SocketAddress;
 class TCPServer;

Userland/Libraries/LibCore/Promise.h

@@ -1,76 +1,127 @@
 /*
  * Copyright (c) 2021, Kyle Pereira <hey@xylepereira.me>
  * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ * Copyright (c) 2021-2023, Ali Mohammad Pur <mpfard@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/Concepts.h>
 #include <LibCore/EventLoop.h>
 #include <LibCore/Object.h>
 
 namespace Core {
 
-template<typename Result>
+template<typename Result, typename TError>
 class Promise : public Object {
     C_OBJECT(Promise);
 
 public:
-    Function<ErrorOr<void>(Result&)> on_resolved;
+    using ErrorType = TError;
 
-    ErrorOr<void> resolve(Result&& result)
+    Function<ErrorOr<void>(Result&)> on_resolution;
+    Function<void(ErrorType&)> on_rejection;
+
+    void resolve(Result&& result)
     {
-        m_pending_or_error = move(result);
+        m_result_or_rejection = move(result);
 
-        if (on_resolved)
-            return on_resolved(m_pending_or_error->value());
-        return {};
+        if (on_resolution) {
+            auto handler_result = on_resolution(m_result_or_rejection->value());
+            possibly_handle_rejection(handler_result);
+        }
     }
 
-    void cancel(Error error)
+    void reject(ErrorType&& error)
     {
-        m_pending_or_error = move(error);
+        m_result_or_rejection = move(error);
+        possibly_handle_rejection(*m_result_or_rejection);
     }
 
-    bool is_canceled()
+    bool is_rejected()
     {
-        return m_pending_or_error.has_value() && m_pending_or_error->is_error();
+        return m_result_or_rejection.has_value() && m_result_or_rejection->is_error();
     }
 
     bool is_resolved() const
     {
-        return m_pending_or_error.has_value() && !m_pending_or_error->is_error();
+        return m_result_or_rejection.has_value() && !m_result_or_rejection->is_error();
     }
 
-    ErrorOr<Result> await()
+    ErrorOr<Result, ErrorType>& await()
     {
-        while (!m_pending_or_error.has_value())
+        while (!m_result_or_rejection.has_value())
             Core::EventLoop::current().pump();
 
-        return m_pending_or_error.release_value();
+        return *m_result_or_rejection;
     }
 
     // Converts a Promise<A> to a Promise<B> using a function func: A -> B
     template<typename T>
-    RefPtr<Promise<T>> map(T func(Result&))
+    RefPtr<Promise<T>> map(Function<T(Result&)> func)
     {
         RefPtr<Promise<T>> new_promise = Promise<T>::construct();
-        on_resolved = [new_promise, func](Result& result) {
-            auto t = func(result);
-            return new_promise->resolve(move(t));
+        on_resolution = [new_promise, func = move(func)](Result& result) -> ErrorOr<void> {
+            new_promise->resolve(func(result));
+            return {};
+        };
+        on_rejection = [new_promise](ErrorType& error) {
+            new_promise->reject(move(error));
         };
         return new_promise;
     }
 
+    template<CallableAs<void, Result&> F>
+    Promise& when_resolved(F handler)
+    {
+        on_resolution = [handler = move(handler)](Result& result) { return handler(result); };
+        if (is_resolved()) {
+            auto handler_result = on_resolution(m_result_or_rejection->value());
+            possibly_handle_rejection(handler_result);
+        }
+
+        return *this;
+    }
+
+    template<CallableAs<ErrorOr<void>, Result&> F>
+    Promise& when_resolved(F handler)
+    {
+        on_resolution = move(handler);
+        if (is_resolved()) {
+            auto handler_result = on_resolution(m_result_or_rejection->value());
+            possibly_handle_rejection(handler_result);
+        }
+
+        return *this;
+    }
+
+    template<CallableAs<void, ErrorType&> F>
+    Promise& when_rejected(F handler)
+    {
+        on_rejection = move(handler);
+        if (is_rejected())
+            on_rejection(m_result_or_rejection->error());
+
+        return *this;
+    }
+
 private:
+    template<typename T>
+    void possibly_handle_rejection(ErrorOr<T>& result)
+    {
+        if (result.is_error() && on_rejection)
+            on_rejection(result.error());
+    }
+
     Promise() = default;
     Promise(Object* parent)
         : Object(parent)
     {
     }
 
-    Optional<ErrorOr<Result>> m_pending_or_error;
+    Optional<ErrorOr<Result, ErrorType>> m_result_or_rejection;
 };
 
 }

Userland/Libraries/LibCore/ThreadEventQueue.cpp

@@ -76,7 +76,7 @@ void ThreadEventQueue::cancel_all_pending_jobs()
 {
     Threading::MutexLocker lock(m_private->mutex);
     for (auto const& promise : m_private->pending_promises)
-        promise->cancel(Error::from_errno(ECANCELED));
+        promise->reject(Error::from_errno(ECANCELED));
 
     m_private->pending_promises.clear();
 }
@@ -87,7 +87,7 @@ size_t ThreadEventQueue::process()
     {
         Threading::MutexLocker locker(m_private->mutex);
         events = move(m_private->queued_events);
-        m_private->pending_promises.remove_all_matching([](auto& job) { return job->is_resolved() || job->is_canceled(); });
+        m_private->pending_promises.remove_all_matching([](auto& job) { return job->is_resolved() || job->is_rejected(); });
     }
 
     size_t processed_events = 0;