[gjs/ewlsh/implicit-mainloop] Implement custom GSource to handle promise queueing

[Evan Welsh <ewlsh src gnome org>]

commit fc2b3578c0b283d90eaf8d548d269efa5f9f5d2a
Author: Evan Welsh <contact evanwelsh com>
Date:   Sat Sep 4 19:58:31 2021 -0700

    Implement custom GSource to handle promise queueing

 gjs/context-private.h |   5 +-
 gjs/context.cpp       |  60 +++++++------------
 gjs/promise.cpp       | 161 ++++++++++++++++++++++++++++++++++++++++++++++++++
 gjs/promise.h         |  90 ++++++++++++++++++++++++++++
 meson.build           |   1 +
 5 files changed, 277 insertions(+), 40 deletions(-)
---
diff --git a/gjs/context-private.h b/gjs/context-private.h
index 75b04bf0..9b8c78e8 100644
--- a/gjs/context-private.h
+++ b/gjs/context-private.h
@@ -17,6 +17,7 @@
 #include <utility>  // for pair
 #include <vector>
 
+#include <gio/gio.h>
 #include <glib-object.h>
 #include <glib.h>
 
@@ -37,6 +38,7 @@
 #include "gjs/jsapi-util.h"
 #include "gjs/macros.h"
 #include "gjs/profiler.h"
+#include "gjs/promise.h"
 
 namespace js {
 class SystemAllocPolicy;
@@ -78,7 +80,7 @@ class GjsContextPrivate : public JS::JobQueue {
     std::vector<std::string> m_args;
 
     JobQueueStorage m_job_queue;
-    unsigned m_idle_drain_handler;
+    Gjs::PromiseJobQueue m_promise_queue_task;
 
     std::vector<std::pair<DestroyNotify, void*>> m_destroy_notifications;
     std::vector<Gjs::Closure::Ptr> m_async_closures;
@@ -134,7 +136,6 @@ class GjsContextPrivate : public JS::JobQueue {
     class SavedQueue;
     void start_draining_job_queue(void);
     void stop_draining_job_queue(void);
-    static gboolean drain_job_queue_idle_handler(void* data);
 
     uint8_t handle_exit_code(const char* type, const char* identifier,
                              GError** error);
diff --git a/gjs/context.cpp b/gjs/context.cpp
index d8992a58..0eeef19f 100644
--- a/gjs/context.cpp
+++ b/gjs/context.cpp
@@ -81,6 +81,7 @@
 #include "gjs/objectbox.h"
 #include "gjs/profiler-private.h"
 #include "gjs/profiler.h"
+#include "gjs/promise.h"
 #include "gjs/text-encoding.h"
 #include "modules/modules.h"
 #include "util/log.h"
@@ -320,9 +321,11 @@ gjs_context_class_init(GjsContextClass *klass)
         char *priv_typelib_dir = g_build_filename (PKGLIBDIR, "girepository-1.0", NULL);
 #endif
         g_irepository_prepend_search_path(priv_typelib_dir);
-    g_free (priv_typelib_dir);
+        g_free(priv_typelib_dir);
     }
 
+    gjs_register_native_module("_promiseNative",
+                               gjs_define_native_promise_stuff);
     gjs_register_native_module("_byteArrayNative", gjs_define_byte_array_stuff);
     gjs_register_native_module("_encodingNative",
                                gjs_define_text_encoding_stuff);
@@ -404,6 +407,8 @@ void GjsContextPrivate::unregister_notifier(DestroyNotify notify_func,
 
 void GjsContextPrivate::dispose(void) {
     if (m_cx) {
+        stop_draining_job_queue();
+
         gjs_debug(GJS_DEBUG_CONTEXT,
                   "Notifying reference holders of GjsContext dispose");
 
@@ -531,8 +536,8 @@ GjsContextPrivate::GjsContextPrivate(JSContext* cx, GjsContext* public_context)
     : m_public_context(public_context),
       m_cx(cx),
       m_owner_thread(std::this_thread::get_id()),
+      m_promise_queue_task(this),
       m_environment_preparer(cx) {
-
     JS_SetGCCallback(
         cx,
         [](JSContext*, JSGCStatus status, JS::GCReason reason, void* data) {
@@ -653,6 +658,8 @@ GjsContextPrivate::GjsContextPrivate(JSContext* cx, GjsContext* public_context)
             cx, "resource:///org/gnome/gjs/modules/esm/_bootstrap/default.js",
             "ESM bootstrap");
     }
+
+    start_draining_job_queue();
 }
 
 void GjsContextPrivate::set_args(std::vector<std::string>&& args) {
@@ -894,32 +901,15 @@ bool GjsContextPrivate::should_exit(uint8_t* exit_code_p) const {
 }
 
 void GjsContextPrivate::start_draining_job_queue(void) {
-    if (!m_idle_drain_handler) {
-        gjs_debug(GJS_DEBUG_CONTEXT, "Starting promise job queue handler");
-        m_idle_drain_handler = g_idle_add_full(
-            G_PRIORITY_DEFAULT, drain_job_queue_idle_handler, this, nullptr);
-    }
+    gjs_debug(GJS_DEBUG_CONTEXT, "Starting promise job queue task");
+    m_promise_queue_task.start();
 }
 
 void GjsContextPrivate::stop_draining_job_queue(void) {
     m_draining_job_queue = false;
-    if (m_idle_drain_handler) {
-        gjs_debug(GJS_DEBUG_CONTEXT, "Stopping promise job queue handler");
-        g_source_remove(m_idle_drain_handler);
-        m_idle_drain_handler = 0;
-    }
-}
 
-gboolean GjsContextPrivate::drain_job_queue_idle_handler(void* data) {
-    gjs_debug(GJS_DEBUG_CONTEXT, "Promise job queue handler");
-    auto* gjs = static_cast<GjsContextPrivate*>(data);
-    gjs->runJobs(gjs->context());
-    /* Uncatchable exceptions are swallowed here - no way to get a handle on
-     * the main loop to exit it from this idle handler */
-    gjs_debug(GJS_DEBUG_CONTEXT, "Promise job queue handler finished");
-    g_assert(gjs->empty() && gjs->m_idle_drain_handler == 0 &&
-             "GjsContextPrivate::runJobs() should have emptied queue");
-    return G_SOURCE_REMOVE;
+    gjs_debug(GJS_DEBUG_CONTEXT, "Stopping promise job queue task");
+    m_promise_queue_task.stop();
 }
 
 JSObject* GjsContextPrivate::getIncumbentGlobal(JSContext* cx) {
@@ -942,18 +932,13 @@ bool GjsContextPrivate::enqueuePromiseJob(JSContext* cx [[maybe_unused]],
               gjs_debug_object(job).c_str(), gjs_debug_object(promise).c_str(),
               gjs_debug_object(allocation_site).c_str());
 
-    if (m_idle_drain_handler)
-        g_assert(!empty());
-    else
-        g_assert(empty());
-
     if (!m_job_queue.append(job)) {
         JS_ReportOutOfMemory(m_cx);
         return false;
     }
 
     JS::JobQueueMayNotBeEmpty(m_cx);
-    start_draining_job_queue();
+    m_promise_queue_task.restart();
     return true;
 }
 
@@ -1031,8 +1016,8 @@ bool GjsContextPrivate::run_jobs_fallible(void) {
         }
     }
 
+    m_draining_job_queue = false;
     m_job_queue.clear();
-    stop_draining_job_queue();
     JS::JobQueueIsEmpty(m_cx);
     return retval;
 }
@@ -1041,14 +1026,12 @@ class GjsContextPrivate::SavedQueue : public JS::JobQueue::SavedJobQueue {
  private:
     GjsContextPrivate* m_gjs;
     JS::PersistentRooted<JobQueueStorage> m_queue;
-    bool m_idle_was_pending : 1;
     bool m_was_draining : 1;
 
  public:
     explicit SavedQueue(GjsContextPrivate* gjs)
         : m_gjs(gjs),
           m_queue(gjs->m_cx, std::move(gjs->m_job_queue)),
-          m_idle_was_pending(gjs->m_idle_drain_handler != 0),
           m_was_draining(gjs->m_draining_job_queue) {
         gjs_debug(GJS_DEBUG_CONTEXT, "Pausing job queue");
         gjs->stop_draining_job_queue();
@@ -1058,8 +1041,7 @@ class GjsContextPrivate::SavedQueue : public JS::JobQueue::SavedJobQueue {
         gjs_debug(GJS_DEBUG_CONTEXT, "Unpausing job queue");
         m_gjs->m_job_queue = std::move(m_queue.get());
         m_gjs->m_draining_job_queue = m_was_draining;
-        if (m_idle_was_pending)
-            m_gjs->start_draining_job_queue();
+        m_gjs->start_draining_job_queue();
     }
 };
 
@@ -1285,13 +1267,17 @@ bool GjsContextPrivate::eval(const char* script, ssize_t script_len,
     }
 
     if (exit_status_p) {
+        uint8_t code;
         if (retval.isInt32()) {
             int code = retval.toInt32();
             gjs_debug(GJS_DEBUG_CONTEXT,
                       "Script returned integer code %d", code);
             *exit_status_p = code;
+        } else if (should_exit(&code)) {
+            *exit_status_p = code;
         } else {
-            /* Assume success if no integer was returned */
+            /* Assume success if no integer was returned and should exit isn't
+             * set */
             *exit_status_p = 0;
         }
     }
@@ -1325,9 +1311,7 @@ bool GjsContextPrivate::eval_module(const char* identifier,
         return false;
     }
 
-    bool ok = true;
-    if (!JS::ModuleEvaluate(m_cx, obj))
-        ok = false;
+    bool ok = JS::ModuleEvaluate(m_cx, obj);
 
     /* The promise job queue should be drained even on error, to finish
      * outstanding async tasks before the context is torn down. Drain after
diff --git a/gjs/promise.cpp b/gjs/promise.cpp
new file mode 100644
index 00000000..9a8392fa
--- /dev/null
+++ b/gjs/promise.cpp
@@ -0,0 +1,161 @@
+/* -*- mode: C++; c-basic-offset: 4; indent-tabs-mode: nil; -*- */
+// SPDX-License-Identifier: MIT OR LGPL-2.0-or-later
+// SPDX-FileCopyrightText: 2021 Evan Welsh <contact evanwelsh com>
+
+#include <config.h>
+
+#include <js/CallArgs.h>
+#include <js/RootingAPI.h>
+#include <js/TypeDecls.h>
+#include <jsapi.h>
+
+#include "gjs/context-private.h"
+#include "gjs/promise.h"
+
+// G_PRIORITY_HIGH is -100, we set -1000 to ensure our source
+// always has the greatest priority. This means our prepare will
+// be called before other sources, and prepare will determine whether
+// we dispatch.
+#define GJS_PROMISE_JOB_QUEUE_SOURCE_PRIORITY -1000
+
+namespace detail {
+typedef gboolean (*PromiseJobQueueSourceFunc)(void* promise_queue_source);
+
+// Called to determine whether the source should run (dispatch) in the
+// next event loop iteration. If the job queue is not empty we return true
+// to schedule a dispatch, if the job queue has been empty we quit the main
+// loop. This should return execution to gjs_spin_event_loop which may
+// restart the loop if additional jobs are added.
+gboolean PromiseJobQueueSource::prepare(GSource* source,
+                                        int* timeout [[maybe_unused]]) {
+    auto promise_queue_source =
+        reinterpret_cast<PromiseJobQueueSource*>(source);
+
+    GjsContextPrivate* cx = promise_queue_source->cx;
+    if (!cx->empty())
+        return true;
+
+    g_main_context_wakeup(promise_queue_source->main_context);
+    return false;
+}
+
+// If the job queue is empty, dispatch will quit the event loop
+// otherwise it will drain the job queue. Dispatch must always
+// return G_SOURCE_CONTINUE, it should never remove the source
+// from the loop.
+gboolean PromiseJobQueueSource::dispatch(GSource* source, GSourceFunc callback,
+                                         void* data [[maybe_unused]]) {
+    auto promise_queue_source =
+        reinterpret_cast<PromiseJobQueueSource*>(source);
+
+    if (g_cancellable_is_cancelled(promise_queue_source->cancellable))
+        return G_SOURCE_REMOVE;
+
+    PromiseJobQueueSourceFunc func =
+        reinterpret_cast<PromiseJobQueueSourceFunc>(callback);
+
+    // The ready time is sometimes set to 0 to kick us out of polling,
+    // we need to reset the value here or this source will always be the
+    // next one to execute. (it will starve the other sources)
+    g_source_set_ready_time(source, -1);
+
+    func(promise_queue_source);
+
+    return G_SOURCE_CONTINUE;
+}
+
+// Removes the GjsPrivateContext reference.
+void PromiseJobQueueSource::finalize(GSource* source) {
+    reinterpret_cast<PromiseJobQueueSource*>(source)->~PromiseJobQueueSource();
+}
+
+gboolean PromiseJobQueueSource::callback(void* source) {
+    auto promise_queue_source =
+        reinterpret_cast<PromiseJobQueueSource*>(source);
+    if (g_cancellable_is_cancelled(promise_queue_source->cancellable))
+        return G_SOURCE_REMOVE;
+
+    GjsContextPrivate* cx = promise_queue_source->cx;
+    if (cx->empty())
+        g_main_context_wakeup(promise_queue_source->main_context);
+
+    // Drain the job queue.
+    cx->runJobs(cx->context());
+
+    return G_SOURCE_CONTINUE;
+}
+
+PromiseJobQueueSource* PromiseJobQueueSource::create(
+    GjsContextPrivate* cx, GMainContext* main_context) {
+    PromiseJobQueueSource* source = reinterpret_cast<PromiseJobQueueSource*>(
+        g_source_new(&source_funcs, sizeof(PromiseJobQueueSource)));
+    g_source_set_priority(source, GJS_PROMISE_JOB_QUEUE_SOURCE_PRIORITY);
+    g_source_set_callback(source, &callback, nullptr, nullptr);
+    g_source_set_name(source, "GjsPromiseJobQueueSource");
+
+    source->cx = cx;
+    source->main_context = g_main_context_ref(main_context);
+    source->cancellable = g_cancellable_new();
+
+    GSource* cancellable_source = g_cancellable_source_new(source->cancellable);
+    g_source_add_child_source(source, cancellable_source);
+    g_source_set_dummy_callback(cancellable_source);
+
+    return source;
+}
+
+void PromiseJobQueueSource::cancel() { g_cancellable_cancel(cancellable); }
+
+PromiseJobQueueSource* PromiseJobQueueSource::ref() {
+    return reinterpret_cast<PromiseJobQueueSource*>(g_source_ref(this));
+}
+void PromiseJobQueueSource::unref() { g_source_unref(this); }
+
+GSourceFuncs PromiseJobQueueSource::source_funcs = {
+    &PromiseJobQueueSource::prepare,  nullptr, &PromiseJobQueueSource::dispatch,
+    &PromiseJobQueueSource::finalize, nullptr, nullptr,
+};
+
+}  // namespace detail
+namespace Gjs {
+// Creates a new GSource with this class' state and source_funcs.
+PromiseJobQueue::PromiseJobQueue(GjsContextPrivate* cx)
+    : main_context(g_main_context_ref_thread_default()),
+      source(detail::PromiseJobQueueSource::create(cx, main_context)->ref()) {}
+
+void PromiseJobQueue::restart() {
+    if (isAttached())
+        return;
+
+    start();
+}
+
+bool PromiseJobQueue::isAttached() { return !!g_source_get_context(source); }
+
+void PromiseJobQueue::start() {
+    g_assert(!isAttached());
+    g_source_attach(source, main_context);
+}
+
+void PromiseJobQueue::stop() { source->cancel(); }
+
+};  // namespace Gjs
+GJS_JSAPI_RETURN_CONVENTION
+bool run_func(JSContext* cx, unsigned argc, JS::Value* vp) {
+    JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
+
+    auto gjs = GjsContextPrivate::from_cx(cx);
+    gjs->runJobs(cx);
+
+    args.rval().setUndefined();
+    return true;
+}
+
+JSFunctionSpec gjs_native_promise_module_funcs[] = {
+    JS_FN("run", run_func, 2, 0), JS_FS_END};
+
+bool gjs_define_native_promise_stuff(JSContext* cx,
+                                     JS::MutableHandleObject module) {
+    module.set(JS_NewPlainObject(cx));
+    return JS_DefineFunctions(cx, module, gjs_native_promise_module_funcs);
+}
diff --git a/gjs/promise.h b/gjs/promise.h
new file mode 100644
index 00000000..1a3612d7
--- /dev/null
+++ b/gjs/promise.h
@@ -0,0 +1,90 @@
+/* -*- mode: C++; c-basic-offset: 4; indent-tabs-mode: nil; -*- */
+/*
+ * SPDX-License-Identifier: MIT OR LGPL-2.0-or-later
+ * SPDX-FileCopyrightText: 2021 Evan Welsh <contact evanwelsh com>
+ */
+
+#ifndef GJS_PROMISE_H_
+#define GJS_PROMISE_H_
+
+#include <config.h>
+
+#include <gio/gio.h>
+#include <glib.h>
+
+#include <js/TypeDecls.h>
+
+class GjsContextPrivate;
+
+namespace detail {
+class PromiseJobQueueSource : public GSource {
+    // The private GJS context this source runs within.
+    GjsContextPrivate* cx;
+    GMainContext* main_context;
+    GjsAutoPointer<GCancellable, void, g_free> cancellable;
+
+    static gboolean prepare(GSource* source, gint* timeout [[maybe_unused]]);
+
+    static gboolean dispatch(GSource* source, GSourceFunc callback,
+                             gpointer data [[maybe_unused]]);
+
+    static void finalize(GSource* source);
+
+    static gboolean callback(void* source);
+
+    static GSourceFuncs source_funcs;
+
+ public:
+    /**
+     * create:
+     *
+     * @brief Creates a new GjsPromiseJobQueueSource GSource with an
+     * optional cancellable.
+     *
+     * @param cx the current JSContext
+     * @param cancellable an optional cancellable
+     *
+     * @returns the created source
+     */
+    static PromiseJobQueueSource* create(GjsContextPrivate* cx,
+                                         GMainContext* main_context);
+    void cancel();
+    PromiseJobQueueSource* ref();
+    void unref();
+};
+};  // namespace detail
+
+namespace Gjs {
+
+/**
+ * A class which holds the state for PromiseJobQueueSource
+ * GSources and the GSourceFuncs for the source behavior.
+ */
+class PromiseJobQueue {
+    // The thread-default GMainContext
+    GjsAutoPointer<GMainContext, GMainContext, g_main_context_unref>
+        main_context;
+    // The custom source.
+    GjsAutoPointer<detail::PromiseJobQueueSource, GSource, g_source_unref>
+        source;
+    unsigned source_id;
+
+ public:
+    /**
+     * @brief Constructs a new GjsPromiseJobQueue
+     *
+     * @param cx the current private context
+     */
+    PromiseJobQueue(GjsContextPrivate* cx);
+
+    void start();
+    void stop();
+    bool isAttached();
+    void restart();
+};
+};  // namespace Gjs
+
+bool gjs_define_native_promise_stuff(JSContext* cx,
+                                     JS::MutableHandleObject module);
+
+#endif  // GJS_PROMISE_H_
diff --git a/meson.build b/meson.build
index b6f6b40e..bbfee34b 100644
--- a/meson.build
+++ b/meson.build
@@ -407,6 +407,7 @@ libgjs_sources = [
     'gjs/objectbox.cpp', 'gjs/objectbox.h',
     'gjs/profiler.cpp', 'gjs/profiler-private.h',
     'gjs/text-encoding.cpp', 'gjs/text-encoding.h',
+    'gjs/promise.cpp', 'gjs/promise.h',
     'gjs/stack.cpp',
     'modules/console.cpp', 'modules/console.h',
     'modules/modules.cpp', 'modules/modules.h',