GObject: Check implementing Gio.AsyncInitable

We want to avoid the situation in #287 altogether by making sure that
you can't implement Gio.AsyncInitable in your object without
implementing vfunc_init_async(), and print an error message that is more
likely to tell you what the problem is.

Since you _also_ can't implement vfunc_init_async() due to #72, we make
the error message more broad than it needs to be; the error message
should be changed to something like "You must define vfunc_init_async()
in order to implement Gio.AsyncInitable."

See #287.

2 files changed, 19 insertions, 0 deletions

diff --git a/installed-tests/js/testGObjectInterface.js b/installed-tests/js/testGObjectInterface.js
index a6e0ffa2..2574d936 100644
--- a/installed-tests/js/testGObjectInterface.js
+++ b/installed-tests/js/testGObjectInterface.js
@@ -300,3 +300,13 @@ describe('GObject interface', function () {
             /\[object instance wrapper GType:Gjs_GObjectImplementingGObjectInterface jsobj@0x[a-f0-9]+ native@0x[a-f0-9]+\]/);
     });
 });
+
+describe('Specific class and interface checks', function () {
+    it('Gio.AsyncInitable must implement vfunc_async_init', function () {
+        expect(() => GObject.registerClass({
+            Implements: [Gio.Initable, Gio.AsyncInitable],
+        }, class BadAsyncInitable extends GObject.Object {
+            vfunc_init() {}
+        })).toThrow();
+    });
+});
diff --git a/modules/overrides/GObject.js b/modules/overrides/GObject.js
index 5cc26082..b22d63b3 100644
--- a/modules/overrides/GObject.js
+++ b/modules/overrides/GObject.js
@@ -199,6 +199,15 @@ function _interfacePresent(required, klass) {
 }
 
 function _checkInterface(iface, proto) {
+    // Checks for specific interfaces
+
+    // Default vfunc_async_init() will run vfunc_init() in a thread and crash.
+    // Change error message when https://gitlab.gnome.org/GNOME/gjs/issues/72
+    // has been solved.
+    if (iface.$gtype.name === 'GAsyncInitable' &&
+        !Object.getOwnPropertyNames(proto).includes('vfunc_init_async'))
+        throw new Error("It's not currently possible to implement Gio.AsyncInitable.");
+
     // Check that proto implements all of this interface's required interfaces.
     // "proto" refers to the object's prototype (which implements the interface)
     // whereas "iface.prototype" is the interface's prototype (which may still