Fix promise transfer settlement stalling on node 26.4+ (#565)

TransferablePromiseHolder::ResolveTask settles the receiving side's promise without running a microtask checkpoint, so under the nodejs isolate's explicit microtasks policy the awaiting continuations stay queued until unrelated JS activity runs one. Node used to mask this with an accidental per-iteration checkpoint; nodejs/node#62969 (26.4.0) removed it.

Author: maksimyaromin

src/module/transferable.cc

@@ -124,6 +124,9 @@ class TransferablePromiseHolder final : public ClassHandle {
 				} else {
 					Unmaybe(resolver->Resolve(context, value->TransferIn()));
 				}
+				// Under the nodejs isolate's explicit microtasks policy the continuations
+				// queued above would wait for an unrelated callback to run a checkpoint.
+				Isolate::GetCurrent()->PerformMicrotaskCheckpoint();
 			}
 
 			RemoteTuple<Promise::Resolver, v8::Context> resolver;

tests/promise-transfer-microtask.js

@@ -0,0 +1,20 @@
+const ivm = require('isolated-vm');
+
+(async () => {
+	const isolate = new ivm.Isolate;
+	const context = await isolate.createContext();
+	await context.evalClosure(
+		'sleep = ms => $0.applySync(undefined, [ ms ], { result: { promise: true } })',
+		[ ms => new Promise(resolve => setTimeout(resolve, ms)) ],
+		{ arguments: { reference: true } },
+	);
+	// The transferred promise must settle on its own, without waiting for
+	// another callback to wake the microtask queue.
+	setTimeout(() => {
+		console.log('fail');
+		process.exit();
+	}, 10000);
+	await context.eval('sleep(5)', { promise: true });
+	console.log('pass');
+	process.exit();
+})();