fix: only marshal promise resolution when created on the runtime loop

NativeScript/runtime/PromiseProxy.cpp

@@ -1,19 +1,36 @@
 #include "PromiseProxy.h"
+
+#include <CoreFoundation/CoreFoundation.h>
+
 #include "Helpers.h"
+#include "Runtime.h"
 
 using namespace v8;
 
 namespace tns {
 
+// Reports whether the calling thread runs the isolate's runtime loop. That loop
+// is where timers fire and is always being pumped, so a promise resolution is
+// marshaled back to its creating thread only when that thread is the runtime
+// loop; a promise created elsewhere settles on whichever thread resolves it.
+static void IsRuntimeRunloopCallback(const FunctionCallbackInfo<Value>& args) {
+    Runtime* runtime = Runtime::GetRuntime(args.GetIsolate());
+    bool isRuntimeLoop = runtime != nullptr && CFRunLoopGetCurrent() == runtime->RuntimeLoop();
+    args.GetReturnValue().Set(isRuntimeLoop);
+}
+
 void PromiseProxy::Init(v8::Local<v8::Context> context) {
     std::string source = R"(
-        // Ensure that Promise callbacks are executed on the
-        // same thread on which they were created
-        (() => {
+        // Run a Promise's callbacks on the thread that created it, but only when
+        // that thread is the runtime loop. A Promise created on a background
+        // thread settles on whichever thread resolves it, because the background
+        // run loop may be dormant and marshaling a resolution to it would hang.
+        (function(isRuntimeRunloop) {
             global.Promise = new Proxy(global.Promise, {
                 construct: function(target, args) {
                     let origFunc = args[0];
                     let runloop = CFRunLoopGetCurrent();
+                    let originIsRuntimeLoop = isRuntimeRunloop();
 
                     let promise = new target(function(resolve, reject) {
                         function isFulfilled() {
@@ -29,7 +46,7 @@ void PromiseProxy::Init(v8::Local<v8::Context> context) {
                                 return;
                             }
                             const resolveCall = resolve.bind(this, value);
-                            if (runloop === CFRunLoopGetCurrent()) {
+                            if (!originIsRuntimeLoop || runloop === CFRunLoopGetCurrent()) {
                                 markFulfilled();
                                 resolveCall();
                             } else {
@@ -42,7 +59,7 @@ void PromiseProxy::Init(v8::Local<v8::Context> context) {
                                 return;
                             }
                             const rejectCall = reject.bind(this, reason);
-                            if (runloop === CFRunLoopGetCurrent()) {
+                            if (!originIsRuntimeLoop || runloop === CFRunLoopGetCurrent()) {
                                 markFulfilled();
                                 rejectCall();
                             } else {
@@ -60,7 +77,7 @@ void PromiseProxy::Init(v8::Local<v8::Context> context) {
                                 return orig.bind(target);
                             }
                             return typeof orig === 'function' ? function(x) {
-                                if (runloop === CFRunLoopGetCurrent()) {
+                                if (!originIsRuntimeLoop || runloop === CFRunLoopGetCurrent()) {
                                     orig.bind(target, x)();
                                     return target;
                                 }
@@ -72,7 +89,7 @@ void PromiseProxy::Init(v8::Local<v8::Context> context) {
                     });
                 }
             });
-        })();
+        })
     )";
 
     Isolate* isolate = context->GetIsolate();
@@ -83,6 +100,17 @@ void PromiseProxy::Init(v8::Local<v8::Context> context) {
 
     Local<Value> result;
     success = script->Run(context).ToLocal(&result);
+    tns::Assert(success && result->IsFunction(), isolate);
+
+    Local<v8::Function> installProxy = result.As<v8::Function>();
+
+    Local<v8::Function> isRuntimeRunloop;
+    success = v8::Function::New(context, IsRuntimeRunloopCallback).ToLocal(&isRuntimeRunloop);
+    tns::Assert(success, isolate);
+
+    Local<Value> installArgs[] = { isRuntimeRunloop };
+    Local<Value> installResult;
+    success = installProxy->Call(context, context->Global(), 1, installArgs).ToLocal(&installResult);
     tns::Assert(success, isolate);
 }
 

TestRunner/app/tests/Promises.js

@@ -130,4 +130,24 @@ describe("Promise scheduling", function () {
             done();
         });
     });
+
+    it("the 'then' callback runs for a Promise created on a background thread and resolved on the runtime loop", done => {
+        // https://github.com/NativeScript/ios/issues/330
+        // The promise is constructed on a background dispatch queue whose run
+        // loop is dormant, then resolve() is invoked on the runtime loop. The
+        // resolution must run there instead of being marshaled back to the
+        // parked background loop, otherwise the promise never settles.
+        const backgroundQueue = dispatch_get_global_queue(qos_class_t.QOS_CLASS_DEFAULT, 0);
+        dispatch_async(backgroundQueue, () => {
+            new Promise(resolve => {
+                NSOperationQueue.mainQueue.addOperationWithBlock(() => resolve("settled"));
+            }).then(value => {
+                expect(value).toBe("settled");
+                done();
+            }).catch(error => {
+                expect(true).toBe(false, "The promise rejected unexpectedly: " + error);
+                done();
+            });
+        });
+    });
 });