Fast forward resolved/rejected promises with fibers await

This makes `await`ing an already resolved promise significantly faster.

Author: WyriHaximus

src/SimpleFiber.php

@@ -29,12 +29,6 @@ public function resume(mixed $value): void
 
     public function throw(mixed $throwable): void
     {
-        if (!$throwable instanceof \Throwable) {
-            $throwable = new \UnexpectedValueException(
-                'Promise rejected with unexpected value of type ' . (is_object($throwable) ? get_class($throwable) : gettype($throwable))
-            );
-        }
-
         if ($this->fiber === null) {
             Loop::futureTick(static fn() => \Fiber::suspend(static fn() => throw $throwable));
             return;

src/functions.php

@@ -80,15 +80,47 @@ function await(PromiseInterface $promise): mixed
 {
     $fiber = FiberFactory::create();
 
+    $resolved = false;
+    $rejected = false;
+    $resolvedValue = null;
+    $rejectedThrowable = null;
+    $useFiber = false;
+
     $promise->then(
-        function (mixed $value) use (&$resolved, $fiber): void {
-            $fiber->resume($value);
+        function (mixed $value) use (&$resolved, &$resolvedValue, &$useFiber, $fiber): void {
+            $resolved = true;
+            $resolvedValue = $value;
+
+            if ($useFiber) {
+                $fiber->resume($value);
+            }
         },
-        function (mixed $throwable) use (&$resolved, $fiber): void {
-            $fiber->throw($throwable);
+        function (mixed $throwable) use (&$rejected, &$rejectedThrowable, &$useFiber, $fiber): void {
+            if (!$throwable instanceof \Throwable) {
+                $throwable = new \UnexpectedValueException(
+                    'Promise rejected with unexpected value of type ' . (is_object($throwable) ? get_class($throwable) : gettype($throwable))
+                );
+            }
+
+            $rejected = true;
+            $rejectedThrowable = $throwable;
+
+            if ($useFiber) {
+                $fiber->throw($throwable);
+            }
         }
     );
 
+    if ($resolved) {
+        return $resolvedValue;
+    }
+
+    if ($rejected) {
+        throw $rejectedThrowable;
+    }
+
+    $useFiber = true;
+
     return $fiber->suspend();
 }
 

tests/AwaitTest.php

@@ -157,6 +157,36 @@ public function testAwaitShouldNotCreateAnyGarbageReferencesForPromiseRejectedWi
         $this->assertEquals(0, gc_collect_cycles());
     }
 
+    /**
+     * @dataProvider provideAwaiters
+     */
+    public function testAlreadyFulfilledPromiseShouldNotSuspendFiber(callable $await)
+    {
+        for ($i = 0; $i < 6; $i++) {
+            $this->assertSame($i, $await(React\Promise\resolve($i)));
+        }
+    }
+
+    /**
+     * @dataProvider provideAwaiters
+     */
+    public function testNestedAwaits(callable $await)
+    {
+        $this->assertTrue($await(new Promise(function ($resolve) use ($await) {
+            $resolve($await(new Promise(function ($resolve) use ($await) {
+                $resolve($await(new Promise(function ($resolve) use ($await) {
+                    $resolve($await(new Promise(function ($resolve) use ($await) {
+                        $resolve($await(new Promise(function ($resolve) use ($await) {
+                            Loop::addTimer(0.01, function () use ($resolve) {
+                                $resolve(true);
+                            });
+                        })));
+                    })));
+                })));
+            })));
+        })));
+    }
+
     public function provideAwaiters(): iterable
     {
         yield 'await' => [static fn (React\Promise\PromiseInterface $promise): mixed => React\Async\await($promise)];