Fast forward resolved/rejected promises with await

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

Ported from: reactphp#18

Author: WyriHaximus

src/functions.php

@@ -53,18 +53,20 @@
 function await(PromiseInterface $promise)
 {
     $wait = true;
-    $resolved = null;
-    $exception = null;
+    $resolved = false;
     $rejected = false;
+    $resolvedValue = null;
+    $rejectedThrowable = null;
 
     $promise->then(
-        function ($c) use (&$resolved, &$wait) {
-            $resolved = $c;
+        function ($c) use (&$resolved, &$resolvedValue, &$wait) {
+            $resolvedValue = $c;
+            $resolved = true;
             $wait = false;
             Loop::stop();
         },
-        function ($error) use (&$exception, &$rejected, &$wait) {
-            $exception = $error;
+        function ($error) use (&$rejected, &$rejectedThrowable, &$wait) {
+            $rejectedThrowable = $error;
             $rejected = true;
             $wait = false;
             Loop::stop();
@@ -75,24 +77,40 @@ function ($error) use (&$exception, &$rejected, &$wait) {
     // argument does not show up in the stack trace in PHP 7+ only.
     $promise = null;
 
+    if ($rejected) {
+        awaitThrow($rejectedThrowable);
+    }
+
+    if ($resolved) {
+        return $resolvedValue;
+    }
+
     while ($wait) {
         Loop::run();
     }
 
     if ($rejected) {
-        // promise is rejected with an unexpected value (Promise API v1 or v2 only)
-        if (!$exception instanceof \Throwable) {
-            $exception = new \UnexpectedValueException(
-                'Promise rejected with unexpected value of type ' . (is_object($exception) ? get_class($exception) : gettype($exception))
-            );
-        }
-
-        throw $exception;
+        awaitThrow($rejectedThrowable);
     }
 
-    return $resolved;
+    return $resolvedValue;
 }
 
+/**
+ * @internal
+ * @param \Exception|\Throwable $rejectedThrowable
+ */
+function awaitThrow($rejectedThrowable)
+{
+    // promise is rejected with an unexpected value (Promise API v1 or v2 only)
+    if (!$rejectedThrowable instanceof \Exception && !$rejectedThrowable instanceof \Throwable) {
+        $rejectedThrowable = new \UnexpectedValueException(
+            'Promise rejected with unexpected value of type ' . (is_object($rejectedThrowable) ? get_class($rejectedThrowable) : gettype($rejectedThrowable))
+        );
+    }
+
+    throw $rejectedThrowable;
+}
 
 /**
  * Execute a Generator-based coroutine to "await" promises.

tests/AwaitTest.php

@@ -122,6 +122,13 @@ public function testAwaitShouldNotCreateAnyGarbageReferencesForRejectedPromise()
         $this->assertEquals(0, gc_collect_cycles());
     }
 
+    public function testAlreadyFulfilledPromiseShouldNotSuspendFiber()
+    {
+        for ($i = 0; $i < 6; $i++) {
+            $this->assertSame($i, React\Async\await(React\Promise\resolve($i)));
+        }
+    }
+
     public function testAwaitShouldNotCreateAnyGarbageReferencesForPromiseRejectedWithNullValue()
     {
         if (!interface_exists('React\Promise\CancellablePromiseInterface')) {