Improve async() by making its promises cancellable

README.md

@@ -204,6 +204,29 @@ $promise->then(function (int $bytes) {
 });
 ```
 
+Promises returned by `async()` can be cancelled, and when done any currently and future awaited promise inside that and 
+any nested fibers with their awaited promises will also be cancelled. As such the following example will only output 
+`ab` as the [`sleep()`](https://reactphp.org/promise-timer/#sleep) between `a` and `b` is cancelled throwing a timeout 
+exception that bubbles up through the fibers ultimately to the end user through the [`await()`](#await) on the last line 
+of the example.
+
+```php
+$promise = async(static function (): int {
+    echo 'a';
+    await(async(static function(): void {
+        echo 'b';
+        await(sleep(2));
+        echo 'c';
+    })());
+    echo 'd';
+
+    return time();
+})();
+
+$promise->cancel();
+await($promise);
+```
+
 ### await()
 
 The `await(PromiseInterface $promise): mixed` function can be used to

composer.json

@@ -31,7 +31,8 @@
         "react/promise": "^2.8 || ^1.2.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.3"
+        "phpunit/phpunit": "^9.3",
+        "react/promise-timer": "^1.8"
     },
     "autoload": {
         "psr-4": {

src/FiberMap.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace React\Async;
+
+use React\Promise\PromiseInterface;
+
+/**
+ * @internal
+ */
+final class FiberMap
+{
+    private static array $status = [];
+    private static array $map   = [];
+
+    public static function register(\Fiber $fiber): void
+    {
+        self::$status[\spl_object_id($fiber)] = false;
+        self::$map[\spl_object_id($fiber)] = [];
+    }
+
+    public static function cancel(\Fiber $fiber): void
+    {
+        self::$status[\spl_object_id($fiber)] = true;
+    }
+
+    public static function isCancelled(\Fiber $fiber): bool
+    {
+        return self::$status[\spl_object_id($fiber)];
+    }
+
+    public static function setPromise(\Fiber $fiber, PromiseInterface $promise): void
+    {
+        self::$map[\spl_object_id($fiber)] = $promise;
+    }
+
+    public static function unsetPromise(\Fiber $fiber, PromiseInterface $promise): void
+    {
+        unset(self::$map[\spl_object_id($fiber)]);
+    }
+
+    public static function has(\Fiber $fiber): bool
+    {
+        return array_key_exists(\spl_object_id($fiber), self::$map);
+    }
+
+    public static function getPromise(\Fiber $fiber): ?PromiseInterface
+    {
+        return self::$map[\spl_object_id($fiber)] ?? null;
+    }
+
+    public static function unregister(\Fiber $fiber): void
+    {
+        unset(self::$status[\spl_object_id($fiber)], self::$map[\spl_object_id($fiber)]);
+    }
+}

src/functions.php

@@ -148,24 +148,69 @@
  * });
  * ```
  *
+ * Promises returned by `async()` can be cancelled, and when done any currently
+ * and future awaited promise inside that and any nested fibers with their
+ * awaited promises will also be cancelled. As such the following example will
+ * only output `ab` as the [`sleep()`](https://reactphp.org/promise-timer/#sleep)
+ * between `a` and `b` is cancelled throwing a timeout exception that bubbles up
+ * through the fibers ultimately to the end user through the [`await()`](#await)
+ * on the last line of the example.
+ *
+ * ```php
+ * $promise = async(static function (): int {
+ *     echo 'a';
+ *     await(async(static function(): void {
+ *         echo 'b';
+ *         await(sleep(2));
+ *         echo 'c';
+ *     })());
+ *     echo 'd';
+ *
+ *     return time();
+ * })();
+ *
+ * $promise->cancel();
+ * await($promise);
+ * ```
+ *
  * @param callable(mixed ...$args):mixed $function
  * @return callable(): PromiseInterface<mixed>
  * @since 4.0.0
  * @see coroutine()
  */
 function async(callable $function): callable
 {
-    return static fn (mixed ...$args): PromiseInterface => new Promise(function (callable $resolve, callable $reject) use ($function, $args): void {
-        $fiber = new \Fiber(function () use ($resolve, $reject, $function, $args): void {
-            try {
-                $resolve($function(...$args));
-            } catch (\Throwable $exception) {
-                $reject($exception);
+    return static function (mixed ...$args) use ($function): PromiseInterface {
+        $fiber = null;
+        $promise = new Promise(function (callable $resolve, callable $reject) use ($function, $args, &$fiber): void {
+            $fiber = new \Fiber(function () use ($resolve, $reject, $function, $args, &$fiber): void {
+                try {
+                    $resolve($function(...$args));
+                } catch (\Throwable $exception) {
+                    $reject($exception);
+                } finally {
+                    FiberMap::unregister($fiber);
+                }
+            });
+
+            FiberMap::register($fiber);
+
+            $fiber->start();
+        }, function () use (&$fiber): void {
+            FiberMap::cancel($fiber);
+            $promise = FiberMap::getPromise($fiber);
+            if ($promise instanceof CancellablePromiseInterface) {
+                $promise->cancel();
             }
         });
 
-        $fiber->start();
-    });
+        $lowLevelFiber = \Fiber::getCurrent();
+        if ($lowLevelFiber !== null) {
+            FiberMap::setPromise($lowLevelFiber, $promise);
+        }
+
+        return $promise;
+    };
 }
 
 
@@ -230,9 +275,18 @@ function await(PromiseInterface $promise): mixed
     $rejected = false;
     $resolvedValue = null;
     $rejectedThrowable = null;
+    $lowLevelFiber = \Fiber::getCurrent();
+
+    if ($lowLevelFiber !== null && FiberMap::isCancelled($lowLevelFiber) && $promise instanceof CancellablePromiseInterface) {
+        $promise->cancel();
+    }
 
     $promise->then(
-        function (mixed $value) use (&$resolved, &$resolvedValue, &$fiber): void {
+        function (mixed $value) use (&$resolved, &$resolvedValue, &$fiber, $lowLevelFiber, $promise): void {
+            if ($lowLevelFiber !== null) {
+                FiberMap::unsetPromise($lowLevelFiber, $promise);
+            }
+
             if ($fiber === null) {
                 $resolved = true;
                 $resolvedValue = $value;
@@ -241,7 +295,11 @@ function (mixed $value) use (&$resolved, &$resolvedValue, &$fiber): void {
 
             $fiber->resume($value);
         },
-        function (mixed $throwable) use (&$rejected, &$rejectedThrowable, &$fiber): void {
+        function (mixed $throwable) use (&$rejected, &$rejectedThrowable, &$fiber, $lowLevelFiber, $promise): void {
+            if ($lowLevelFiber !== null) {
+                FiberMap::unsetPromise($lowLevelFiber, $promise);
+            }
+
             if (!$throwable instanceof \Throwable) {
                 $throwable = new \UnexpectedValueException(
                     'Promise rejected with unexpected value of type ' . (is_object($throwable) ? get_class($throwable) : gettype($throwable))
@@ -285,6 +343,10 @@ function (mixed $throwable) use (&$rejected, &$rejectedThrowable, &$fiber): void
         throw $rejectedThrowable;
     }
 
+    if ($lowLevelFiber !== null) {
+        FiberMap::setPromise($lowLevelFiber, $promise);
+    }
+
     $fiber = FiberFactory::create();
 
     return $fiber->suspend();

tests/AsyncTest.php

@@ -11,6 +11,7 @@
 use function React\Promise\all;
 use function React\Promise\reject;
 use function React\Promise\resolve;
+use function React\Promise\Timer\sleep;
 
 class AsyncTest extends TestCase
 {
@@ -185,4 +186,108 @@ public function testAsyncReturnsPromiseThatFulfillsWithValueWhenCallbackReturnsA
         $this->assertGreaterThan(0.1, $time);
         $this->assertLessThan(0.12, $time);
     }
+
+    public function testCancel()
+    {
+        self::expectOutputString('a');
+        $this->expectException(\Exception::class);
+        $this->expectExceptionMessage('Timer cancelled');
+
+        $promise = async(static function (): int {
+            echo 'a';
+            await(sleep(2));
+            echo 'b';
+
+            return time();
+        })();
+
+        $promise->cancel();
+        await($promise);
+    }
+
+    public function testCancelTryCatch()
+    {
+        self::expectOutputString('ab');
+
+        $promise = async(static function (): int {
+            echo 'a';
+            try {
+                await(sleep(2));
+            } catch (\Throwable) {
+                // No-Op
+            }
+            echo 'b';
+
+            return time();
+        })();
+
+        $promise->cancel();
+        await($promise);
+    }
+
+    public function testNestedCancel()
+    {
+        self::expectOutputString('abc');
+        $this->expectException(\Exception::class);
+        $this->expectExceptionMessage('Timer cancelled');
+
+        $promise = async(static function (): int {
+            echo 'a';
+            await(async(static function(): void {
+                echo 'b';
+                await(async(static function(): void {
+                    echo 'c';
+                    await(sleep(2));
+                    echo 'd';
+                })());
+                echo 'e';
+            })());
+            echo 'f';
+
+            return time();
+        })();
+
+        $promise->cancel();
+        await($promise);
+    }
+
+    public function testCancelFiberThatCatchesExceptions()
+    {
+        self::expectOutputString('ab');
+        $this->expectException(\Exception::class);
+        $this->expectExceptionMessage('Timer cancelled');
+
+        $promise = async(static function (): int {
+            echo 'a';
+            try {
+                await(sleep(2));
+            } catch (\Throwable) {
+                // No-Op
+            }
+            echo 'b';
+            await(sleep(0.1));
+            echo 'c';
+
+            return time();
+        })();
+
+        $promise->cancel();
+        await($promise);
+    }
+
+    public function testNotAwaitedPromiseWillNotBeCanceled()
+    {
+        self::expectOutputString('acb');
+
+        async(static function (): int {
+            echo 'a';
+            sleep(0.001)->then(static function (): void {
+                echo 'b';
+            });
+            echo 'c';
+
+            return time();
+        })()->cancel();
+        Loop::run();
+    }
 }

tests/AwaitTest.php

@@ -332,6 +332,42 @@ public function testNestedAwaits(callable $await)
         })));
     }
 
+    /**
+     * @dataProvider provideAwaiters
+     */
+    public function testResolvedPromisesShouldBeDetached(callable $await)
+    {
+        $await(async(function () use ($await): int {
+            $fiber = \Fiber::getCurrent();
+            $await(React\Promise\Timer\sleep(0.01));
+            $this->assertNull(React\Async\FiberMap::getPromise($fiber));
+
+            return time();
+        })());
+    }
+
+    /**
+     * @dataProvider provideAwaiters
+     */
+    public function testRejectedPromisesShouldBeDetached(callable $await)
+    {
+        $this->expectException(\Exception::class);
+        $this->expectExceptionMessage('Boom!');
+
+        $await(async(function () use ($await): int {
+            $fiber = \Fiber::getCurrent();
+            try {
+                $await(React\Promise\reject(new \Exception('Boom!')));
+            } catch (\Throwable $throwable) {
+                throw $throwable;
+            } finally {
+                $this->assertNull(React\Async\FiberMap::getPromise($fiber));
+            }
+
+            return time();
+        })());
+    }
+
     public function provideAwaiters(): iterable
     {
         yield 'await' => [static fn (React\Promise\PromiseInterface $promise): mixed => React\Async\await($promise)];