Improve async() by making its promises cancelable

Since `async()` returns a promise and those are normally cancelable, implementing this puts them in line with the rest of our ecosystem. As such the following example will throw a timeout exception from the canceled `sleep()` call. ```php $promise = async(static function (): int { echo 'a'; await(sleep(2)); echo 'b'; return time(); })(); $promise->cancel(); await($promise); ```` This builds on top of reactphp#15, reactphp#18, reactphp#19, reactphp#26, reactphp#28, reactphp#30, and reactphp#32.
WyriHaximus-labs · Feb 20, 2022 · 1dd9138 · 1dd9138
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diff --git a/README.md b/README.md
@@ -204,6 +204,108 @@ $promise->then(function (int $bytes) {
 });
 ```
 
+Promises returned by the `async` function can be cancelled and when done they will cancel any recursive `async` call 
+and any currently awaited promise using the `await` function. In the following example `echo 'b';` will never 
+be reached, and the `await` function at the bottom will also throw an exception with the following message 
+`Timer cancelled`.
+
+```php
+$promise = async(static function (): int {
+    echo 'a';
+    await(sleep(2));
+    echo 'b';
+
+    return time();
+})();
+
+$promise->cancel();
+await($promise);
+````
+
+If you however decide to try and catch that `await` you will reach `echo 'b';`. The exception you caught however 
+isn't thrown by the bottom await function. Just as with synchronous code catching it lets you ignore the exception or 
+error that is thrown.
+
+```php
+$promise = async(static function (): int {
+    echo 'a';
+    try {
+        await(sleep(2));
+    } catch (\Throwable) {
+        // No-Op
+    }
+    echo 'b';
+
+    return time();
+})();
+
+$promise->cancel();
+await($promise);
+```
+
+When a fiber is cancelled, all currently pending and future awaited promises will be cancelled. As such the following 
+example will never output `c` and a timeout exception will be thrown.
+
+```php
+$promise = async(static function (): int {
+    echo 'a';
+    try {
+        await(sleep(2));
+    } catch (\Throwable) {
+        // No-Op
+    }
+    echo 'b';
+    await(sleep(0.1));
+    echo 'c';
+
+    return time();
+})();
+
+$promise->cancel();
+await($promise);
+```
+
+Any nested `async` and `await` calls are also canceled. You can nest this as deep as you want. As long as you await 
+every promise yielding function you call. The following example will output `abc`.
+
+```php
+$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);
+```
+
+Be very much aware that if you call a promise yielding function and not await it, it will not be cancelled. The 
+following example will output `acb`.
+
+```php
+$promise = async(static function (): int {
+    echo 'a';
+    sleep(0.001)->then(static function (): void {
+        echo 'b';
+    });
+    echo 'c';
+
+    return time();
+})();
+
+$promise->cancel();
+```
+
 ### await()
 
 The `await(PromiseInterface $promise): mixed` function can be used to

diff --git a/composer.json b/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": {

diff --git a/src/FiberMap.php b/src/FiberMap.php
@@ -0,0 +1,51 @@
+<?php
+
+namespace React\Async;
+
+use Fiber;
+use React\Promise\PromiseInterface;
+
+/**
+ * @internal
+ */
+final class FiberMap
+{
+    private array $status = [];
+    private array $map   = [];
+
+    public function register(Fiber $fiber): void
+    {
+        $this->status[spl_object_hash($fiber)] = false;
+        $this->map[spl_object_hash($fiber)] = [];
+    }
+
+    public function cancel(Fiber $fiber): void
+    {
+        $this->status[spl_object_hash($fiber)] = true;
+    }
+
+    public function isCanceled(Fiber $fiber): bool
+    {
+        return $this->status[spl_object_hash($fiber)];
+    }
+
+    public function attachPromise(Fiber $fiber, PromiseInterface $promise): void
+    {
+        $this->map[spl_object_hash($fiber)][spl_object_hash($promise)] = $promise;
+    }
+
+    public function has(Fiber $fiber): bool
+    {
+        return array_key_exists(spl_object_hash($fiber), $this->map);
+    }
+
+    public function getPromises(Fiber $fiber): array
+    {
+        return $this->map[spl_object_hash($fiber)];
+    }
+
+    public function unregister(Fiber $fiber): void
+    {
+        unset($this->status[spl_object_hash($fiber)], $this->map[spl_object_hash($fiber)]);
+    }
+}
diff --git a/src/functions.php b/src/functions.php
@@ -2,6 +2,7 @@
 
 namespace React\Async;
 
+use Fiber;
 use React\EventLoop\Loop;
 use React\Promise\CancellablePromiseInterface;
 use React\Promise\Deferred;
@@ -155,17 +156,40 @@
  */
 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 {
+            if ($fiber instanceof Fiber) {
+                fiberMap()->cancel($fiber);
+                foreach (fiberMap()->getPromises($fiber) as $promise) {
+                    if (method_exists($promise, 'cancel')) {
+                        $promise->cancel();
+                    }
+                }
             }
         });
 
-        $fiber->start();
-    });
+        $lowLevelFiber = \Fiber::getCurrent();
+        if ($lowLevelFiber !== null) {
+            fiberMap()->attachPromise($lowLevelFiber, $promise);
+        }
+
+        return $promise;
+    };
 }
 
 
@@ -230,6 +254,13 @@ function await(PromiseInterface $promise): mixed
     $rejected = false;
     $resolvedValue = null;
     $rejectedThrowable = null;
+    $lowLevelFiber = \Fiber::getCurrent();
+
+    if ($lowLevelFiber !== null) {
+        if (fiberMap()->isCanceled($lowLevelFiber) && $promise instanceof CancellablePromiseInterface) {
+            $promise->cancel();
+        }
+    }
 
     $promise->then(
         function (mixed $value) use (&$resolved, &$resolvedValue, &$fiber): void {
@@ -285,6 +316,10 @@ function (mixed $throwable) use (&$rejected, &$rejectedThrowable, &$fiber): void
         throw $rejectedThrowable;
     }
 
+    if ($lowLevelFiber !== null) {
+        fiberMap()->attachPromise($lowLevelFiber, $promise);
+    }
+
     $fiber = FiberFactory::create();
 
     return $fiber->suspend();
@@ -601,3 +636,17 @@ function waterfall(array $tasks): PromiseInterface
 
     return $deferred->promise();
 }
+
+/**
+ * @internal
+ */
+function fiberMap(): FiberMap
+{
+    static $wm = null;
+
+    if ($wm === null) {
+        $wm = new FiberMap();
+    }
+
+    return $wm;
+}
diff --git a/tests/AsyncTest.php b/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 +186,110 @@ 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');
+//        $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';
+
+            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();
+    }
 }