Add fiber interoperability support

While there is no technical need to add this for this, we also don't want to block other projects creating adapters. However, this interoperability support is undocumented and as such unsupported. Use at your own risk.

src/FiberFactory.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace React\Async;
+
+/**
+ * This factory its only purpose is interoperability. Where with
+ * event loops one could simply wrap another event loop. But with fibers
+ * that has become impossible and as such we provide this factory and the
+ * FiberInterface.
+ *
+ * Usage is not documented and as such not supported and might chang without
+ * notice. Use at your own risk.
+ *
+ * @internal
+ */
+final class FiberFactory
+{
+    private static ?\Closure $factory = null;
+
+    public static function create(): FiberInterface
+    {
+        return (self::factory())();
+    }
+
+    public static function factory(\Closure $factory = null): \Closure
+    {
+        if ($factory !== null) {
+            self::$factory = $factory;
+        }
+
+        return self::$factory ?? static fn (): FiberInterface => new SimpleFiber();
+    }
+}

src/FiberInterface.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace React\Async;
+
+/**
+ * This interface its only purpose is interoperability. Where with
+ * event loops one could simply wrap another event loop. But with fibers
+ * that has become impossible and as such we provide this interface and the
+ * FiberFactory.
+ *
+ * Usage is not documented and as such not supported and might chang without
+ * notice. Use at your own risk.
+ *
+ * @internal
+ */
+interface FiberInterface
+{
+    public function resume(mixed $value): void;
+
+    public function throw(mixed $throwable): void;
+
+    public function suspend(): mixed;
+}

src/SimpleFiber.php

@@ -7,7 +7,7 @@
 /**
  * @internal
  */
-final class SimpleFiber
+final class SimpleFiber implements FiberInterface
 {
     private static ?\Fiber $scheduler = null;
     private ?\Fiber $fiber = null;

src/functions.php

@@ -78,7 +78,7 @@ function async(callable $coroutine, mixed ...$args): PromiseInterface
  */
 function await(PromiseInterface $promise): mixed
 {
-    $fiber = new SimpleFiber();
+    $fiber = FiberFactory::create();
 
     $promise->then(
         function (mixed $value) use (&$resolved, $fiber): void {