Improve Condition:

1. Introduce the ADT type ConditionState to replace ?Awaitable<T>, suppressing HH_FIXME[4110]
2. Added a Finish state to remove the reference from the child awaitable to the Condition, in case of memory leak
3. Added wait_for_notification_async and ConditionNotifyee to hide the setState function

src/_Private/ConditionState.php

@@ -0,0 +1,133 @@
+<?hh
+/*
+ *  Copyright (c) 2004-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the hphp/hsl/ subdirectory of this source tree.
+ *
+ */
+
+namespace HH\Lib\_Private;
+
+use type HH\Lib\Async\Condition;
+
+interface ConditionState<T> {
+  public function waitForNotificationAsync(
+    Condition<T> $condition,
+    Awaitable<void> $notifiers,
+  ): Awaitable<T>;
+  public function trySucceed(Condition<T> $condition, T $result): bool;
+  public function tryFail(Condition<T> $condition, \Exception $exception): bool;
+}
+
+final class NotStarted<T> implements ConditionState<T> {
+  private function __construct() {}
+  <<__Memoize>>
+  public static function getInstance(): this {
+    return new self();
+  }
+
+  public async function waitForNotificationAsync(
+    Condition<T> $condition,
+    Awaitable<void> $notifiers,
+  ): Awaitable<T> {
+    $handle = ConditionWaitHandle::create($notifiers);
+    $condition->setState(new AsyncResult($handle));
+    try {
+      return await $handle;
+    } finally {
+      $condition->setState(Finished::getInstance());
+    }
+  }
+
+  public function trySucceed(Condition<T> $condition, T $result): bool {
+    $condition->setState(
+      new SyncResult(
+        async {
+          return $result;
+        },
+      ),
+    );
+    return true;
+  }
+  public function tryFail(
+    Condition<T> $condition,
+    \Exception $exception,
+  ): bool {
+    $condition->setState(
+      new SyncResult(
+        async {
+          throw $exception;
+        },
+      ),
+    );
+    return true;
+  }
+}
+
+final class AsyncResult<T> implements ConditionState<T> {
+  public function __construct(private ConditionWaitHandle<T> $resultHandle) {}
+  public function waitForNotificationAsync(
+    Condition<T> $_state_ref,
+    Awaitable<void> $_notifiers,
+  ): Awaitable<T> {
+    invariant_violation('Unable to wait for notification twice');
+  }
+  public function trySucceed(Condition<T> $condition, T $result): bool {
+    $this->resultHandle->succeed($result);
+    return true;
+  }
+  public function tryFail(
+    Condition<T> $condition,
+    \Exception $exception,
+  ): bool {
+    $this->resultHandle->fail($exception);
+    return true;
+  }
+
+}
+
+final class SyncResult<T> implements ConditionState<T> {
+  public function __construct(private Awaitable<T> $resultAwaitable) {}
+  public function waitForNotificationAsync(
+    Condition<T> $condition,
+    Awaitable<void> $_notifiers,
+  ): Awaitable<T> {
+    $condition->setState(Finished::getInstance());
+    return $this->resultAwaitable;
+  }
+
+  public function trySucceed(Condition<T> $condition, T $result): bool {
+    return false;
+  }
+  public function tryFail(
+    Condition<T> $condition,
+    \Exception $exception,
+  ): bool {
+    return false;
+  }
+}
+
+final class Finished<T> implements ConditionState<T> {
+  private function __construct() {}
+  <<__Memoize>>
+  public static function getInstance(): this {
+    return new self();
+  }
+  public function waitForNotificationAsync(
+    Condition<T> $_state_ref,
+    Awaitable<void> $_notifiers,
+  ): Awaitable<T> {
+    invariant_violation('Unable to wait for notification twice');
+  }
+  public function trySucceed(Condition<T> $condition, T $result): bool {
+    return false;
+  }
+  public function tryFail(
+    Condition<T> $condition,
+    \Exception $exception,
+  ): bool {
+    return false;
+  }
+}

src/async/Condition.php

@@ -10,48 +10,43 @@
 
 namespace HH\Lib\Async;
 
+use namespace HH\Lib\_Private;
+
 /**
  * A wrapper around ConditionWaitHandle that allows notification events
  * to occur before the condition is awaited.
  */
-class Condition<T> {
-  private ?Awaitable<T> $condition = null;
+class Condition<T> implements ConditionNotifyee<T> {
+  private _Private\ConditionState<T> $state;
+  public function __construct() {
+    $this->state = _Private\NotStarted::getInstance();
+  }
+
+  public function setState(_Private\ConditionState<T> $state): void {
+    $this->state = $state;
+  }
 
-  /**
-   * Notify the condition variable of success and set the result.
-   */
   final public function succeed(T $result): void {
-    if ($this->condition === null) {
-      $this->condition = async {
-        return $result;
-      };
-    } else {
-      invariant(
-        $this->condition is ConditionWaitHandle<_>,
-        'Unable to notify AsyncCondition twice',
-      );
-      /* HH_FIXME[4110]: Type error revealed by type-safe instanceof feature. See https://fburl.com/instanceof */
-      $this->condition->succeed($result);
+    if (!$this->state->trySucceed($this, $result)) {
+      invariant_violation('Unable to notify Condition twice');
     }
   }
 
-  /**
-   * Notify the condition variable of failure and set the exception.
-   */
   final public function fail(\Exception $exception): void {
-    if ($this->condition === null) {
-      $this->condition = async {
-        throw $exception;
-      };
-    } else {
-      invariant(
-        $this->condition is ConditionWaitHandle<_>,
-        'Unable to notify AsyncCondition twice',
-      );
-      $this->condition->fail($exception);
+    if (!$this->state->tryFail($this, $exception)) {
+      invariant_violation('Unable to notify Condition twice');
+
     }
   }
 
+  final public function trySucceed(T $result): bool {
+    return $this->state->trySucceed($this, $result);
+  }
+
+  final public function tryFail(\Exception $exception): bool {
+    return $this->state->tryFail($this, $exception);
+  }
+
   /**
    * Asynchronously wait for the condition variable to be notified and
    * return the result or throw the exception received via notification.
@@ -66,9 +61,54 @@ final public function fail(\Exception $exception): void {
   final public async function waitForNotificationAsync(
     Awaitable<void> $notifiers,
   ): Awaitable<T> {
-    if ($this->condition === null) {
-      $this->condition = ConditionWaitHandle::create($notifiers);
-    }
-    return await $this->condition;
+    return await $this->state->waitForNotificationAsync($this, $notifiers);
   }
 }
+
+
+/**
+ * Asynchronously wait for the condition variable to be notified and
+ * return the result or throw the exception received via notification.
+ *
+ * The caller must provide an Awaitable $notifiers (which must be a
+ * WaitHandle) that must not finish before the notification is received.
+ * This means $notifiers must represent work that is guaranteed to
+ * eventually trigger the notification. As long as the notification is
+ * issued only once, asynchronous execution unrelated to $notifiers is
+ * allowed to trigger the notification.
+ */
+function wait_for_notification_async<T>(
+  (function(ConditionNotifyee<T>): Awaitable<void>) $notifiers,
+): Awaitable<T> {
+  $condition = new Condition();
+  return $condition->waitForNotificationAsync($notifiers($condition));
+}
+
+interface ConditionNotifyee<-T> {
+
+  /**
+   * Notify the condition variable of success and set the $result.
+   */
+  public function succeed(T $result): void;
+  /**
+   * Notify the condition variable of success and set the $result.
+   *
+   * @return
+   *   true if the condition is set to $result successfully, false if the
+   *   condition was previously set to another result or exception.
+   */
+  public function trySucceed(T $result): bool;
+
+  /**
+   * Notify the condition variable of failure and set the exception.
+   */
+  public function fail(\Exception $exception): void;
+  /**
+   * Notify the condition variable of failure and set the $exception.
+   *
+   * @return
+   *   true if the condition is set to $exception successfully, false if the
+   *   condition was previously set to another result or exception.
+   */
+  public function tryFail(\Exception $exception): bool;
+}