Add Condition->trySucceed and Condition->tryFail (facebook#185)

Summary:
It would allow for use cases like this:

```
async function outer(): Awaitable<void> {
  await wait_for_notification_async(
    async $notifyee ==> {
      concurrent {
        await async {
          await gen_usleep(100);
          $notifyee->trySucceed("fast condition");
        }
        await async {
          await gen_usleep(10000000000);
          $notifyee->trySucceed("slow condition");
        }
      }
    }
  );
}
```

X-link: hhvm/hsl#185

Reviewed By: fredemmott

Differential Revision: D34903436

Pulled By: Atry

fbshipit-source-id: 859d72707fa338df22522c778c35f47c23497746

Author: Atry

hphp/hsl/src/async/Condition.php

@@ -21,34 +21,58 @@ class Condition<T> {
    * Notify the condition variable of success and set the result.
    */
   final public function succeed(T $result): void {
+    invariant($this->trySucceed($result), 'Unable to notify Condition twice');
+  }
+
+  /**
+   * Notify the condition variable of failure and set the exception.
+   */
+  final public function fail(\Exception $exception): void {
+    invariant($this->tryFail($exception), 'Unable to notify Condition twice');
+  }
+
+  /**
+   * 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.
+   */
+  final public function trySucceed(T $result): bool {
     if ($this->condition === null) {
       $this->condition = async {
         return $result;
       };
+      return true;
     } else {
-      invariant(
-        $this->condition is ConditionWaitHandle<_>,
-        'Unable to notify AsyncCondition twice',
-      );
+      if (!($this->condition is ConditionWaitHandle<_>)) {
+        return false;
+      }
       /* HH_FIXME[4110]: Type error revealed by type-safe instanceof feature. See https://fburl.com/instanceof */
       $this->condition->succeed($result);
+      return true;
     }
   }
 
   /**
-   * Notify the condition variable of failure and set the exception.
+   * 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.
    */
-  final public function fail(\Exception $exception): void {
+  final public function tryFail(\Exception $exception): bool {
     if ($this->condition === null) {
       $this->condition = async {
         throw $exception;
       };
+      return true;
     } else {
-      invariant(
-        $this->condition is ConditionWaitHandle<_>,
-        'Unable to notify AsyncCondition twice',
-      );
+      if (!($this->condition is ConditionWaitHandle<_>)) {
+        return false;
+      }
       $this->condition->fail($exception);
+      return true;
     }
   }