Asynchronous Operations: Exceptions

In general, an async operation has the following pattern:

  • Call an async function
  • Get an awaitable back
  • await the awaitable to get a result

However, sometimes an async function can throw an exception. The good news is that the same exception object that would be thrown in the non-async version of the code will be returned when we await the awaitable.

<?hh // strict

namespace Hack\UserDocumentation\AsyncOps\Exceptions\Examples\BasicException;

async function exception_thrower(): Awaitable<void> {
  throw new \Exception("Return exception handle");
}

async function basic_exception(): Awaitable<void> {
  // the handle does not throw, but result will be an Exception objection.
  // Remember, this is the same as:
  //   $handle = exception_thrower();
  //   await $handle;
  await exception_thrower();
}

<<__EntryPoint>>
function main(): void {
  \HH\Asio\join(basic_exception());
}
Output
Fatal error: Uncaught exception 'Exception' with message 'Return exception handle' in /home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/basic-exception.php:6
Stack trace:
#0 /home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/basic-exception.php(14): Hack\UserDocumentation\AsyncOps\Exceptions\Examples\BasicException\exception_thrower()
#1 /home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/basic-exception.php(19): Hack\UserDocumentation\AsyncOps\Exceptions\Examples\BasicException\basic_exception()
#2 (): Hack\UserDocumentation\AsyncOps\Exceptions\Examples\BasicException\main()
#3 {main}

The use of from_async ignores any successful awaitable results and just throw an exception of one of the awaitable results, if one of the results was an exception.

<?hh // strict

namespace Hack\UserDocumentation\AsyncOps\Exceptions\Examples\MultipleAwaitable;
use namespace HH\Lib\Vec;

async function exception_thrower(): Awaitable<void> {
  throw new \Exception("Return exception handle");
}

async function non_exception_thrower(): Awaitable<int> {
  return 2;
}

async function multiple_waithandle_exception(): Awaitable<void> {
  $handles = vec[exception_thrower(), non_exception_thrower()];
  // You will get a fatal error here with the exception thrown
  $results = await Vec\from_async($handles);
  // This won't happen
  \var_dump($results);
}

<<__EntryPoint>>
function main(): void {
  \HH\Asio\join(multiple_waithandle_exception());
}
Output
Fatal error: Uncaught exception 'Exception' with message 'Return exception handle' in /home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/multiple-awaitable-exception.php:7
Stack trace:
#0 /home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/multiple-awaitable-exception.php(15): Hack\UserDocumentation\AsyncOps\Exceptions\Examples\MultipleAwaitable\exception_thrower()
#1 /home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/multiple-awaitable-exception.php(24): Hack\UserDocumentation\AsyncOps\Exceptions\Examples\MultipleAwaitable\multiple_waithandle_exception()
#2 (): Hack\UserDocumentation\AsyncOps\Exceptions\Examples\MultipleAwaitable\main()
#3 {main}

To get around this, and get the successful results as well, we can use the utility function HH\Asio\wrap. It takes an awaitable and returns the expected result or the exception if one was thrown. The exception it gives back is of the type ResultOrExceptionWrapper.

namespace HH\Asio {
  interface ResultOrExceptionWrapper<T> {
    public function isSucceeded(): bool;
    public function isFailed(): bool;
    public function getResult(): T;
    public function getException(): \Exception;
  }
}

Taking the example above and using the wrapping mechanism, this is what the code looks like:

<?hh // strict

namespace Hack\UserDocumentation\AsyncOps\Exceptions\Examples\Wrapping;
use namespace HH\Lib\Vec;

require __DIR__ . "/../../../../vendor/hh_autoload.php";

async function exception_thrower(): Awaitable<void> {
  throw new \Exception();
}

async function non_exception_thrower(): Awaitable<int> {
  return 2;
}

async function wrapping_exceptions(): Awaitable<void> {
  $handles = vec[\HH\Asio\wrap(exception_thrower()),
              \HH\Asio\wrap(non_exception_thrower())];
  // Since we wrapped, the results will contain both the exception and the
  // integer result
  $results = await Vec\from_async($handles);
  \var_dump($results);
}

<<__EntryPoint>>
function main(): void {
  \HH\Asio\join(wrapping_exceptions());
}
Output
vec(2) {
  object(HH\Asio\WrappedException)#5 (1) {
    ["exception":"HH\Asio\WrappedException":private]=>
    object(Exception)#3 (8) {
      ["message":protected]=>
      string(0) ""
      ["string":"Exception":private]=>
      string(0) ""
      ["code":protected]=>
      int(0)
      ["file":protected]=>
      string(128) "/home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/wrapping-exceptions.php"
      ["line":protected]=>
      int(9)
      ["trace":"Exception":private]=>
      array(3) {
        [0]=>
        array(4) {
          ["file"]=>
          string(128) "/home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/wrapping-exceptions.php"
          ["line"]=>
          int(17)
          ["function"]=>
          string(78) "Hack\UserDocumentation\AsyncOps\Exceptions\Examples\Wrapping\exception_thrower"
          ["args"]=>
          array(0) {
          }
        }
        [1]=>
        array(4) {
          ["file"]=>
          string(128) "/home/ubuntu/user-documentation-2018-09-07/guides/hack/09-asynchronous-operations/10-exceptions-examples/wrapping-exceptions.php"
          ["line"]=>
          int(27)
          ["function"]=>
          string(80) "Hack\UserDocumentation\AsyncOps\Exceptions\Examples\Wrapping\wrapping_exceptions"
          ["args"]=>
          array(0) {
          }
        }
        [2]=>
        array(2) {
          ["function"]=>
          string(65) "Hack\UserDocumentation\AsyncOps\Exceptions\Examples\Wrapping\main"
          ["args"]=>
          array(0) {
          }
        }
      }
      ["previous":"Exception":private]=>
      NULL
      ["userMetadata":protected]=>
      NULL
    }
  }
  object(HH\Asio\WrappedResult)#8 (1) {
    ["result":"HH\Asio\WrappedResult":private]=>
    int(2)
  }
}