2068874 - Failed event dispatch can reply with a non-cloneable error and mask the real failure

Status: RESOLVED FIXED

(Remote Protocol :: Marionette, defect, P3)

Description

[Henrik Skupin [:whimboo][⌚️UTC+2]]

MarionetteCommandsChild.#dispatchEvent converts a failed event synthesis into an AbortError so the actor proxy can retry it:

    } catch (e) {
      if (e.message.includes("NS_ERROR_FAILURE")) {
        // Event dispatch failed. Re-throwing as AbortError to allow retrying
        // to dispatch the event.
        throw new DOMException(
          `Failed to dispatch event "${eventName}": ${e.message}`,
          "AbortError"
        );
      }

      throw e;

The check only looks at message, but an XPCOM exception carries the result code in name and can have an empty message. Such an error is not converted, falls through to receiveMessage, and is returned raw as { error: e, isWebDriverError: false }. An nsIException is not structured-cloneable, so the reply itself fails and the caller only sees:

    unknown error (500): [Exception... "MarionetteCommands:MarionetteCommandsParent:_dispatchEvent:
    message reply cannot be cloned." nsresult: "0x80004005 (DataCloneError)" location: "<unknown>" data: no]

The real error is lost. Instrumenting the catch shows name=NS_ERROR_FAILURE, result=2147500037 and an empty message.

Steps to reproduce: dispatch a touch pointerMove against a parent process page with system access enabled and remote.events.async.touch.enabled disabled. Changing the condition to also accept e.name === "NS_ERROR_FAILURE" turns the reply into:

    unknown error (500): AbortError: Failed to dispatch event "synthesizeTouchAtPoint": NS_ERROR_FAILURE

which is actionable. This only unmasks the error and does not make the dispatch succeed, so the underlying NS_ERROR_FAILURE from synthesizing a touch event in a parent process page needs to be looked at separately.

Bug 1983780 fails with the same DataCloneError signature and could maybe become diagnosable. We could reopen if the fix is easy enough.

Comment 1

[Henrik Skupin [:whimboo][⌚️UTC+2]]

Attachment: Bug 2068874 - [marionette] Detect XPCOM exceptions when event dispatching fails.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/c9ccc78915ec
https://hg.mozilla.org/integration/autoland/rev/6b49efb97177
[marionette] Detect XPCOM exceptions when event dispatching fails. r=Sasha

Comment 3

[Silaghi Andreea]

https://hg.mozilla.org/mozilla-central/rev/6b49efb97177