1460719 - Unnecessary (unconditonal) cloning when using "async function" syntax in child/ext-*.js

Status: RESOLVED WONTFIX

(WebExtensions :: General, enhancement)

Description

[Rob Wu [:robwu]]

async functions in child/ext-*.js are handled by wrapPromise in ExtensionCommon.jsm

When the async function syntax is used, the returned promise does not belong to context.cloneScope, and hence wrapPromise defaults to cloning the return value: https://searchfox.org/mozilla-central/rev/eb6c5214a63e20a3fff455e92c876287a8d2e188/toolkit/components/extensions/ExtensionCommon.jsm#416-417

This cloning becomes a problem if the returned value has functions, e.g.:
- https://searchfox.org/mozilla-central/rev/eb6c5214a63e20a3fff455e92c876287a8d2e188/browser/components/extensions/child/ext-devtools-panels.js#260
- https://searchfox.org/mozilla-central/rev/eb6c5214a63e20a3fff455e92c876287a8d2e188/browser/components/extensions/child/ext-devtools-panels.js#279
- https://searchfox.org/mozilla-central/rev/eb6c5214a63e20a3fff455e92c876287a8d2e188/toolkit/components/extensions/child/ext-contentScripts.js#56

To work around this automatic cloning, those implementations in our code base do this:

methodName() {
  return new context.cloneScope.Promise().resolve(() => {
    return somehowGenerateObjectInCloneScope();
  });
}

Ideally this verbose syntax should not be necessary, and we should just be able to do this:

async methodName() {
  return somehowGenerateObbjectInCloneScope();
}


There are two ways to do this.
1. Modify wrapPromise to check whether the resolved promise value is in cloneScope, and if so, skip cloning.
2. Modify the above functions to do this:

async methodName() {
  let val = somehowGenerateObbjectInCloneScope();
  return new ExtensionCommon.NoCloneSpreadArgs([val]);
}

The latter relies on the existing special logic for NoCloneSpreadArgs: https://searchfox.org/mozilla-central/rev/eb6c5214a63e20a3fff455e92c876287a8d2e188/toolkit/components/extensions/ExtensionCommon.jsm#461-464

Comment 1

[Kris Maglione [:kmag]]

Cloning is the expected default behavior. The current method of returning a Promise is the intended way to signal that we don't want cloning at the moment, and it's used rarely enough that it's not an issue.