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worker: avoid potential deadlock on NearHeapLimit

Rodrigo Muino Tomonari requested to merge github/fork/santigimeno/santi/fix_38208 into master

It can happen that the NearHeapLimit callback is called while calling the oninit() function on MessagePort construction causing a deadlock when the Worker::Exit() method is called, as the mutex_ was already held on the CreateEnvMessagePort() method. To fix it, just use the mutex_ to protect the child_port_data_ variable and avoid holding it when creating the MessagePort. Also, return early from Worker::Run() if the worker message port could not be created.

Fixes: https://github.com/nodejs/node/issues/38208.

A couple of things I'd like to point out:

  • I've only been able to reproduce the issue from the original report in the v12.x branch, but I think that potentially it could happen on newer branches too.
  • I've implemented this solution on the assumption that the mutex is only needed to protect child_port_data_ based on this comment, if that's not the case, it won't work. Another option I've tried that also works was using a recursive mutex, but I don't know if that solution would be acceptable.

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