Re: Two fixes for module bug when interleaving threads

[Juri Linkov <juri@linkov.net>]

> I just pushed two fixes for the same bug that appears when
> interleaving module calls in separate threads:
> cdc632fbe6e149318147a98cccf1b7af191f2ce8 for the release branch and
> 23974cfa48b9245658667eff81d132b3aecd2618 for master. The intention was
> that a simpler and more localized fix should go to the release branch,
> the "proper" fix to master. Now I've realized that arguably the
> simpler fix is overall better: it's more localized, touches fewer
> moving pieces, and is probably not significantly slower (though I
> haven't benchmarked it). Any opinions? If people agree, we can
> overwrite the fix on master with the fix on the release branch.

Unfortunately, 23974cfa48b9245658667eff81d132b3aecd2618 in master
broke module loading: while loading a module, Emacs prints the
message "Loading (module)..." and hangs, doesn't react to input,
becomes completely unresponsive to C-g.

Here's is the shortest reproducible test case:

M-x package-install RET zmq RET
(load-file "~/.emacs.d/elpa/zmq-20200912.1126/emacs-zmq.so")

It hangs after printing to stderr:

  double free or corruption (!prev)
  Fatal error 6: Aborted

Fortunately, this problem doesn't exist in the 27 release branch.