But I think we’d have to be lucky for it to trigger the deadlock (lucky
because the two substitutes should be fetched at the same instant,
roughly).

Given that it’s a nasty issue, I’d prefer to have an (ideally automated)
test.

Thoughts? Ideas?

Anyway, thanks for thinking through it and for explaining your reasoning
and coming up with a patch!

And: merry end-of-year holidays too.  :-)

Ludo’.