It's not necessary because the broadcasters hold a weak_ptr (*) to it,
and will delete the weak_ptr next time they try to lock it. Doing this
prevents recursion in RemoveListener, where the function can end up
holding the only shared_ptr to a listener, and its destruction can
trigger another call to RemoveListener -- which will mess up the state
of the first instance.
This is the same bug that we've have fixed in
https://reviews.llvm.org/D23406, but it was effectively undone in
https://reviews.llvm.org/D157556. With the addition of a primary
listener, a fix like D23406 becomes unwieldy (and it has already shown
itself to be fragile), which is why this patch attempts a different
approach.
Like in 2016, I don't know a good way to unit test this bug, since it
depends on precise timing, but the thing I like about this approach is
that it enables us to change the broadcaster mutex into a non-recursive
one. While that doesn't prevent the bug from happening again, it will
make it much easier to spot in the future, as the code will hang with a
smoking gun (instead of crashing a little while later). I'm going to
attempt that in a separate patch to minimize disruption.
(*) Technically a broadcaster holds the *primary* listener as a
shared_ptr, but that's still ok as it means that listener will not get
destroyed until it is explicitly removed.
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