The Link plugin uses an inherently racy protocol to communicate with
applications: shared memory with no means of synchronization. This
commit makes no changes to this protocol and shouldn't break anything
that was working before. That means it doesn't eliminate race
conditions, but it does allow the plugin to work without the risk of
*data races* specifically, which are undefined behavior.
This is done by using 32-bit atomic operations to copy the LinkedMem
struct out of shared memory before reading from it. This should work
well, since every field of LinkedMem has a size that's a multiple of
32 bits, and 32-bit atomics are widely supported.
With this change, applications can avoid data races with the Link
plugin by also accessing shared memory exclusively with 32-bit atomic
operations. Applications that already work with the plugin should
continue to work.
