supabase / pg_net / 39748967387

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fix: a race condition in which a backend might segfault

In the old code SetLatch was called on the shared_latch only after checking that
it was not NULL. But there was a narrow race condition in which the shared_latch
variable might be assigned to NULL after the if condition succeeded but before
the SetLatch was called if the worker exited in that narrow window and
net_on_exit function set the shared_latch to NULL. The fix is to make the
shared_latch pointer volatile and then to load it once before checking it for
NULL and calling SetLatch.

This commit also removes the pg_write_barrier calls because these are redundant:
SetLatch and ConditionVariableBroadcast functions already have a
pg_write_barrier call inside them.

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