Is this the same as using “bracket accept hClose $ blabla” (modulo correct types)?
Sort of (and you could turn this pattern into a similar combinator), except here you want to accept the socket and hand it off to a child thread. In order to make that pattern safe you have to use mask:
foo = mask $ \unmask -> do
(s, a, _) <- unmask $ accept sock
-- here async exceptions are blocked which gives you time to install a signal
-- handler, you only unmask exceptions once the handler is installed
forkIOWithUnmask $ \restore -> ((restore $ reply state s a) `catch` handler) `finally` (cleanup s a)