Re: [Haskell-cafe] addFinalizer in GHC 7.10

On Jan 29, 2018, at 10:25 PM, Kazu Yamamoto (山本和彦) wrote:

socket family stype protocol = do fd <- c_socket ... ... let s = Socket fd addFinalizer s $ close s ruturn s

For the record, I think I've convinced Kazu Yamamoto that this is an anti-pattern. Such a finalizer would easily end up closing already closed sockets, whose file-descriptors may already be associated with other open files or sockets. That way lie all sorts of difficult to isolate race-condition bugs. To make this safe, the close function would need to mutate the socket, invalidating the enclosed file-descriptor, and would then need to be a NOP or just raise an exception if the socket is closed again (the finalizer should invoke a close variant that just returns without raising exceptions if the socket is already closed). There is, AFAIK still an unresolved bug along these lines somewhere in http-client and its dependencies. So far no reproducing cases have been provided. No very recent reports either, perhaps it went away, or people have just been more lucky lately: https://github.com/snoyberg/http-client/issues/252 https://github.com/vincenthz/hs-tls/issues/179 All that said, the original question about addFinalizer vs. GHC 7.10 may still be worth exploring, even if its use-case for Sockets goes away. So, please don't take this poset to mean that the original question should be ignored. -- Viktor.