An OTP library for cutting the Gordian knot of stuck OTP processes.
One of the hardest problems to debug in Erlang/OTP is why a gen_server/gen_statem/etc is deadlocked. Often this is due to a blocking call cycle where A calls B who calls C who calls A. A is waiting for the return from B when the second call comes in, so it cannot respond to it, which means all the processes deadlock until the call timeout hits, or forever if the timeouts are infinite.
When a process is stuck like this, you can't use sys:get_status or install a debug trace, those use messages, which cannot be handled. All you can do is look at erlang:process_info for the process, which often does not contain enough information to understand why it is deadlocked.
Alexander, the Great, original cutter of the Gordian knot, is a parse transform that instruments blocking OTP calls by writing the blocking call's information to the process dictionary before the blocking call and then removing it afterwards. It can then use that information to detect call cycles because other processes can read the process dictionary via erlang:process_info. So if A calls B, A first records it is calling B (and is thus unavailable to handle a call until it returns) and then examines B's process dictionary to see if B is blocked making a call of its own. If B is blocked we follow the chain of processes until we run out of processes to follow, or we find a loop. If a loop is detected, an exception is thrown.
Simply enable the parse transform on the modules you want to check for call cycles:
-compile([{parse_transform, alexander}]).Or use it in your Erlang compiler flags:
{parse_transform, alexander}
Alexander can only see call cycles between processes it has transformed at compile time, so you might want to use something like rebar3's overrides feature:
{overrides, [{add, [{erl_opts, [{parse_transform, alexander}]}]}]}.For this to work you likely need a compiled copy of alexander in your OTP code path somewhere. If you are using other parse transforms, Alexander will try to run them for you before applying its own transform. This should work for most use cases but it is not well tested.