function_clause in lists:foldl during live upgrade

Hi ,

I need help. I can’t figure it out.
We do live upgrades, and during the testing a weird error occurred.

{function_clause, [{lists,foldl, [#Fun<wep_workflow_executor.8.100719303>, #{{{mddb_wfm_check_endpoint,1},mddb_wfm_check_endpoint} => #{}, {{mddb_wfm_convert_input,2},mddb_wfm_convert_input} => #{}, {{mddb_wfm_gather_input,4},mddb_wfm_gather_input} => #{}, {{mddb_wfm_get,5},mddb_wfm_get} => #{}, {{mddb_wfm_redirect,3},mddb_wfm_redirect} => #{}}, []], [{file,"lists.erl"},{line,1266}]},

If I understand it correctly at some point is_function(F, 2) should return false to get this error.

foldl(F, Accu, [Hd|Tail]) -> foldl(F, F(Hd, Accu), Tail); foldl(F, Accu, []) when is_function(F, 2) -> Accu.

The thing is that is_function/2 gave me true even if the function not yet exists, hard purged from the vm.

I cannot reproduce this error, but already happened twice during live upgrade.
Any guess or idea what i am doing wrong?

1 Like

may be you need show your code. one of the reason may be you sotre the Fun but when code upgraded the fun code has been clean

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It looks like your foldl is using a local function from the wep_workflow_executor module during the live upgrade. That’s a recipe for problems. Use export funs if you need them to live across live upgrades. (You may need to declosurize the fun, but since it’s passed to a foldl that’s easy.)

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The thing that bothers me that i’m getting function_clause instead of bad fun.

In sequential code where no full qualified call i thought i should not be worried about code change. I mean it is not hard purged until there is a process which using that version of the module.

Security team do not let us share code, but looks something like this:

fun_with_the_problem(Param1, Param2) ->
  Map = local_fun(Param1),
  lists:foldl(fun(Ref, Acc) ->
                maps:merge(Acc, #{Ref => other_module:magic_stuff(Param2)})
              end, #{}, Map).
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I using these code to test:

make_fun() ->
    fun() -> io:format("aaa~n", []) end.% modify aaa to bbb in 10000

load once will use the old version code, everything is ok
load twice get an error, just like what you say before

172> spawn(fun() -> F = test:make_fun(), timer:sleep(10000), io:format("===~p~n", [erlang:is_function(F,0)]), F() end).
173> c(test).
174> c(test).
175> ===true
=ERROR REPORT==== 29-Jun-2022::17:15:30 ===
Error in process <0.608.0> with exit value:

so, If it’s possible:

  1. F point address X
  2. reload code twice, but process will not be kill(not calling function in test)
  3. memory not enought, reuse address X
  4. F change to something X point to
  5. is_function/2 return false

but I don’t know how to test it, just a conjecture
hope it can help you :slightly_smiling_face: