erlang ets select bad arg

Question

erlang version 18.3

Got an strange error with Erlang ets:select/1

the following code will do select element from table and take them .

if I do

save(10), %% insert 10 data

remove(3) %% remove 3 data per time

it works

if I do

save(6007), %% insert  more datas

remove(400) %% remove 400 data per time

it was bad arg in ets:select(Cont) also, it was not the in the first or second loop, but was always there.

any suggestion?

 -record(item, {name, age}).
 %% start the table
 start() ->
 ets:new(example_table, [public, {keypos, 2},
 named_table,
 {read_concurrency, true},
 {write_concurrency, true}]).

 %% insert n demo data
 save(Limit) ->
 All = lists:seq(1 ,Limit),
 All_rec = [#item{name = {<<"demo">>, integer_to_binary(V)} , age = V} || V <- All],
 ets:insert(example_table, All_rec).

 %% remove all data, n data per select
 remove(Limit) ->
   M_head   = #item{name = '$1', _ = '_'},
 M_guards = [],
 M_result = ['$1'],
 M_spec   = [{M_head, M_guards, M_result}],

 case ets:select(example_table, M_spec, Limit) of
      '$end_of_table' ->
      0;
      {Keys, Cont} ->

      remove(example_table, Keys, Cont, 0, [])
      end.

      remove(Table, [],  Cont, Count, _Acc) ->

        case ets:select(Cont) of
          '$end_of_table' ->
            Count;
          {Keys, Cont_1} ->

            remove(Table, Keys,  Cont_1, Count, [])
        end;
      remove(Table,[Key | T],  Cont, Count, Acc) ->
        case ets:take(example_table, Key) of
          [] ->
       remove(Table, T,  Cont, Count, Acc);
          [Rec] ->
       io:format("Rec [~p] ~n", [Rec]),
       remove(Table, T,  Cont, Count + 1, [Rec | Acc])
        end.

stack trace

     4> example_remove:save(6007).
      true
     5> example_remove:remove(500).
    ** exception error: bad argument
   in function  ets:select/1
    called as ets:select({example_table,304,500,<<>>,
                             [{<<"demo">>,<<"2826">>},
                              {<<"demo">>,<<"3837">>},
                              {<<"demo">>,<<"5120">>},
                              {<<"demo">>,<<"878">>},
                              {<<"demo">>,<<"1195">>},
                              {<<"demo">>,<<"1256">>},
                              {<<"demo">>,<<"1449">>},
                              {<<"demo">>,<<"5621">>},
                              {<<"demo">>,<<"5768">>}],
                             9})
 in call from example_remove:remove/5 (d:/workspace/simple-cache/src/example_remove.erl, line 47)

Answer 1

I believe this happens because you simultaneously iterate over the table and modify it.

I suggest wrapping main remove cycle with guards of safe_fixtable

remove(Limit) ->
    ets:safe_fixtable(example_table, true),
    M_head   = #item{name = '$1', _ = '_'},
    M_guards = [],
    M_result = ['$1'],
    M_spec   = [{M_head, M_guards, M_result}],

    R = case ets:select(example_table, M_spec, Limit) of
        '$end_of_table' ->
            0;
        {Keys, Cont} ->
            remove(example_table, Keys, Cont, 0, [])
    end,
    ets:safe_fixtable(example_table, false),
    R.