erlang ets select bad arg - erlang

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.