[erlang-bugs] dets hangs sometimes

[Hans Bolinder]

Hi again!

[John Hughes:]
> QuickCheck found another dets bug at Klarna. The following test makes dets hang:
>  
> run() ->
>     dets:stop(),
>     file:delete(dets_table),
>     {ok,dets_table} = dets:open_file(dets_table,[{type,bag}]),
>     ok = dets:insert(dets_table,[{0,21},{0,0},{1,0},{2,0},{2,1}]),
>     ok = dets:insert(dets_table,[{1,21}]),
>     true = dets:insert_new(dets_table,[{3,0}]),
>     {[{1,21}],Cont} = dets:match_object(dets_table,{'$1',21},0),
>     {[{0,21}],Cont2} = dets:match_object(Cont),
>     ok = dets:match_delete(dets_table,{'$1','$2'}),
>     true = dets:insert_new(dets_table,[{31,0}]), 
>     ok = dets:insert(dets_table,[{2,1},{18,0},{31,1}]),
>     ok = dets:insert(dets_table,[{1,0},{1,1},{2,0}]),
>     [] = dets:match_object(dets_table,{0,0}),
>     io:format("Here it comes..."),
>     %% This call hangs
>     dets:match_object(Cont2),
>     io:format("now!\n").
>  
> When I run it:
>  
> 2> c(dets_stopped).
> {ok,dets_stopped}
> 3> dets_stopped:run().
> Here it comes...
>  
> Make sure to run it in a newly started node... dets has some state which can change the behaviour.
>  
> Maybe it could be simplified some more, but most changes I make to it cause it to pass.

It's not a bug. You can try adding the line

   dets:verbose(),

It'll then output 

=ERROR REPORT==== 17-May-2011::15:20:42 ===
** dets: traversal of dets_table needs safe_fixtable

The manual states that 

  The  table  should  always  be protected using safe_fixtable/2
  before calling match_object/3, or errors may occur when call-
  ing match_object/1.

Adding 

   dets:safe_fixtable(dets_table, true),

before match_object seems to make it work.

Best regards,

Hans Bolinder, Erlang/OTP team, Ericsson