Question

I’m trying to write a basic Erlang program that reads in a file in one process and writes the file in another process. I’m finding that sometimes the output file gets truncated.

For instance I wrote eunit test. If I run the single test drive_test:write_file_test() the output is correctly written. But running drive_test:test() truncates the output file in a different place each time.

Do I need to do something special to make sure that the process finishes writing before it closes?

drive.erl:

-module(drive).
-include("library.hrl").
-export([init/1]).

init(Drive) ->
  loop(Drive).

loop(Drive) ->
  receive
    {write, Data} ->
      write(Drive,Data),
      loop(Drive);
    close -> 
      close(Drive)
  end.

write(Drive,Data) ->
  %io:format("~p", [Data]),
  Handler = Drive#drive.volume,
  file:write(Handler, [Data]).

close(Drive) ->
  Handler = Drive#drive.volume,
  file:close(Handler),
  io:format("closed ~w~n", [Drive]).

drive_test.erl

-module(drive_test).
-include_lib("eunit/include/eunit.hrl").
-include("library.hrl").

startupShutdown_test() ->
  DrivePid = spawn(drive,init,[#drive{number=1}]),
  DrivePid ! close.

write_basic_test() ->
  {ok, F} =file:open("test/library/eunit.txt", write),
  DrivePid = spawn(drive,init,[#drive{number=1,volume=F}]),
  DrivePid ! {write, "Some Data"},
  DrivePid ! close.

write_file_test() ->
  {ok, Fin} = file:open("cathedral.pdf", read),
  {ok, Fout} =file:open("test/library/eunit2.txt", write),
  DrivePid = spawn(drive,init,[#drive{number=1,volume=Fout}]),
  write_file( Fin, DrivePid),
  DrivePid ! close.

write_file(F, DrivePid ) ->
  Rd = file:read(F, 256),
  case Rd of
    {ok, Data} -> 
      DrivePid ! {write, Data}, 
      write_file(F, DrivePid );
    eof -> file:close(F);
    _ -> ?_assert(false)
  end.

truncated file:

$ ls -l cathedral.pdf test/library/eunit2.txt
-rwx------+ 1 218879 Sep 16 22:21 cathedral.pdf
-rwxr-xr-x  1  60928 Dec 17 09:31 test/library/eunit2.txt

Answer 1

It is most probably a “timing” related problem. I suspect it is related to how “Eunit” performs its processing: “EUnit” probably doesn’t give enough time to your module to close before exiting and thus terminating all processes.