multithreading - TThreadedQueue access violation with multiple consumers -

couple of days ago user (@victoria) gave me example how send email multiple recipients, putting each message (idmessage) in queue, , send in next several threads.

she said example going present trouble when more 1 thread stays in access queue.

she had here reasons. right. in tests happened, access violation when used more 1 thread (without resort sleep() function).

i have read possible bug in ttreadedqueue class, can seen here, did not understand thing.

my question: - example below fixable or ttreadedqueue have supposed bug since creation?

uses   system.types, generics.collections, idmessage;  type   tthreaditem = class;   tthreadlist = tobjectlist<tthreaditem>;   tmessageitem = tidmessage;   tmessagequeue = tthreadedqueue<tmessageitem>;    tthreadpool = class   private     fqueue: tmessagequeue;     fthreads: tthreadlist;   public     constructor create(count: integer);     destructor destroy; override;     procedure shutdown;     property queue: tmessagequeue read fqueue;   end;    tthreaditem = class(tthread)   private     fqueue: tmessagequeue;   protected     procedure execute; override;   public     constructor create(queue: tmessagequeue); reintroduce;   end;  implementation  { tthreadpool }  constructor tthreadpool.create(count: integer); var   i: integer;   thread: tthreaditem; begin   inherited create;   { create thread queue wait push , pop of items infinite     time; that's useful thread sleeping }   fqueue := tmessagequeue.create;   fthreads := tthreadlist.create;    := 0 count-1   begin     thread := tthreaditem.create(fqueue);     fthreads.add(thread);   end; end;  destructor tthreadpool.destroy; begin   shutdown;   fthreads.free;   fqueue.free;   inherited; end;  procedure tthreadpool.shutdown; var   thread: tthreaditem;   message: tmessageitem; begin   { signal threads termination }   thread in fthreads     thread.terminate;   { shutdown queue "unlock" sleeping threads }   fqueue.doshutdown;   { free unprocessed enqueued message items }   message := fqueue.popitem;   while assigned(message)   begin     message.free;     message := fqueue.popitem;   end; end;  { tthreaditem }  constructor tthreaditem.create(queue: tmessagequeue); begin   inherited create;   fqueue := queue; end;  procedure tthreaditem.execute; var   message: tmessageitem; begin   { <- create , setup indy sending object here }   try     while not terminated       { here we'll wait infinite time item or until queue shutted down;         should consider checking error state }       if fqueue.popitem(message) = wrsignaled       try         { <- send message through indy sending object here }               message.free;       end;       { <- destroy indy sending object here }   end; end;  ////////////////////////////////////////////////////////////////////////////////////////////////////////////  { possible usage: }  pool := tthreadpool.create(2); { <- create 2 threads } := 0 99 begin   message := tmessageitem.create(nil);   message.subject := 'message subject';   ...   pool.queue.pushitem(message); end;