catch(...) and forced unwind

[Jason Merrill]

Yes, it's that topic again.  The previous discussion can be found here:

  http://gcc.gnu.org/ml/gcc-patches/2003-04/threads.html#02246
  http://gcc.gnu.org/ml/gcc-patches/2003-05/threads.html#00000

I've tried to CC everyone who contributed to the earlier discussion.  

It's coming up again because it turns out that the obvious implementation
of the iostream inserters and extractors involves a catch(...) which does
not rethrow, so the question of what to do with such a catch block has
significant practical consequences.

To recap, some folks thought that such a catch block should rethrow
automatically, and some thought that it should work just like it does for
any other exception, effectively deferring the cancellation until the next
call to a cancel point.

The argument for rethrow is that it's probably the right behavior.
The argument for continue is that it's what the user wrote and we shouldn't
  be changing the semantics of their code behind their back.
The current compromise behavior is to call terminate so that the user is
  aware of the problem.

The relevant verbiage from the C++ standard:

  27.6.1.1  Class template basic_istream                   [lib.istream]

4 If one of these called functions  throws  an  exception,  then  unless
  explicitly  noted  otherwise,  the input function sets badbit in error
  state.  If badbit is on in exceptions(), the  input  function rethrows
  the exception without completing its actions, otherwise  it  does  not
  throw anything and proceeds as  if  the called function had returned a
  failure indication.

As a result, the following testcase will abort under the compromise
behavior:

  #include <pthread.h>
  #include <iostream>

  void *
  tf (void *)
  {
    while (true)
      std::cout << "." << std::flush;
    return NULL;
  }

  int
  main (void)
  {
    pthread_t t;
    while (true)
      {
        if (pthread_create (&t, NULL, tf, NULL))
          break;
        if (pthread_cancel (t))
          break;
        if (pthread_join (t, NULL))
          break;
      }
    return 1;
  }

Clearly, this high-level behavior is wrong.  iostream code should be
cancelable.  

Either of the proposed behaviors would make this particular testcase work;
the rethrow option would cause cancellation to finish sooner, but the
continue option would only defer cancellation until the next flush.  If we
left out the flush, however, the continue option would never actually
cancel because the only cancel point in the loop is guarded by the
catch(...) in the string inserter.

I believe this is a bug in the library: it thinks it knows what sorts of
exceptions it will see and wants to swallow all of them, but it is wrong.

I see three ways to fix this:

1) automatically rethrow cancellation.
2) explicitly rethrow cancellation--this would require some way to catch it
   specifically, but we really ought to have that anyway.
3) don't catch and swallow all exceptions; changing the catch(...) to
   catch(std::exception) would catch all exceptions thrown by other parts
   of the library, which AFAIK is what we're trying to accomplish.

Thoughts?

Jason