Re: [PATCH v4 7/8] QUIT processing w/ explicit throw for gdb_exception_forced_quit

[Pedro Alves <pedro@palves.net>]

On 2023-01-12 1:56 a.m., Kevin Buettner wrote:
> This commit contains changes which have an explicit throw for
> gdb_exception_forced_quit, or, in a couple of cases for gdb_exception,
> but with a throw following a check to see if 'reason' is
> RETURN_FORCED_QUIT.
> 
> Most of these are straightforward - it made sense to continue to allow
> an existing catch of gdb_exception to also catch gdb_exception_quit;
> in these cases, a catch/throw for gdb_exception_forced_quit was added.
> 
> There are two cases, however, which deserve a more detailed
> explanation.
> 
> 1) remote_fileio_request in gdb/remote-fileio.c:
> 
> The try block calls do_remote_fileio_request which can (in turn)
> call one of the functions in remote_fio_func_map[].  Taking the
> first one, remote_fileio_func_open(), we have the following call
> path to maybe_quit():
> 
>   remote_fileio_func_open(remote_target*, char*)
>     -> target_read_memory(unsigned long, unsigned char*, long)
>     -> target_read(target_ops*, target_object, char const*, unsigned char*, unsigned long, long)
>     -> maybe_quit()
> 
> Since there is a path to maybe_quit(), we must ensure that the
> catch block is not permitted to swallow a QUIT representing a
> SIGTERM.
> 
> However, for this case, we must take care not to change the way that
> Ctrl-C / SIGINT is handled; we want to send a suitable EINTR reply to
> the remote target should that happen.  That being the case, I added a
> catch/throw for gdb_exception_forced_quit.  I also did a bit of
> rewriting here, adding a catch for gdb_exception_quit in favor of
> checking the 'reason' code in the catch block for gdb_exception.
> 
> 2) mi_execute_command in gdb/mi/mi-main.c:
> 
> The try block calls captured_mi_execute_command(); there exists
> a call path to maybe_quit():
> 
>   captured_mi_execute_command(ui_out*, mi_parse*)
>     -> mi_cmd_execute(mi_parse*)
>     -> get_current_frame()
>     -> get_prev_frame_always_1(frame_info*)
>     -> frame_register_unwind_location(frame_info*, int, int*, lval_type*, unsigned long*, int*)
>     -> frame_register_unwind(frame_info*, int, int*, int*, lval_type*, unsigned long*, int*, unsigned char*)
>     -> value_entirely_available(value*)
>     -> value_fetch_lazy(value*)
>     -> value_fetch_lazy_memory(value*)
>     -> read_value_memory(value*, long, int, unsigned long, unsigned char*, unsigned long)
>     -> maybe_quit()
> 
> That being the case, we can't allow the exception handler (catch block)
> to swallow a gdb_exception_quit for SIGTERM.  However, it does seem
> reasonable to output the exception via the mi interface so that some
> suitable message regarding SIGTERM might be printed; therefore, I
> check the exception's 'reason' field for RETURN_FORCED_QUIT and
> do a throw for this case.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26761
> ---
>  gdb/event-top.c     |  2 ++
>  gdb/mi/mi-main.c    |  4 ++++
>  gdb/remote-fileio.c | 15 ++++++++++-----
>  gdb/tui/tui.c       |  4 ++++
>  4 files changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/gdb/event-top.c b/gdb/event-top.c
> index 4a46e1b9346..9d34e574653 100644
> --- a/gdb/event-top.c
> +++ b/gdb/event-top.c
> @@ -1281,6 +1281,8 @@ async_disconnect (gdb_client_data arg)
>        gdb_puts ("Could not kill the program being debugged",
>  		gdb_stderr);
>        exception_print (gdb_stderr, exception);
> +      if (exception.reason == RETURN_FORCED_QUIT)
> +        throw;
>      }
>  
>    for (inferior *inf : all_inferiors ())
> diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
> index e0cade2edc3..d7ca7843d6f 100644
> --- a/gdb/mi/mi-main.c
> +++ b/gdb/mi/mi-main.c
> @@ -1952,6 +1952,10 @@ mi_execute_command (const char *cmd, int from_tty)
>  	     somewhere.  */
>  	  mi_print_exception (command->token, result);
>  	  mi_out_rewind (current_uiout);
> +
> +	  /* Throw to a higher level catch for SIGTERM sent to GDB.  */
> +	  if (result.reason == RETURN_FORCED_QUIT)
> +	    throw;
>  	}
>  
>        bpstat_do_actions ();
> diff --git a/gdb/remote-fileio.c b/gdb/remote-fileio.c
> index 3ff2a65b0ec..c804b0b2098 100644
> --- a/gdb/remote-fileio.c
> +++ b/gdb/remote-fileio.c
> @@ -1191,12 +1191,17 @@ remote_fileio_request (remote_target *remote, char *buf, int ctrlc_pending_p)
>  	{
>  	  do_remote_fileio_request (remote, buf);
>  	}
> -      catch (const gdb_exception &ex)
> +      catch (const gdb_exception_forced_quit &ex)
> +        {
> +	  throw;
> +	}

Tabs vs spaces above.

> +      catch (const gdb_exception_quit &ex)
>  	{
> -	  if (ex.reason == RETURN_QUIT)
> -	    remote_fileio_reply (remote, -1, FILEIO_EINTR);
> -	  else
> -	    remote_fileio_reply (remote, -1, FILEIO_EIO);
> +	  remote_fileio_reply (remote, -1, FILEIO_EINTR);
> +	}
> +      catch (const gdb_exception &ex)
> +        {
> +	  remote_fileio_reply (remote, -1, FILEIO_EIO);
>  	}

Ditto.

>      }
>  
> diff --git a/gdb/tui/tui.c b/gdb/tui/tui.c
> index cdae9ffe02b..23b47ee163c 100644
> --- a/gdb/tui/tui.c
> +++ b/gdb/tui/tui.c
> @@ -122,6 +122,10 @@ tui_rl_switch_mode (int notused1, int notused2)
>  	  tui_enable ();
>  	}
>      }
> +  catch (const gdb_exception_forced_quit &ex)
> +    {
> +      throw;
> +    }

We can't let the exception propagate here.  See comment just above, which reads:

  /* Don't let exceptions escape.  We're in the middle of a readline
     callback that isn't prepared for that.  */
  try
    {
  ...


I think we need to use the set_quit_flag()-to-rethrow-later trick here.

>    catch (const gdb_exception &ex)
>      {
>        exception_print (gdb_stderr, ex);
>