Re: [PATCH 1/4 v8] gdb: Buffer output streams during events that might download debuginfo

[Tom de Vries <tdevries@suse.de>]

On 1/19/24 18:13, Aaron Merey wrote:
> Hi Luis,
> 
> On Fri, Jan 19, 2024 at 9:35 AM Luis Machado <luis.machado@arm.com> wrote:
>>
>> I bisected a number of failures in gdb.dwarf2/dw2-dir-file-name.exp to commit 519d634396592c5698add4a327468e6e6920576e.
>>
>> In particular, my testing was done on aarch64-linux (Ubuntu 20.04).
> 
> Thanks for finding this.  These regressions are due to a last-minute
> decision to replace throw_exception with throw in one of the functions
> added in commit 519d634.
> 
> I'm going to merge the patch below that restores throw_exception.
> Regression tested on Fedora 39 x86_64.
> 
> Aaron
> 
> ---
> gdb/ui-out.h: Fix exception handling in do_with_buffered_output
> 
> Replace throw with throw_exeception in do_with_buffered_output.
> 
> This patch fixes regressions in gdb.dwarf2/dw2-dir-file-name.exp
> caused by commit 519d63439.
> 
> do_with_buffered_output needs to use throw_exception instead of
> throw to ensure that exceptions of the correct type are thrown.
> If using throw, gdb_exception_error may be wrongly converted into
> gdb_exception during print_frame_info.  This prevents the exception
> from being caught in print_stack_frame.

Hi Aaron,

what you're describing here sounds like exception slicing.

The easiest way to fix this is by rethrowing using "throw" without argument.

See this ( https://en.cppreference.com/w/cpp/language/throw ):
...
When rethrowing exceptions, the second form must be used to avoid object 
slicing in the (typical) case where exception objects use inheritance: ...

Thanks,
- Tom

> ---
>   gdb/ui-out.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/ui-out.h b/gdb/ui-out.h
> index 71add908a84..5b6ddd55063 100644
> --- a/gdb/ui-out.h
> +++ b/gdb/ui-out.h
> @@ -543,7 +543,7 @@ do_with_buffered_output (F func, ui_out *uiout, Arg... args)
>       {
>         func (uiout, std::forward<Arg> (args)...);
>       }
> -  catch (const gdb_exception &ex)
> +  catch (gdb_exception &ex)
>       {
>         /* Ideally flush would be called in the destructor of buffer_group,
>   	 however flushing might cause an exception to be thrown.  Catch it
> @@ -556,7 +556,7 @@ do_with_buffered_output (F func, ui_out *uiout, Arg... args)
>   	{
>   	}
>   
> -      throw (std::move (ex));
> +      throw_exception (std::move (ex));
>       }
>   
>     /* Try was successful.  Let any further exceptions propagate.  */