can show engine status ever return an error?

[Zardosht Kasheff]

Hello all,

While implementing our storage engine's version of show engine status,
I noticed that if the handlerton function show_status ever returns an
error, this causes a crash in a debug build of mysqld, in protocol.cc
line 416, in the following code:

  case Diagnostics_area::DA_EMPTY:
  default:
    DBUG_ASSERT(0);

The reason is that the variable thd->main_da.status() is not set when
this function is called.

In the function ha_show_status, in handler.cc, I see the following
code to end the function:

  if (!result)
    my_eof(thd);
  return result;
}

So, if the handlerton's show_status function successfully returns,
my_eof(thd) is called, and the assert is not hit in protocol.cc.
However, if show_status returns an error, then nothing is called here,
causing the assert.

That being said, what is the expected behavior here? Is the assumption
that show_status can never return an error (and if so, then shouldn't
show_status be a void function and not return a bool)? Is there
something simple and safe that can be done that allows show_status to
return an error in some cases, but not causing a crash? Perhaps:

if (!result)
  my_eof(thd);
else
  something_else;
return result;
}

Thanks
-Zardosht