Re: [PATCH] gdb: Remove LANG_MAGIC

[Pedro Alves <palves@redhat.com>]

On 4/12/19 12:08 PM, Andrew Burgess wrote:
> * Pedro Alves <palves@redhat.com> [2019-04-12 10:50:00 +0100]:
> 
>> On 4/12/19 12:23 AM, Andrew Burgess wrote:
>>> The language_defn structure has an la_magic field, this used to be
>>> used as a basic check that the language_defn structure had the
>>> expected layout - at least the end of the structure was where we
>>> expected it to be.
>>>
>>> This feature only really makes sense if we imagine GDB dynamically
>>> loading language support from dynamic libraries, where a version
>>> mismatch might cause problems.
>>
>> We used to have more magic fields like these in other structures,
>> like target_ops, for example.  IIRC, their intended purpose was to
>> catch miscompilation, before we had Makefile auto-dependencies.  Before
>> that, it was common to change the structure's definition, rebuild, and
>> then find out that gdb hit a "failed internal consistency check" assertion,
>> because some of the .o files hadn't been rebuilt to reflect the new layout.
>>
>> For example, the target_ops magic said:
>>
>>  /* Magic number for checking ops size.  If a struct doesn't end with this
>>     number, somebody changed the declaration but didn't change all the
>>     places that initialize one.  */
>>
>>  #define        OPS_MAGIC       3840
>>
>> With auto-dependencies, the problem this "solved" is largely a
>> thing of the past.
> 
> Thanks for the explanation, that makes sense.
> 
> I assume you're happy that these days we should "assume a working
> build system", and that if any errors like the above cropped up we
> should fix those in the build dependencies, not by adding magic fields
> to gdb.

Yes, of course.  I was the one who removed OPS_MAGIC (in f6ac5f3d63e0)  :-)

Thanks,
Pedro Alves