[PATCH] lib: fix warnings when building with clang

[Tomi Ollila]

On Sun, Oct 21 2012, Jani Nikula <jani at nikula.org> wrote:

> On Sun, 21 Oct 2012, Ethan Glasser-Camp <ethan.glasser.camp at gmail.com> wrote:
>> Jani Nikula <jani at nikula.org> writes:
>>
>>> Building notmuch with CC=clang and CXX=clang++ produces the warnings:
>>>
>>> CC -O2 lib/tags.o
>>> lib/tags.c:43:5: warning: expression result unused [-Wunused-value]
>>>     talloc_steal (tags, list);
>>>     ^~~~~~~~~~~~~~~~~~~~~~~~~
>>> /usr/include/talloc.h:345:143: note: expanded from:
>>>   ...__location__); __talloc_steal_ret; })
>>>                     ^~~~~~~~~~~~~~~~~~
>>> 1 warning generated.
>>>
>>> CXX -O2 lib/message.o
>>> lib/message.cc:791:5: warning: expression result unused [-Wunused-value]
>>>     talloc_reference (message, message->tag_list);
>>>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> /usr/include/talloc.h:932:36: note: expanded from:
>>>   ...(_TALLOC_TYPEOF(ptr))_talloc_reference_loc((ctx),(ptr), __location__)
>>>      ^                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> 1 warning generated.
>>>
>>> Check talloc_reference() return value, and explicitly ignore
>>> talloc_steal() return value as it has no failure modes, to silence the
>>> warnings.
>>> ---
>>>  lib/message.cc |    4 +++-
>>>  lib/tags.c     |    2 +-
>>>  2 files changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/lib/message.cc b/lib/message.cc
>>> index 978de06..320901f 100644
>>> --- a/lib/message.cc
>>> +++ b/lib/message.cc
>>> @@ -788,7 +788,9 @@ notmuch_message_get_tags (notmuch_message_t *message)
>>>       * possible to modify the message tags (which talloc_unlink's the
>>>       * current list from the message) while still iterating because
>>>       * the iterator will keep the current list alive. */
>>> -    talloc_reference (message, message->tag_list);
>>> +    if (!talloc_reference (message, message->tag_list))
>>> +	return NULL;
>>> +
>>>      return tags;
>>>  }
>>
>> Hi! What you did with talloc_steal is obviously fine. 
>>
>> I'd be happier about what you did with talloc_reference() if there were
>> precedent, or a clearly-articulated convention for notmuch. Instead this
>> is the third use in the codebase that I can see, and the other two are
>> each unique to themselves. In mime-node.c we print an "out-of-memory"
>> error and in lib/filenames.c we cast (void) talloc_reference (...), I
>> guess figuring that we're pretty much hosed anyhow if we run out of
>> memory.
>>
>> Why return NULL here? It seems like if talloc_reference fails, we're
>> going to crash eventually, so we should print an error to explain our
>> impending doom. I'd guess you're uneasy printing anything from lib/, but
>> still want to signal an error, and the only way you can do so is to
>> return NULL. I guess that silences the compiler warning, but it's not
>> really the correct way to handle the error IMO. On the other hand, it's
>> such a weird corner case that I don't even think it merits a FIXME
>> comment.
>>
>> How about an assert instead of a return NULL?
>
> No. I don't think a library should assert, exit, or print to stderr on
> this sort of thing. It's up to the calling application. Even if it
> probably doesn't have many choices left, given how much memory
> talloc_reference needs (not much).
>
> Ignoring the talloc_reference return value with (void) is just wrong,
> and the caller of notmuch_message_get_tags should anticipate a NULL
> return. So IMHO that's the pragmatic thing to do in this mostly
> theoretical situation, the biggest change being silencing the warning.

I agree that the best library can do is to return NULL (if talloc had
a place in ctx to store error indication that could be used but I did
not see any in quick look -- and using global there is not a good idea)

but, before returning NULL should 'tags' be freed.

Additionally, should lib/filenames.c be changed to have code:

if (unlikely (talloc_reference(filenames, list) == NULL)) {
    talloc_free (filenames);
    return NULL;
}

> BR,
> Jani.

(btw, what are the chances that program crashes before returning NULL
due to page fault in stack frame allocation ???)

Tomi