Bug#1000974: copy_move_algo.hpp:1083:10: error: ‘__fallthrough__’ was not declared in this scope; did you mean ‘fallthrough’?

[Thomas Goirand <zigo@debian.org>]

On 12/5/21 4:50 PM, Giovanni Mascellani wrote:
> reassign 1000974 xfslibs-dev
> severity 1000974 important
> retitle 1000974 xfs/linux.h defines common word "fallthrough" breaking
> unrelated headers
> thanks
> 
> Hi,
> 
> On 04/12/21 23:36, Thomas Goirand wrote:
>> On 12/4/21 5:11 PM, Giovanni Mascellani wrote:
>>> Could you try running that compilation command with g++ -E, so you can
>>> see what BOOST_FALLTHROUGH is actually begin replaced with?
>>
>> Oh, sorry, now I get what you meant (did a c++ --help to understand what
>> -E was doing). Here's the result in
>> CMakeFiles/seastar.dir/src/core/file.cc.o:
>>
>> __attribute__((__attribute__((__fallthrough__))));
>>
>> Probably, there's a mistake, and it should really be:
>>
>> __attribute__((__fallthrough__));
>>
>> instead, which is the source of the trouble?
> 
> It seems that the problem goes this way:
> 
>  * Boost defines in /usr/include/boost/config/compiler/gcc.hpp:
> 
> #define BOOST_FALLTHROUGH __attribute__((fallthrough))
> 
>  * But /usr/include/xfs/linux.h defines:
> 
> #define fallthrough __attribute__((__fallthrough__))
> 
> So the "fallthrough" in Boost's definition is expanded again and causes
> the problem.
> 
> I think xfs/linux.h is at fault here, because it aggressively defines a
> common word (while Boost namespaces all its definitions with a BOOST_
> prefix). Unfortunately the C/C++ preprocessor makes it easy to break
> other headers if you define stuff too liberally. This wold also, for
> example, break any program that use the name "fallthrough" for a
> variable, which is a completely reasonable name to use. A simple example
> could be:
> ---
> #include <xfs/linux.h>
> 
> int main() {
>     int fallthrough = 0;
>     return fallthrough;
> }
> ---
> which fails compilation with:
> ---
> $ LANG=C gcc test.c
> test.c: In function 'main':
> test.c:4:5: warning: 'fallthrough' attribute not followed by ';'
> [-Wattributes]
>     4 |     int fallthrough = 0;
>       |     ^~~
> test.c:4:21: error: expected identifier or '(' before '=' token
>     4 |     int fallthrough = 0;
>       |                     ^
> In file included from test.c:1:
> test.c:5:12: error: expected expression before '__attribute__'
>     5 |     return fallthrough;
>       |            ^~~~~~~~~~~
> ---
> 
> You can probably work around this problem by undefining "fallthrough"
> just after the xfs/linux.h header. In the meantime I am reassigning this
> bug to xfslibs-dev.
> 
> Giovanni.


Hi,

I can confirm that commenting away line 373 to 381 of xfs/linux.h solve
the troubles when building Ceph. Downgrading to 5.13.0-1 (using
snapshot.d.o) also solved the trouble, showing that 5.14.0 really is the
trouble here.

Thanks Giovanni for finding this out.

Cheers,

Thomas Goirand (zigo)