Bug#909778: libsdl2-dev: SDL_config.h no longer in cflags provided by pkg-config/sdl2-config

[Simon McVittie]

On Fri, 28 Sep 2018 at 08:30:27 +0300, Adrian Bunk wrote:
> In file included from /usr/include/SDL2/SDL_main.h:25,
>                  from /usr/include/SDL2/SDL.h:32,
>                  from /build/1st/fifechan-0.1.4+git20180904/include/fifechan/sdl/sdlgraphics.hpp:68,
>                  from /build/1st/fifechan-0.1.4+git20180904/include/fifechan/sdl.hpp:68,
>                  from /build/1st/fifechan-0.1.4+git20180904/src/sdl/sdl.cpp:69:
> /usr/include/SDL2/SDL_stdinc.h:31:10: fatal error: SDL_config.h: No such file or directory
>  #include "SDL_config.h"
>           ^~~~~~~~~~~~~~
> 
> Root cause:
> 
> $ pkg-config --cflags sdl2
> -D_REENTRANT -I/usr/include/SDL2
> $ sdl2-config --cflags
> -I/usr/include/SDL2 -D_REENTRANT
> $ dpkg -S SDL_config.h
> libsdl2-dev:amd64: /usr/include/x86_64-linux-gnu/SDL2/SDL_config.h

This is a regression caused by fixing #909740. Hugh McMaster correctly
pointed out that /usr/include/<triplet> is one of the default search
paths for headers. However, SDL_config.h is designed to be included as
<SDL_config.h> after -I/usr/include/SDL2, not as <SDL2/SDL_config.h>,
according to the design documented in <https://ometer.com/parallel.html>
and popularized by packages like GLib and GTK+; so moving it to
/usr/include/<triplet>/SDL2 does not make it includable. How was the
patch in #909740 tested?

If sdl2-config didn't exist, then this would be easy to fix by adding
-I/usr/include/<triplet>/SDL2 to the pkg-config file (similar to what
happens in GLib); but doing the same thing in sdl2-config would give it
per-architecture variation and make it non-co-installable, so that isn't
an option here.

Here is a possible solution that I think respects all the constraints,
although I have not tested it:

* move SDL_config.h to /usr/include/<triplet>/SDL2/_real_SDL_config.h
  or similar (so that it has the same level of parallel-installability
  as SDL2 itself, and is clearly not part of the SDL API)

* create /usr/include/SDL2/SDL_config.h containing:

  /* Debian-specific, for multiarch support */
  #include <SDL2/_real_SDL_config.h>

To avoid regressions like this in future, I would recommend adding an
autopkgtest that compiles C code that calls some trivial function from
SDL in the most recommended way, and checks that it works correctly,
similar to debian/tests/build in packages like dbus and glib2.0. In the
case of SDL2, it should probably be compiled twice: once using
pkg-config to get the required CFLAGS and libraries, and once using
sdl2-config, to make sure both work.

    smcv