[PATCH] libstdc++: don't use #include_next in c_global headers

[Jonathan Wakely]

On 23/04/20 09:23 +0100, Jonathan Wakely wrote:
>On 23/04/20 06:32 +0200, Helmut Grohne wrote:
>>Hi,
>>
>>On Mon, Apr 20, 2020 at 10:12:37AM +0100, Jonathan Wakely wrote:
>>>> Now you are probably going to say that "-isystem /usr/include" is a bad
>>>> idea and that you shouldn't do that.
>>>
>>>Right.
>>>
>>>> I'm inclined to agree. This isn't a
>>>> problem just yet. Debian wants to move /usr/include/stdlib.h to
>>>> /usr/include/<multiarch>/stdlib.h. After that move, the problematic flag
>>>> becomes "-isystem /usr/include/<multiarch>". Unfortunately, around 30
>>>> Debian packages[1] do pass exactly that flag. Regardless whether doing
>>>> so is a bad idea, I guess we will have to support that.
>>>
>>>Or Debian should fix what they're going to break.
>>
>>This is not quite precise. The offending -isystem
>>/usr/include/<multiarch> flag is already being passed. According to what
>>you write later, doing so is broken today. It just happens to work by
>>accident. So all we do is making the present breakage visible.
>>
>>>> I am proposing to replace those two #include_next with plain #include.
>>>> That'll solve the problem described above, but it is not entirely
>>>> obvious that doing so doesn't break something else.
>>>>
>>>> After switching those #include_next to #include,
>>>> libstdc++-v3/include/c_global/cstdlib will continue to temporarily
>>>> will #include <stdlib.h>. Now, it'll search all include directories. It
>>>> may find libstdc++-v3/include/c_comaptibility/stdlib.h or the libc's
>>>> version. We cannot tell which. If it finds the one from libstdc++-v3,
>>>> the header will notice the _GLIBCXX_INCLUDE_NEXT_C_HEADERS macro and
>>>> immediately #include_next <stdlib.h> skipping the rest of the header.
>>>> That in turn will find the libc version. So in both cases, it ends up
>>>> using the right one. Precisely what we wanted.
>>>
>>>As Marc said, this doesn't work.
>>
>>That is not very precise either. Marc said that it won't fix all cases.
>>In practice, it would make those work that don't #include <stdlib.h> but
>>use #include <cstdlib> instead.
>>
>>Marc also indicated that using include_next for a header of a different
>>name is wrong. So this is a bug in libstdc++ regardless of whether it
>>breaks or unbreaks other pieces of software.
>
>He said he doesn't like it, that doesn't mean it's a bug or actually
>causes incorrect results.
>
>Whereas using -isystem provably *does* break the implementation,
>making it impossible for #include <stdlib.h> to meet the requirements
>of the C++ standard. And your proposed patch doesn't prevent that.
>
>
>>>If a program tries to include <stdlib.h> it needs to get the libstdc++
>>>version, otherwise only the libc versions of certain functions are
>>>defined. That means the additional C++ overloads such as ::abs(long)
>>>and ::abs(long long) won't be defined. That is the reason why
>>>libstdc++ provides its own <stdlib.h>.
>>>
>>>And if you do -isystem /usr/include (or any other option that causes
>>>libstdc++'s <stdlib.h> to be skipped) that doesn't work. Only
>>>::abs(int) gets defined.
>>>
>>>So -isystem /usr/include breaks code, with or without your patch.
>>
>>It is very difficult to disagree with -isystem /usr/include or -isystem
>>/usr/include/<triplet> being broken and unsupported. Having you state it
>>that clearly does help with communicating to other upstreams. For this
>>reason, I've looked into the remaining cases. It turns out that there
>>aren't that many left. In particular chromium, opencv and vtk got fixed
>>in the mean time. Basically all remaining failures could be attributed
>>to qmake, which passes all directories below /usr/include (including
>>/usr/include and /usr/include/<triplet> if a .pc file mentions them)
>>using -isystem. I've sent a patch https://bugs.debian.org/958479 to make
>>qmake stop doing that.
>>
>>I therefore agree with you that the patch I sent for libstdc++ is not
>>necessary to make packages build on Debian. Removing the offending
>>-isystem flags from the respective builds is a manageable option and has
>>already happened to a large extend.
>
>Yes, I introduced the current <stdlib.h> and <math.h> wrappers years
>ago in GCC 6, and so I'm surprised to see it coming up again now.
>Several packages had problems and already fixed them.
>
>>We can conclude that the motivation for my patch is not a good one,
>>because it embraces broken behaviour. However, the use of include_next
>>remains a bug, because the name of the including and the name of the
>>included header differ, and it should be fixed on that ground.
>
>Not liking something is not a bug.
>
>You need to demonstrate an actual bug (e.g. failure to compile,
>non-conformance to the C++ standard) that is not caused by user error
>(like misuse of -isystem) to argue for fixing something.

N.B. the GCC docs are quite clear that reordering include directories
risk breaking GCC's necessary use of #include_next:

   If a standard system include directory, or a directory specified
   with -isystem, is also specified with -I, the -I option is ignored.
   The directory is still searched but as a system directory at its
   normal position in the system include chain.  This is to ensure that
   GCC's procedure to fix buggy system headers and the ordering for the
   "#include_next" directive are not inadvertently changed.  If you
   really need to change the search order for system directories, use
   the -nostdinc and/or -isystem options.

Why do people think they "really need to change the search order"? If
they're going to play silly games, they should be prepared to do it
properly, and also put the standard C++ dirs back in the right order.

It seems to be this is simple user error, and those packages should be
fixed to stop misusing an option that is documented to have a risk of
breaking things.