as-needed and libstdc++ and pulling libpthread

[Samuel Thibault <samuel dot thibault at ens-lyon dot org>]

Hello,

We are having an issue with pulling libpthread with the as-needed
linking feature, which seems to be half-working only by luck on
GNU/Linux, and not working on GNU/Hurd. Basically, if I try to build:

    #include <mutex>
    
    void f(void) { }
    
    std::once_flag flag;
    int main(void) {
    	std::call_once(flag, f);
    }

with

    g++ test.cpp -o test -pthread
or
    g++ test.cpp -o test -lpthread

the link does not actually add libpthread (even if libpthread.so does
get read by the linker, as seen in -Wl,-verbose), and I am getting a
runtime error:

    terminate called after throwing an instance of 'std::system_error'
      what():  Error in unknown error system: FFFFFFFF

because libstdc++'s call_once checks that the libpthread got linked in.

The same issue can be seen on GNU/Linux when using 

    g++ test.cpp -o test -lpthread

but not with

    g++ test.cpp -o test -pthread

Because in the former case, libpthread is before libstdc++ on the link
command.


The "as-needed" feature is here on purpose avoiding to link against
libpthread, because it thinks it's not needed, because it doesn't know
that calling std::call_once will later on need libpthread. And libstdc++
itself doesn't link against libpthread on purpose, making all its
references to pthread function weak.

AFAICT, on GNU/Linux things work (by luck) with -pthread because
libstdc++ not only calls pthread functions, but also __errno_location.
printf("%p\n", errno); is indeed enough to emit a reference to
__errno_location, and then linking with -pthread will indeed bring
in libpthread. But that seems pure luck to me: should we remove
__errno_location from libpthread, we'd have the same situation. And it
doesn't work with -lpthread, programmers will wonder why.

I plan to "fix" the issue on GNU/Hurd by adding errno_location in
libpthread, so it gets the same behavior as on GNU/Linux to avoid being
exposed to such situations while GNU/Linux is not, but that's really
worrying for GNU/Linux too.

Samuel