[Bug 1876958] Re: Ubuntu-specific pthread issue

[David Rogers]

Update: this does not involve threads (see new minimal version below).
Also, adding -O2 to the above version (with threading) gives no output,
but creates a SEGV with the version below.  Finally, the bug occurs at
compile-time, not link time, since the object file (".o") created from
this also errors when run on CentOS.

#include <functional>
#include <stdio.h>

struct run {
    const std::function<void()> &fn;
    run(const std::function<void()> &fn_) : fn(fn_) {}

    void operator()() {
        fn();
    }
};

int main() {
    auto fn1 = [](){ printf("fn1\n"); };
    auto r1 = run(fn1);
    auto fn2 = [](){ printf("fn2\n"); };
    auto r2 = run(fn2);
    r1();
    r2();
}

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https://bugs.launchpad.net/bugs/1876958

Title:
  Ubuntu-specific pthread issue

Status in gcc-defaults package in Ubuntu:
  New

Bug description:
  The following c++11 program works properly (prints fn1 and fn2) on
  every system and compiler I have checked except for Ubuntu - where it
  prints (fn2 and fn2).  This happens on both laptop and server installs
  of Ubuntu 18.04.

  All system gcc packages on my Ubuntu 18.04 system show the same (incorrect) output:
  g++-5 (Ubuntu 5.5.0-12ubuntu1) 5.5.0 20171010
  g++-6 (Ubuntu 6.5.0-2ubuntu1~18.04) 6.5.0 20181026
  g++-7 (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
  g++-8 (Ubuntu 8.3.0-26ubuntu1~18.04) 8.3.0

  Notes:
   - Re-compiling gcc from source eliminates the problem.
   - I have not checked any Ubuntu versions other than 18.04.

  Off-topic note:
   - this may be a [libpthread bug](https://bugs.launchpad.net/ubuntu/+source/libpthread-stubs/)?

  /* Minimal bug-reproducer for gcc on ubuntu */
  #include <functional>
  #include <thread>
  #include <stdio.h>

  struct run {
      const std::function<void()> &fn;
      run(const std::function<void()> &fn_) : fn(fn_) {}

      void operator()() {
          fn();
      }
  };

  int main() {
      auto fn1 = [](){ printf("fn1\n"); };
      auto r1 = run(fn1);
      auto fn2 = [](){ printf("fn2\n"); };
      auto r2 = run(fn2);
      std::thread t1( r1 );
      std::thread t2( r2 );
      t1.join();
      t2.join();
  }

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