ptrace: SIGTRAP and EXIT race

[Konstantin Belousov]

On Thu, Feb 21, 2019 at 08:43:20PM -0800, Robert Ayrapetyan wrote:
> Hello.
> 
> Before creating a bug and providing some test code, would ask a community
> here.
> When tracing a process using ptrace and there are multiple threads in the
> tracing process hitting the same breakpoint, sometimes main thread exits
> (WIFEXITED(status) is TRUE) before last queued TRAP_BKPT signal(s) have
> been delivered to the tracing process. So a final breakpoint hits counter
> is less than it should be.
> 
> So in the example below:
> 
> #include <iostream>
> #include <thread>
> 
> #include <pthread_np.h>
> 
> static const int num_threads = 2;
> 
> void foo() {
>     for (int i = 0; i < 2; ++i) {
>         printf("hi: %d (tid: %d)\n", i, pthread_getthreadid_np());
>     }
> }
> 
> int main() {
>     std::thread t[num_threads];
> 
>     for (int i = 0; i < num_threads; ++i) {
>         t[i] = std::thread(foo);
>     }
> 
>     for (int i = 0; i < num_threads; ++i) {
>         t[i].join();
>     }
> 
>     return 0;
> }
> 
> If we set breakpoint to printf, it should be triggered 4 times (tracing
> process should receive TRAP_BKPT 4 times). However, in ~1 of 5 runs, it
> receives TRAP_BKPT just 2 or 3 times.
> 
> Is this expected? Thanks.

I indeed would expect that all four breakpoints triggered before the
main thread exits, assuming that the breakpoints were installed before
the threads are created.  Please provide the stand-alone (and preferrably
non-interactive) test to reproduce the issue.