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Date:      Thu, 21 Feb 2019 20:43:20 -0800
From:      Robert Ayrapetyan <robert.ayrapetyan@gmail.com>
To:        FreeBSD <freebsd-hackers@freebsd.org>
Subject:   ptrace: SIGTRAP and EXIT race
Message-ID:  <CAAboi9tT==FFXdqq9XG2v8Lxf8RBuTMx5ns4puZ-hjD5KecFsA@mail.gmail.com>
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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.

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