ptrace group stop signal number not reset before PTRACE_INTERRUPT is delivered?

[Keno Fischer]

Consider below test case (not all of it is necessary for reproducing
the behavior in question, but I wanted to cover related cases as well
to make sure they behave as expected). In this test case, the last
group-stop (after PTRACE_INTERRUPT) is delivered with a
WSTOPSIG(status) of SIGTTIN, which was the signr of the previous group
stop. From reading the man-page, I would have expected SIGTRAP. Now, I
understand that if there is another stop pending, PTRACE_INTERRUPT
will simply piggy-backs off that one, but I don't believe that is
happening in this case. Further, the current behavior seems to make it
very hard (impossible?) to reliably tell a true group-stop from a
PTRACE_INTERRUPT generated one. Is this behavior intended? If so, I
would be happy to update the man page accordingly, but would like some
guidance on what the intended semantics are.

```
#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <linux/ptrace.h>

int main() {
    pid_t child, ret;
    int err;
    int status;
    if (0 == (child = fork())) {
       kill(getpid(), SIGSTOP);
       kill(getpid(), SIGSTOP);
       kill(getpid(), SIGTTIN);
       sleep(1000);
       exit(0);
    }
    ret = waitpid(child, &status, WSTOPPED);
    assert(ret == child);
    err = ptrace(PTRACE_SEIZE, child, NULL, NULL);
    assert(err == 0);
    err = ptrace(PTRACE_CONT, child, NULL, NULL);
    assert(err == 0);
    // Should now hit SIGSTOP signal-stop
    ret = waitpid(child, &status, 0);
    assert(ret == child && WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);
    err = ptrace(PTRACE_CONT, child, NULL, (void*)SIGSTOP);
    assert(err == 0);
    // Should now hit SIGSTOP group-stop
    ret = waitpid(child, &status, 0);
    assert(ret == child && (status>>16 == PTRACE_EVENT_STOP) &&
WSTOPSIG(status) == SIGSTOP);
    err = ptrace(PTRACE_CONT, child, NULL, NULL);
    assert(err == 0);
    // Should now hit SIGTTIN signal-stop
    ret = waitpid(child, &status, 0);
    assert(ret == child && WIFSTOPPED(status) && WSTOPSIG(status) == SIGTTIN);
    err = ptrace(PTRACE_CONT, child, NULL, (void*)SIGTTIN);
    assert(err == 0);
    // Should now hit SIGTTIN group-stop
    ret = waitpid(child, &status, 0);
    assert(ret == child && (status>>16 == PTRACE_EVENT_STOP) &&
WSTOPSIG(status) == SIGTTIN);
    err = ptrace(PTRACE_CONT, child, NULL, NULL);
    assert(err == 0);
    // Now interrupt it
    err = ptrace(PTRACE_INTERRUPT, child, NULL, NULL);
    assert(err == 0);
    // Should now hit interrupt group-stop
    ret = waitpid(child, &status, 0);
    printf("Interrupt group-stop delivered with signal %d\n", WSTOPSIG(status));
    assert(ret == child && (status>>16 == PTRACE_EVENT_STOP) &&
WSTOPSIG(status) == SIGTRAP);
    exit(0);
}
```