Do I have to block signals on the main thread to handle cancel point on another thread?

Question

When I was working on a TCP server running in a dedicated thread, I noticed strange behavior in signal handling. I have prepared the following MWE (I've used cerr to avoid race condition on debug printing):

#include <signal.h>
#include <unistd.h>

#include <iostream>
#include <thread>
#include <chrono>

using namespace std;

#undef THREAD

class RaiiObject
{
public:
    RaiiObject() { cerr << "RaiiObject ctor" << endl; }
    ~RaiiObject() { cerr << "RaiiObject dtor" << endl; }
};

static void signalHandler(int sig)
{
    write(2, "Signal\n", 7);
}

static void blockSigint()
{
    sigset_t blockset;

    sigemptyset(&blockset);
    sigaddset(&blockset, SIGINT);
    sigprocmask(SIG_BLOCK, &blockset, NULL);
}

static void setSigintHandler()
{
    struct sigaction sa;
    sa.sa_handler = signalHandler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGINT, &sa, NULL);
}

void runSelect()
{
    sigset_t emptyset;
    sigemptyset(&emptyset);

    setSigintHandler();

    RaiiObject RaiiObject{};
    fd_set fdRead;

    while (true) {
        cerr << "Loop iteration" << endl;
        FD_ZERO(&fdRead);
        FD_SET(0, &fdRead);
        while (true) {
            if (pselect(FD_SETSIZE, &fdRead, NULL, NULL, NULL, &emptyset) > 0) {
                cerr << "Select" << endl;
            } else {
                cerr << "Select break" << endl;
                return;
            }
        }
    }
}

int main()
{
    cerr << "Main start" << endl;

#ifdef THREAD
    cerr << "Thread start" << endl;
    //blockSigint();
    thread{runSelect}.join();
#else
    runSelect();
#endif

    cerr << "Main exit" << endl;

    return EXIT_SUCCESS;
}

When I compile a single-threaded program (#undef THREAD), I can correctly terminate the runSelect() function with Ctrl-C:

Main start
RaiiObject ctor
Loop iteration
^CSignal
Select break
RaiiObject dtor
Main exit

But when I compile a multithreaded (#define THREAD) program, it hangs at the signal handler:

Main start
RaiiObject ctor
Loop iteration
^CSignal

Only when I block the signal on the main thread with blockSigint() the program again work as I want.

I've examined the program with strace -tt -f and I noticed that working versions use pselect6() with ERESTARTNOHAND:

14:46:53.543360 write(2, "Loop iteration", 14Loop iteration) = 14
14:46:53.543482 write(2, "\n", 1
)       = 1
14:46:53.543586 pselect6(1024, [0], NULL, NULL, NULL, {[], 8}) = ? ERESTARTNOHAND (To be restarted if no handler)
14:46:55.286989 --- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=2707461, si_uid=1000} ---
14:46:55.287120 write(2, "Signal\n", 7Signal
) = 7
14:46:55.287327 rt_sigreturn({mask=[]}) = -1 EINTR (Interrupted system call)
14:46:55.287569 write(2, "Select break", 12Select break) = 12
14:46:55.287760 write(2, "\n", 1

but broken version uses futex():

[pid 3469011] 14:48:37.211792 write(2, "Loop iteration", 14Loop iteration) = 14
[pid 3469011] 14:48:37.211916 write(2, "\n", 1
) = 1
[pid 3469011] 14:48:37.212031 pselect6(1024, [0], NULL, NULL, NULL, {[], 8} <unfinished ...>
[pid 3469010] 14:48:40.046146 <... futex resumed>) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
[pid 3469010] 14:48:40.046256 --- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=2707461, si_uid=1000} ---
[pid 3469010] 14:48:40.046354 write(2, "Signal\n", 7Signal
) = 7
[pid 3469010] 14:48:40.046588 rt_sigreturn({mask=[]}) = -1 EINTR (Interrupted system call)
[pid 3469010] 14:48:40.046821 futex(0x7f4e5c16b9d0, FUTEX_WAIT, 3469011, NULL) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)

Answer 1

As you noticed, my problem was that sigaction() has been connect signal handler to both main() thread and runSelect() thread so SIGINT signal could be caught by main().

Now I have prepared a version in which only the main thread handles SIGINT signal and sends SIGUSR1 signal to specific threads with pthread_kill().

#include <signal.h>
#include <unistd.h>

#include <iostream>
#include <thread>
#include <chrono>

using namespace std;

pthread_t nativeHandle;

class RaiiObject
{
public:
    RaiiObject() { cerr << "RaiiObject ctor" << endl; }
    ~RaiiObject() { cerr << "RaiiObject dtor" << endl; }
};

static void sigintHandler(int)
{
    write(2, "INT\n", 4);
    pthread_kill(nativeHandle, SIGUSR1);
}

static void sigusrHandler(int)
{
    write(2, "USR\n", 4);
}

static void blockSigint()
{
    sigset_t blockset;

    sigemptyset(&blockset);
    sigaddset(&blockset, SIGINT);
    sigprocmask(SIG_BLOCK, &blockset, NULL);
}

static void setSigintHandler()
{
    struct sigaction sa;
    sa.sa_handler = sigintHandler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGINT, &sa, NULL);
}

static void setSigusrHandler()
{
    struct sigaction sa;
    sa.sa_handler = sigusrHandler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGUSR1, &sa, NULL);
}

void runSelect()
{
    sigset_t emptyset;
    sigemptyset(&emptyset);

    blockSigint();
    setSigusrHandler();

    RaiiObject RaiiObject{};
    fd_set fdRead;

    while (true) {
        cerr << "Loop iteration" << endl;
        FD_ZERO(&fdRead);
        FD_SET(0, &fdRead);
        while (true) {
            if (pselect(FD_SETSIZE, &fdRead, NULL, NULL, NULL, &emptyset) > 0) {
                cerr << "Select" << endl;
                return;
            } else {
                cerr << "Select break" << endl;
                return;
            }
        }
    }
}

int main()
{
    cerr << "Main start" << endl;

    cerr << "Thread start" << endl;
    thread runSelectThread{runSelect};
    nativeHandle = runSelectThread.native_handle();
    setSigintHandler();
    runSelectThread.join();

    cerr << "Main exit" << endl;

    return EXIT_SUCCESS;
}

Main start
Thread start
RaiiObject ctor
Loop iteration
^CINT
USR
Select break
RaiiObject dtor
Main exit