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Catch signals on Linux with signalfd (follow-up to #98 and #132) #142

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Catch signals on Linux with signalfd (follow-up to #98 and #132) #142

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59 changes: 32 additions & 27 deletions ttyplot.c
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,9 @@
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#ifdef __linux__
#include <sys/signalfd.h>
#endif
#include <fcntl.h>

#ifdef __OpenBSD__
Expand Down Expand Up @@ -287,32 +290,6 @@ static void finish(int signum) {
sigint_pending = 1;
}

static int pselect_without_signal_starvation(
int nfds,
fd_set * readfds,
fd_set * writefds,
fd_set * exceptfds,
const struct timespec * timeout,
const sigset_t * sigmask) {
// With high pressure on file descriptors (e.g. with "ttyplot < /dev/zero")
// a call to `pselect` could stall signal delivery for 20+ seconds on Linux.
// To avoid that situation, we first do a call to `pselect` that is dedicated
// to signal delivery and that only.
// (Related: https://stackoverflow.com/q/62315082)
const struct timespec zero_timeout = { .tv_sec = 0, .tv_nsec = 0 };

// First call, signal delivery only
const int select_ret = pselect(0, NULL, NULL, NULL, &zero_timeout, sigmask);

const bool signal_received = ((select_ret == -1) && (errno == EINTR));
if (signal_received) {
return select_ret;
}

// Second call
return pselect(nfds, readfds, writefds, exceptfds, timeout, sigmask);
}

static void redraw_screen(const char * errstr) {
if (window_big_enough_to_draw()) {
paint_plot();
Expand Down Expand Up @@ -591,6 +568,15 @@ int main(int argc, char *argv[]) {
signal(SIGWINCH, resize);
signal(SIGINT, finish);

// On Linux, with high pressure on file descriptors (e.g. with "ttyplot < /dev/zero"),
// a call to `pselect` could stall signal delivery for 20+ seconds. signalfd() is then
// a more reliable way of catching the signals.
// (Related: https://stackoverflow.com/q/62315082)
int signals = -1;
#ifdef __linux__
signals = signalfd(-1, &block_sigset, 0);
#endif

while(1) {
// Block until (a) we receive a signal or (b) stdin can be read without blocking
// or (c) timeout expires, in order to reduce use of CPU and power while idle
Expand All @@ -601,20 +587,39 @@ int main(int argc, char *argv[]) {
FD_SET(STDIN_FILENO, &read_fds);
select_nfds = STDIN_FILENO + 1;
}
if (signals != -1) {
FD_SET(signals, &read_fds);
if (signals >= select_nfds)
select_nfds = signals + 1;
}
if (tty != -1) {
FD_SET(tty, &read_fds);
if (tty >= select_nfds)
select_nfds = tty + 1;
}
struct timespec timeout = { .tv_sec = 0, .tv_nsec = 500e6 }; // 500 milliseconds for refreshing the clock
const int select_ret = pselect_without_signal_starvation(select_nfds, &read_fds, NULL, NULL, &timeout, &empty_sigset);
const int select_ret = pselect(select_nfds, &read_fds, NULL, NULL, &timeout, &empty_sigset);

gettimeofday(&now, NULL);

// Refresh the clock on timeouts.
if (select_ret == 0)
redraw_needed = true;

// On Linux, catch signals that came through the signalfd file descriptor.
#ifdef __linux__
if (select_ret > 0 && FD_ISSET(signals, &read_fds)) {
struct signalfd_siginfo siginfo;
int count = read(signals, &siginfo, sizeof(siginfo));
if (count > 0) {
if (siginfo.ssi_signo == SIGINT)
sigint_pending = 1;
else if (siginfo.ssi_signo == SIGWINCH)
sigwinch_pending = 1;
}
}
#endif

// Handle signals.
if (sigint_pending) {
break;
Expand Down