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Improve reliability of signal handling in child processes

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fish forks child processes when (for example) writing out builtin output.
After fork it resets signal handlers, but if a signal is delivered before
the signal handlers are reset, it will inherit fish's default handlers,
which do things like swallow SIGINT. Teach fish's default signal handlers
to detect this case and re-raise signals with default handlers.

This improves the reliability of control-C in the face of builtins.
This commit is contained in:
ridiculousfish 2019-02-10 12:54:42 -08:00
parent 28e2cfeb4b
commit 83884c2c13
1 changed files with 24 additions and 2 deletions
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26
src/signal.cpp
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@ -182,6 +182,23 @@ const wchar_t *signal_get_desc(int sig) {
return _(L"Unknown");
}
/// Store the "main" pid. This allows us to reliably determine if we are in a forked child.
static const pid_t s_main_pid = getpid();
/// It's possible that we receive a signal after we have forked, but before we have reset the signal
/// handlers (or even run the pthread_atfork calls). In that event we will do something dumb like
/// swallow SIGINT. Ensure that doesn't happen. Check if we are the main fish process; if not reset
/// and re-raise the signal. \return whether we re-raised the signal.
static bool reraise_if_forked_child(int sig) {
// Don't use is_forked_child, that relies on atfork handlers which maybe have not run yet.
if (getpid() == s_main_pid) {
return false;
}
signal(sig, SIG_DFL);
raise(sig);
return true;
}
/// Standard signal handler.
static void default_handler(int signal, siginfo_t *info, void *context) {
UNUSED(info);
@ -196,6 +213,7 @@ static void default_handler(int signal, siginfo_t *info, void *context) {
static void handle_winch(int sig, siginfo_t *info, void *context) {
UNUSED(info);
UNUSED(context);
if (reraise_if_forked_child(sig)) return;
common_handle_winch(sig);
default_handler(sig, 0, 0);
}
@ -206,6 +224,7 @@ static void handle_winch(int sig, siginfo_t *info, void *context) {
static void handle_hup(int sig, siginfo_t *info, void *context) {
UNUSED(info);
UNUSED(context);
if (reraise_if_forked_child(sig)) return;
if (event_is_signal_observed(SIGHUP)) {
default_handler(sig, 0, 0);
} else {
@ -215,9 +234,9 @@ static void handle_hup(int sig, siginfo_t *info, void *context) {
/// Handle sigterm. The only thing we do is restore the front process ID, then die.
static void handle_sigterm(int sig, siginfo_t *info, void *context) {
UNUSED(sig);
UNUSED(info);
UNUSED(context);
if (reraise_if_forked_child(sig)) return;
restore_term_foreground_process_group();
signal(SIGTERM, SIG_DFL);
raise(SIGTERM);
@ -226,18 +245,21 @@ static void handle_sigterm(int sig, siginfo_t *info, void *context) {
/// Interactive mode ^C handler. Respond to int signal by setting interrupted-flag and stopping all
/// loops and conditionals.
static void handle_int(int sig, siginfo_t *info, void *context) {
if (reraise_if_forked_child(sig)) return;
reader_handle_sigint();
default_handler(sig, info, context);
}
/// Non-interactive ^C handler.
static void handle_int_notinteractive(int sig, siginfo_t *info, void *context) {
if (reraise_if_forked_child(sig)) return;
parser_t::skip_all_blocks();
default_handler(sig, info, context);
}
/// sigchld handler. Does notification and calls the handler in proc.c.
static void handle_chld(int sig, siginfo_t *info, void *context) {
if (reraise_if_forked_child(sig)) return;
job_handle_signal(sig, info, context);
default_handler(sig, info, context);
}
@ -245,9 +267,9 @@ static void handle_chld(int sig, siginfo_t *info, void *context) {
// We have a sigalarm handler that does nothing. This is used in the signal torture test, to verify
// that we behave correctly when receiving lots of irrelevant signals.
static void handle_sigalarm(int sig, siginfo_t *info, void *context) {
UNUSED(sig);
UNUSED(info);
UNUSED(context);
if (reraise_if_forked_child(sig)) return;
}
void signal_reset_handlers() {