add pending state to jl_thread_suspend_and_get_state-machine

Fixes an issue with #55500, where signals may abruptly abort the process
as they observe it is still processing the resume SIGUSR2 message and
are not able to wait for that processing to end before setting the new
message to exit.

src/signals-unix.c

@@ -448,6 +448,7 @@ static int signal_caught_cond = -1;
 
 int jl_thread_suspend_and_get_state(int tid, int timeout, bt_context_t *ctx)
 {
+    int err;
     pthread_mutex_lock(&in_signal_lock);
     jl_ptls_t ptls2 = jl_atomic_load_relaxed(&jl_all_tls_states)[tid];
     jl_task_t *ct2 = ptls2 ? jl_atomic_load_relaxed(&ptls2->current_task) : NULL;
@@ -456,22 +457,51 @@ int jl_thread_suspend_and_get_state(int tid, int timeout, bt_context_t *ctx)
         pthread_mutex_unlock(&in_signal_lock);
         return 0;
     }
-    sig_atomic_t request = 0;
-    if (!jl_atomic_cmpswap(&ptls2->signal_request, &request, 1)) {
+    if (jl_atomic_load(&ptls2->signal_request) != 0) {
         // something is wrong, or there is already a usr2 in flight elsewhere
+        // try to wait for it to finish or wait for timeout
+        struct pollfd event = {signal_caught_cond, POLLIN, 0};
+        do {
+            err = poll(&event, 1, timeout * 1000);
+        } while (err == -1 && errno == EINTR);
+        if (err == -1 || (event.revents & POLLIN) == 0) {
+            // not ready after timeout: cancel this request
+            pthread_mutex_unlock(&in_signal_lock);
+            return 0;
+        }
+    }
+    // check for  any stale signal_caught_cond events
+    struct pollfd event = {signal_caught_cond, POLLIN, 0};
+    do {
+        err = poll(&event, 1, 0);
+    } while (err == -1 && errno == EINTR);
+    if (err == -1) {
         pthread_mutex_unlock(&in_signal_lock);
         return 0;
     }
+    if ((event.revents & POLLIN) != 0) {
+        // consume it before continuing
+        eventfd_t got;
+        do {
+            err = read(signal_caught_cond, &got, sizeof(eventfd_t));
+        } while (err == -1 && errno == EINTR);
+        if (err != sizeof(eventfd_t)) abort();
+        assert(got == 1); (void) got;
+    }
+    sig_atomic_t request = jl_atomic_exchange(&ptls2->signal_request, 1);
+    assert(request == 0 || request == -1);
     request = 1;
-    int err = pthread_kill(ptls2->system_id, SIGUSR2);
-    // wait for thread to acknowledge or timeout
-    struct pollfd event = {signal_caught_cond, POLLIN, 0};
+    err = pthread_kill(ptls2->system_id, SIGUSR2);
     if (err == 0) {
+        // wait for thread to acknowledge or timeout
+        struct pollfd event = {signal_caught_cond, POLLIN, 0};
         do {
             err = poll(&event, 1, timeout * 1000);
         } while (err == -1 && errno == EINTR);
+        if (err != 1 || (event.revents & POLLIN) == 0)
+            err = -1;
     }
-    if ((event.revents & POLLIN) == 0) {
+    if (err == -1) {
         // not ready after timeout: try to cancel this request
         if (jl_atomic_cmpswap(&ptls2->signal_request, &request, 0)) {
             pthread_mutex_unlock(&in_signal_lock);
@@ -487,7 +517,7 @@ int jl_thread_suspend_and_get_state(int tid, int timeout, bt_context_t *ctx)
     // Now the other thread is waiting on exit_signal_cond (verify that here by
     // checking it is 0, and add an acquire barrier for good measure)
     request = jl_atomic_load_acquire(&ptls2->signal_request);
-    assert(request == 0); (void) request;
+    assert(request == 0 || request == -1); (void) request;
     jl_atomic_store_release(&ptls2->signal_request, 4); // prepare to resume normally, but later code may change this
     *ctx = *signal_context;
     return 1;
@@ -546,6 +576,7 @@ static void jl_exit_thread0(int signo, jl_bt_element_t *bt_data, size_t bt_size)
 }
 
 // request:
+// -1: processing
 //  0: nothing [not from here]
 //  1: get state & wait for request
 //  2: throw sigint if `!defer_signal && io_wait` or if force throw threshold
@@ -561,22 +592,36 @@ void usr2_handler(int sig, siginfo_t *info, void *ctx)
     if (ptls == NULL)
         return;
     int errno_save = errno;
-    // acknowledge that we saw the signal_request
-    sig_atomic_t request = jl_atomic_exchange(&ptls->signal_request, 0);
+    sig_atomic_t request = jl_atomic_load(&ptls->signal_request);
+    if (request == 0)
+        return;
+    if (!jl_atomic_cmpswap(&ptls->signal_request, &request, -1))
+        return;
     if (request == 1) {
         signal_context = jl_to_bt_context(ctx);
+        // acknowledge that we saw the signal_request and set signal_context
         int err;
         eventfd_t got = 1;
         err = write(signal_caught_cond, &got, sizeof(eventfd_t));
         if (err != sizeof(eventfd_t)) abort();
+        sig_atomic_t processing = -1;
+        jl_atomic_cmpswap(&ptls->signal_request, &processing, 0);
+        // wait for exit signal
         do {
             err = read(exit_signal_cond, &got, sizeof(eventfd_t));
         } while (err == -1 && errno == EINTR);
         if (err != sizeof(eventfd_t)) abort();
         assert(got == 1);
-        request = jl_atomic_exchange(&ptls->signal_request, 0);
+        request = jl_atomic_exchange(&ptls->signal_request, -1);
+        signal_context = NULL;
         assert(request == 2 || request == 3 || request == 4);
     }
+    int err;
+    eventfd_t got = 1;
+    err = write(signal_caught_cond, &got, sizeof(eventfd_t));
+    if (err != sizeof(eventfd_t)) abort();
+    sig_atomic_t processing = -1;
+    jl_atomic_cmpswap(&ptls->signal_request, &processing, 0);
     if (request == 2) {
         int force = jl_check_force_sigint();
         if (force || (!ptls->defer_signal && ptls->io_wait)) {