Write more tests attempting to break windows

This time I haven't succeeded in breaking anything which is a good sign.

test/posix/signal_latency_async_test.c

@@ -0,0 +1,148 @@
+// Copyright 2024 Justine Alexandra Roberts Tunney
+//
+// Permission to use, copy, modify, and/or distribute this software for
+// any purpose with or without fee is hereby granted, provided that the
+// above copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
+// WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
+// AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
+// DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
+// PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
+// TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include <pthread.h>
+#include <signal.h>
+#include <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <unistd.h>
+
+#define ITERATIONS 10000
+
+pthread_t sender_thread;
+pthread_t receiver_thread;
+struct timespec send_time;
+atomic_int sender_got_signal;
+double latencies[ITERATIONS];
+
+void sender_signal_handler(int signo) {
+  sender_got_signal = 1;
+}
+
+void receiver_signal_handler(int signo) {
+  struct timespec receive_time;
+  clock_gettime(CLOCK_MONOTONIC, &receive_time);
+
+  long sec_diff = receive_time.tv_sec - send_time.tv_sec;
+  long nsec_diff = receive_time.tv_nsec - send_time.tv_nsec;
+  double latency_ns = sec_diff * 1e9 + nsec_diff;
+
+  static int iteration = 0;
+  if (iteration < ITERATIONS)
+    latencies[iteration++] = latency_ns;
+
+  // Pong sender
+  if (pthread_kill(sender_thread, SIGUSR2))
+    exit(2);
+
+  // Exit if done
+  if (iteration >= ITERATIONS)
+    pthread_exit(0);
+}
+
+void *sender_func(void *arg) {
+
+  for (int i = 0; i < ITERATIONS; i++) {
+
+    // Wait a bit sometimes
+    if (rand() % 2 == 1) {
+      volatile unsigned v = 0;
+      for (;;)
+        if (++v == 4000)
+          break;
+    }
+
+    // Ping receiver
+    clock_gettime(CLOCK_MONOTONIC, &send_time);
+    if (pthread_kill(receiver_thread, SIGUSR1))
+      exit(6);
+
+    // Wait for pong
+    for (;;)
+      if (atomic_load_explicit(&sender_got_signal, memory_order_relaxed))
+        break;
+    sender_got_signal = 0;
+  }
+
+  return 0;
+}
+
+void *receiver_func(void *arg) {
+
+  // Wait for asynchronous signals
+  volatile unsigned v = 0;
+  for (;;)
+    ++v;
+
+  return 0;
+}
+
+int compare(const void *a, const void *b) {
+  const double *x = a, *y = b;
+  if (*x < *y)
+    return -1;
+  else if (*x > *y)
+    return 1;
+  else
+    return 0;
+}
+
+int main() {
+
+  // Install signal handlers
+  struct sigaction sa;
+  sa.sa_handler = receiver_signal_handler;
+  sa.sa_flags = 0;
+  sigemptyset(&sa.sa_mask);
+  sigaction(SIGUSR1, &sa, 0);
+  sa.sa_handler = sender_signal_handler;
+  sigaction(SIGUSR2, &sa, 0);
+
+  // Create receiver thread first
+  if (pthread_create(&receiver_thread, 0, receiver_func, 0))
+    exit(11);
+
+  // Create sender thread
+  if (pthread_create(&sender_thread, 0, sender_func, 0))
+    exit(12);
+
+  // Wait for threads to finish
+  if (pthread_join(sender_thread, 0))
+    exit(13);
+  if (pthread_join(receiver_thread, 0))
+    exit(14);
+
+  // Compute mean latency
+  double total_latency = 0;
+  for (int i = 0; i < ITERATIONS; i++)
+    total_latency += latencies[i];
+  double mean_latency = total_latency / ITERATIONS;
+
+  // Sort latencies to compute percentiles
+  qsort(latencies, ITERATIONS, sizeof(double), compare);
+
+  double p50 = latencies[(int)(0.50 * ITERATIONS)];
+  double p90 = latencies[(int)(0.90 * ITERATIONS)];
+  double p95 = latencies[(int)(0.95 * ITERATIONS)];
+  double p99 = latencies[(int)(0.99 * ITERATIONS)];
+
+  printf("Mean latency: %.2f ns\n", mean_latency);
+  printf("50th percentile latency: %.2f ns\n", p50);
+  printf("90th percentile latency: %.2f ns\n", p90);
+  printf("95th percentile latency: %.2f ns\n", p95);
+  printf("99th percentile latency: %.2f ns\n", p99);
+}