Write more tests attempting to break windows

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

test/posix/pipe_write_eagain_test.c

@@ -0,0 +1,107 @@
+// 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 <errno.h>
+#include <poll.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+/**
+ * @fileoverview Tests that EAGAIN won't corrupt pipe.
+ *
+ * This is a real bug when using CancelIoEx() on winsock writes, so we
+ * need to make sure it doesn't happen on pipes too.
+ */
+
+#define ITERATIONS 100000
+#define ASYMMETRY  3
+
+int fds[2];
+int got_read_eagains;
+int got_write_eagains;
+
+void *worker(void *arg) {
+  for (int expect = 0; expect < ITERATIONS;) {
+    int number;
+    ssize_t rc = read(fds[0], &number, sizeof(number));
+    if (rc == -1) {
+      if (errno == EAGAIN) {
+        ++got_read_eagains;
+        if (poll(&(struct pollfd){fds[0], POLLIN}, 1, -1) == -1)
+          exit(11);
+        continue;
+      }
+      perror("read");
+      exit(8);
+    }
+    size_t got = rc;
+    if (got != sizeof(int))
+      exit(9);
+    if (expect != number)
+      exit(10);
+    ++expect;
+  }
+  return 0;
+}
+
+int main(int argc, char *argv[]) {
+
+  if (pipe2(fds, O_NONBLOCK))
+    return 1;
+
+  pthread_t th;
+  if (pthread_create(&th, 0, worker, 0))
+    return 2;
+
+  int number = 0;
+  for (;;) {
+    int chunk = 0;
+    int numbers[ASYMMETRY];
+    for (;;) {
+      numbers[chunk] = number + chunk;
+      if (++chunk == ASYMMETRY)
+        break;
+      if (number + chunk == ITERATIONS)
+        break;
+    }
+    for (;;) {
+      ssize_t rc = write(fds[1], numbers, chunk * sizeof(int));
+      if (rc == -1) {
+        if (errno == EAGAIN) {
+          ++got_write_eagains;
+          if (poll(&(struct pollfd){fds[1], POLLOUT}, 1, -1) == -1)
+            return 10;
+          continue;
+        }
+        return 3;
+      }
+      if (rc % sizeof(int))
+        return 4;
+      chunk = rc / sizeof(int);
+      number += chunk;
+      break;
+    }
+    if (number == ITERATIONS)
+      break;
+  }
+
+  if (pthread_join(th, 0))
+    return 5;
+
+  if (!got_read_eagains && !got_write_eagains)
+    return 7;
+}