Fix file descriptor assignment issues on Windows

Author: jart

libc/intrin/reservefd.c

@@ -72,7 +72,7 @@ int __reservefd_unlocked(int start) {
     if (_cmpxchg(&g_fds.p[fd].kind, kFdEmpty, kFdReserved)) {
       // g_fds.f isn't guarded by our mutex
       do {
-        f2 = MAX(fd + 1, f1);
+        f2 = MIN(fd + 1, f1);
       } while (!atomic_compare_exchange_weak_explicit(
           &g_fds.f, &f1, f2, memory_order_release, memory_order_relaxed));
       return fd;

test/posix/lowest_fd_test.c

@@ -0,0 +1,45 @@
+// 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 <fcntl.h>
+#include <unistd.h>
+
+int main(int argc, char *argv[]) {
+
+  // ensure most file descriptors are closed
+  for (int fildes = 3; fildes < 80; ++fildes)
+    close(fildes);
+
+  // create new file descriptor
+  if (open("/dev/urandom", O_RDONLY) != 3)
+    return 2;
+
+  // copy file descriptor to higher number
+  if (fcntl(3, F_DUPFD, 70) != 70)
+    return 3;
+
+  // new file descriptor should go for lowest number
+  int fd;
+  if ((fd = open("/dev/urandom", O_RDONLY)) != 4)
+    return 4;
+
+  // move file descriptor to higher number
+  if (close(3))
+    return 5;
+
+  // new file descriptor should go for lowest number
+  if (open("/dev/urandom", O_RDONLY) != 3)
+    return 6;
+}