Fix ECHILD with WNOHANG on Windows

This change along with a patch for rsync's safe_write() function that'll
that'll soon be added to superconfigure, gets rsync working. There's one
remaining issue (which isn't a blocker) which is how rsync logs an error
about abnormal process termination since there's currently no way for us
to send non-fatal signals between processes. rsync in cosmos is restored

Fixes #1240

libc/proc/wait4-nt.c

@@ -88,18 +88,20 @@ static textwindows int __proc_wait(int pid, int *wstatus, int options,
     __proc_lock();
   CheckForZombies:
     int rc = __proc_check(pid, wstatus, rusage);
-    if (rc || (options & WNOHANG)) {
-      __proc_unlock();
-      return rc;
-    }
 
-    // there's no zombies left
+    // if there's no zombies left
     // check if there's any living processes
-    if (dll_is_empty(__proc.list)) {
+    if (!rc && dll_is_empty(__proc.list)) {
       __proc_unlock();
       return echild();
     }
 
+    // otherwise return zombie or zero
+    if (rc || (options & WNOHANG)) {
+      __proc_unlock();
+      return rc;
+    }
+
     // get appropriate wait object
     // register ourself as waiting
     struct Proc *pr = 0;

test/posix/wait2x_test.c

@@ -0,0 +1,70 @@
+// 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 <stdatomic.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+int main(int argc, char *argv[]) {
+  int ws, rc;
+
+  // create shared memory for synchronization
+  atomic_int *ready =
+      mmap(0, 4, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+
+  if ((rc = waitpid(-1, &ws, WNOHANG)) != -1)
+    return 1;
+  if (errno != ECHILD)
+    return 2;
+
+  // create process
+  int pid = fork();
+  if (pid == -1)
+    return 3;
+  if (!pid) {
+    for (;;)
+      if (*ready)
+        break;
+    _Exit(0);
+  }
+
+  // wait on process
+  if ((rc = waitpid(pid, &ws, WNOHANG)) == -1)
+    return 4;
+  if (rc != 0)
+    return 5;
+  if (ws)
+    return 6;
+
+  // signal subprocess
+  *ready = 1;
+
+  if ((rc = waitpid(pid, &ws, 0)) == -1)
+    return 7;
+  if (rc != pid)
+    return 8;
+  if (ws)
+    return 9;
+
+  // wait again
+  if ((rc = waitpid(pid, &ws, WNOHANG)) != -1)
+    return 10;
+  if (errno != ECHILD)
+    return 11;
+}