libs/libc/dirent: preserve errno on readdir() end-of-directory

readdir() returned NULL at end-of-directory without ensuring errno was
clean.  POSIX requires errno to be unchanged at EOF, but the underlying
read() path may leave a stale errno value.  Callers that follow the
POSIX idiom (set errno to 0 before the call, test it after a NULL
return), such as readdir_r() and scandir(), then misread this as a
readdir() failure.

Save errno on entry and restore it on the end-of-directory return so
that EOF no longer reports a spurious error, while genuine errors
(read() returning a negative value) still propagate.

Signed-off-by: yushuailong <yyyusl@qq.com>

libs/libc/dirent/lib_readdir.c

@@ -59,6 +59,7 @@
 
 FAR struct dirent *readdir(DIR *dirp)
 {
+  int errcode;
   int ret;
 
   if (!dirp)
@@ -67,11 +68,22 @@ FAR struct dirent *readdir(DIR *dirp)
       return NULL;
     }
 
+  /* Save errno so it can be restored on end-of-directory.  POSIX requires
+   * errno to be unchanged at EOF, but the read() path may not preserve it.
+   */
+
+  errcode = get_errno();
+
   ret = read(dirp->fd, &dirp->entry, sizeof(struct dirent));
-  if (ret <= 0)
+  if (ret == 0)
     {
+      set_errno(errcode);  /* EOF: restore errno (not an error) */
       return NULL;
     }
+  else if (ret < 0)
+    {
+      return NULL;         /* error: read() already set errno */
+    }
 
   return &dirp->entry;
 }