core: Refresh cached file info from the open fd in default_handler. - #865

r->finfo was left as populated by the directory walk's stat() on the pathname, even after the file is opened. A rename(2) racing with that open() leaves the fd pointing at a different file than the one that was stat()'d, so Last-Modified, ETag, Content-Length and the byte range actually sent end up describing the old file while the content comes from the new one.

Refresh r->finfo from the already-open file descriptor right after open(), before any of that is computed.

PR 54962.

• This PR does not report or address a security vulnerabilityhttps://bz.apache.org/bugzilla/show_bug.cgi?id=54962
  A rename() of the requested file racing with the handler's open() leaves the fd pointing at a different file than the one the directory walk stat()'d, so Last-Modified, ETag, Content-Length and the Range handling all end up describing the old file while the body comes from the new one. Refreshes r->finfo from the already-open fd before any of that is computed.
  (see SECURITY.md and
  https://www.apache.org/security/#reporting-a-vulnerability).
• Code follows the httpd style guide.
• New log messages (level debug or higher) use an empty APLOGNO() tag;
  numbers are assigned by a committer at merge time (see docs/log-message-tags/README).
• If the change is user-visible, a changes-entries/*.txt file is included,
  following the template in README.CHANGES (not needed otherwise).
• Test cases based on pyhttpd are included where appropriate, in the
  test/modules/xxx directory matching the modules/xxx module source,
  or test/modules/core for core server changes. On Unix, verify these
  pass locally with e.g. make check-pytest PYTEST_DIRS=test/modules/xxx.