The ETag sidecar describes a specific snapshot of the cache file. If the
cache file is replaced or edited out-of-band (manual edit, restore from
backup, another tool overwriting it), the sidecar's recorded ETag no
longer matches the bytes on disk -- using it would cause the server to
return 304 and we'd serve the wrong content from cache.
- _write_etag now os.utime()'s the sidecar to share the cache file's
st_mtime_ns immediately after writing it.
- _read_etag compares the two mtimes; if they don't match it returns
None and removes the sidecar so subsequent calls don't re-check it.
J. Nick Koston
5ae259ccba