Upstream: better handling of invalid headers in cache files. If cache file is truncated, it is possible that u->process_header() will return NGX_AGAIN. Added appropriate handling of this case by changing the error to NGX_HTTP_UPSTREAM_INVALID_HEADER. Also, added appropriate logging of this and NGX_HTTP_UPSTREAM_INVALID_HEADER cases at the "crit" level. Note that this will result in duplicate logging in case of NGX_HTTP_UPSTREAM_INVALID_HEADER. While this is something better to avoid, it is considered to be an overkill to implement cache-specific error logging in u->process_header(). Additionally, u->buffer.start is now reset to be able to receive a new response, and u->cache_status set to MISS to provide the value in the $upstream_cache_status variable, much like it happens on other cache file errors detected by ngx_http_file_cache_read(), instead of HIT, which is believed to be misleading.