Как понимаю, нужно посмотреть функцию gx_http_read_client_request_body?

В самом ngx_http_dav_module.c такой код:

case NGX_HTTP_PUT:

if (r->uri.data[r->uri.len - 1] == '/') {
ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
"cannot PUT to a collection");
return NGX_HTTP_CONFLICT;
}

r->request_body_in_file_only = 1;
r->request_body_in_persistent_file = 1;
r->request_body_in_clean_file = 1;
r->request_body_file_group_access = 1;
r->request_body_file_log_level = 0;

rc = ngx_http_read_client_request_body(r, ngx_http_dav_put_han

if (rc >= NGX_HTTP_SPECIAL_RESPONSE) {
return rc;
}

return NGX_DONE;