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.

--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -582,6 +582,8 @@ ngx_http_upstream_init_request(ngx_http_
             if (rc == NGX_HTTP_UPSTREAM_INVALID_HEADER) {
                 rc = NGX_DECLINED;
                 r->cached = 0;
+                u->buffer.start = NULL;
+                u->cache_status = NGX_HTTP_CACHE_MISS;
             }
 
             if (ngx_http_upstream_cache_background_update(r, u) != NGX_OK) {
@@ -1059,8 +1061,16 @@ ngx_http_upstream_cache_send(ngx_http_re
         return NGX_ERROR;
     }
 
+    if (rc == NGX_AGAIN) {
+        rc = NGX_HTTP_UPSTREAM_INVALID_HEADER;
+    }
+
     /* rc == NGX_HTTP_UPSTREAM_INVALID_HEADER */
 
+    ngx_log_error(NGX_LOG_CRIT, r->connection->log, 0,
+                  "cache file \"%s\" contains invalid header",
+                  c->file.name.data);
+
     /* TODO: delete file */
 
     return rc;