HTTP: added route logging.

-  Configuration: added "/config/settings/http/log_route".

   Type: bool
   Default: false

   This adds configurability to the error log.  It allows enabling and
   disabling logs related to how the router performs selection of the
   routes.

-  HTTP: logging request line.

   Log level: [notice]

   The request line is essential to understand which logs correspond to
   which request when reading the logs.

-  HTTP: logging route that's been discarded.

   Log level: [info]

-  HTTP: logging route whose action is selected.

   Log level: [notice]

-  HTTP: logging when "fallback" action is taken.

   Log level: [notice]

Closes: <https://github.com/nginx/unit/issues/758>
Link: <https://github.com/nginx/unit/pull/824>
Link: <https://github.com/nginx/unit/pull/839>
Suggested-by: Timo Stark <t.stark@nginx.com>
Suggested-by: Mark L Wood-Patrick <mwoodpatrick@gmail.com>
Suggested-by: Liam Crilly <liam@nginx.com>
Tested-by: Liam Crilly <liam@nginx.com>
Acked-by: Artem Konev <a.konev@f5.com>
Cc: Andrew Clayton <a.clayton@nginx.com>
Cc: Andrei Zeliankou <zelenkov@nginx.com>
Reviewed-by: Zhidao Hong <z.hong@f5.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

src/nxt_http_route.c

@@ -1548,6 +1548,18 @@ nxt_http_route_handler(nxt_task_t *task, nxt_http_request_t *r,
 
     for (i = 0; i < route->items; i++) {
         action = nxt_http_route_match(task, r, route->match[i]);
+
+        if (nxt_slow_path(r->log_route)) {
+            uint32_t    lvl = (action == NULL) ? NXT_LOG_INFO : NXT_LOG_NOTICE;
+            const char  *sel = (action == NULL) ? "discarded" : "selected";
+
+            if (route->name.length == 0) {
+                nxt_log(task, lvl, "\"routes/%z\" %s", i, sel);
+            } else {
+                nxt_log(task, lvl, "\"routes/%V/%z\" %s", &route->name, i, sel);
+            }
+        }
+
         if (action != NULL) {
             return action;
         }