update r3945 with more descriptive error message

src/http/modules/ngx_http_fastcgi_module.c

@@ -2003,6 +2003,8 @@ ngx_http_fastcgi_create_loc_conf(ngx_conf_t *cf)
 
     conf->catch_stderr = NGX_CONF_UNSET_PTR;
 
+    ngx_str_set(&conf->upstream.module, "fastcgi");
+
     return conf;
 }
 

src/http/modules/ngx_http_proxy_module.c

@@ -1707,6 +1707,8 @@ ngx_http_proxy_create_loc_conf(ngx_conf_t *cf)
     conf->headers_hash_max_size = NGX_CONF_UNSET_UINT;
     conf->headers_hash_bucket_size = NGX_CONF_UNSET_UINT;
 
+    ngx_str_set(&conf->upstream.module, "proxy");
+
     return conf;
 }
 

src/http/modules/ngx_http_scgi_module.c

@@ -1030,6 +1030,8 @@ ngx_http_scgi_create_loc_conf(ngx_conf_t *cf)
     /* "scgi_cyclic_temp_file" is disabled */
     conf->upstream.cyclic_temp_file = 0;
 
+    ngx_str_set(&conf->upstream.module, "scgi");
+
     return conf;
 }
 

src/http/modules/ngx_http_uwsgi_module.c

@@ -1083,6 +1083,8 @@ ngx_http_uwsgi_create_loc_conf(ngx_conf_t *cf)
     /* "uwsgi_cyclic_temp_file" is disabled */
     conf->upstream.cyclic_temp_file = 0;
 
+    ngx_str_set(&conf->upstream.module, "uwsgi");
+
     return conf;
 }
 

src/http/ngx_http_upstream.c

@@ -661,10 +661,12 @@ ngx_http_upstream_cache(ngx_http_request_t *r, ngx_http_upstream_t *u)
 
         ngx_http_file_cache_create_key(r);
 
-        if (r->cache->header_start >= u->conf->buffer_size) {
+        if (r->cache->header_start + 256 >= u->conf->buffer_size) {
             ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
-                "cache key too large, increase upstream buffer size %uz",
-                u->conf->buffer_size);
+                          "%V_buffer_size %uz is not enough for cache key, "
+                          "it should increased at least to %uz",
+                          &u->conf->module, u->conf->buffer_size,
+                          ngx_align(r->cache->header_start + 256, 1024));
 
             r->cache = NULL;
             return NGX_DECLINED;

src/http/ngx_http_upstream.h

@@ -179,6 +179,7 @@ typedef struct {
     ngx_flag_t                       ssl_session_reuse;
 #endif
 
+    ngx_str_t                        module;
 } ngx_http_upstream_conf_t;