update r3945 with more descriptive error message

src/http/modules/ngx_http_fastcgi_module.c

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 12d5b1d..71194fd 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -2003,6 +2003,8 @@
 
     conf->catch_stderr = NGX_CONF_UNSET_PTR;
 
+    ngx_str_set(&conf->upstream.module, "fastcgi");
+
     return conf;
 }
 

src/http/modules/ngx_http_proxy_module.c

diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index 214fe6a..147422a 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -1707,6 +1707,8 @@
     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

diff --git a/src/http/modules/ngx_http_scgi_module.c b/src/http/modules/ngx_http_scgi_module.c
index d745551..c83d770 100644
--- a/src/http/modules/ngx_http_scgi_module.c
+++ b/src/http/modules/ngx_http_scgi_module.c
@@ -1030,6 +1030,8 @@
     /* "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

diff --git a/src/http/modules/ngx_http_uwsgi_module.c b/src/http/modules/ngx_http_uwsgi_module.c
index cd3c7c1..a907e53 100644
--- a/src/http/modules/ngx_http_uwsgi_module.c
+++ b/src/http/modules/ngx_http_uwsgi_module.c
@@ -1083,6 +1083,8 @@
     /* "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

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 42b351c..29432dc 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -661,10 +661,12 @@
 
         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

diff --git a/src/http/ngx_http_upstream.h b/src/http/ngx_http_upstream.h
index 01e2e1e..fa848c0 100644
--- a/src/http/ngx_http_upstream.h
+++ b/src/http/ngx_http_upstream.h
@@ -179,6 +179,7 @@
     ngx_flag_t                       ssl_session_reuse;
 #endif
 
+    ngx_str_t                        module;
 } ngx_http_upstream_conf_t;