Added warning about unset cache keys.

In fastcgi, scgi and uwsgi modules there are no default cache keys, and
using a cache without a cache key set is likely meaningless.

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 8016f5b..7be137a 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -2582,6 +2582,11 @@
         conf->cache_key = prev->cache_key;
     }
 
+    if (conf->upstream.cache && conf->cache_key.value.data == NULL) {
+        ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
+                           "no \"fastcgi_cache_key\" for \"fastcgi_cache\"");
+    }
+
     ngx_conf_merge_value(conf->upstream.cache_lock,
                               prev->upstream.cache_lock, 0);
 

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 1626984..b4b89ff 100644
--- a/src/http/modules/ngx_http_scgi_module.c
+++ b/src/http/modules/ngx_http_scgi_module.c
@@ -1337,6 +1337,11 @@
         conf->cache_key = prev->cache_key;
     }
 
+    if (conf->upstream.cache && conf->cache_key.value.data == NULL) {
+        ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
+                           "no \"scgi_cache_key\" for \"scgi_cache\"");
+    }
+
     ngx_conf_merge_value(conf->upstream.cache_lock,
                               prev->upstream.cache_lock, 0);
 

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 3956036..0185cba 100644
--- a/src/http/modules/ngx_http_uwsgi_module.c
+++ b/src/http/modules/ngx_http_uwsgi_module.c
@@ -1524,6 +1524,11 @@
         conf->cache_key = prev->cache_key;
     }
 
+    if (conf->upstream.cache && conf->cache_key.value.data == NULL) {
+        ngx_conf_log_error(NGX_LOG_WARN, cf, 0,
+                           "no \"uwsgi_cache_key\" for \"uwsgi_cache\"");
+    }
+
     ngx_conf_merge_value(conf->upstream.cache_lock,
                               prev->upstream.cache_lock, 0);