revert r3935 and fix "stalled cache updating" alert
by freeing cache at upstream finalize phase
patch by Maxim Dounin

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 22187a5..f9f09cb 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2031,15 +2031,6 @@
             c->error = 1;
 
         } else {
-
-#if (NGX_HTTP_CACHE)
-
-            if (r->cache) {
-                ngx_http_file_cache_free(r->cache, u->pipe->temp_file);
-            }
-
-#endif
-
             ngx_http_upstream_finalize_request(r, u, rc);
             return;
         }
@@ -2991,16 +2982,19 @@
 
 #if (NGX_HTTP_CACHE)
 
-    if (u->cacheable && r->cache) {
-        time_t  valid;
+    if (r->cache) {
 
-        if (rc == NGX_HTTP_BAD_GATEWAY || rc == NGX_HTTP_GATEWAY_TIME_OUT) {
+        if (u->cacheable) {
 
-            valid = ngx_http_file_cache_valid(u->conf->cache_valid, rc);
+            if (rc == NGX_HTTP_BAD_GATEWAY || rc == NGX_HTTP_GATEWAY_TIME_OUT) {
+                time_t  valid;
 
-            if (valid) {
-                r->cache->valid_sec = ngx_time() + valid;
-                r->cache->error = rc;
+                valid = ngx_http_file_cache_valid(u->conf->cache_valid, rc);
+
+                if (valid) {
+                    r->cache->valid_sec = ngx_time() + valid;
+                    r->cache->error = rc;
+                }
             }
         }