Cache: report error if slab allocator fails during cache loading.

src/http/ngx_http_cache.h

diff --git a/src/http/ngx_http_cache.h b/src/http/ngx_http_cache.h
index d36fa77..e93278d 100644
--- a/src/http/ngx_http_cache.h
+++ b/src/http/ngx_http_cache.h
@@ -153,6 +153,8 @@
 
     time_t                           inactive;
 
+    time_t                           fail_time;
+
     ngx_uint_t                       files;
     ngx_uint_t                       loader_files;
     ngx_msec_t                       last;

src/http/ngx_http_file_cache.c

diff --git a/src/http/ngx_http_file_cache.c b/src/http/ngx_http_file_cache.c
index 37c4754..c8b19e1 100644
--- a/src/http/ngx_http_file_cache.c
+++ b/src/http/ngx_http_file_cache.c
@@ -2094,6 +2094,12 @@
         fcn = ngx_slab_calloc_locked(cache->shpool,
                                      sizeof(ngx_http_file_cache_node_t));
         if (fcn == NULL) {
+            if (cache->fail_time != ngx_time()) {
+                cache->fail_time = ngx_time();
+                ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, 0,
+                           "could not allocate node%s", cache->shpool->log_ctx);
+            }
+
             ngx_shmtx_unlock(&cache->shpool->mutex);
             return NGX_ERROR;
         }