SSL: reduced logging of session cache failures (ticket #621).

Session cache allocations might fail as long as the new session is different
in size from the one least recently used (and freed when the first allocation
fails).  In particular, it might not be possible to allocate space for
sessions with client certificates, since they are noticeably bigger than
normal sessions.

To ensure such allocation failures won't clutter logs, logging level changed
to "warn", and logging is now limited to at most one warning per second.

src/event/ngx_event_openssl.c

@@ -3770,6 +3770,8 @@ ngx_ssl_session_cache_init(ngx_shm_zone_t *shm_zone, void *data)
 
     ngx_queue_init(&cache->expire_queue);
 
+    cache->fail_time = 0;
+
     len = sizeof(" in SSL session shared cache \"\"") + shm_zone->shm.name.len;
 
     shpool->log_ctx = ngx_slab_alloc(shpool, len);
@@ -3953,8 +3955,11 @@ failed:
 
     ngx_shmtx_unlock(&shpool->mutex);
 
-    ngx_log_error(NGX_LOG_ALERT, c->log, 0,
-                  "could not allocate new session%s", shpool->log_ctx);
+    if (cache->fail_time != ngx_time()) {
+        cache->fail_time = ngx_time();
+        ngx_log_error(NGX_LOG_WARN, c->log, 0,
+                      "could not allocate new session%s", shpool->log_ctx);
+    }
 
     return 0;
 }

src/event/ngx_event_openssl.h

@@ -150,6 +150,7 @@ typedef struct {
     ngx_rbtree_t                session_rbtree;
     ngx_rbtree_node_t           sentinel;
     ngx_queue_t                 expire_queue;
+    time_t                      fail_time;
 } ngx_ssl_session_cache_t;