Core: slab log_nomem flag.

The flag allows to suppress "ngx_slab_alloc() failed: no memory" messages
from a slab allocator, e.g., if an LRU expiration is used by a consumer
and allocation failures aren't fatal.

The flag is now used in the SSL session cache code, and in the limit_req
module.

src/core/ngx_slab.c

@@ -129,6 +129,7 @@ ngx_slab_init(ngx_slab_pool_t *pool)
         pool->pages->slab = pages;
     }
 
+    pool->log_nomem = 1;
     pool->log_ctx = &pool->zero;
     pool->zero = '\0';
 }
@@ -658,7 +659,10 @@ ngx_slab_alloc_pages(ngx_slab_pool_t *pool, ngx_uint_t pages)
         }
     }
 
-    ngx_slab_error(pool, NGX_LOG_CRIT, "ngx_slab_alloc() failed: no memory");
+    if (pool->log_nomem) {
+        ngx_slab_error(pool, NGX_LOG_CRIT,
+                       "ngx_slab_alloc() failed: no memory");
+    }
 
     return NULL;
 }

src/core/ngx_slab.h

@@ -39,6 +39,8 @@ typedef struct {
     u_char           *log_ctx;
     u_char            zero;
 
+    unsigned          log_nomem:1;
+
     void             *data;
     void             *addr;
 } ngx_slab_pool_t;

src/event/ngx_event_openssl.c

@@ -1834,6 +1834,8 @@ ngx_ssl_session_cache_init(ngx_shm_zone_t *shm_zone, void *data)
     ngx_sprintf(shpool->log_ctx, " in SSL session shared cache \"%V\"%Z",
                 &shm_zone->shm.name);
 
+    shpool->log_nomem = 0;
+
     return NGX_OK;
 }
 
@@ -1986,7 +1988,7 @@ failed:
     ngx_shmtx_unlock(&shpool->mutex);
 
     ngx_log_error(NGX_LOG_ALERT, c->log, 0,
-                  "could not add new SSL session to the session cache");
+                  "could not allocate new session%s", shpool->log_ctx);
 
     return 0;
 }

src/http/modules/ngx_http_limit_req_module.c

@@ -451,6 +451,8 @@ ngx_http_limit_req_lookup(ngx_http_limit_req_limit_t *limit, ngx_uint_t hash,
 
         node = ngx_slab_alloc_locked(ctx->shpool, size);
         if (node == NULL) {
+            ngx_log_error(NGX_LOG_ALERT, ngx_cycle->log, 0,
+                          "could not allocate node%s", ctx->shpool->log_ctx);
             return NGX_ERROR;
         }
     }
@@ -674,6 +676,8 @@ ngx_http_limit_req_init_zone(ngx_shm_zone_t *shm_zone, void *data)
     ngx_sprintf(ctx->shpool->log_ctx, " in limit_req zone \"%V\"%Z",
                 &shm_zone->shm.name);
 
+    ctx->shpool->log_nomem = 0;
+
     return NGX_OK;
 }