new file mode 100644
--- /dev/null
+++ b/src/event/ngx_event_busy_lock.h
@@ -0,0 +1,57 @@
+#ifndef _NGX_EVENT_BUSY_LOCK_H_INCLUDED_
+#define _NGX_EVENT_BUSY_LOCK_H_INCLUDED_
+
+
+#include <ngx_config.h>
+#include <ngx_core.h>
+#include <ngx_event.h>
+
+typedef struct ngx_event_busy_lock_ctx_s  ngx_event_busy_lock_ctx_t;
+
+struct ngx_event_busy_lock_ctx_s {
+    ngx_event_t                *event;
+    ngx_event_handler_pt        handler;
+    void                       *data;
+    ngx_msec_t                  timer;
+
+    unsigned                    locked:1;
+    unsigned                    cache_updated:1;
+
+    char                       *md5;
+    ngx_int_t                   slot;
+
+    ngx_event_busy_lock_ctx_t  *next;
+};
+
+
+typedef struct {
+    u_char                     *md5_mask;
+    char                       *md5;
+    ngx_uint_t                  cachable;
+
+    ngx_uint_t                  busy;
+    ngx_uint_t                  max_busy;
+
+    ngx_uint_t                  waiting;
+    ngx_uint_t                  max_waiting;
+
+    ngx_event_busy_lock_ctx_t  *events;
+    ngx_event_busy_lock_ctx_t  *last;
+
+#if (NGX_THREADS)
+    ngx_mutex_t                *mutex;
+#endif
+} ngx_event_busy_lock_t;
+
+
+ngx_int_t ngx_event_busy_lock(ngx_event_busy_lock_t *bl,
+                              ngx_event_busy_lock_ctx_t *ctx);
+ngx_int_t ngx_event_busy_lock_cachable(ngx_event_busy_lock_t *bl,
+                                       ngx_event_busy_lock_ctx_t *ctx);
+ngx_int_t ngx_event_busy_unlock(ngx_event_busy_lock_t *bl,
+                                ngx_event_busy_lock_ctx_t *ctx);
+ngx_int_t ngx_event_busy_lock_cancel(ngx_event_busy_lock_t *bl,
+                                     ngx_event_busy_lock_ctx_t *ctx);
+
+
+#endif /* _NGX_EVENT_BUSY_LOCK_H_INCLUDED_ */