fix plan core whiling concurrently deleting plan

src/sql/plan_cache/ob_i_lib_cache_node.h

@@ -154,7 +154,7 @@ public:
   int unlock() { return rwlock_.unlock(); }
   int64_t inc_ref_count(const CacheRefHandleID ref_handle);
   int64_t dec_ref_count(const CacheRefHandleID ref_handle);
-  int64_t get_ref_count() const { return ref_count_; }
+  int64_t get_ref_count() const { return ATOMIC_LOAD(&ref_count_); }
   common::ObIAllocator *get_allocator() { return &allocator_; }
   common::ObIAllocator &get_allocator_ref() { return allocator_; }
   lib::MemoryContext &get_mem_context() { return mem_context_; }

src/sql/plan_cache/ob_plan_cache.cpp

@@ -1187,6 +1187,8 @@ int ObPlanCache::ref_cache_obj(const ObCacheObjID obj_id, ObCacheObjGuard& guard
   ObGlobalReqTimeService::check_req_timeinfo();
   if (OB_FAIL(co_mgr_.atomic_get_alloc_cache_obj(obj_id, op))) {
     SQL_PC_LOG(WARN, "failed to get update plan statistic", K(obj_id), K(ret));
+  } else if (NULL == op.get_value()) {
+    ret = OB_HASH_NOT_EXIST;
   } else {
     guard.cache_obj_ = op.get_value();
   }

src/sql/plan_cache/ob_plan_cache_callback.cpp

@@ -49,11 +49,26 @@ int ObLibCacheAtomicOp::get_value(ObILibCacheNode *&cache_node)
   return ret;
 }
 
+/*
+worker thread:                   |  evict thread
+                                 |  get all plan id array(contains plan id x)
+deleting .... remove plan id x   |
+from map                         |
+dec ref cnt => ref_cnt=0         |
+                                 | ref plan id x. inc_ref=1
+deleting plan x                  |
+                                 | acess plan x --> cause core!
+*/
+
 void ObCacheObjAtomicOp::operator()(ObjKV &entry)
 {
   if (NULL != entry.second) {
+    if (0 == entry.second->get_ref_count()) {
+      // do nothing
+    } else {
       cache_obj_ = entry.second;
       cache_obj_->inc_ref_count(ref_handle_);
+    }
     SQL_PC_LOG(DEBUG, "succ to get plan", "ref_count", cache_obj_->get_ref_count());
   } else {
     // do nothing