check callbacks existence in onAboveWriteBufferHighWatermark and onBelowWriteBufferLowWatermark (envoyproxy#41521)

I got an envoy crash in my production
environment and the crash stack points to ActiveTcpClient::
onBelowWriteBufferLowWatermark. The envoy is using generic proxy filter
and using tcp connection pool to proxy customised protocol data. This
crash can be frequently reproduced when I benchmark with large body
request & response and kill the client connections during the test. I
check around other files, seems like check callbacks_ existence before
calling it is quite common, and I try use this fix in my environment
seems crash is solved.

Signed-off-by: liulanyi <[email protected]>

Author: liulanyi

source/common/tcp/conn_pool.h

@@ -94,8 +94,16 @@ class ActiveTcpClient : public Envoy::ConnectionPool::ActiveClient {
   // Override the default's of Envoy::ConnectionPool::ActiveClient for class-specific functions.
   // Network::ConnectionCallbacks
   void onEvent(Network::ConnectionEvent event) override;
-  void onAboveWriteBufferHighWatermark() override { callbacks_->onAboveWriteBufferHighWatermark(); }
-  void onBelowWriteBufferLowWatermark() override { callbacks_->onBelowWriteBufferLowWatermark(); }
+  void onAboveWriteBufferHighWatermark() override {
+    if (callbacks_) {
+      callbacks_->onAboveWriteBufferHighWatermark();
+    }
+  }
+  void onBelowWriteBufferLowWatermark() override {
+    if (callbacks_) {
+      callbacks_->onBelowWriteBufferLowWatermark();
+    }
+  }
 
   // Undos the readDisable done in onEvent(Connected)
   void readEnableIfNew();