HBASE-24686 [LOG] Log improvement in Connection#close (#2219)

Signed-off-by: Pankaj Kumar <pankajkumar@apache.org>
Signed-off-by: Viraj Jasani <vjasani@apache.org>
Signed-off-by: Sean Busbey <busbey@apache.org>

hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncConnectionImpl.java

@@ -192,6 +192,10 @@ public void close() {
     if (closed) {
       return;
     }
+    LOG.info("Connection has been closed by {}.", Thread.currentThread().getName());
+    if(LOG.isDebugEnabled()){
+      logCallStack(Thread.currentThread().getStackTrace());
+    }
     IOUtils.closeQuietly(clusterStatusListener);
     IOUtils.closeQuietly(rpcClient);
     IOUtils.closeQuietly(registry);
@@ -202,6 +206,16 @@ public void close() {
     closed = true;
   }
 
+  private void logCallStack(StackTraceElement[] stackTraceElements) {
+    StringBuilder stackBuilder = new StringBuilder("Call stack:");
+    for (StackTraceElement element : stackTraceElements) {
+      stackBuilder.append("\n    at ");
+      stackBuilder.append(element);
+    }
+    stackBuilder.append("\n");
+    LOG.debug(stackBuilder.toString());
+  }
+
   @Override
   public boolean isClosed() {
     return closed;