fixes spurious race condition in client close (#5318)

When the close the accumulo client the following could happen.

 1. THREAD1 sets closed atomic boolean to true in client as part of
    close() call
 2. THREAD2 a background thread in transport pool tries to read property from
    client and gets exception
 3. THREAD1 shuts down the transport pool object as part of closing
    client

This change avoids throwing an exception in step 2 above.  This bug was
causing MetaFateOpsCommandsIT to sometimes fail because it would have
a random excpetion in the json the command printing.

core/src/main/java/org/apache/accumulo/core/clientImpl/ClientContext.java

@@ -46,6 +46,7 @@
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Function;
+import java.util.function.LongSupplier;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
 
@@ -73,6 +74,7 @@
 import org.apache.accumulo.core.client.security.tokens.AuthenticationToken;
 import org.apache.accumulo.core.conf.AccumuloConfiguration;
 import org.apache.accumulo.core.conf.ClientProperty;
+import org.apache.accumulo.core.conf.ConfigurationTypeHelper;
 import org.apache.accumulo.core.conf.Property;
 import org.apache.accumulo.core.data.InstanceId;
 import org.apache.accumulo.core.data.KeyValue;
@@ -1038,7 +1040,21 @@ protected long getTransportPoolMaxAgeMillis() {
   public synchronized ThriftTransportPool getTransportPool() {
     ensureOpen();
     if (thriftTransportPool == null) {
-      thriftTransportPool = ThriftTransportPool.startNew(this::getTransportPoolMaxAgeMillis);
+      LongSupplier maxAgeSupplier = () -> {
+        try {
+          return getTransportPoolMaxAgeMillis();
+        } catch (IllegalStateException e) {
+          if (closed.get()) {
+            // The transport pool has a background thread that may call this supplier in the middle
+            // of closing. This is here to avoid spurious exceptions from race conditions that
+            // happen when closing a client.
+            return ConfigurationTypeHelper
+                .getTimeInMillis(ClientProperty.RPC_TRANSPORT_IDLE_TIMEOUT.getDefaultValue());
+          }
+          throw e;
+        }
+      };
+      thriftTransportPool = ThriftTransportPool.startNew(maxAgeSupplier);
     }
     return thriftTransportPool;
   }