HADOOP-12901. Add warning log when KMSClientProvider cannot create a connection to the KMS server. (Xiao Chen via wang)

(cherry picked from commit 391da36d93358038c50c15d91543f6c765fa0471)
(cherry picked from commit fed2339e285e84a40514f877703b2712a2305a6f)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/crypto/key/kms/KMSClientProvider.java

@@ -42,6 +42,8 @@ import org.apache.hadoop.security.token.delegation.web.DelegationTokenAuthentica
 import org.apache.hadoop.util.HttpExceptionUtils;
 import org.apache.http.client.utils.URIBuilder;
 import org.codehaus.jackson.map.ObjectMapper;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 import javax.net.ssl.HttpsURLConnection;
 
@@ -85,6 +87,9 @@ import com.google.common.base.Strings;
 public class KMSClientProvider extends KeyProvider implements CryptoExtension,
     KeyProviderDelegationTokenExtension.DelegationTokenExtension {
 
+  private static final Logger LOG =
+      LoggerFactory.getLogger(KMSClientProvider.class);
+
   private static final String INVALID_SIGNATURE = "Invalid signature";
 
   private static final String ANONYMOUS_REQUESTS_DISALLOWED = "Anonymous requests are disallowed";
@@ -491,6 +496,9 @@ public class KMSClientProvider extends KeyProvider implements CryptoExtension,
         }
       });
     } catch (IOException ex) {
+      if (ex instanceof SocketTimeoutException) {
+        LOG.warn("Failed to connect to {}:{}", url.getHost(), url.getPort());
+      }
       throw ex;
     } catch (UndeclaredThrowableException ex) {
       throw new IOException(ex.getUndeclaredThrowable());