google-cloud-nio: retry on 502 errors, and do a channel reopen on Unk…

…nownHostException

We've frequently encountered both of these errors transiently in the wild (see
broadinstitute/gatk#4888 and
broadinstitute/gatk#5094

I also increased the maximum depth when inspecting nested exceptions looking for
reopenable errors from 10 to 20, as we've seen chains of exceptions that come very
close to the current limit of 10.

google-cloud-clients/google-cloud-contrib/google-cloud-nio/src/main/java/com/google/cloud/storage/contrib/nio/CloudStorageRetryHandler.java

@@ -21,6 +21,7 @@
 import java.io.EOFException;
 import java.net.SocketException;
 import java.net.SocketTimeoutException;
+import java.net.UnknownHostException;
 import javax.net.ssl.SSLException;
 
 /**
@@ -144,7 +145,7 @@ void sleepForAttempt(int attempt) {
    * @return true if exs is a retryable error, otherwise false
    */
   private static boolean isRetryable(final StorageException exs) {
-    return exs.isRetryable() || exs.getCode() == 500 || exs.getCode() == 503;
+    return exs.isRetryable() || exs.getCode() == 500 || exs.getCode() == 502 || exs.getCode() == 503;
   }
 
   /**
@@ -154,14 +155,15 @@ private static boolean isRetryable(final StorageException exs) {
   private static boolean isReopenable(final StorageException exs) {
     Throwable throwable = exs;
     // ensures finite iteration
-    int maxDepth = 10;
+    int maxDepth = 20;
     while (throwable != null && maxDepth-- > 0) {
       if ((throwable.getMessage() != null
           && throwable.getMessage().contains("Connection closed prematurely"))
           || throwable instanceof SSLException
           || throwable instanceof EOFException
           || throwable instanceof SocketException
-          || throwable instanceof SocketTimeoutException) {
+          || throwable instanceof SocketTimeoutException
+          || throwable instanceof UnknownHostException) {
         return true;
       }
       throwable = throwable.getCause();