test: retry backup operation 20 times

The backup operations can still fail because of too many pending
operations. It would retry 10 times with waiting 60 seconds between
each attempt, but that does not seem to be enough. The max attempts
has therefore been increased to 20 and better logging has been
created by throwing a custom exception when it happens. The previous
log was not visible, because the sample runners are redirecting the
standard out to a string.

Fixes #1019

samples/snippets/src/test/java/com/example/spanner/EncryptionKeyIT.java

@@ -22,6 +22,7 @@
 import com.google.cloud.spanner.ErrorCode;
 import com.google.cloud.spanner.Spanner;
 import com.google.cloud.spanner.SpannerException;
+import com.google.cloud.spanner.SpannerExceptionFactory;
 import com.google.cloud.spanner.SpannerOptions;
 import com.google.common.base.Preconditions;
 import com.google.common.util.concurrent.Uninterruptibles;
@@ -138,8 +139,8 @@ public void testEncryptedDatabaseAndBackupAndRestore() throws Exception {
             + "/backups/" + backupId + " using encryption key " + key);
   }
 
-  private static class ShouldRetryBackupOperation implements Predicate<SpannerException> {
-    private static final int MAX_ATTEMPTS = 10;
+  static class ShouldRetryBackupOperation implements Predicate<SpannerException> {
+    private static final int MAX_ATTEMPTS = 20;
     private int attempts = 0;
 
     @Override
@@ -148,9 +149,11 @@ public boolean test(SpannerException e) {
           && e.getMessage().contains("Please retry the operation once the pending")) {
         attempts++;
         if (attempts == MAX_ATTEMPTS) {
-          System.out.printf("Operation failed %d times because of other pending operations. "
-              + "Giving up operation.\n", attempts);
-          return false;
+          // Throw custom exception so it is easier to locate in the log why it went wrong.
+          throw SpannerExceptionFactory.newSpannerException(ErrorCode.DEADLINE_EXCEEDED,
+              String.format("Operation failed %d times because of other pending operations. "
+                  + "Giving up operation.\n", attempts),
+              e);
         }
         // Wait one minute before retrying.
         Uninterruptibles.sleepUninterruptibly(60L, TimeUnit.SECONDS);