Fix flaky tests in CopyProcessorTests (opensearch-project#12885)

* Fix flaky test CopyProcessorTests#testCopyWithRemoveSource

Signed-off-by: Gao Binlong <gbinlong@amazon.com>

* Remove an existing field to get a non-existing field name

Signed-off-by: Gao Binlong <gbinlong@amazon.com>

* Add a new randomFieldName method

Signed-off-by: Gao Binlong <gbinlong@amazon.com>

---------

Signed-off-by: Gao Binlong <gbinlong@amazon.com>

Author: gaobinlong

modules/ingest-common/src/test/java/org/opensearch/ingest/common/CopyProcessorTests.java

@@ -24,7 +24,7 @@ public class CopyProcessorTests extends OpenSearchTestCase {
     public void testCopyExistingField() throws Exception {
         IngestDocument ingestDocument = RandomDocumentPicks.randomIngestDocument(random());
         String sourceFieldName = RandomDocumentPicks.randomExistingFieldName(random(), ingestDocument);
-        String targetFieldName = RandomDocumentPicks.randomFieldName(random());
+        String targetFieldName = RandomDocumentPicks.randomNonExistingFieldName(random(), ingestDocument);
         Processor processor = createCopyProcessor(sourceFieldName, targetFieldName, false, false, false);
         processor.execute(ingestDocument);
         assertThat(ingestDocument.hasField(targetFieldName), equalTo(true));
@@ -71,7 +71,8 @@ public void testCopyWithIgnoreMissing() throws Exception {
     public void testCopyWithRemoveSource() throws Exception {
         IngestDocument ingestDocument = RandomDocumentPicks.randomIngestDocument(random());
         String sourceFieldName = RandomDocumentPicks.randomExistingFieldName(random(), ingestDocument);
-        String targetFieldName = RandomDocumentPicks.randomFieldName(random());
+        String targetFieldName = RandomDocumentPicks.randomNonExistingFieldName(random(), ingestDocument);
+
         Object sourceValue = ingestDocument.getFieldValue(sourceFieldName, Object.class);
 
         Processor processor = createCopyProcessor(sourceFieldName, targetFieldName, false, true, false);

test/framework/src/main/java/org/opensearch/ingest/RandomDocumentPicks.java

@@ -71,6 +71,17 @@ public static String randomFieldName(Random random) {
         return fieldName.toString();
     }
 
+    /**
+     * Returns a random field name that doesn't exist in the document.
+     */
+    public static String randomNonExistingFieldName(Random random, IngestDocument ingestDocument) {
+        String fieldName;
+        do {
+            fieldName = randomFieldName(random);
+        } while (canAddField(fieldName, ingestDocument) == false);
+        return fieldName;
+    }
+
     /**
      * Returns a random leaf field name.
      */