Ingest: RandomDocumentPicks.randomExistingFieldName doesn't return _v…

…ersion (elastic#88183)

When `RandomDocumentPicks.randomExistingFieldName` selects a field, the
caller expects to be able to manipulate that field in any manner.  `_version` has
special validation, must fit in an `long` and may not be `null`.

This change no longer returns `_version` from `randomExistingFieldName` unless it is the only field in the document.

Related to change: elastic#88102

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

@@ -19,6 +19,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Random;
+import java.util.Set;
 import java.util.TreeMap;
 
 public final class RandomDocumentPicks {
@@ -57,11 +58,12 @@ public static String randomLeafFieldName(Random random) {
 
     /**
      * Returns a randomly selected existing field name out of the fields that are contained
-     * in the document provided as an argument.
+     * in the document provided as an argument.  Does not return the _version field unless it is the only
+     * field.
      */
     public static String randomExistingFieldName(Random random, IngestDocument ingestDocument) {
         Map<String, Object> source = new TreeMap<>(ingestDocument.getSourceAndMetadata());
-        Map.Entry<String, Object> randomEntry = RandomPicks.randomFrom(random, source.entrySet());
+        Map.Entry<String, Object> randomEntry = getRandomEntry(random, source.entrySet());
         String key = randomEntry.getKey();
         while (randomEntry.getValue() instanceof Map) {
             @SuppressWarnings("unchecked")
@@ -74,6 +76,20 @@ public static String randomExistingFieldName(Random random, IngestDocument inges
         return key;
     }
 
+    /**
+     * Return a random entry from a set as long as the entry is not _version.  Returns _version only if it is the only entry.
+     * Since _verison has special validation, tests should test it explicitly rather than randomly
+     */
+    static Map.Entry<String, Object> getRandomEntry(Random random, Set<Map.Entry<String, Object>> entrySet) {
+        Map.Entry<String, Object> randomEntry = RandomPicks.randomFrom(random, entrySet);
+        String key = randomEntry.getKey();
+        while (IngestDocument.Metadata.VERSION.getFieldName().equals(key) && entrySet.size() > 1) {
+            randomEntry = RandomPicks.randomFrom(random, entrySet);
+            key = randomEntry.getKey();
+        }
+        return randomEntry;
+    }
+
     /**
      * Adds a random non existing field to the provided document and associates it
      * with the provided value. The field will be added at a random position within the document,