Restore WildcardFieldMapperTests#testBWCIndexVersion

This test has been removed with elastic#112570 but with
the coming read-only support for v7 we need it back.

Author: cbuescher

x-pack/plugin/wildcard/src/test/java/org/elasticsearch/xpack/wildcard/mapper/WildcardFieldMapperTests.java

@@ -51,6 +51,7 @@
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexVersion;
+import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.cache.bitset.BitsetFilterCache;
 import org.elasticsearch.index.fielddata.FieldDataContext;
 import org.elasticsearch.index.fielddata.IndexFieldData;
@@ -106,6 +107,7 @@ static SearchExecutionContext createMockSearchExecutionContext(boolean allowExpe
 
     static final int MAX_FIELD_LENGTH = 30;
     static WildcardFieldMapper wildcardFieldType;
+    static WildcardFieldMapper wildcardFieldType79;
     static KeywordFieldMapper keywordFieldType;
     private DirectoryReader rewriteReader;
     private BaseDirectoryWrapper rewriteDir;
@@ -127,6 +129,9 @@ public void setUp() throws Exception {
         builder.ignoreAbove(MAX_FIELD_LENGTH);
         wildcardFieldType = builder.build(MapperBuilderContext.root(false, false));
 
+        Builder builder79 = new WildcardFieldMapper.Builder(WILDCARD_FIELD_NAME, IndexVersions.V_7_9_0);
+        wildcardFieldType79 = builder79.build(MapperBuilderContext.root(false, false));
+
         org.elasticsearch.index.mapper.KeywordFieldMapper.Builder kwBuilder = new KeywordFieldMapper.Builder(
             KEYWORD_FIELD_NAME,
             IndexVersion.current()
@@ -207,6 +212,37 @@ public void testIgnoreAbove() throws IOException {
         assertTrue(fields.stream().anyMatch(field -> "field".equals(field.stringValue())));
     }
 
+    public void testBWCIndexVersion() throws IOException {
+        // Create old format index using wildcard ngram analyzer used in 7.9 launch
+        Directory dir = newDirectory();
+        IndexWriterConfig iwc = newIndexWriterConfig(WildcardFieldMapper.WILDCARD_ANALYZER_7_9);
+        iwc.setMergePolicy(newTieredMergePolicy(random()));
+        RandomIndexWriter iw = new RandomIndexWriter(random(), dir, iwc);
+
+        Document doc = new Document();
+        LuceneDocument parseDoc = new LuceneDocument();
+        addFields(parseDoc, doc, "a b");
+        indexDoc(parseDoc, doc, iw);
+
+        iw.forceMerge(1);
+        DirectoryReader reader = iw.getReader();
+        IndexSearcher searcher = newSearcher(reader);
+        iw.close();
+
+        // Unnatural circumstance - testing we fail if we were to use the new analyzer on old index
+        Query oldWildcardFieldQuery = wildcardFieldType.fieldType().wildcardQuery("a b", null, null);
+        TopDocs oldWildcardFieldTopDocs = searcher.search(oldWildcardFieldQuery, 10, Sort.INDEXORDER);
+        assertThat(oldWildcardFieldTopDocs.totalHits.value(), equalTo(0L));
+
+        // Natural circumstance test we revert to the old analyzer for old indices
+        Query wildcardFieldQuery = wildcardFieldType79.fieldType().wildcardQuery("a b", null, null);
+        TopDocs wildcardFieldTopDocs = searcher.search(wildcardFieldQuery, 10, Sort.INDEXORDER);
+        assertThat(wildcardFieldTopDocs.totalHits.value(), equalTo(1L));
+
+        reader.close();
+        dir.close();
+    }
+
     // Test long query strings don't cause exceptions
     public void testTooBigQueryField() throws IOException {
         Directory dir = newDirectory();