Make "german2" an alias for "german" snowball stemmer

docs/changelog/113614.yaml

@@ -0,0 +1,18 @@
+pr: 113614
+summary: The 'german2' stemmer is now an alias for the 'german' snowball stemmer
+area: Analysis
+type: breaking
+issues: []
+breaking:
+  title: The "german2" snowball stemmer is now an alias for the "german" stemmer
+  area: Analysis
+  details: >-
+    Lucene 10 has merged the improved "german2" snowball language stemmer with the
+    "german" stemmer. For Elasticsearch, "german2" is now a deprecated alias for
+    "german". This may results in slightly different tokens being generated for
+    terms with umlaut substitution (like "ue" for "ü" etc...)
+  impact: >-
+    Replace usages of "german2" with "german" in analysis configuration. Old
+    indices that use the "german" stemmer should be reindexed if possible.
+  notable: false
+

docs/reference/analysis/tokenfilters/stemmer-tokenfilter.asciidoc

@@ -173,7 +173,6 @@ http://bvg.udc.es/recursos_lingua/stemming.jsp[`minimal_galician`] (Plural step
 German::
 https://dl.acm.org/citation.cfm?id=1141523[*`light_german`*],
 https://snowballstem.org/algorithms/german/stemmer.html[`german`],
-https://snowballstem.org/algorithms/german2/stemmer.html[`german2`],
 http://members.unine.ch/jacques.savoy/clef/morpho.pdf[`minimal_german`]
 
 Greek::

modules/analysis-common/src/main/java/org/elasticsearch/analysis/common/StemmerTokenFilterFactory.java

@@ -89,6 +89,8 @@ public class StemmerTokenFilterFactory extends AbstractTokenFilterFactory {
 
     private final String language;
 
+    private static final DeprecationLogger DEPRECATION_LOGGER = DeprecationLogger.getLogger(StemmerTokenFilterFactory.class);
+
     StemmerTokenFilterFactory(IndexSettings indexSettings, Environment environment, String name, Settings settings) throws IOException {
         super(name, settings);
         this.language = Strings.capitalize(settings.get("language", settings.get("name", "porter")));
@@ -190,7 +192,12 @@ public boolean incrementToken() {
             } else if ("german".equalsIgnoreCase(language)) {
                 return new SnowballFilter(tokenStream, new GermanStemmer());
             } else if ("german2".equalsIgnoreCase(language)) {
-                // TODO Lucene 10 upgrade: how about bw comp for users relying on german2 stemmer that is now folded into german stemmer?
+                DEPRECATION_LOGGER.critical(
+                    DeprecationCategory.ANALYSIS,
+                    "german2_stemmer_deprecation",
+                    "The 'german2' stemmer has been deprecated and folded into the 'german' Stemmer. "
+                        + "Replace all usages of 'german2' with 'german'."
+                );
                 return new SnowballFilter(tokenStream, new GermanStemmer());
             } else if ("light_german".equalsIgnoreCase(language) || "lightGerman".equalsIgnoreCase(language)) {
                 return new GermanLightStemFilter(tokenStream);

modules/analysis-common/src/test/java/org/elasticsearch/analysis/common/StemmerTokenFilterFactoryTests.java

@@ -8,6 +8,7 @@
  */
 package org.elasticsearch.analysis.common;
 
+import org.apache.lucene.analysis.Analyzer;
 import org.apache.lucene.analysis.TokenStream;
 import org.apache.lucene.analysis.Tokenizer;
 import org.apache.lucene.analysis.core.WhitespaceTokenizer;
@@ -16,6 +17,7 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.env.Environment;
 import org.elasticsearch.index.IndexVersion;
+import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.analysis.AnalysisTestsHelper;
 import org.elasticsearch.index.analysis.IndexAnalyzers;
 import org.elasticsearch.index.analysis.NamedAnalyzer;
@@ -103,6 +105,42 @@ public void testMultipleLanguagesThrowsException() throws IOException {
         assertEquals("Invalid stemmer class specified: [english, light_english]", e.getMessage());
     }
 
+    public void testGermanAndGerman2Stemmer() throws IOException {
+        IndexVersion v = IndexVersionUtils.randomVersionBetween(random(), IndexVersions.UPGRADE_TO_LUCENE_10_0_0, IndexVersion.current());
+        Analyzer analyzer = createGermanStemmer("german", v);
+        assertAnalyzesTo(analyzer, "Buecher Bücher", new String[] { "Buch", "Buch" });
+
+        analyzer = createGermanStemmer("german2", v);
+        assertAnalyzesTo(analyzer, "Buecher Bücher", new String[] { "Buch", "Buch" });
+        assertWarnings(
+            "The 'german2' stemmer has been deprecated and folded into the 'german' Stemmer. "
+                + "Replace all usages of 'german2' with 'german'."
+        );
+    }
+
+    private static Analyzer createGermanStemmer(String variant, IndexVersion v) throws IOException {
+
+        Settings settings = Settings.builder()
+            .put("index.analysis.filter.my_german.type", "stemmer")
+            .put("index.analysis.filter.my_german.language", variant)
+            .put("index.analysis.analyzer.my_german.tokenizer", "whitespace")
+            .put("index.analysis.analyzer.my_german.filter", "my_german")
+            .put(SETTING_VERSION_CREATED, v)
+            .put(Environment.PATH_HOME_SETTING.getKey(), createTempDir().toString())
+            .build();
+
+        ESTestCase.TestAnalysis analysis = AnalysisTestsHelper.createTestAnalysisFromSettings(settings, PLUGIN);
+        TokenFilterFactory tokenFilter = analysis.tokenFilter.get("my_german");
+        assertThat(tokenFilter, instanceOf(StemmerTokenFilterFactory.class));
+        Tokenizer tokenizer = new WhitespaceTokenizer();
+        tokenizer.setReader(new StringReader("Buecher oder Bücher"));
+        TokenStream create = tokenFilter.create(tokenizer);
+        assertThat(create, instanceOf(SnowballFilter.class));
+        IndexAnalyzers indexAnalyzers = analysis.indexAnalyzers;
+        NamedAnalyzer analyzer = indexAnalyzers.get("my_german");
+        return analyzer;
+    }
+
     public void testKpDeprecation() throws IOException {
         IndexVersion v = IndexVersionUtils.randomVersion(random());
         Settings settings = Settings.builder()