feat(server): allow A/B test with a restricted set of rules

languagetool-server/src/main/java/org/languagetool/server/TextChecker.java

@@ -104,6 +104,22 @@ protected abstract DetectedLanguage getLanguage(String text, Map<String, String>
   private long pingsCleanDateMillis = System.currentTimeMillis();
   PipelinePool pipelinePool; // mocked in test -> package-private / not final
 
+  private final static List<String> onlyUsers;
+  private final static List<String> onlyRules;
+
+  static {
+    String onlyUsersEnv = System.getenv("LT_TEST_ONLY_USERS");
+    if (onlyUsersEnv == null) {
+      onlyUsersEnv = "";
+    }
+    String onlyRulesEnv = System.getenv("LT_TEST_ONLY_RULES");
+    if (onlyRulesEnv == null) {
+      onlyRulesEnv = "";
+    }
+    onlyUsers = Arrays.asList(onlyUsersEnv.split(","));
+    onlyRules = Arrays.asList(onlyRulesEnv.split(","));
+  }
+
   TextChecker(HTTPServerConfig config, boolean internalServer, Queue<Runnable> workQueue, RequestCounter reqCounter) {
     this.config = config;
     this.workQueue = workQueue;
@@ -464,6 +480,12 @@ public RuleMatch[] match(AnalyzedSentence sentence) throws IOException {
     }
     List<String> enabledRules = getEnabledRuleIds(params);
 
+    if (onlyUsers.contains(params.getOrDefault("username", "")) ||
+      (abTest != null && abTest.contains("only"))) {
+      useEnabledOnly = true;
+      enabledRules = onlyRules;
+    }
+
     List<String> disabledRules = getDisabledRuleIds(params);
     List<CategoryId> enabledCategories = getCategoryIds("enabledCategories", params);
     List<CategoryId> disabledCategories = getCategoryIds("disabledCategories", params);