Do not abort on empty tag collections (#4241)

monticore-grammar/src/main/java/de/monticore/tagging/SimpleSymbolTagger.java

@@ -1,7 +1,6 @@
 /* (c) https://github.com/MontiCore/monticore */
 package de.monticore.tagging;
 
-import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
 import com.google.common.cache.CacheLoader;
 import com.google.common.cache.LoadingCache;
@@ -99,13 +98,14 @@ protected Stream<ASTTargetElement> findTagTargets(@Nonnull ISymbol symbol) {
     Iterator<ASTTargetElement> iterator = new ProgressiveIterator<ASTTargetElement, ASTTagUnit>
             (backingTagUnits.get().iterator()) {
       @Override
-      Collection<? extends ASTTargetElement> doWork(ASTTagUnit tagUnit) {
+      @Nonnull
+      protected Collection<? extends ASTTargetElement> doWork(ASTTagUnit tagUnit) {
         // Add the matching ASTTargetElement from #findTagTargetsOfTagUnit to the buffer
         return findTagTargetsOfTagUnit(tagUnit, fqn);
       }
 
       @Override
-      void cleanup() {
+      protected void cleanup() {
         // cleanup (remove unloaded TagUnits)
         tagUnitMapping.cleanUp();
       }
@@ -237,7 +237,13 @@ void tryToAdvance() {
         return;
       }
       // Actually do the tagging related logic
-      targetElementBuffer.addAll(this.doWork(backing.next()));
+      while (backing.hasNext() && targetElementBuffer.isEmpty()) {
+        // Repeat the work until the buffer is no longer empty (or the backing iterator consumed)
+        targetElementBuffer.addAll(this.doWork(backing.next()));
+      }
+      if (!backing.hasNext()) {
+        this.cleanup();
+      }
     }
 
     /**
@@ -247,9 +253,9 @@ void tryToAdvance() {
      * @return the values to be added to a buffer
      */
     @Nonnull
-    abstract Collection<? extends A> doWork(B next);
+    protected abstract Collection<? extends A> doWork(B next);
 
-    abstract void cleanup();
+    protected abstract void cleanup();
   }
 
   protected static class TagFQNMapping {

monticore-test/01.experiments/tagging/src/test/java/TagTest.java

@@ -41,9 +41,12 @@ public static void init() throws Exception {
     Log.enableFailQuick(false);
 
     // Load all relevant models
-    Optional opt = TagRepository.loadTagModel(new File("src/test/resources/models/Simple.tags"));
-    if (opt.isEmpty())
-      Assert.fail("Failed to load Simple.tags");
+    var emptyTagsOpt1 = TagRepository.loadTagModel(new File("src/test/resources/models/Empty.tags"));
+    Assert.assertTrue("Failed to load Empty.tags", emptyTagsOpt1.isPresent());
+    var opt = TagRepository.loadTagModel(new File("src/test/resources/models/Simple.tags"));
+    Assert.assertTrue("Failed to load Simple.tags", opt.isPresent());
+    var emptyTagsOpt2 = TagRepository.loadTagModel(new File("src/test/resources/models/Empty.tags"));
+    Assert.assertTrue("Failed to load Empty.tags", emptyTagsOpt2.isPresent());
 
     tagger = new SimpleSymbolTagger(TagRepository::getLoadedTagUnits);
 

monticore-test/01.experiments/tagging/src/test/resources/models/Empty.tags

@@ -0,0 +1,5 @@
+/* (c) https://github.com/MontiCore/monticore */
+
+tags Empty {
+  // no tags, empty
+}