vcs: handle lightweight tags in Git

vcs/src/main/java/org/openjdk/skara/vcs/git/GitRepository.java

@@ -1399,11 +1399,19 @@ public Optional<Tag.Annotated> annotate(Tag tag) throws IOException {
             var lines = await(p).stdout();
             if (lines.size() >= 4) {
                 var name = lines.get(0);
-                var target = new Hash(lines.get(1));
-                var author = Author.fromString(lines.get(2));
+                var targetLine = lines.get(1);
+                var authorLine = lines.get(2);
+                var dateLine = lines.get(3);
 
+                if (targetLine.isEmpty() && authorLine.equals(" ") && dateLine.isEmpty()) {
+                    // Must be a lightweight tag, no metadata present
+                    return Optional.empty();
+                }
+
+                var target = new Hash(targetLine);
+                var author = Author.fromString(authorLine);
                 var formatter = DateTimeFormatter.ISO_OFFSET_DATE_TIME;
-                var date = ZonedDateTime.parse(lines.get(3), formatter);
+                var date = ZonedDateTime.parse(dateLine, formatter);
                 var message = String.join("\n", lines.subList(4, lines.size()));
 
                 return Optional.of(new Tag.Annotated(name, target, author, date, message));

vcs/src/test/java/org/openjdk/skara/vcs/RepositoryTests.java

@@ -2323,4 +2323,31 @@ void testCommitterDate() throws IOException {
             assertEquals(committed, commit.committed());
         }
     }
+
+    @Test
+    void testLightweightTags() throws IOException, InterruptedException {
+        try (var dir = new TemporaryDirectory()) {
+            var repo = Repository.init(dir.path(), VCS.GIT);
+            var readme = dir.path().resolve("README");
+            Files.write(readme, List.of("Hello, readme!"));
+
+            repo.add(readme);
+            var head = repo.commit("Add README", "author", "author@openjdk.java.net");
+
+            // We don't want to expose making lightweight tags via the Repository class,
+            // so use a ProcessBuilder and invoke git directly here
+            var pb = new ProcessBuilder("git", "tag", "test-tag", head.hex());
+            pb.directory(repo.root().toFile());
+            assertEquals(0, pb.start().waitFor());
+
+            var tags = repo.tags();
+            assertEquals(1, tags.size());
+
+            var tag = tags.get(0);
+            assertEquals("test-tag", tag.name());
+
+            // Lightweight tags can't be annotated
+            assertEquals(Optional.empty(), repo.annotate(tag));
+        }
+    }
 }