Honor tag creation date properly

pom.xml

@@ -53,7 +53,7 @@
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
         <artifactId>bom-${jenkins.baseline}.x</artifactId>
-        <version>5857.vb_f3dd0731f44</version>
+        <version>5933.vcf06f7b_5d1a_2</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>
@@ -227,5 +227,4 @@
       <url>https://repo.jenkins-ci.org/public/</url>
     </pluginRepository>
   </pluginRepositories>
-
 </project>

src/main/java/jenkins/plugins/git/AbstractGitSCMSource.java

@@ -110,6 +110,7 @@
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevTag;
 import org.eclipse.jgit.revwalk.RevTree;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.transport.RefSpec;
@@ -425,6 +426,34 @@
         }
     }
 
+    private long getTagTimestamp(RevWalk walk, ObjectId objectId) throws IOException {
+        try {
+            // Annotated tag object
+            RevTag tag = walk.parseTag(objectId);
+
+            if (tag.getTaggerIdent() != null
+                    && tag.getTaggerIdent().getWhen() != null) {
+                return tag.getTaggerIdent().getWhen().getTime();
+            }
+
+            // No tagger ident (or weird tag) — walk the tag chain to a commit, if any
+            Object target = tag.getObject();
+            for (int i = 0; i < 32 && target instanceof RevTag; i++) {
+                target = ((RevTag) target).getObject();
+            }
+
+            if (target instanceof RevCommit) {
+                return TimeUnit.SECONDS.toMillis(((RevCommit) target).getCommitTime());
+            }
+
+            throw new IOException("Tag does not ultimately reference a commit: " + objectId.name());
+        } catch (org.eclipse.jgit.errors.IncorrectObjectTypeException e) {
+            // Lightweight tag (or direct commit id)
+            RevCommit commit = walk.parseCommit(objectId);
+            return TimeUnit.SECONDS.toMillis(commit.getCommitTime());
+        }
+    }
+
     /**
      * {@inheritDoc}
      */
@@ -786,8 +815,7 @@
                     }
                     count++;
                     final String tagName = StringUtils.removeStart(ref.getKey(), Constants.R_TAGS);
-                    RevCommit commit = walk.parseCommit(ref.getValue());
-                    final long lastModified = TimeUnit.SECONDS.toMillis(commit.getCommitTime());
+                    final long lastModified = getTagTimestamp(walk, ref.getValue());
                     if (request.process(new GitTagSCMHead(tagName, lastModified),
                             new SCMSourceRequest.IntermediateLambda<ObjectId>() {
                                 @Nullable
@@ -804,7 +832,7 @@
                                                                       @Nullable ObjectId revisionInfo)
                                         throws IOException, InterruptedException {
                                     RevCommit commit = walk.parseCommit(revisionInfo);
-                                    final long lastModified = TimeUnit.SECONDS.toMillis(commit.getCommitTime());
+                                    final long lastModified = getTagTimestamp(walk, revisionInfo);
                                     final RevTree tree = commit.getTree();
                                     return new TreeWalkingSCMProbe(tagName, lastModified, repository, tree);
                                 }
@@ -1012,8 +1040,7 @@
                                            final Repository repository = client.getRepository();
                                            RevWalk walk = new RevWalk(repository)) {
                                           ObjectId ref = client.revParse(tagRef);
-                                          RevCommit commit = walk.parseCommit(ref);
-                                          long lastModified = TimeUnit.SECONDS.toMillis(commit.getCommitTime());
+                                          long lastModified = getTagTimestamp(walk, ref);
                                           listener.getLogger().printf("Resolved tag %s revision %s%n", revision,
                                                   ref.getName());
                                           return new GitTagSCMRevision(new GitTagSCMHead(revision, lastModified),

src/test/java/jenkins/plugins/git/AbstractGitSCMSourceWantTagsTest.java

@@ -3,6 +3,7 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.empty;
+import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.is;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -169,6 +170,25 @@ void indexingHasBranchAndTagDiscoveryTraitIgnoreTagDiscoveryTrait() throws Excep
         assertTrue(tagsFetched);
     }
 
+    @Test
+    public void tagTimestampsAreValid() throws Exception {
+        source.setTraits(Collections.singletonList(new TagDiscoveryTrait()));
+        Set<SCMHead> heads = source.fetch(LISTENER);
+
+        Set<GitTagSCMHead> tags = heads.stream()
+                .filter(h -> h instanceof GitTagSCMHead)
+                .map(h -> (GitTagSCMHead) h)
+                .collect(Collectors.toSet());
+
+        assertThat("Should discover both tags", tags.size(), is(2));
+
+        long year2000 = 946684800000L; // Jan 1, 2000
+        for (GitTagSCMHead tag : tags) {
+            assertThat("Tag " + tag.getName() + " should have valid timestamp",
+                    tag.getTimestamp(), greaterThan(year2000));
+        }
+    }
+
     static boolean tagsFetched;
 
     public static class MockGitClientForTags extends TestJGitAPIImpl {