Handle Gerrit Change-Ids for merge commits

Otherwise successful, non-conflicting merges will never get a
Gerrit Change-Id.

Bug: 358206
Change-Id: I9b599ad01d9f7332200c1d81a1ba6ce5ef990ab5
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/MergeCommandTest.java

@@ -54,6 +54,7 @@ import static org.junit.Assume.assumeTrue;
 
 import java.io.File;
 import java.util.Iterator;
+import java.util.regex.Pattern;
 
 import org.eclipse.jgit.api.MergeCommand.FastForwardMode;
 import org.eclipse.jgit.api.MergeResult.MergeStatus;
@@ -1584,9 +1585,7 @@ public class MergeCommandTest extends RepositoryTestCase {
 		assertEquals(MergeStatus.CONFLICTING, result.getMergeStatus());
 	}
 
-	@Test
-	public void testMergeWithMessageOption() throws Exception {
-		try (Git git = new Git(db)) {
+	private Ref prepareSuccessfulMerge(Git git) throws Exception {
 		writeTrashFile("a", "1\na\n3\n");
 		git.add().addFilepattern("a").call();
 		RevCommit initialCommit = git.commit().setMessage("initial").call();
@@ -1604,7 +1603,13 @@ public class MergeCommandTest extends RepositoryTestCase {
 		git.add().addFilepattern("c").call();
 		git.commit().setMessage("main").call();
 
-			Ref sideBranch = db.exactRef("refs/heads/side");
+		return db.exactRef("refs/heads/side");
+	}
+
+	@Test
+	public void testMergeWithMessageOption() throws Exception {
+		try (Git git = new Git(db)) {
+			Ref sideBranch = prepareSuccessfulMerge(git);
 
 			git.merge().include(sideBranch).setStrategy(MergeStrategy.RESOLVE)
 					.setMessage("user message").call();
@@ -1617,6 +1622,43 @@ public class MergeCommandTest extends RepositoryTestCase {
 		}
 	}
 
+	@Test
+	public void testMergeWithChangeId() throws Exception {
+		try (Git git = new Git(db)) {
+			Ref sideBranch = prepareSuccessfulMerge(git);
+
+			git.merge().include(sideBranch).setStrategy(MergeStrategy.RESOLVE)
+					.setInsertChangeId(true).call();
+
+			assertNull(db.readMergeCommitMsg());
+
+			Iterator<RevCommit> it = git.log().call().iterator();
+			RevCommit newHead = it.next();
+			String commitMessage = newHead.getFullMessage();
+			assertTrue(Pattern.compile("\nChange-Id: I[0-9a-fA-F]{40}\n")
+					.matcher(commitMessage).find());
+		}
+	}
+
+	@Test
+	public void testMergeWithMessageAndChangeId() throws Exception {
+		try (Git git = new Git(db)) {
+			Ref sideBranch = prepareSuccessfulMerge(git);
+
+			git.merge().include(sideBranch).setStrategy(MergeStrategy.RESOLVE)
+					.setMessage("user message").setInsertChangeId(true).call();
+
+			assertNull(db.readMergeCommitMsg());
+
+			Iterator<RevCommit> it = git.log().call().iterator();
+			RevCommit newHead = it.next();
+			String commitMessage = newHead.getFullMessage();
+			assertTrue(commitMessage.startsWith("user message\n\n"));
+			assertTrue(Pattern.compile("\nChange-Id: I[0-9a-fA-F]{40}\n")
+					.matcher(commitMessage).find());
+		}
+	}
+
 	@Test
 	public void testMergeConflictWithMessageOption() throws Exception {
 		try (Git git = new Git(db)) {

org.eclipse.jgit/src/org/eclipse/jgit/api/MergeCommand.java

@@ -112,6 +112,8 @@ public class MergeCommand extends GitCommand<MergeResult> {
 
 	private String message;
 
+	private boolean insertChangeId;
+
 	private ProgressMonitor monitor = NullProgressMonitor.INSTANCE;
 
 	/**
@@ -392,6 +394,7 @@ public class MergeCommand extends GitCommand<MergeResult> {
 						try (Git git = new Git(getRepository())) {
 							newHeadId = git.commit()
 									.setReflogComment(refLogMessage.toString())
+									.setInsertChangeId(insertChangeId)
 									.call().getId();
 						}
 						mergeStatus = MergeStatus.MERGED;
@@ -618,6 +621,23 @@ public class MergeCommand extends GitCommand<MergeResult> {
 		return this;
 	}
 
+	/**
+	 * If set to true a change id will be inserted into the commit message
+	 *
+	 * An existing change id is not replaced. An initial change id (I000...)
+	 * will be replaced by the change id.
+	 *
+	 * @param insertChangeId
+	 *            whether to insert a change id
+	 * @return {@code this}
+	 * @since 5.0
+	 */
+	public MergeCommand setInsertChangeId(boolean insertChangeId) {
+		checkCallable();
+		this.insertChangeId = insertChangeId;
+		return this;
+	}
+
 	/**
 	 * The progress monitor associated with the diff operation. By default, this
 	 * is set to <code>NullProgressMonitor</code>