Add tests for updating single refs to missing objects

The reader may find it surprising that this succeeds without incident
unless there is peeling or a fast-forward check involved. This behavior
may be changed in the future, but for now, just document the current
behavior.

Change-Id: I348b37e93e0264dc0905c4d58ce881852d1dfe5e

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/RefUpdateTest.java

@@ -58,10 +58,12 @@ import static org.junit.Assert.fail;
 
 import java.io.File;
 import java.io.IOException;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 
+import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
@@ -959,6 +961,92 @@ public class RefUpdateTest extends SampleDataRepositoryTestCase {
 				"HEAD").getReverseEntries().get(0).getComment());
 	}
 
+	@Test
+	public void testCreateMissingObject() throws IOException {
+		String name = "refs/heads/abc";
+		ObjectId bad =
+				ObjectId.fromString("deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
+		RefUpdate ru = db.updateRef(name);
+		ru.setNewObjectId(bad);
+		Result update = ru.update();
+		assertEquals(Result.NEW, update);
+
+		Ref ref = db.exactRef(name);
+		assertNotNull(ref);
+		assertFalse(ref.isPeeled());
+		assertEquals(bad, ref.getObjectId());
+
+		try (RevWalk rw = new RevWalk(db)) {
+			rw.parseAny(ref.getObjectId());
+			fail("Expected MissingObjectException");
+		} catch (MissingObjectException expected) {
+			assertEquals(bad, expected.getObjectId());
+		}
+
+		RefDirectory refdir = (RefDirectory) db.getRefDatabase();
+		try {
+			// Packing requires peeling, which fails.
+			refdir.pack(Arrays.asList(name));
+		} catch (MissingObjectException expected) {
+			assertEquals(bad, expected.getObjectId());
+		}
+	}
+
+	@Test
+	public void testUpdateMissingObject() throws IOException {
+		String name = "refs/heads/abc";
+		RefUpdate ru = updateRef(name);
+		Result update = ru.update();
+		assertEquals(Result.NEW, update);
+		ObjectId oldId = ru.getNewObjectId();
+
+		ObjectId bad =
+				ObjectId.fromString("deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
+		ru = db.updateRef(name);
+		ru.setNewObjectId(bad);
+		update = ru.update();
+		assertEquals(Result.REJECTED, update);
+
+		Ref ref = db.exactRef(name);
+		assertNotNull(ref);
+		assertEquals(oldId, ref.getObjectId());
+	}
+
+	@Test
+	public void testForceUpdateMissingObject() throws IOException {
+		String name = "refs/heads/abc";
+		RefUpdate ru = updateRef(name);
+		Result update = ru.update();
+		assertEquals(Result.NEW, update);
+
+		ObjectId bad =
+				ObjectId.fromString("deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
+		ru = db.updateRef(name);
+		ru.setNewObjectId(bad);
+		update = ru.forceUpdate();
+		assertEquals(Result.FORCED, update);
+
+		Ref ref = db.exactRef(name);
+		assertNotNull(ref);
+		assertFalse(ref.isPeeled());
+		assertEquals(bad, ref.getObjectId());
+
+		try (RevWalk rw = new RevWalk(db)) {
+			rw.parseAny(ref.getObjectId());
+			fail("Expected MissingObjectException");
+		} catch (MissingObjectException expected) {
+			assertEquals(bad, expected.getObjectId());
+		}
+
+		RefDirectory refdir = (RefDirectory) db.getRefDatabase();
+		try {
+			// Packing requires peeling, which fails.
+			refdir.pack(Arrays.asList(name));
+		} catch (MissingObjectException expected) {
+			assertEquals(bad, expected.getObjectId());
+		}
+	}
+
 	private static void writeReflog(Repository db, ObjectId newId, String msg,
 			String refName) throws IOException {
 		RefDirectory refs = (RefDirectory) db.getRefDatabase();