ObjectIdSerializer: Support serialization of known non-null ObjectId

The implementation of ObjectIdSerializer, added in change I7599cf8bd,
is not equivalent to the original implementation in Gerrit [1].

The Gerrit implementation provides separate methods to (de)serialize
instances of ObjectId that are known to be non-null. In these methods,
no "marker" is written to the stream. Replacing Gerrit's implementation
with ObjectIdSerializer [2] broke persistent caches because it started
writing markers where they were not expected [3].

Since ObjectIdSerializer is included in JGit 4.11 we can't change the
existing #write and #read methods. Keep those as-is, but extend the
Javadoc to clarify that they support possibly null ObjectId instances.

Add new methods #writeWithoutMarker and #readWithoutMarker to support
the cases where the ObjectId is known to be non-null and the marker
should not be written to the serialization stream.

Also:

- Replace the hard-coded `0` and `1` markers with constants that can
  be linked from the Javadocs.

- Include the marker value in the "Invalid flag before ObjectId"
  exception message.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/9792
[2] https://gerrit-review.googlesource.com/c/gerrit/+/165851
[3] https://gerrit-review.googlesource.com/c/gerrit/+/165952

Change-Id: Iaf84c3ec32ecf83efffb306fdb4940cc85740f3f
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSerializerTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSerializerTest.java
index 24bc40eee8..d98b792d75 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSerializerTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ObjectIdSerializerTest.java
@@ -79,10 +79,18 @@ public class ObjectIdSerializerTest {
 			throws Exception {
 		File file = File.createTempFile("ObjectIdSerializerTest_", "");
 		try (OutputStream out = new FileOutputStream(file)) {
-			ObjectIdSerializer.write(out, objectId);
+			if (objectId == null) {
+				ObjectIdSerializer.write(out, objectId);
+			} else {
+				ObjectIdSerializer.writeWithoutMarker(out, objectId);
+			}
 		}
 		try (InputStream in = new FileInputStream(file)) {
-			return ObjectIdSerializer.read(in);
+			if (objectId == null) {
+				return ObjectIdSerializer.read(in);
+			} else {
+				return ObjectIdSerializer.readWithoutMarker(in);
+			}
 		}
 	}
 }