Add a public RevTag.parse() method

Callers might have a canonical tag encoding on hand that they
wish to convert into a clean structure for presentation purposes,
and the object may not be available in a repository.  (E.g. maybe
its a "draft" tag being written in an editor.)

Change-Id: I387a462afb70754aa7ee20891e6c0262438fdf32
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevTagParseTest.java

@@ -45,10 +45,13 @@ package org.eclipse.jgit.revwalk;
 
 import java.io.ByteArrayOutputStream;
 
+import org.eclipse.jgit.errors.CorruptObjectException;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectInserter;
 import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.lib.RepositoryTestCase;
+import org.eclipse.jgit.lib.Tag;
 
 public class RevTagParseTest extends RepositoryTestCase {
 	public void testTagBlob() throws Exception {
@@ -395,6 +398,23 @@ public class RevTagParseTest extends RepositoryTestCase {
 		assertEquals(shortMsg, c.getShortMessage());
 	}
 
+	public void testParse_PublicParseMethod() throws CorruptObjectException {
+		ObjectInserter.Formatter fmt = new ObjectInserter.Formatter();
+		Tag src = new Tag();
+		src.setObjectId(fmt.idFor(Constants.OBJ_TREE, new byte[] {}),
+				Constants.OBJ_TREE);
+		src.setTagger(committer);
+		src.setTag("a.test");
+		src.setMessage("Test tag\n\nThis is a test.\n");
+
+		RevTag p = RevTag.parse(fmt.format(src));
+		assertEquals(src.getObjectId(), p.getObject());
+		assertEquals(committer, p.getTaggerIdent());
+		assertEquals("a.test", p.getTagName());
+		assertEquals("Test tag", p.getShortMessage());
+		assertEquals(src.getMessage(), p.getFullMessage());
+	}
+
 	private static ObjectId id(final String str) {
 		return ObjectId.fromString(str);
 	}

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevTag.java

@@ -53,12 +53,65 @@ import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.util.MutableInteger;
 import org.eclipse.jgit.util.RawParseUtils;
 
 /** An annotated tag. */
 public class RevTag extends RevObject {
+	/**
+	 * Parse an annotated tag from its canonical format.
+	 *
+	 * This method constructs a temporary revision pool, parses the tag as
+	 * supplied, and returns it to the caller. Since the tag was built inside of
+	 * a private revision pool its object pointer will be initialized, but will
+	 * not have its headers loaded.
+	 *
+	 * Applications are discouraged from using this API. Callers usually need
+	 * more than one object. Use {@link RevWalk#parseTag(AnyObjectId)} to obtain
+	 * a RevTag from an existing repository.
+	 *
+	 * @param raw
+	 *            the canonical formatted tag to be parsed.
+	 * @return the parsed tag, in an isolated revision pool that is not
+	 *         available to the caller.
+	 * @throws CorruptObjectException
+	 *             the tag contains a malformed header that cannot be handled.
+	 */
+	public static RevTag parse(byte[] raw) throws CorruptObjectException {
+		return parse(new RevWalk((ObjectReader) null), raw);
+	}
+
+	/**
+	 * Parse an annotated tag from its canonical format.
+	 *
+	 * This method inserts the tag directly into the caller supplied revision
+	 * pool, making it appear as though the tag exists in the repository, even
+	 * if it doesn't. The repository under the pool is not affected.
+	 *
+	 * @param rw
+	 *            the revision pool to allocate the tag within. The tag's object
+	 *            pointer will be obtained from this pool.
+	 * @param raw
+	 *            the canonical formatted tag to be parsed.
+	 * @return the parsed tag, in an isolated revision pool that is not
+	 *         available to the caller.
+	 * @throws CorruptObjectException
+	 *             the tag contains a malformed header that cannot be handled.
+	 */
+	public static RevTag parse(RevWalk rw, byte[] raw)
+			throws CorruptObjectException {
+		ObjectInserter.Formatter fmt = new ObjectInserter.Formatter();
+		boolean retain = rw.isRetainBody();
+		rw.setRetainBody(true);
+		RevTag r = rw.lookupTag(fmt.idFor(Constants.OBJ_TAG, raw));
+		r.parseCanonical(rw, raw);
+		rw.setRetainBody(retain);
+		return r;
+	}
+
 	private RevObject object;
 
 	private byte[] buffer;