upload-pack: send symbolic refs as capabilities

cgit has this feature for some time. This will teach JGit to send symbolic refs,
too.

Change-Id: I7cb2ab4e6d31a838a0af92eac64535fdb66ed74a
Signed-off-by: Yuxuan 'fishy' Wang <fishywang@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java

@@ -129,6 +129,13 @@ public class GitProtocolConstants {
 	 */
 	public static final String OPTION_ALLOW_TIP_SHA1_IN_WANT = "allow-tip-sha1-in-want"; //$NON-NLS-1$
 
+	/**
+	 * Symbolic reference support for better negotiation.
+	 *
+	 * @since 3.6
+	 */
+	public static final String OPTION_SYMREF = "symref"; //$NON-NLS-1$
+
 	/**
 	 * The client supports atomic pushes. If this option is used, the server
 	 * will update all refs within one atomic transaction.

org.eclipse.jgit/src/org/eclipse/jgit/transport/RefAdvertiser.java

@@ -43,6 +43,8 @@
 
 package org.eclipse.jgit.transport;
 
+import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_SYMREF;
+
 import java.io.IOException;
 import java.util.HashSet;
 import java.util.LinkedHashSet;
@@ -145,6 +147,27 @@ public abstract class RefAdvertiser {
 		capablities.add(name);
 	}
 
+	/**
+	 * Add a symbolic ref to capabilities.
+	 * <p>
+	 * This method must be invoked prior to any of the following:
+	 * <ul>
+	 * <li>{@link #send(Map)}
+	 * <li>{@link #advertiseHave(AnyObjectId)}
+	 * </ul>
+	 *
+	 * @param from
+	 *            The symbolic ref, e.g. "HEAD"
+	 * @param to
+	 *            The real ref it points to, e.g. "refs/heads/master"
+	 *
+	 * @since 3.6
+	 */
+	public void addSymref(String from, String to) {
+		String symref = String.format("%s=%s:%s", OPTION_SYMREF, from, to); //$NON-NLS-1$
+		advertiseCapability(symref);
+	}
+
 	/**
 	 * Format an advertisement for the supplied refs.
 	 *

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java

@@ -807,7 +807,9 @@ public class UploadPack {
 				|| policy == null)
 			adv.advertiseCapability(OPTION_ALLOW_TIP_SHA1_IN_WANT);
 		adv.setDerefTags(true);
-		advertised = adv.send(getAdvertisedOrDefaultRefs());
+		Map<String, Ref> refs = getAdvertisedOrDefaultRefs();
+		findSymrefs(adv, refs);
+		advertised = adv.send(refs);
 		if (adv.isEmpty())
 			adv.advertiseId(ObjectId.zeroId(), "capabilities^{}"); //$NON-NLS-1$
 		adv.end();
@@ -1430,4 +1432,12 @@ public class UploadPack {
 		if (sideband)
 			pckOut.end();
 	}
+
+	private void findSymrefs(
+			final RefAdvertiser adv, final Map<String, Ref> refs) {
+		Ref head = refs.get(Constants.HEAD);
+		if (head != null && head.isSymbolic()) {
+			adv.addSymref(Constants.HEAD, head.getLeaf().getName());
+		}
+	}
 }