Fix NPE in openFetch on Transport without local repository

Setting the walk and other fields to null will result in NPEs when the
user e.g. calls fetch on the connection, but at least the advertised
refs can be read like that without having a local repository.

Bug: 413389
Change-Id: I39c8363e81a1c7e6cb3412ba88542ead669e69ed
Signed-off-by: Robin Stocker <robin@nibor.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

org.eclipse.jgit.http.test/tst/org/eclipse/jgit/http/test/HttpClientTests.java

@@ -376,4 +376,20 @@ public class HttpClientTests extends HttpTestCase {
 			t.close();
 		}
 	}
+
+	@Test
+	public void testListRemoteWithoutLocalRepository() throws Exception {
+		Transport t = Transport.open(smartAuthNoneURI);
+		try {
+			FetchConnection c = t.openFetch();
+			try {
+				Ref head = c.getRef(Constants.HEAD);
+				assertNotNull(head);
+			} finally {
+				c.close();
+			}
+		} finally {
+			t.close();
+		}
+	}
 }

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

@@ -239,11 +239,16 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 	public BasePackFetchConnection(final PackTransport packTransport) {
 		super(packTransport);
 
+		if (local != null) {
 			final FetchConfig cfg = local.getConfig().get(FetchConfig.KEY);
+			allowOfsDelta = cfg.allowOfsDelta;
+		} else {
+			allowOfsDelta = true;
+		}
 		includeTags = transport.getTagOpt() != TagOpt.NO_TAGS;
 		thinPack = transport.isFetchThin();
-		allowOfsDelta = cfg.allowOfsDelta;
 
+		if (local != null) {
 			walk = new RevWalk(local);
 			reachableCommits = new RevCommitList<RevCommit>();
 			REACHABLE = walk.newFlag("REACHABLE"); //$NON-NLS-1$
@@ -254,6 +259,13 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 			walk.carry(COMMON);
 			walk.carry(REACHABLE);
 			walk.carry(ADVERTISED);
+		} else {
+			walk = null;
+			REACHABLE = null;
+			COMMON = null;
+			STATE = null;
+			ADVERTISED = null;
+		}
 	}
 
 	private static class FetchConfig {
@@ -357,6 +369,7 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 
 	@Override
 	public void close() {
+		if (walk != null)
 			walk.release();
 		super.close();
 	}

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

@@ -559,6 +559,9 @@ public abstract class Transport {
 
 	/**
 	 * Open a new transport with no local repository.
+	 * <p>
+	 * Note that the resulting transport instance can not be used for fetching
+	 * or pushing, but only for reading remote refs.
 	 *
 	 * @param uri
 	 * @return new Transport instance
@@ -1238,6 +1241,9 @@ public abstract class Transport {
 
 	/**
 	 * Begins a new connection for fetching from the remote repository.
+	 * <p>
+	 * If the transport has no local repository, the fetch connection can only
+	 * be used for reading remote refs.
 	 *
 	 * @return a fresh connection to fetch from the remote repository.
 	 * @throws NotSupportedException