RewriteGenerator: avoid adding null parent

Prevent adding a null parent to a commit's parent array. Doing so
can cause NPEs later on.

Bug: 552160
Change-Id: Ib24b7b9b7b08e0b6f246006b4a4cade7eeb830b9
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

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

@@ -49,6 +49,7 @@ import static org.junit.Assert.assertNull;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.revwalk.filter.MessageRevFilter;
 import org.eclipse.jgit.revwalk.filter.RevFilter;
+import org.eclipse.jgit.treewalk.filter.PathFilterGroup;
 import org.junit.Test;
 
 public class FirstParentRevWalkTest extends RevWalkTestCase {
@@ -425,4 +426,39 @@ public class FirstParentRevWalkTest extends RevWalkTestCase {
 		markStart(a);
 		assertNull(rw.next());
 	}
+
+	@Test
+	public void testWithTopoSortAndTreeFilter() throws Exception {
+		RevCommit a = commit();
+		RevCommit b = commit(tree(file("0", blob("b"))), a);
+		RevCommit c = commit(tree(file("0", blob("c"))), b, a);
+		RevCommit d = commit(tree(file("0", blob("d"))), c);
+
+		rw.reset();
+		rw.setFirstParent(true);
+		rw.sort(RevSort.TOPO, true);
+		rw.setTreeFilter(PathFilterGroup.createFromStrings("0"));
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertCommit(c, rw.next());
+		assertCommit(b, rw.next());
+		assertNull(rw.next());
+	}
+
+	@Test
+	public void testWithTopoSortAndTreeFilter2() throws Exception {
+		RevCommit a = commit();
+		RevCommit b = commit(tree(file("0", blob("b"))), a);
+		RevCommit c = commit(tree(file("0", blob("c"))), a, b);
+		RevCommit d = commit(tree(file("0", blob("d"))), c);
+
+		rw.reset();
+		rw.setFirstParent(true);
+		rw.sort(RevSort.TOPO, true);
+		rw.setTreeFilter(PathFilterGroup.createFromStrings("0"));
+		markStart(d);
+		assertCommit(d, rw.next());
+		assertCommit(c, rw.next());
+		assertNull(rw.next());
+	}
 }

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

@@ -103,11 +103,15 @@ class RewriteGenerator extends Generator {
 		final int nParents = pList.length;
 		for (int i = 0; i < nParents; i++) {
 			final RevCommit oldp = pList[i];
-			if (firstParent && i > 0) {
+			final RevCommit newp = rewrite(oldp);
-				c.parents = new RevCommit[] { rewrite(oldp) };
+			if (firstParent) {
+				if (newp == null) {
+					c.parents = RevCommit.NO_PARENTS;
+				} else {
+					c.parents = new RevCommit[] { newp };
+				}
 				return c;
 			}
-			final RevCommit newp = rewrite(oldp);
 			if (oldp != newp) {
 				pList[i] = newp;
 				rewrote = true;