Merge "Only throw MissingObjectException when necessary"

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriterBitmapWalker.java

@@ -117,6 +117,7 @@ final class PackWriterBitmapWalker {
 						new AddUnseenToBitmapFilter(seen, bitmapResult));
 			}
 
+			try {
 				while (walker.next() != null) {
 					// Iterate through all of the commits. The BitmapRevFilter does
 					// the work.
@@ -136,6 +137,20 @@ final class PackWriterBitmapWalker {
 					bitmapResult.addObject(ro, ro.getType());
 					pm.update(1);
 				}
+			} catch (MissingObjectException e) {
+				if (!ignoreMissingStart) {
+					throw e;
+				}
+				// Even when none of the objects we started the walk from is missing,
+				// an object reachable from one can be. RevWalk and ObjectWalk don't
+				// provide a way to ignore the missing object and continue, so bail
+				// out early with an undersized bitmap.
+				//
+				// The resulting packfile is likely to be much too large, but that's
+				// better than serving an error.
+				//
+				// TODO(czhen): Resume the walk instead once RevWalk supports that.
+			}
 		}
 
 		return bitmapResult;