Fix a bug when w.Resume is set.

If w.Resume is set but there is no file to resume, then we erronously
return io.EOF.  Also, while we're in here, remove a dependency on the
old object listing code.

Fixes #59.

b2/integration_test.go

@@ -281,6 +281,20 @@ func TestResumeWriter(t *testing.T) {
 	}
 }
 
+func TestResumeWriterWithoutExtantFile(t *testing.T) {
+	ctx := context.Background()
+	bucket, done := startLiveTest(ctx, t)
+	defer done()
+
+	r := io.LimitReader(zReader{}, 15e6)
+	w := bucket.Object("foo").NewWriter(ctx)
+	w.ChunkSize = 5e6
+	w.Resume = true
+	if _, err := io.Copy(w, r); err != nil {
+		t.Fatalf("io.Copy: %v", err)
+	}
+}
+
 func TestAttrs(t *testing.T) {
 	ctx := context.Background()
 	ctx, cancel := context.WithTimeout(ctx, 10*time.Minute)

b2/writer.go

@@ -303,21 +303,28 @@ func (w *Writer) getLargeFile() (beLargeFileInterface, error) {
 		}
 		return w.o.b.b.startLargeFile(w.ctx, w.name, ctype, w.info)
 	}
+	var got bool
+	iter := w.o.b.List(w.ctx, ListPrefix(w.name), ListUnfinished())
+	var fi beFileInterface
+	for iter.Next() {
+		obj := iter.Object()
+		if obj.Name() == w.name {
+			got = true
+			fi = obj.f
+		}
+	}
+	if iter.Err() != nil {
+		return nil, iter.Err()
+	}
+	if !got {
+		w.Resume = false
+		return w.getLargeFile()
+	}
+
 	next := 1
 	seen := make(map[int]string)
 	var size int64
-	var fi beFileInterface
 	for {
-		cur := &Cursor{name: w.name}
-		objs, _, err := w.o.b.ListObjects(w.ctx, 1, cur)
-		if err != nil {
-			return nil, err
-		}
-		if len(objs) < 1 || objs[0].name != w.name {
-			w.Resume = false
-			return w.getLargeFile()
-		}
-		fi = objs[0].f
 		parts, n, err := fi.listParts(w.ctx, next, 100)
 		if err != nil {
 			return nil, err