fs/object: fix memory object out of bounds Seek

fs/object/object.go

@@ -279,19 +279,26 @@ func (o *MemoryObject) SetModTime(ctx context.Context, modTime time.Time) error
 
 // Open opens the file for read.  Call Close() on the returned io.ReadCloser
 func (o *MemoryObject) Open(ctx context.Context, options ...fs.OpenOption) (io.ReadCloser, error) {
-	content := o.content
+	var offset, limit int64 = 0, -1
 	for _, option := range options {
 		switch x := option.(type) {
 		case *fs.RangeOption:
-			content = o.content[x.Start:x.End]
+			offset, limit = x.Decode(o.Size())
 		case *fs.SeekOption:
-			content = o.content[x.Offset:]
+			offset = x.Offset
 		default:
 			if option.Mandatory() {
 				fs.Logf(o, "Unsupported mandatory option: %v", option)
 			}
 		}
 	}
+	content := o.content
+	offset = max(offset, 0)
+	if limit < 0 {
+		content = content[offset:]
+	} else {
+		content = content[offset:min(offset+limit, int64(len(content)))]
+	}
 	return io.NopCloser(bytes.NewBuffer(content)), nil
 }
 

fs/object/object_test.go

@@ -110,6 +110,7 @@ func TestMemoryObject(t *testing.T) {
 	assert.Equal(t, newNow, o.ModTime(context.Background()))
 
 	checkOpen := func(rc io.ReadCloser, expected string) {
+		t.Helper()
 		actual, err := io.ReadAll(rc)
 		assert.NoError(t, err)
 		err = rc.Close()
@@ -118,6 +119,7 @@ func TestMemoryObject(t *testing.T) {
 	}
 
 	checkContent := func(o fs.Object, expected string) {
+		t.Helper()
 		rc, err := o.Open(context.Background())
 		assert.NoError(t, err)
 		checkOpen(rc, expected)
@@ -127,12 +129,28 @@ func TestMemoryObject(t *testing.T) {
 
 	rc, err := o.Open(context.Background(), &fs.RangeOption{Start: 1, End: 3})
 	assert.NoError(t, err)
-	checkOpen(rc, "ot")
+	checkOpen(rc, "ota")
+
+	rc, err = o.Open(context.Background(), &fs.RangeOption{Start: 1, End: -1})
+	assert.NoError(t, err)
+	checkOpen(rc, "otato")
+
+	rc, err = o.Open(context.Background(), &fs.RangeOption{Start: 1, End: 4096})
+	assert.NoError(t, err)
+	checkOpen(rc, "otato")
+
+	rc, err = o.Open(context.Background(), &fs.RangeOption{Start: -1, End: 4})
+	assert.NoError(t, err)
+	checkOpen(rc, "tato")
 
 	rc, err = o.Open(context.Background(), &fs.SeekOption{Offset: 3})
 	assert.NoError(t, err)
 	checkOpen(rc, "ato")
 
+	rc, err = o.Open(context.Background(), &fs.SeekOption{Offset: -100})
+	assert.NoError(t, err)
+	checkOpen(rc, "potato")
+
 	// check it fits within the buffer
 	newNow = now.Add(2 * time.Minute)
 	newContent := bytes.NewBufferString("Rutabaga")