Add tests for Load() with negative offset

src/restic/backend/local/backend_test.go

@@ -51,6 +51,13 @@ func TestLocalBackendLoad(t *testing.T) {
 	test.TestLoad(t)
 }
 
+func TestLocalBackendLoadNegativeOffset(t *testing.T) {
+	if SkipMessage != "" {
+		t.Skip(SkipMessage)
+	}
+	test.TestLoadNegativeOffset(t)
+}
+
 func TestLocalBackendSave(t *testing.T) {
 	if SkipMessage != "" {
 		t.Skip(SkipMessage)

src/restic/backend/mem/backend_test.go

@@ -51,6 +51,13 @@ func TestMemBackendLoad(t *testing.T) {
 	test.TestLoad(t)
 }
 
+func TestMemBackendLoadNegativeOffset(t *testing.T) {
+	if SkipMessage != "" {
+		t.Skip(SkipMessage)
+	}
+	test.TestLoadNegativeOffset(t)
+}
+
 func TestMemBackendSave(t *testing.T) {
 	if SkipMessage != "" {
 		t.Skip(SkipMessage)

src/restic/backend/mem/mem_backend.go

@@ -120,7 +120,7 @@ func memLoad(be *MemoryBackend, h backend.Handle, p []byte, off int64) (int, err
 	case off > int64(len(buf)):
 		return 0, errors.New("offset beyond end of file")
 	case off < -int64(len(buf)):
-		return 0, errors.New("offset beyond beginning of file")
+		off = 0
 	case off < 0:
 		off = int64(len(buf)) + off
 	}

src/restic/backend/rest/backend_test.go

@@ -51,6 +51,13 @@ func TestRestBackendLoad(t *testing.T) {
 	test.TestLoad(t)
 }
 
+func TestRestBackendLoadNegativeOffset(t *testing.T) {
+	if SkipMessage != "" {
+		t.Skip(SkipMessage)
+	}
+	test.TestLoadNegativeOffset(t)
+}
+
 func TestRestBackendSave(t *testing.T) {
 	if SkipMessage != "" {
 		t.Skip(SkipMessage)

src/restic/backend/rest/rest.go

@@ -82,12 +82,12 @@ func (b *restBackend) Load(h backend.Handle, p []byte, off int64) (n int, err er
 			return 0, err
 		}
 
-		if off > -info.Size {
-			return 0, errors.New("offset before beginning of file")
-		}
-
+		if -off > info.Size {
+			off = 0
+		} else {
 			off = info.Size + off
 		}
+	}
 
 	req, err := http.NewRequest("GET", restPath(b.url, h), nil)
 	if err != nil {

src/restic/backend/s3/backend_test.go

@@ -51,6 +51,13 @@ func TestS3BackendLoad(t *testing.T) {
 	test.TestLoad(t)
 }
 
+func TestS3BackendLoadNegativeOffset(t *testing.T) {
+	if SkipMessage != "" {
+		t.Skip(SkipMessage)
+	}
+	test.TestLoadNegativeOffset(t)
+}
+
 func TestS3BackendSave(t *testing.T) {
 	if SkipMessage != "" {
 		t.Skip(SkipMessage)

src/restic/backend/sftp/backend_test.go

@@ -51,6 +51,13 @@ func TestSftpBackendLoad(t *testing.T) {
 	test.TestLoad(t)
 }
 
+func TestSftpBackendLoadNegativeOffset(t *testing.T) {
+	if SkipMessage != "" {
+		t.Skip(SkipMessage)
+	}
+	test.TestLoadNegativeOffset(t)
+}
+
 func TestSftpBackendSave(t *testing.T) {
 	if SkipMessage != "" {
 		t.Skip(SkipMessage)

src/restic/backend/test/backend_test.go

@@ -51,6 +51,13 @@ func TestTestBackendLoad(t *testing.T) {
 	test.TestLoad(t)
 }
 
+func TestTestBackendLoadNegativeOffset(t *testing.T) {
+	if SkipMessage != "" {
+		t.Skip(SkipMessage)
+	}
+	test.TestLoadNegativeOffset(t)
+}
+
 func TestTestBackendSave(t *testing.T) {
 	if SkipMessage != "" {
 		t.Skip(SkipMessage)

src/restic/backend/test/tests.go

@@ -220,9 +220,59 @@ func TestLoad(t testing.TB) {
 		buf := make([]byte, l)
 		n, err := b.Load(handle, buf, int64(o))
 
-		// if we requested data beyond the end of the file, ignore
+		// if we requested data beyond the end of the file, require
 		// ErrUnexpectedEOF error
-		if l > len(d) && err == io.ErrUnexpectedEOF {
+		if l > len(d) {
+			if err != io.ErrUnexpectedEOF {
+				t.Errorf("Load(%d, %d) did not return io.ErrUnexpectedEOF", len(buf), int64(o))
+			}
+			err = nil
+			buf = buf[:len(d)]
+		}
+
+		if err != nil {
+			t.Errorf("Load(%d, %d): unexpected error: %v", len(buf), int64(o), err)
+			continue
+		}
+
+		if n != len(buf) {
+			t.Errorf("Load(%d, %d): wrong length returned, want %d, got %d",
+				len(buf), int64(o), len(buf), n)
+			continue
+		}
+
+		buf = buf[:n]
+		if !bytes.Equal(buf, d) {
+			t.Errorf("Load(%d, %d) returned wrong bytes", len(buf), int64(o))
+			continue
+		}
+	}
+
+	// test with negative offset
+	for i := 0; i < 50; i++ {
+		l := rand.Intn(length + 2000)
+		o := rand.Intn(length + 2000)
+
+		d := data
+		if o < len(d) {
+			d = d[len(d)-o:]
+		} else {
+			o = 0
+		}
+
+		if l > 0 && l < len(d) {
+			d = d[:l]
+		}
+
+		buf := make([]byte, l)
+		n, err := b.Load(handle, buf, -int64(o))
+
+		// if we requested data beyond the end of the file, require
+		// ErrUnexpectedEOF error
+		if l > len(d) {
+			if err != io.ErrUnexpectedEOF {
+				t.Errorf("Load(%d, %d) did not return io.ErrUnexpectedEOF", len(buf), int64(o))
+			}
 			err = nil
 			buf = buf[:len(d)]
 		}
@@ -259,6 +309,61 @@ func TestLoad(t testing.TB) {
 	OK(t, b.Remove(backend.Data, id.String()))
 }
 
+// TestLoadNegativeOffset tests the backend's Load function with negative offsets.
+func TestLoadNegativeOffset(t testing.TB) {
+	b := open(t)
+	defer close(t)
+
+	length := rand.Intn(1<<24) + 2000
+
+	data := Random(23, length)
+	id := backend.Hash(data)
+
+	handle := backend.Handle{Type: backend.Data, Name: id.String()}
+	err := b.Save(handle, data)
+	if err != nil {
+		t.Fatalf("Save() error: %v", err)
+	}
+
+	// test normal reads
+	for i := 0; i < 50; i++ {
+		l := rand.Intn(length + 2000)
+		o := -rand.Intn(length + 2000)
+
+		buf := make([]byte, l)
+		n, err := b.Load(handle, buf, int64(o))
+		t.Logf("data %v, load(%v, %v) -> %v %v",
+			len(data), len(buf), o, n, err)
+
+		// if we requested data beyond the end of the file, require
+		// ErrUnexpectedEOF error
+		if len(buf) > -o {
+			if err != io.ErrUnexpectedEOF {
+				t.Errorf("Load(%d, %d) did not return io.ErrUnexpectedEOF", len(buf), o)
+			}
+			err = nil
+			buf = buf[:-o]
+		}
+
+		if err != nil {
+			t.Errorf("Load(%d, %d) returned error: %v", len(buf), o, err)
+		}
+
+		if n != len(buf) {
+			t.Errorf("Load(%d, %d) returned short read, only got %d bytes", len(buf), o, n)
+		}
+
+		p := len(data) + o
+		if !bytes.Equal(buf, data[p:p+len(buf)]) {
+			t.Errorf("Load(%d, %d) returned wrong bytes", len(buf), o)
+			continue
+		}
+
+	}
+
+	OK(t, b.Remove(backend.Data, id.String()))
+}
+
 // TestSave tests saving data in the backend.
 func TestSave(t testing.TB) {
 	b := open(t)