From 822422ef03880db40b6fbd3e24d59e763537ec87 Mon Sep 17 00:00:00 2001
From: Michael Eischer <michael.eischer@fau.de>
Date: Sun, 19 Sep 2021 19:58:47 +0200
Subject: [PATCH] retry key loading on hash mismatch

---
 internal/backend/utils.go      | 15 ++++++++++++
 internal/backend/utils_test.go | 42 ++++++++++++++++++++++++++++++++++
 internal/cache/backend_test.go |  1 -
 3 files changed, 57 insertions(+), 1 deletion(-)

diff --git a/internal/backend/utils.go b/internal/backend/utils.go
index be1c2a9e0..d2ac44670 100644
--- a/internal/backend/utils.go
+++ b/internal/backend/utils.go
@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"io"
 
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/restic"
 )
 
@@ -13,6 +15,7 @@ import (
 // buffer, which is truncated. If the buffer is not large enough or nil, a new
 // one is allocated.
 func LoadAll(ctx context.Context, buf []byte, be restic.Backend, h restic.Handle) ([]byte, error) {
+	retriedInvalidData := false
 	err := be.Load(ctx, h, 0, 0, func(rd io.Reader) error {
 		// make sure this is idempotent, in case an error occurs this function may be called multiple times!
 		wr := bytes.NewBuffer(buf[:0])
@@ -21,6 +24,18 @@ func LoadAll(ctx context.Context, buf []byte, be restic.Backend, h restic.Handle
 			return cerr
 		}
 		buf = wr.Bytes()
+
+		// retry loading damaged data only once. If a file fails to download correctly
+		// the second time, then it  is likely corrupted at the backend. Return the data
+		// to the caller in that case to let it decide what to do with the data.
+		if !retriedInvalidData && h.Type != restic.ConfigFile {
+			id, err := restic.ParseID(h.Name)
+			if err == nil && !restic.Hash(buf).Equal(id) {
+				debug.Log("retry loading broken blob %v", h)
+				retriedInvalidData = true
+				return errors.Errorf("loadAll(%v): invalid data returned", h)
+			}
+		}
 		return nil
 	})
 
diff --git a/internal/backend/utils_test.go b/internal/backend/utils_test.go
index 2e77fa9bd..3fb570eba 100644
--- a/internal/backend/utils_test.go
+++ b/internal/backend/utils_test.go
@@ -5,6 +5,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"math/rand"
 	"testing"
 
@@ -56,6 +57,47 @@ func save(t testing.TB, be restic.Backend, buf []byte) restic.Handle {
 	return h
 }
 
+type quickRetryBackend struct {
+	restic.Backend
+}
+
+func (be *quickRetryBackend) Load(ctx context.Context, h restic.Handle, length int, offset int64, fn func(rd io.Reader) error) error {
+	err := be.Backend.Load(ctx, h, length, offset, fn)
+	if err != nil {
+		// retry
+		err = be.Backend.Load(ctx, h, length, offset, fn)
+	}
+	return err
+}
+
+func TestLoadAllBroken(t *testing.T) {
+	b := mock.NewBackend()
+
+	data := rtest.Random(23, rand.Intn(MiB)+500*KiB)
+	id := restic.Hash(data)
+	// damage buffer
+	data[0] ^= 0xff
+
+	b.OpenReaderFn = func(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+		return ioutil.NopCloser(bytes.NewReader(data)), nil
+	}
+
+	// must fail on first try
+	_, err := backend.LoadAll(context.TODO(), nil, b, restic.Handle{Type: restic.PackFile, Name: id.String()})
+	if err == nil {
+		t.Fatalf("missing expected error")
+	}
+
+	// must return the broken data after a retry
+	be := &quickRetryBackend{Backend: b}
+	buf, err := backend.LoadAll(context.TODO(), nil, be, restic.Handle{Type: restic.PackFile, Name: id.String()})
+	rtest.OK(t, err)
+
+	if !bytes.Equal(buf, data) {
+		t.Fatalf("wrong data returned")
+	}
+}
+
 func TestLoadAllAppend(t *testing.T) {
 	b := mem.New()
 
diff --git a/internal/cache/backend_test.go b/internal/cache/backend_test.go
index 79b838eb2..e9f0c4d2b 100644
--- a/internal/cache/backend_test.go
+++ b/internal/cache/backend_test.go
@@ -48,7 +48,6 @@ func remove(t testing.TB, be restic.Backend, h restic.Handle) {
 func randomData(n int) (restic.Handle, []byte) {
 	data := test.Random(rand.Int(), n)
 	id := restic.Hash(data)
-	copy(id[:], data)
 	h := restic.Handle{
 		Type: restic.IndexFile,
 		Name: id.String(),