cache: Drop cache entry if it cannot be processed

Failing to process data requested from the cache usually indicates a
problem with the returned data. Assume that the cache entry is somehow
damaged and retry downloading it once.

internal/cache/backend.go

@@ -161,7 +161,12 @@ func (b *Backend) Load(ctx context.Context, h restic.Handle, length int, offset
 	// try loading from cache without checking that the handle is actually cached
 	inCache, err := b.loadFromCache(ctx, h, length, offset, consumer)
 	if inCache {
-		return err
+		if err == nil {
+			return nil
+		}
+
+		// drop from cache and retry once
+		_ = b.Cache.remove(h)
 	}
 	debug.Log("error loading %v from cache: %v", h, err)
 

internal/cache/backend_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"io"
+	"io/ioutil"
 	"math/rand"
 	"sync"
 	"testing"
@@ -171,3 +172,37 @@ func TestErrorBackend(t *testing.T) {
 
 	wg.Wait()
 }
+
+func TestBackendRemoveBroken(t *testing.T) {
+	be := mem.New()
+
+	c, cleanup := TestNewCache(t)
+	defer cleanup()
+
+	h, data := randomData(5234142)
+	// save directly in backend
+	save(t, be, h, data)
+
+	// prime cache with broken copy
+	broken := append([]byte{}, data...)
+	broken[0] ^= 0xff
+	err := c.Save(h, bytes.NewReader(broken))
+	test.OK(t, err)
+
+	// loadall retries if broken data was returned
+	buf, err := backend.LoadAll(context.TODO(), nil, c.Wrap(be), h)
+	test.OK(t, err)
+
+	if !bytes.Equal(buf, data) {
+		t.Fatalf("wrong data returned")
+	}
+
+	// check that the cache now contains the correct data
+	rd, err := c.load(h, 0, 0)
+	test.OK(t, err)
+	cached, err := ioutil.ReadAll(rd)
+	test.OK(t, err)
+	if !bytes.Equal(cached, data) {
+		t.Fatalf("wrong data cache")
+	}
+}