check: Allow filling the cache during check

Closes #1665

internal/checker/checker.go

@@ -164,7 +164,11 @@ func (c *Checker) LoadIndex(ctx context.Context) (hints []error, errs []error) {
 		}
 	}
 
-	c.repo.SetIndex(c.masterIndex)
+	err := c.repo.SetIndex(c.masterIndex)
+	if err != nil {
+		debug.Log("SetIndex returned error: %v", err)
+		errs = append(errs, err)
+	}
 
 	return hints, errs
 }

internal/mock/repository.go

@@ -11,7 +11,7 @@ type Repository struct {
 
 	KeyFn func() *crypto.Key
 
-	SetIndexFn func(restic.Index)
+	SetIndexFn func(restic.Index) error
 
 	IndexFn         func() restic.Index
 	SaveFullIndexFn func() error
@@ -51,8 +51,8 @@ func (repo Repository) Key() *crypto.Key {
 }
 
 // SetIndex is a stub method.
-func (repo Repository) SetIndex(idx restic.Index) {
-	repo.SetIndexFn(idx)
+func (repo Repository) SetIndex(idx restic.Index) error {
+	return repo.SetIndexFn(idx)
 }
 
 // Index is a stub method.

internal/repository/repository.go

@@ -331,8 +331,20 @@ func (r *Repository) Index() restic.Index {
 }
 
 // SetIndex instructs the repository to use the given index.
-func (r *Repository) SetIndex(i restic.Index) {
+func (r *Repository) SetIndex(i restic.Index) error {
 	r.idx = i.(*MasterIndex)
+
+	ids := restic.NewIDSet()
+	for _, idx := range r.idx.All() {
+		id, err := idx.ID()
+		if err != nil {
+			debug.Log("not using index, ID() returned error %v", err)
+			continue
+		}
+		ids.Insert(id)
+	}
+
+	return r.PrepareCache(ids)
 }
 
 // SaveIndex saves an index in the repository.
@@ -428,6 +440,8 @@ func (r *Repository) PrepareCache(indexIDs restic.IDSet) error {
 		return nil
 	}
 
+	debug.Log("prepare cache with %d index files", len(indexIDs))
+
 	// clear old index files
 	err := r.Cache.Clear(restic.IndexFile, indexIDs)
 	if err != nil {

internal/restic/repository.go

@@ -15,7 +15,7 @@ type Repository interface {
 
 	Key() *crypto.Key
 
-	SetIndex(Index)
+	SetIndex(Index) error
 
 	Index() Index
 	SaveFullIndex(context.Context) error