FindUsedBlobs: Test that seen blobs are skipped

This also copies the TreeLoader interface from internal/walker to allow
stubbing the repository in the call to `FindUsedBlobs`.

internal/restic/find.go

@@ -2,9 +2,14 @@ package restic
 
 import "context"
 
+// TreeLoader loads a tree from a repository.
+type TreeLoader interface {
+	LoadTree(context.Context, ID) (*Tree, error)
+}
+
 // FindUsedBlobs traverses the tree ID and adds all seen blobs (trees and data
 // blobs) to the set blobs. Already seen tree blobs will not be visited again.
-func FindUsedBlobs(ctx context.Context, repo Repository, treeID ID, blobs BlobSet) error {
+func FindUsedBlobs(ctx context.Context, repo TreeLoader, treeID ID, blobs BlobSet) error {
 	h := BlobHandle{ID: treeID, Type: TreeBlob}
 	if blobs.Has(h) {
 		return nil

internal/restic/find_test.go

@@ -12,6 +12,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
 )
@@ -118,6 +119,31 @@ func TestFindUsedBlobs(t *testing.T) {
 	}
 }
 
+type ForbiddenRepo struct{}
+
+func (r ForbiddenRepo) LoadTree(ctx context.Context, id restic.ID) (*restic.Tree, error) {
+	return nil, errors.New("should not be called")
+}
+
+func TestFindUsedBlobsSkipsSeenBlobs(t *testing.T) {
+	repo, cleanup := repository.TestRepository(t)
+	defer cleanup()
+
+	snapshot := restic.TestCreateSnapshot(t, repo, findTestTime, findTestDepth, 0)
+	t.Logf("snapshot %v saved, tree %v", snapshot.ID().Str(), snapshot.Tree.Str())
+
+	usedBlobs := restic.NewBlobSet()
+	err := restic.FindUsedBlobs(context.TODO(), repo, *snapshot.Tree, usedBlobs)
+	if err != nil {
+		t.Fatalf("FindUsedBlobs returned error: %v", err)
+	}
+
+	err = restic.FindUsedBlobs(context.TODO(), ForbiddenRepo{}, *snapshot.Tree, usedBlobs)
+	if err != nil {
+		t.Fatalf("FindUsedBlobs returned error: %v", err)
+	}
+}
+
 func BenchmarkFindUsedBlobs(b *testing.B) {
 	repo, cleanup := repository.TestRepository(b)
 	defer cleanup()