Don't use transactions when direct db access is enough

For read-only action transactions are not necessary and they risk to deadlock
if multiple go-routines try to read the database.

deb/local.go

@@ -237,12 +237,13 @@ func (collection *LocalRepoCollection) Len() int {
 
 // Drop removes remote repo from collection
 func (collection *LocalRepoCollection) Drop(repo *LocalRepo) error {
-	transaction, err := collection.db.OpenTransaction()
+	if _, err := collection.db.Get(repo.Key()); err != nil {
-	if err != nil {
+		if err == database.ErrNotFound {
+			return errors.New("local repo not found")
+		}
+
 		return err
 	}
-	defer transaction.Discard()
-
 	delete(collection.cache, repo.UUID)
 
 	batch := collection.db.CreateBatch()

deb/local_test.go

@@ -201,3 +201,13 @@ func (s *LocalRepoCollectionSuite) TestDrop(c *C) {
 
 	c.Check(s.collection.Drop(repo1), ErrorMatches, "local repo not found")
 }
+
+func (s *LocalRepoCollectionSuite) TestDropNonExisting(c *C) {
+	repo := NewLocalRepo("local3", "Comment 3")
+
+	_, err := s.collection.ByUUID(repo.UUID)
+	c.Check(err, ErrorMatches, "local repo .* not found")
+
+	err = s.collection.Drop(repo)
+	c.Check(s.collection.Drop(repo), ErrorMatches, "local repo not found")
+}

deb/remote.go

@@ -898,13 +898,7 @@ func (collection *RemoteRepoCollection) Len() int {
 
 // Drop removes remote repo from collection
 func (collection *RemoteRepoCollection) Drop(repo *RemoteRepo) error {
-	transaction, err := collection.db.OpenTransaction()
+	if _, err := collection.db.Get(repo.Key()); err != nil {
-	if err != nil {
-		return err
-	}
-	defer transaction.Discard()
-
-	if _, err = transaction.Get(repo.Key()); err != nil {
 		if err == database.ErrNotFound {
 			return errors.New("repo not found")
 		}

deb/snapshot.go

@@ -389,13 +389,7 @@ func (collection *SnapshotCollection) Len() int {
 
 // Drop removes snapshot from collection
 func (collection *SnapshotCollection) Drop(snapshot *Snapshot) error {
-	transaction, err := collection.db.OpenTransaction()
+	if _, err := collection.db.Get(snapshot.Key()); err != nil {
-	if err != nil {
-		return err
-	}
-	defer transaction.Discard()
-
-	if _, err = transaction.Get(snapshot.Key()); err != nil {
 		if err == database.ErrNotFound {
 			return errors.New("snapshot not found")
 		}