advisor: add a small unittest

Now that we have the codecov metrics back I noticed that the
advisor/backend.go code has no tests for the rollback functionality.

This commit adds a simple testcase.

advisor/backend_test.go

@@ -0,0 +1,76 @@
+// -*- Mode: Go; indent-tabs-mode: t -*-
+
+/*
+ * Copyright (C) 2023 Canonical Ltd
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+package advisor_test
+
+import (
+	"os"
+
+	. "gopkg.in/check.v1"
+
+	"github.com/snapcore/snapd/advisor"
+	"github.com/snapcore/snapd/dirs"
+)
+
+type backendSuite struct{}
+
+var _ = Suite(&backendSuite{})
+
+func (s *backendSuite) SetUpTest(c *C) {
+	dirs.SetRootDir(c.MkDir())
+	c.Assert(os.MkdirAll(dirs.SnapCacheDir, 0755), IsNil)
+
+	// create an empty DB
+	db, err := advisor.Create()
+	c.Assert(err, IsNil)
+	err = db.Commit()
+	c.Assert(err, IsNil)
+}
+
+func dumpCommands(c *C) map[string]string {
+	cmds, err := advisor.DumpCommands()
+	c.Assert(err, IsNil)
+	return cmds
+}
+
+func (s *backendSuite) TestCreateCommit(c *C) {
+	expectedCommands := map[string]string{
+		"meh": `[{"snap":"foo","version":"1.0"}]`,
+		"foo": `[{"snap":"foo","version":"1.0"}]`,
+	}
+
+	db, err := advisor.Create()
+	c.Assert(err, IsNil)
+	c.Assert(db.AddSnap("foo", "1.0", "foo summary", []string{"foo", "meh"}), IsNil)
+	// adding does not change the DB
+	c.Check(dumpCommands(c), DeepEquals, map[string]string{})
+	// but commit does
+	c.Assert(db.Commit(), IsNil)
+	c.Check(dumpCommands(c), DeepEquals, expectedCommands)
+}
+
+func (s *backendSuite) TestCreateRollback(c *C) {
+	db, err := advisor.Create()
+	c.Assert(err, IsNil)
+	// adding does not change the DB
+	c.Assert(db.AddSnap("foo", "1.0", "foo summary", []string{"foo", "meh"}), IsNil)
+	// and rollback ensures any change is reverted
+	c.Assert(db.Rollback(), IsNil)
+	c.Check(dumpCommands(c), DeepEquals, map[string]string{})
+}