channeldb: add optional meta and migration30

This commit enables the db to run optional migrations that are specified
by config flags. To achieve this, an optional meta is introduced to
manage the optional migrations. We distinguish the two types of
migrations here so it's easier to manage them for the concern a future
migration can cause trouble for us to determine the db version if we
don't.

channeldb/meta_test.go

@@ -44,7 +44,7 @@ func applyMigration(t *testing.T, beforeMigration, afterMigration func(d *DB),
 		t.Fatalf("unable to store meta data: %v", err)
 	}
 
-	versions := []version{
+	versions := []mandatoryVersion{
 		{
 			number:    0,
 			migration: nil,
@@ -124,7 +124,7 @@ func TestOrderOfMigrations(t *testing.T) {
 	t.Parallel()
 
 	appliedMigration := -1
-	versions := []version{
+	versions := []mandatoryVersion{
 		{0, nil},
 		{1, nil},
 		{2, func(tx kvdb.RwTx) error {
@@ -498,3 +498,85 @@ func TestMigrationDryRun(t *testing.T) {
 		true,
 		true)
 }
+
+// TestOptionalMeta checks the basic read and write for the optional meta.
+func TestOptionalMeta(t *testing.T) {
+	t.Parallel()
+
+	db, cleanUp, err := MakeTestDB()
+	defer cleanUp()
+	require.NoError(t, err)
+
+	// Test read an empty optional meta.
+	om, err := db.fetchOptionalMeta()
+	require.NoError(t, err, "error getting optional meta")
+	require.Empty(t, om.Versions, "expected empty versions")
+
+	// Test write an optional meta.
+	om = &OptionalMeta{
+		Versions: map[uint64]string{
+			0: optionalVersions[0].name,
+		},
+	}
+	err = db.putOptionalMeta(om)
+	require.NoError(t, err, "error putting optional meta")
+
+	om1, err := db.fetchOptionalMeta()
+	require.NoError(t, err, "error getting optional meta")
+	require.Equal(t, om, om1, "unexpected empty versions")
+	require.Equal(t, "0: prune revocation log", om.String())
+}
+
+// TestApplyOptionalVersions checks that the optional migration is applied as
+// expected based on the config.
+func TestApplyOptionalVersions(t *testing.T) {
+	t.Parallel()
+
+	db, cleanUp, err := MakeTestDB()
+	defer cleanUp()
+	require.NoError(t, err)
+
+	// Overwrite the migration function so we can count how many times the
+	// migration has happened.
+	migrateCount := 0
+	optionalVersions[0].migration = func(_ kvdb.Backend) error {
+		migrateCount++
+		return nil
+	}
+
+	// Test that when the flag is false, no migration happens.
+	cfg := OptionalMiragtionConfig{}
+	err = db.applyOptionalVersions(cfg)
+	require.NoError(t, err, "failed to apply optional migration")
+	require.Equal(t, 0, migrateCount, "expected no migration")
+
+	// Check the optional meta is not updated.
+	om, err := db.fetchOptionalMeta()
+	require.NoError(t, err, "error getting optional meta")
+	require.Empty(t, om.Versions, "expected empty versions")
+
+	// Test that when specified, the optional migration is applied.
+	cfg.PruneRevocationLog = true
+	err = db.applyOptionalVersions(cfg)
+	require.NoError(t, err, "failed to apply optional migration")
+	require.Equal(t, 1, migrateCount, "expected migration")
+
+	// Fetch the updated optional meta.
+	om, err = db.fetchOptionalMeta()
+	require.NoError(t, err, "error getting optional meta")
+
+	// Verify that the optional meta is updated as expected.
+	omExpected := &OptionalMeta{
+		Versions: map[uint64]string{
+			0: optionalVersions[0].name,
+		},
+	}
+	require.Equal(t, omExpected, om, "unexpected empty versions")
+
+	// Test that though specified, the optional migration is not run since
+	// it's already been applied.
+	cfg.PruneRevocationLog = true
+	err = db.applyOptionalVersions(cfg)
+	require.NoError(t, err, "failed to apply optional migration")
+	require.Equal(t, 1, migrateCount, "expected no migration")
+}