watchtower/wtdb: add tower-to-session index entry for all towers

In this commit, a small migration is added to the watchtower client DB
to ensure that there is an entry in the towerID-to-sessionID index for
all towers in the db regardless of if they have sessions or not. This is
required as a follow up to migration 1 since that migration only created
entries in the index for towers that had associated sessions which would
lead to "tower not found" errors on start up.

watchtower/wtdb/migration5/client_db_test.go

@@ -0,0 +1,162 @@
+package migration5
+
+import (
+	"testing"
+
+	"github.com/lightningnetwork/lnd/channeldb/migtest"
+	"github.com/lightningnetwork/lnd/kvdb"
+)
+
+var (
+	// preIndex is the data in the tower-to-session index before the
+	// migration.
+	preIndex = map[string]interface{}{
+		towerIDString(1): map[string]interface{}{
+			sessionIDString("1"): string([]byte{1}),
+			sessionIDString("3"): string([]byte{1}),
+		},
+		towerIDString(3): map[string]interface{}{
+			sessionIDString("4"): string([]byte{1}),
+		},
+	}
+
+	// preIndexBadTowerID has an invalid TowerID. This is used to test that
+	// the migration correctly rolls back on failure.
+	preIndexBadTowerID = map[string]interface{}{
+		"1": map[string]interface{}{},
+	}
+
+	// towerDBEmpty is the data in an empty tower bucket before the
+	// migration.
+	towerDBEmpty = map[string]interface{}{}
+
+	towerDBMatchIndex = map[string]interface{}{
+		towerIDString(1): map[string]interface{}{},
+		towerIDString(3): map[string]interface{}{},
+	}
+
+	towerDBWithExtraEntries = map[string]interface{}{
+		towerIDString(1): map[string]interface{}{},
+		towerIDString(3): map[string]interface{}{},
+		towerIDString(4): map[string]interface{}{},
+	}
+
+	// post is the expected data after migration.
+	postIndex = map[string]interface{}{
+		towerIDString(1): map[string]interface{}{
+			sessionIDString("1"): string([]byte{1}),
+			sessionIDString("3"): string([]byte{1}),
+		},
+		towerIDString(3): map[string]interface{}{
+			sessionIDString("4"): string([]byte{1}),
+		},
+		towerIDString(4): map[string]interface{}{},
+	}
+)
+
+// TestCompleteTowerToSessionIndex tests that the
+// MigrateCompleteTowerToSessionIndex function correctly completes the
+// towerID-to-sessionID index in the tower client db.
+func TestCompleteTowerToSessionIndex(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		shouldFail bool
+		towerDB    map[string]interface{}
+		pre        map[string]interface{}
+		post       map[string]interface{}
+	}{
+		{
+			name:    "no changes - empty tower db",
+			towerDB: towerDBEmpty,
+			pre:     preIndex,
+			post:    preIndex,
+		},
+		{
+			name:    "no changes - tower db matches index",
+			towerDB: towerDBMatchIndex,
+			pre:     preIndex,
+			post:    preIndex,
+		},
+		{
+			name:    "fill in missing towers",
+			towerDB: towerDBWithExtraEntries,
+			pre:     preIndex,
+			post:    postIndex,
+		},
+		{
+			name:       "fail due to corrupt db",
+			shouldFail: true,
+			towerDB:    preIndexBadTowerID,
+			pre:        preIndex,
+			post:       preIndex,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			// Before the migration we have a tower bucket and an
+			// initial tower-to-session index bucket.
+			before := func(tx kvdb.RwTx) error {
+				err := migtest.RestoreDB(
+					tx, cTowerBkt, test.towerDB,
+				)
+				if err != nil {
+					return err
+				}
+
+				return migtest.RestoreDB(
+					tx, cTowerIDToSessionIDIndexBkt,
+					test.pre,
+				)
+			}
+
+			// After the migration, we should have an untouched
+			// tower bucket and a possibly tweaked tower-to-session
+			// index bucket.
+			after := func(tx kvdb.RwTx) error {
+				if err := migtest.VerifyDB(
+					tx, cTowerBkt, test.towerDB,
+				); err != nil {
+					return err
+				}
+
+				// If we expect our migration to fail, we don't
+				// expect our index bucket to be unchanged.
+				if test.shouldFail {
+					return migtest.VerifyDB(
+						tx, cTowerIDToSessionIDIndexBkt,
+						test.pre,
+					)
+				}
+
+				return migtest.VerifyDB(
+					tx, cTowerIDToSessionIDIndexBkt,
+					test.post,
+				)
+			}
+
+			migtest.ApplyMigration(
+				t, before, after,
+				MigrateCompleteTowerToSessionIndex,
+				test.shouldFail,
+			)
+		})
+	}
+}
+
+func towerIDString(id int) string {
+	towerID := TowerID(id)
+	return string(towerID.Bytes())
+}
+
+func sessionIDString(id string) string {
+	var sessID SessionID
+	copy(sessID[:], id)
+	return string(sessID[:])
+}