channeldb: recover missing db version

diff --git a/channeldb/db.go b/channeldb/db.go
index d801de6..3ed623e 100644
--- a/channeldb/db.go
+++ b/channeldb/db.go
@@ -43,6 +43,14 @@ import (
 
 const (
 	dbName = "channel.db"
+
+	// missingDBVersionRecoveryVersion is the latest mandatory DB
+	// version before the init ordering regression that could create a
+	// DB without writing the DB version key. Affected DBs are therefore
+	// already at least this version, so recovery starts here to run the
+	// v0.21 waiting proof migration without replaying older migrations
+	// against a modern DB.
+	missingDBVersionRecoveryVersion = 33
 )
 
 var (
@@ -1869,16 +1877,43 @@ func (c *ChannelStateDB) DeleteChannelOpeningState(outPoint []byte) error {
 // applies migration functions to the current database and recovers the
 // previous state of db if at least one error/panic appeared during migration.
 func (d *DB) syncVersions(versions []mandatoryVersion) error {
+	latestVersion := getLatestDBVersion(versions)
+
 	meta, err := d.FetchMeta()
 	if err != nil {
-		if err == ErrMetaNotFound {
+		switch {
+		case errors.Is(err, ErrMetaNotFound):
 			meta = &Meta{}
-		} else {
+
+		case errors.Is(err, ErrDBVersionNotFound):
+			recoveryVersion := uint32(
+				missingDBVersionRecoveryVersion,
+			)
+
+			// Missing DB version recovery is only valid for DBs
+			// created after the init ordering regression. Older DBs
+			// wrote the DB version before init returned, so a
+			// missing version key on a sub-33 DB is not a valid
+			// state to infer from.
+			if latestVersion < recoveryVersion {
+				return fmt.Errorf("unable to recover missing "+
+					"DB version key: latest_version=%v "+
+					"recovery_version=%v", latestVersion,
+					recoveryVersion)
+			}
+
+			log.Warnf("DB version key missing, recovering from "+
+				"db_version=%v", recoveryVersion)
+
+			meta = &Meta{
+				DbVersionNumber: recoveryVersion,
+			}
+
+		default:
 			return err
 		}
 	}
 
-	latestVersion := getLatestDBVersion(versions)
 	log.Infof("Checking for schema update: latest_version=%v, "+
 		"db_version=%v", latestVersion, meta.DbVersionNumber)
 
diff --git a/channeldb/meta_test.go b/channeldb/meta_test.go
index ca25328..a7a0b4f 100644
--- a/channeldb/meta_test.go
+++ b/channeldb/meta_test.go
@@ -2,12 +2,14 @@ package channeldb
 
 import (
 	"bytes"
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"testing"
 
 	"github.com/btcsuite/btcwallet/walletdb"
 	"github.com/lightningnetwork/lnd/kvdb"
+	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/stretchr/testify/require"
 )
 
@@ -687,6 +689,135 @@ func TestInitChannelDBCreatesMissingTopLevelBuckets(t *testing.T) {
 	require.NoError(t, err)
 }
 
+// TestMissingDBVersionRunsWaitingProofMigration asserts that a DB initialized
+// without a version key is recovered from the last v0.20 mandatory version so
+// migration 35 can migrate legacy waiting proof records.
+func TestMissingDBVersionRunsWaitingProofMigration(t *testing.T) {
+	t.Parallel()
+
+	backend, cleanup, err := kvdb.GetTestBackend(t.TempDir(), "cdb")
+	require.NoError(t, err)
+	t.Cleanup(cleanup)
+
+	const scid = 101
+	ann := &lnwire.AnnounceSignatures1{
+		ChannelID:        lnwire.ChannelID{1, 2, 3},
+		ShortChannelID:   lnwire.NewShortChanIDFromInt(scid),
+		NodeSignature:    wireSig,
+		BitcoinSignature: wireSig,
+		ExtraOpaqueData:  []byte{4, 5, 6},
+	}
+
+	legacyKey, legacyValue := encodeLegacyWaitingProof(t, true, ann)
+
+	err = kvdb.Update(backend, func(tx kvdb.RwTx) error {
+		_, err := tx.CreateTopLevelBucket(metaBucket)
+		if err != nil {
+			return err
+		}
+
+		bucket, err := tx.CreateTopLevelBucket(waitingProofsBucketKey)
+		if err != nil {
+			return err
+		}
+
+		return bucket.Put(legacyKey[:], legacyValue)
+	}, func() {})
+	require.NoError(t, err)
+
+	db, err := CreateWithBackend(backend)
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		require.NoError(t, db.Close())
+	})
+
+	err = db.View(func(tx kvdb.RTx) error {
+		metaBucket := tx.ReadBucket(metaBucket)
+		require.NotNil(t, metaBucket)
+
+		versionBytes := metaBucket.Get(dbVersionKey)
+		require.Len(t, versionBytes, 4)
+		require.Equal(
+			t, LatestDBVersion(), byteOrder.Uint32(versionBytes),
+		)
+
+		bucket := tx.ReadBucket(waitingProofsBucketKey)
+		require.NotNil(t, bucket)
+		require.Nil(t, bucket.Get(legacyKey[:]))
+
+		proof := NewWaitingProof(true, ann)
+		typedKey := proof.Key()
+		require.NotNil(t, bucket.Get(typedKey[:]))
+
+		return nil
+	}, func() {})
+	require.NoError(t, err)
+
+	store, err := NewWaitingProofStore(db)
+	require.NoError(t, err)
+
+	proof := NewWaitingProof(true, ann)
+	migratedProof, err := store.Get(proof.Key())
+	require.NoError(t, err)
+	require.Equal(t, proof.Key(), migratedProof.Key())
+}
+
+// TestMissingDBVersionRecoveryRequiresBaseline asserts that a missing version
+// key cannot be recovered if the target version list does not include the
+// recovery baseline.
+func TestMissingDBVersionRecoveryRequiresBaseline(t *testing.T) {
+	t.Parallel()
+
+	backend, cleanup, err := kvdb.GetTestBackend(t.TempDir(), "cdb")
+	require.NoError(t, err)
+	t.Cleanup(cleanup)
+
+	err = kvdb.Update(backend, func(tx kvdb.RwTx) error {
+		_, err := tx.CreateTopLevelBucket(metaBucket)
+
+		return err
+	}, func() {})
+	require.NoError(t, err)
+
+	db := &DB{
+		Backend: backend,
+	}
+
+	versions := []mandatoryVersion{
+		{
+			number:    0,
+			migration: nil,
+		},
+		{
+			number:    1,
+			migration: nil,
+		},
+	}
+
+	err = db.syncVersions(versions)
+	require.ErrorContains(t, err, "unable to recover missing DB version")
+}
+
+// encodeLegacyWaitingProof encodes a waiting proof using the pre-migration
+// format.
+func encodeLegacyWaitingProof(t *testing.T, isRemote bool,
+	ann *lnwire.AnnounceSignatures1) ([9]byte, []byte) {
+
+	t.Helper()
+
+	var key [9]byte
+	binary.BigEndian.PutUint64(key[:8], ann.ShortChannelID.ToUint64())
+	if isRemote {
+		key[8] = 1
+	}
+
+	var value bytes.Buffer
+	require.NoError(t, binary.Write(&value, byteOrder, isRemote))
+	require.NoError(t, ann.Encode(&value, 0))
+
+	return key, value.Bytes()
+}
+
 // TestMarkerAndTombstone tests that markers like a tombstone can be added to a
 // DB.
 func TestMarkerAndTombstone(t *testing.T) {
diff --git a/docs/release-notes/release-notes-0.21.2.md b/docs/release-notes/release-notes-0.21.2.md
index ccf4822..d4bee4f 100644
--- a/docs/release-notes/release-notes-0.21.2.md
+++ b/docs/release-notes/release-notes-0.21.2.md
@@ -36,6 +36,13 @@
   combination. This also affects callers replaying an affected historical
   route returned by `ListPayments` or `TrackPayment`.
 
+* [Fixed a channeldb migration
+  bug](https://github.com/lightningnetwork/lnd/pull/10985) where databases
+  initialized without a persisted `metadata/dbp` version key could skip later
+  mandatory migrations. This recovers such databases from the last known
+  v0.20-era mandatory version so the v0.21 waiting proof migration runs
+  without replaying older migrations against an already-initialized database.
+
 # New Features
 
 ## Functional Enhancements