channeldb: add tombstone option and isOutpointClosed helper

diff --git a/channeldb/channel.go b/channeldb/channel.go
index c842050..973ea22 100644
--- a/channeldb/channel.go
+++ b/channeldb/channel.go
@@ -368,6 +368,37 @@ const (
 	outpointClosed indexStatus = 1
 )
 
+// isOutpointClosed reports whether the supplied chanKey has been flipped to
+// outpointClosed in the supplied outpointBucket. The flip is performed in the
+// same transaction as the rest of CloseChannel (sync and tombstone paths
+// alike), so a true result is the authoritative "this channel went through
+// CloseChannel" signal. On tombstone-enabled backends the chanBucket may still
+// exist on disk; readers consult this helper to skip those entries. Callers
+// fetch outpointBucket once and pass it in, which lets loop-style readers
+// hoist the bucket lookup out of the inner loop.
+func isOutpointClosed(opBucket kvdb.RBucket, chanKey []byte) (bool, error) {
+	if opBucket == nil {
+		return false, nil
+	}
+	raw := opBucket.Get(chanKey)
+	if raw == nil {
+		return false, nil
+	}
+
+	var status uint8
+	statusRecord := tlv.MakePrimitiveRecord(indexStatusType, &status)
+	stream, err := tlv.NewStream(statusRecord)
+	if err != nil {
+		return false, err
+	}
+	if err := stream.Decode(bytes.NewReader(raw)); err != nil {
+		return false, fmt.Errorf("decode outpoint status for "+
+			"chan_key=%x: %w", chanKey, err)
+	}
+
+	return indexStatus(status) == outpointClosed, nil
+}
+
 // ChannelType is an enum-like type that describes one of several possible
 // channel types. Each open channel is associated with a particular type as the
 // channel type may determine how higher level operations are conducted such as
diff --git a/channeldb/db.go b/channeldb/db.go
index 999d2cd..d1a7ad0 100644
--- a/channeldb/db.go
+++ b/channeldb/db.go
@@ -418,7 +418,8 @@ func CreateWithBackend(backend kvdb.Backend, modifiers ...OptionModifier) (*DB,
 			linkNodeDB: &LinkNodeDB{
 				backend: backend,
 			},
-			backend: backend,
+			backend:                 backend,
+			tombstoneClosedChannels: opts.tombstoneClosedChannels,
 		},
 		clock:                     opts.clock,
 		dryRun:                    opts.dryRun,
@@ -548,6 +549,12 @@ type ChannelStateDB struct {
 	// backend points to the actual backend holding the channel state
 	// database. This may be a real backend or a cache middleware.
 	backend kvdb.Backend
+
+	// tombstoneClosedChannels is set by OptionTombstoneClosedChannels.
+	// When true, CloseChannel skips deleting nested per-channel state and
+	// relies on the outpointBucket flip to outpointClosed as the
+	// authoritative closed-channel signal.
+	tombstoneClosedChannels bool
 }
 
 // GetParentDB returns the "main" channeldb.DB object that is the owner of this
diff --git a/channeldb/options.go b/channeldb/options.go
index a8ec8cf..eec3b54 100644
--- a/channeldb/options.go
+++ b/channeldb/options.go
@@ -71,6 +71,16 @@ type Options struct {
 	// storeFinalHtlcResolutions determines whether to persistently store
 	// the final resolution of incoming htlcs.
 	storeFinalHtlcResolutions bool
+
+	// tombstoneClosedChannels, when true, instructs CloseChannel to skip
+	// the cascading deletion of nested per-channel state and rely on the
+	// outpoint-index flip to mark the channel as closed. KV-over-SQL
+	// backends (sqlite, postgres) opt in because nested-bucket deletes
+	// inside a write transaction translate into a long-running
+	// ON DELETE CASCADE that holds the database write-lock for many
+	// seconds on long-lived channels. bbolt and etcd leave this off; the
+	// synchronous delete is already cheap there.
+	tombstoneClosedChannels bool
 }
 
 // DefaultOptions returns an Options populated with default values.
@@ -151,3 +161,14 @@ func OptionGcDecayedLog(noGc bool) OptionModifier {
 		o.OptionalMiragtionConfig.MigrationFlags[1] = !noGc
 	}
 }
+
+// OptionTombstoneClosedChannels controls whether CloseChannel skips the
+// cascading deletion of nested per-channel state and relies on the
+// outpoint-index flip to mark the channel as closed. Set this to true on
+// KV-over-SQL backends (sqlite, postgres); leave it false for bbolt and
+// etcd.
+func OptionTombstoneClosedChannels(enabled bool) OptionModifier {
+	return func(o *Options) {
+		o.tombstoneClosedChannels = enabled
+	}
+}