channeldb: add NoMigration mode

In case the channeldb package is used as a library in external tools, it
can be useful to allow read-only access to a DB. This allows such a
tool to access a DB even if not all migrations were executed, which can
be useful for recovery purposes.
To make it possible to even start the DB with a read-only backend, we
need to disable the automatic migration step.

channeldb/db.go

@@ -263,15 +263,17 @@ func Open(dbPath string, modifiers ...OptionModifier) (*DB, error) {
 // CreateWithBackend creates channeldb instance using the passed kvdb.Backend.
 // Any necessary schemas migrations due to updates will take place as necessary.
 func CreateWithBackend(backend kvdb.Backend, modifiers ...OptionModifier) (*DB, error) {
-	if err := initChannelDB(backend); err != nil {
-		return nil, err
-	}
-
 	opts := DefaultOptions()
 	for _, modifier := range modifiers {
 		modifier(&opts)
 	}
 
+	if !opts.NoMigration {
+		if err := initChannelDB(backend); err != nil {
+			return nil, err
+		}
+	}
+
 	chanDB := &DB{
 		Backend: backend,
 		channelStateDB: &ChannelStateDB{
@@ -291,17 +293,19 @@ func CreateWithBackend(backend kvdb.Backend, modifiers ...OptionModifier) (*DB,
 	chanDB.graph, err = NewChannelGraph(
 		backend, opts.RejectCacheSize, opts.ChannelCacheSize,
 		opts.BatchCommitInterval, opts.PreAllocCacheNumNodes,
-		opts.UseGraphCache,
+		opts.UseGraphCache, opts.NoMigration,
 	)
 	if err != nil {
 		return nil, err
 	}
 
 	// Synchronize the version of database and apply migrations if needed.
+	if !opts.NoMigration {
 		if err := chanDB.syncVersions(dbVersions); err != nil {
 			backend.Close()
 			return nil, err
 		}
+	}
 
 	return chanDB, nil
 }

channeldb/graph.go

@@ -189,11 +189,13 @@ type ChannelGraph struct {
 // returned instance has its own unique reject cache and channel cache.
 func NewChannelGraph(db kvdb.Backend, rejectCacheSize, chanCacheSize int,
 	batchCommitInterval time.Duration, preAllocCacheNumNodes int,
-	useGraphCache bool) (*ChannelGraph, error) {
+	useGraphCache, noMigrations bool) (*ChannelGraph, error) {
 
+	if !noMigrations {
 		if err := initChannelGraph(db); err != nil {
 			return nil, err
 		}
+	}
 
 	g := &ChannelGraph{
 		db:          db,

channeldb/graph_test.go

@@ -77,7 +77,7 @@ func MakeTestGraph(modifiers ...OptionModifier) (*ChannelGraph, func(), error) {
 	graph, err := NewChannelGraph(
 		backend, opts.RejectCacheSize, opts.ChannelCacheSize,
 		opts.BatchCommitInterval, opts.PreAllocCacheNumNodes,
-		true,
+		true, false,
 	)
 	if err != nil {
 		backendCleanup()

channeldb/options.go

@@ -50,6 +50,11 @@ type Options struct {
 	// path finding.
 	UseGraphCache bool
 
+	// NoMigration specifies that underlying backend was opened in read-only
+	// mode and migrations shouldn't be performed. This can be useful for
+	// applications that use the channeldb package as a library.
+	NoMigration bool
+
 	// clock is the time source used by the database.
 	clock clock.Clock
 
@@ -71,6 +76,7 @@ func DefaultOptions() Options {
 		ChannelCacheSize:      DefaultChannelCacheSize,
 		PreAllocCacheNumNodes: DefaultPreAllocCacheNumNodes,
 		UseGraphCache:         true,
+		NoMigration:           false,
 		clock:                 clock.NewDefaultClock(),
 	}
 }
@@ -136,6 +142,14 @@ func OptionSetBatchCommitInterval(interval time.Duration) OptionModifier {
 	}
 }
 
+// OptionNoMigration allows the database to be opened in read only mode by
+// disabling migrations.
+func OptionNoMigration(b bool) OptionModifier {
+	return func(o *Options) {
+		o.NoMigration = b
+	}
+}
+
 // OptionClock sets a non-default clock dependency.
 func OptionClock(clock clock.Clock) OptionModifier {
 	return func(o *Options) {

routing/pathfind_test.go

@@ -175,7 +175,7 @@ func makeTestGraph(useCache bool) (*channeldb.ChannelGraph, kvdb.Backend,
 	graph, err := channeldb.NewChannelGraph(
 		backend, opts.RejectCacheSize, opts.ChannelCacheSize,
 		opts.BatchCommitInterval, opts.PreAllocCacheNumNodes,
-		useCache,
+		useCache, false,
 	)
 	if err != nil {
 		cleanUp()