sqldb+lncfg: use SQLite-appropriate default for max connections

Both the sqldb and kvdb SQLite layers were not using a sensible default
for MaxConnections. The sqldb store used defaultMaxConns (25) which is
meant for Postgres, and the kvdb path passed 0 (unlimited) to
sqlbase.Init when unconfigured.

Add a MaxConns() method on SqliteConfig that returns the configured
value or defaults to 2, appropriate for SQLite's single-writer model.
Use it in both sqldb/sqlite.go and lncfg/db.go so both layers share
the same default.

lncfg/db.go

@@ -207,7 +207,7 @@ func (db *DB) Init(ctx context.Context, dbPath string) error {
 		sqlbase.Init(db.Postgres.MaxConnections)
 
 	case db.Backend == SqliteBackend:
-		sqlbase.Init(db.Sqlite.MaxConnections)
+		sqlbase.Init(db.Sqlite.MaxConns())
 	}
 
 	return nil

sqldb/config.go

@@ -32,6 +32,12 @@ type SqliteConfig struct {
 }
 
 const (
+	// defaultSqliteMaxConns is the default number of maximum open
+	// connections for SQLite. SQLite only supports a single writer, so a
+	// low default reduces contention on the busy_timeout and limits
+	// resource usage, especially on mobile.
+	defaultSqliteMaxConns = 2
+
 	// defaultBusyTimeoutMs is the default busy_timeout value in
 	// milliseconds, used when no BusyTimeout is configured.
 	defaultBusyTimeoutMs = 5000
@@ -47,6 +53,18 @@ func (s *SqliteConfig) busyTimeoutMs() int64 {
 	return defaultBusyTimeoutMs
 }
 
+// MaxConns returns the effective maximum number of open connections. If
+// MaxConnections is not set, it returns a default of 2. This low default is
+// chosen because SQLite only supports a single writer, which helps reduce
+// contention and resource usage.
+func (s *SqliteConfig) MaxConns() int {
+	if s.MaxConnections > 0 {
+		return s.MaxConnections
+	}
+
+	return defaultSqliteMaxConns
+}
+
 // Validate checks that the SqliteConfig values are valid.
 func (p *SqliteConfig) Validate() error {
 	if err := p.QueryConfig.Validate(true); err != nil {

sqldb/sqlite.go

@@ -136,8 +136,8 @@ func NewSqliteStore(cfg *SqliteConfig, dbPath string) (*SqliteStore, error) {
 			err)
 	}
 
-	db.SetMaxOpenConns(defaultMaxConns)
+	db.SetMaxOpenConns(cfg.MaxConns())
-	db.SetMaxIdleConns(defaultMaxConns)
+	db.SetMaxIdleConns(cfg.MaxConns())
 	db.SetConnMaxLifetime(connIdleLifetime)
 	queries := sqlc.New(db)