node: fix instance dir locking and improve error message

The lock file was ineffective because opening leveldb storage in
read-only mode doesn't really take the lock. Fix it by including a
dedicated flock library (which is actually split out of goleveldb).

node/errors.go

@@ -17,10 +17,28 @@
 package node
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
+	"syscall"
 )
 
+var (
+	ErrDatadirUsed    = errors.New("datadir already used by another process")
+	ErrNodeStopped    = errors.New("node not started")
+	ErrNodeRunning    = errors.New("node already running")
+	ErrServiceUnknown = errors.New("unknown service")
+
+	datadirInUseErrnos = map[uint]bool{11: true, 32: true, 35: true}
+)
+
+func convertFileLockError(err error) error {
+	if errno, ok := err.(syscall.Errno); ok && datadirInUseErrnos[uint(errno)] {
+		return ErrDatadirUsed
+	}
+	return err
+}
+
 // DuplicateServiceError is returned during Node startup if a registered service
 // constructor returns a service of the same type that was already started.
 type DuplicateServiceError struct {