virtcontainers: store: Add a ItemLock API

The ItemLock API allows for taking shared and exclusive locks on all
items.
For virtcontainers, this is specialized into taking locks on the Lock
item, and will be used for sandbox locking.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

virtcontainers/store/filesystem_backend.go

@@ -12,7 +12,9 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"syscall"
 
+	"github.com/kata-containers/runtime/virtcontainers/pkg/uuid"
 	opentracing "github.com/opentracing/opentracing-go"
 	"github.com/sirupsen/logrus"
 )
@@ -103,6 +105,8 @@ type filesystem struct {
 
 	path    string
 	rawPath string
+
+	lockTokens map[string]*os.File
 }
 
 // Logger returns a logrus logger appropriate for logging Store filesystem messages
@@ -174,6 +178,7 @@ func (f *filesystem) new(ctx context.Context, path string, host string) error {
 	f.ctx = ctx
 	f.path = path
 	f.rawPath = filepath.Join(f.path, "raw")
+	f.lockTokens = make(map[string]*os.File)
 
 	f.logger().Debugf("New filesystem store backend for %s", path)
 
@@ -259,3 +264,48 @@ func (f *filesystem) raw(id string) (string, error) {
 
 	return filesystemScheme + "://" + file.Name(), nil
 }
+
+func (f *filesystem) lock(item Item, exclusive bool) (string, error) {
+	itemPath, err := f.itemToPath(item)
+	if err != nil {
+		return "", err
+	}
+
+	itemFile, err := os.Open(itemPath)
+	if err != nil {
+		return "", err
+	}
+
+	var lockType int
+	if exclusive {
+		lockType = syscall.LOCK_EX
+	} else {
+		lockType = syscall.LOCK_SH
+	}
+
+	if err := syscall.Flock(int(itemFile.Fd()), lockType); err != nil {
+		itemFile.Close()
+		return "", err
+	}
+
+	token := uuid.Generate().String()
+	f.lockTokens[token] = itemFile
+
+	return token, nil
+}
+
+func (f *filesystem) unlock(item Item, token string) error {
+	itemFile := f.lockTokens[token]
+	if itemFile == nil {
+		return fmt.Errorf("No lock for token %s", token)
+	}
+
+	if err := syscall.Flock(int(itemFile.Fd()), syscall.LOCK_UN); err != nil {
+		return err
+	}
+
+	itemFile.Close()
+	delete(f.lockTokens, token)
+
+	return nil
+}