DockerCLI/vendor/github.com/Microsoft/hcsshim/internal/wclayer/preparelayer.go

package wclayer

import (
	"sync"

	"github.com/Microsoft/hcsshim/internal/hcserror"
	"github.com/sirupsen/logrus"
)

var prepareLayerLock sync.Mutex

// PrepareLayer finds a mounted read-write layer matching path and enables the
// the filesystem filter for use on that layer.  This requires the paths to all
// parent layers, and is necessary in order to view or interact with the layer
// as an actual filesystem (reading and writing files, creating directories, etc).
// Disabling the filter must be done via UnprepareLayer.
func PrepareLayer(path string, parentLayerPaths []string) error {
	title := "hcsshim::PrepareLayer "
	logrus.Debugf(title+"path %s", path)

	// Generate layer descriptors
	layers, err := layerPathsToDescriptors(parentLayerPaths)
	if err != nil {
		return err
	}

	// This lock is a temporary workaround for a Windows bug. Only allowing one
	// call to prepareLayer at a time vastly reduces the chance of a timeout.
	prepareLayerLock.Lock()
	defer prepareLayerLock.Unlock()
	err = prepareLayer(&stdDriverInfo, path, layers)
	if err != nil {
		err = hcserror.Errorf(err, title, "path=%s", path)
		logrus.Error(err)
		return err
	}

	logrus.Debugf(title+"succeeded path=%s", path)
	return nil
}