agent: write OCI spec to config.json in bundle path

In order to comply with the OCI specification, the
spec must be written to a file named 'config.json'
located in the bundle directory.

Fixes: #349

Signed-off-by: Edward Guzman <eguzman@nvidia.com>

grpc.go

@@ -13,6 +13,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"path"
 	"path/filepath"
 	"regexp"
 	"strconv"
@@ -524,6 +525,47 @@ func (a *agentGRPC) rollbackFailingContainerCreation(ctr *container) {
 	}
 }
 
+func changeToBundlePath(spec *specs.Spec) (string, error) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return cwd, err
+	}
+
+	rootfsPath := spec.Root.Path
+	if !filepath.IsAbs(rootfsPath) {
+		rootfsPath = filepath.Join(cwd, rootfsPath)
+	}
+
+	bundlePath := filepath.Dir(rootfsPath)
+
+	err = os.Chdir(bundlePath)
+	if err != nil {
+		return cwd, err
+	}
+
+	return cwd, nil
+}
+
+func writeSpecToFile(spec *specs.Spec) error {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return err
+	}
+
+	configPath := path.Join(cwd, "config.json")
+	f, err := os.OpenFile(configPath, os.O_RDWR|os.O_CREATE, 0666)
+	if err != nil {
+		return err
+	}
+
+	defer f.Close()
+	if err = json.NewEncoder(f).Encode(&spec); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func (a *agentGRPC) CreateContainer(ctx context.Context, req *pb.CreateContainerRequest) (resp *gpb.Empty, err error) {
 	if err := a.createContainerChecks(req); err != nil {
 		return emptyResp, err
@@ -579,6 +621,12 @@ func (a *agentGRPC) CreateContainer(ctx context.Context, req *pb.CreateContainer
 		return emptyResp, err
 	}
 
+	// Change cwd because runc sets the bundle path to cwd
+	oldcwd, err := changeToBundlePath(ociSpec)
+	if err != nil {
+		return emptyResp, err
+	}
+
 	// Convert the OCI specification into a libcontainer configuration.
 	config, err := specconv.CreateLibcontainerConfig(&specconv.CreateOpts{
 		CgroupName:   req.ContainerId,
@@ -590,6 +638,20 @@ func (a *agentGRPC) CreateContainer(ctx context.Context, req *pb.CreateContainer
 		return emptyResp, err
 	}
 
+	// In order to comply with the OCI specification, the spec must be
+	// written to a file named 'config.json' located in the bundle
+	// directory. https://github.com/opencontainers/runtime-spec
+	err = writeSpecToFile(ociSpec)
+	if err != nil {
+		return nil, err
+	}
+
+	// Change back to root dir
+	err = os.Chdir(oldcwd)
+	if err != nil {
+		return nil, err
+	}
+
 	// Update libcontainer configuration for specific cases not handled
 	// by the specconv converter.
 	if err = a.updateContainerConfig(ociSpec, config, ctr); err != nil {