Migrate can move containers to a new runtime

This is a horrible hack to work around issues with Fedora 31, but
other distros might need it to, so we'll move it upstream.

I do not recommend this functionality for general use, and the
manpages and other documentation will reflect this. But for some
upgrade cases, it will be the only thing that allows for a
working system.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/runtime_migrate.go

@@ -5,14 +5,15 @@ package libpod
 import (
 	"context"
 	"fmt"
-	"github.com/containers/libpod/pkg/util"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strconv"
 	"syscall"
 
+	"github.com/containers/libpod/libpod/define"
 	"github.com/containers/libpod/pkg/rootless"
+	"github.com/containers/libpod/pkg/util"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 )
@@ -63,11 +64,34 @@ func (r *Runtime) migrate(ctx context.Context) error {
 		}
 	}
 
+	// Did the user request a new runtime?
+	runtimeChangeRequested := r.migrateRuntime != ""
+	requestedRuntime, runtimeExists := r.ociRuntimes[r.migrateRuntime]
+	if !runtimeExists && runtimeChangeRequested {
+		return errors.Wrapf(define.ErrInvalidArg, "change to runtime %q requested but no such runtime is defined", r.migrateRuntime)
+	}
+
 	for _, ctr := range allCtrs {
+		needsWrite := false
+
+		// Reset pause process location
 		oldLocation := filepath.Join(ctr.state.RunDir, "conmon.pid")
 		if ctr.config.ConmonPidFile == oldLocation {
 			logrus.Infof("changing conmon PID file for %s", ctr.ID())
 			ctr.config.ConmonPidFile = filepath.Join(ctr.config.StaticDir, "conmon.pid")
+			needsWrite = true
+		}
+
+		// Reset runtime
+		if runtimeChangeRequested {
+			logrus.Infof("Resetting container %s runtime to runtime %s", ctr.ID(), r.migrateRuntime)
+			ctr.config.OCIRuntime = r.migrateRuntime
+			ctr.ociRuntime = requestedRuntime
+
+			needsWrite = true
+		}
+
+		if needsWrite {
 			if err := r.state.RewriteContainerConfig(ctr, ctr.config); err != nil {
 				return errors.Wrapf(err, "error rewriting config for container %s", ctr.ID())
 			}