extend machine start to support snapshots

By adding functional options to Start(), we can call Start() with the
WithSnapshot parameter to load a snapshot before starting.

Signed-off-by: Gabriel Ionescu <gbi@amazon.com>
Signed-off-by: Gabriel Ionescu <plp.github@gmail.com>

Author: Gabriel Ionescu

machine.go

@@ -385,7 +385,7 @@ func NewMachine(ctx context.Context, cfg Config, opts ...Opt) (*Machine, error)
 // handlers succeed, then this will start the VMM instance.
 // Start may only be called once per Machine.  Subsequent calls will return
 // ErrAlreadyStarted.
-func (m *Machine) Start(ctx context.Context) error {
+func (m *Machine) Start(ctx context.Context, opts ...Opt) error {
 	m.logger.Debug("Called Machine.Start()")
 	alreadyStarted := true
 	m.startOnce.Do(func() {
@@ -406,6 +406,10 @@ func (m *Machine) Start(ctx context.Context) error {
 		}
 	}()
 
+	for _, opt := range opts {
+		opt(m)
+	}
+
 	err = m.Handlers.Run(ctx, m)
 	if err != nil {
 		return err

machineiface.go

@@ -23,7 +23,7 @@ var _ MachineIface = (*Machine)(nil)
 // MachineIface can be used for mocking and testing of the Machine. The Machine
 // is subject to change, meaning this interface would change.
 type MachineIface interface {
-	Start(context.Context) error
+	Start(context.Context, ...Opt) error
 	StopVMM() error
 	Shutdown(context.Context) error
 	Wait(context.Context) error

opts.go

@@ -14,6 +14,7 @@
 package firecracker
 
 import (
+	"context"
 	"os/exec"
 
 	"github.com/sirupsen/logrus"
@@ -47,3 +48,13 @@ func WithProcessRunner(cmd *exec.Cmd) Opt {
 		machine.cmd = cmd
 	}
 }
+
+// WithSnapshot will allow for the machine to start using a given snapshot.
+func WithSnapshot(ctx context.Context, memFilePath, snapshotPath string) Opt {
+	return func(machine *Machine) {
+		err := machine.LoadSnapshot(ctx, memFilePath, snapshotPath)
+		if err != nil {
+			machine.logger.Errorf("LoadSnapshot failed with %s", err)
+		}
+	}
+}