Add panic recovery to Create method for drivers

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>

libmachine/drivers/rpc/server_driver.go

@@ -3,6 +3,8 @@ package rpcdriver
 import (
 	"encoding/gob"
 	"encoding/json"
+	"fmt"
+	"runtime/debug"
 
 	"github.com/docker/machine/libmachine/drivers"
 	"github.com/docker/machine/libmachine/log"
@@ -11,6 +13,20 @@ import (
 	"github.com/docker/machine/libmachine/version"
 )
 
+type Stacker interface {
+	Stack() []byte
+}
+
+type StandardStack struct{}
+
+func (ss *StandardStack) Stack() []byte {
+	return debug.Stack()
+}
+
+var (
+	stdStacker Stacker = &StandardStack{}
+)
+
 func init() {
 	gob.Register(new(RPCFlags))
 	gob.Register(new(mcnflag.IntFlag))
@@ -108,8 +124,22 @@ func (r *RPCServerDriver) SetConfigRaw(data []byte, _ *struct{}) error {
 	return json.Unmarshal(data, &r.ActualDriver)
 }
 
-func (r *RPCServerDriver) Create(_, _ *struct{}) error {
-	return r.ActualDriver.Create()
+func trapPanic(err *error) {
+	if r := recover(); r != nil {
+		*err = fmt.Errorf("Panic in the driver: %s\n%s", r.(error), stdStacker.Stack())
+	}
+}
+
+func (r *RPCServerDriver) Create(_, _ *struct{}) (err error) {
+	// In an ideal world, plugins wouldn't ever panic.  However, panics
+	// have been known to happen and cause issues.  Therefore, we recover
+	// and do not crash the RPC server completely in the case of a panic
+	// during create.
+	defer trapPanic(&err)
+
+	err = r.ActualDriver.Create()
+
+	return err
 }
 
 func (r *RPCServerDriver) DriverName(_ *struct{}, reply *string) error {

libmachine/drivers/rpc/server_driver_test.go

@@ -0,0 +1,75 @@
+package rpcdriver
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/docker/machine/drivers/fakedriver"
+	"github.com/stretchr/testify/assert"
+)
+
+type panicDriver struct {
+	*fakedriver.Driver
+	panicErr  error
+	returnErr error
+}
+
+type FakeStacker struct {
+	trace []byte
+}
+
+func (fs *FakeStacker) Stack() []byte {
+	return fs.trace
+}
+
+func (p *panicDriver) Create() error {
+	if p.panicErr != nil {
+		panic(p.panicErr)
+	}
+	return p.returnErr
+}
+
+func TestRPCServerDriverCreate(t *testing.T) {
+	testCases := []struct {
+		description  string
+		expectedErr  error
+		serverDriver *RPCServerDriver
+		stacker      Stacker
+	}{
+		{
+			description: "Happy path",
+			expectedErr: nil,
+			serverDriver: &RPCServerDriver{
+				ActualDriver: &panicDriver{
+					returnErr: nil,
+				},
+			},
+		},
+		{
+			description: "Normal error, no panic",
+			expectedErr: errors.New("API not available"),
+			serverDriver: &RPCServerDriver{
+				ActualDriver: &panicDriver{
+					returnErr: errors.New("API not available"),
+				},
+			},
+		},
+		{
+			description: "Panic happened during create",
+			expectedErr: errors.New("Panic in the driver: index out of range\nSTACK TRACE"),
+			serverDriver: &RPCServerDriver{
+				ActualDriver: &panicDriver{
+					panicErr: errors.New("index out of range"),
+				},
+			},
+			stacker: &FakeStacker{
+				trace: []byte("STACK TRACE"),
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		stdStacker = tc.stacker
+		assert.Equal(t, tc.expectedErr, tc.serverDriver.Create(nil, nil))
+	}
+}