Require configuration to create Environment

packer.go

@@ -3,10 +3,16 @@ package main
 
 import (
 	"github.com/mitchellh/packer/packer"
+	"fmt"
 	"os"
 )
 
 func main() {
-	env := packer.NewEnvironment(nil)
+	env, err := packer.NewEnvironment(nil)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Packer initialization error: \n\n%s\n", err)
+		os.Exit(1)
+	}
+
 	os.Exit(env.Cli(os.Args[1:]))
 }

packer/environment.go

@@ -2,6 +2,7 @@
 package packer
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"sort"
@@ -46,33 +47,36 @@ type EnvironmentConfig struct {
 	Ui      Ui
 }
 
+// DefaultEnvironmentConfig returns a default EnvironmentConfig that can
+// be used to create a new enviroment with NewEnvironment with sane defaults.
+func DefaultEnvironmentConfig() *EnvironmentConfig {
+	config := &EnvironmentConfig{}
+	config.Ui = &ReaderWriterUi{os.Stdin, os.Stdout}
+	return config
+}
+
 // This creates a new environment
-func NewEnvironment(config *EnvironmentConfig) *Environment {
-	env := &Environment{}
+func NewEnvironment(config *EnvironmentConfig) (env *Environment, err error) {
+	if config == nil {
+		err = errors.New("config must be given to initialize environment")
+		return
+	}
+
+	env = &Environment{}
+	env.builderFactory = config.BuilderFactory
 	env.command = make(map[string]Command)
+	env.ui = config.Ui
 
-	if config != nil {
-		for k, v := range config.Command {
-			env.command[k] = v
-		}
-
-		env.builderFactory = config.BuilderFactory
-		env.ui = config.Ui
-	}
-
-	if _, ok := env.command["build"]; !ok {
-		env.command["build"] = new(buildCommand)
+	for k, v := range config.Command {
+		env.command[k] = v
 	}
 
+	// TODO: Should "version" be allowed to be overriden?
 	if _, ok := env.command["version"]; !ok {
 		env.command["version"] = new(versionCommand)
 	}
 
-	if env.ui == nil {
-		env.ui = &ReaderWriterUi{os.Stdin, os.Stdout}
-	}
-
-	return env
+	return
 }
 
 // Executes a command as if it was typed on the command-line interface.

packer/environment_test.go

@@ -32,7 +32,32 @@ func testEnvironment() *Environment {
 		new(bytes.Buffer),
 	}
 
-	return NewEnvironment(config)
+	env, err := NewEnvironment(config)
+	if err != nil {
+		panic(err)
+	}
+
+	return env
+}
+
+func TestEnvironment_DefaultConfig_Ui(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	config := DefaultEnvironmentConfig()
+	assert.NotNil(config.Ui, "default UI should not be nil")
+
+	rwUi, ok := config.Ui.(*ReaderWriterUi)
+	assert.True(ok, "default UI should be ReaderWriterUi")
+	assert.Equal(rwUi.Writer, os.Stdout, "default UI should go to stdout")
+	assert.Equal(rwUi.Reader, os.Stdin, "default UI should read from stdin")
+}
+
+func TestNewEnvironment_NoConfig(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	env, err := NewEnvironment(nil)
+	assert.Nil(env, "env should be nil")
+	assert.NotNil(err, "should be an error")
 }
 
 func TestEnvironment_Cli_CallsRun(t *testing.T) {
@@ -44,7 +69,7 @@ func TestEnvironment_Cli_CallsRun(t *testing.T) {
 	config.Command = make(map[string]Command)
 	config.Command["foo"] = command
 
-	env := NewEnvironment(config)
+	env, _ := NewEnvironment(config)
 	assert.Equal(env.Cli([]string{"foo", "bar", "baz"}), 0, "runs foo command")
 	assert.True(command.runCalled, "run should've been called")
 	assert.Equal(command.runEnv, env, "should've ran with env")
@@ -99,18 +124,6 @@ func TestEnvironment_DefaultCli_Version(t *testing.T) {
 	assert.Equal(defaultEnv.Cli([]string{"bad", "version"}), 1, "version should NOT work anywhere")
 }
 
-func TestEnvironment_DefaultUi(t *testing.T) {
-	assert := asserts.NewTestingAsserts(t, true)
-
-	defaultEnv := NewEnvironment(nil)
-	assert.NotNil(defaultEnv.Ui(), "default UI should not be nil")
-
-	rwUi, ok := defaultEnv.Ui().(*ReaderWriterUi)
-	assert.True(ok, "default UI should be ReaderWriterUi")
-	assert.Equal(rwUi.Writer, os.Stdout, "default UI should go to stdout")
-	assert.Equal(rwUi.Reader, os.Stdin, "default UI should read from stdin")
-}
-
 func TestEnvironment_PrintHelp(t *testing.T) {
 	// Just call the function and verify that no panics occur
 	testEnvironment().PrintHelp()
@@ -124,7 +137,7 @@ func TestEnvironment_SettingUi(t *testing.T) {
 	config := &EnvironmentConfig{}
 	config.Ui = ui
 
-	env := NewEnvironment(config)
+	env, _ := NewEnvironment(config)
 
 	assert.Equal(env.Ui(), ui, "UIs should be equal")
 }