run terraform init when plugins are missing

Many environments remove the `./terraform/.terraform` directory from the
bbl state directory to reduce the bbl-state size.

bbl replaces these plugins on subsequent "up" and "destroy" operations
as needed, but as of bbl v9, these plugins are also required for "bbl
outputs" and "bbl jumpbox-address" commands.

This change runs "terraform init --upgrade" when terraform outputs are
requested and the terraform/.terraform directory is missing.

This allow environments to omit the .terraform directory to still reduce
the bbl-state size and also supports consuming bbl-state across
different architectures (e.g. linux <=> darwin) without having to
manually run "terraform init".

terraform/executor.go

@@ -286,12 +286,25 @@ func (e Executor) Output(outputName string) (string, error) {
 	return strings.TrimSuffix(buffer.String(), "\n"), nil
 }
 
+func (e Executor) terraformInitIfNeeded(terraformDir string) error {
+	_, err := e.fs.Stat(filepath.Join(terraformDir, ".terraform"))
+	if err == nil {
+		return nil
+	}
+
+	return e.Init()
+}
+
 func (e Executor) Outputs() (map[string]interface{}, error) {
 	terraformDir, err := e.stateStore.GetTerraformDir()
 	if err != nil {
 		return map[string]interface{}{}, err
 	}
 
+	if err = e.terraformInitIfNeeded(terraformDir); err != nil {
+		return nil, err
+	}
+
 	varsDir, err := e.stateStore.GetVarsDir()
 	if err != nil {
 		return map[string]interface{}{}, err

terraform/executor_test.go

@@ -629,6 +629,45 @@ var _ = Describe("Executor", func() {
 			}))
 		})
 
+		When("the .terraform diretory is present", func() {
+			It("does not run an unnecessary terraform init", func() {
+				_, err := executor.Outputs()
+				Expect(err).NotTo(HaveOccurred())
+
+				Expect(cli.RunCall.CallCount).To(Equal(0))
+			})
+		})
+
+		When("the .terraform directory is missing", func() {
+			BeforeEach(func() {
+				fileIO.StatCall.Fake = func(s string) (os.FileInfo, error) {
+					if strings.HasSuffix(s, ".terraform") {
+						return nil, os.ErrNotExist
+					}
+					return nil, nil
+				}
+			})
+
+			It("runs terraform init ", func() {
+				_, err := executor.Outputs()
+				Expect(err).NotTo(HaveOccurred())
+
+				Expect(cli.RunCall.CallCount).To(Equal(1))
+				Expect(cli.RunCall.Receives.Args).To(Equal([]string{"init", "--upgrade"}))
+			})
+
+			When("running terraform init fails", func() {
+				It("returns an error", func() {
+					cli.RunCall.Returns.Errors = []error{
+						errors.New("kiwi"),
+					}
+
+					_, err := executor.Outputs()
+					Expect(err).To(MatchError("Run terraform init --upgrade: kiwi"))
+				})
+			})
+		})
+
 		Context("when an error occurs", func() {
 			Context("when it fails to get vars dir", func() {
 				BeforeEach(func() {