Prompt for input variables before context validate

Also adds a regression test using Mock UI. Fixes #3767.

command/plan.go

@@ -68,14 +68,16 @@ func (c *PlanCommand) Run(args []string) int {
 		c.Ui.Error(err.Error())
 		return 1
 	}
-	if !validateContext(ctx, c.Ui) {
-		return 1
-	}
+
 	if err := ctx.Input(c.InputMode()); err != nil {
 		c.Ui.Error(fmt.Sprintf("Error configuring: %s", err))
 		return 1
 	}
 
+	if !validateContext(ctx, c.Ui) {
+		return 1
+	}
+
 	if refresh {
 		c.Ui.Output("Refreshing Terraform state prior to plan...\n")
 		state, err := ctx.Refresh()

command/plan_test.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"bytes"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -330,6 +331,30 @@ func TestPlan_vars(t *testing.T) {
 	}
 }
 
+func TestPlan_varsUnset(t *testing.T) {
+	// Disable test mode so input would be asked
+	test = false
+	defer func() { test = true }()
+
+	defaultInputReader = bytes.NewBufferString("bar\n")
+
+	p := testProvider()
+	ui := new(cli.MockUi)
+	c := &PlanCommand{
+		Meta: Meta{
+			ContextOpts: testCtxConfig(p),
+			Ui:          ui,
+		},
+	}
+
+	args := []string{
+		testFixturePath("plan-vars"),
+	}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: %d\n\n%s", code, ui.ErrorWriter.String())
+	}
+}
+
 func TestPlan_varFile(t *testing.T) {
 	varFilePath := testTempFile(t)
 	if err := ioutil.WriteFile(varFilePath, []byte(planVarFile), 0644); err != nil {