Add missing validation and tests for Spot Instance requests

builder/amazon/common/run_config.go

@@ -112,6 +112,13 @@ func (c *RunConfig) Prepare(ctx *interpolate.Context) []error {
 		}
 	}
 
+	if c.SpotPriceAutoProduct != "" {
+		if c.SpotPrice != "auto" {
+			errs = append(errs, errors.New(
+				"spot_price should be set to auto when spot_price_auto_product is specified"))
+		}
+	}
+
 	if c.UserData != "" && c.UserDataFile != "" {
 		errs = append(errs, fmt.Errorf("Only one of user_data or user_data_file can be specified."))
 	} else if c.UserDataFile != "" {

builder/amazon/common/run_config_test.go

@@ -118,13 +118,19 @@ func TestRunConfigPrepare_SpotAuto(t *testing.T) {
 	c := testConfig()
 	c.SpotPrice = "auto"
 	if err := c.Prepare(nil); len(err) != 1 {
-		t.Fatalf("err: %s", err)
+		t.Fatalf("spot_price_auto_product should be set when spot_price is set to auto")
 	}
 
+	// Good - SpotPrice and SpotPriceAutoProduct are correctly set
 	c.SpotPriceAutoProduct = "foo"
 	if err := c.Prepare(nil); len(err) != 0 {
 		t.Fatalf("err: %s", err)
 	}
+
+	c.SpotPrice = ""
+	if err := c.Prepare(nil); len(err) != 1 {
+		t.Fatalf("spot_price should be set to auto when spot_price_auto_product is set")
+	}
 }
 
 func TestRunConfigPrepare_SSHPort(t *testing.T) {