merge config.Terraform fields in config.Append

Ensure that fields set in an earlier Terraform config block aren't
removed by Append when encountering another Terraform block. When
multiple blocks contain the same field, the later one still wins.

config/append.go

@@ -35,8 +35,13 @@ func Append(c1, c2 *Config) (*Config, error) {
 		c.Atlas = c2.Atlas
 	}
 
-	c.Terraform = c1.Terraform
-	if c2.Terraform != nil {
+	// merge Terraform blocks
+	if c1.Terraform != nil {
+		c.Terraform = c1.Terraform
+		if c2.Terraform != nil {
+			c.Terraform.Merge(c2.Terraform)
+		}
+	} else {
 		c.Terraform = c2.Terraform
 	}
 

config/append_test.go

@@ -118,6 +118,30 @@ func TestAppend(t *testing.T) {
 			},
 			false,
 		},
+
+		{
+			&Config{
+				Terraform: &Terraform{
+					RequiredVersion: "A",
+				},
+			},
+			&Config{
+				Terraform: &Terraform{
+					Backend: &Backend{
+						Type: "test",
+					},
+				},
+			},
+			&Config{
+				Terraform: &Terraform{
+					RequiredVersion: "A",
+					Backend: &Backend{
+						Type: "test",
+					},
+				},
+			},
+			false,
+		},
 	}
 
 	for i, tc := range cases {

config/config_terraform.go

@@ -47,6 +47,18 @@ func (t *Terraform) Validate() []error {
 	return errs
 }
 
+// Merge t with t2.
+// Any conflicting fields are overwritten by t2.
+func (t *Terraform) Merge(t2 *Terraform) {
+	if t2.RequiredVersion != "" {
+		t.RequiredVersion = t2.RequiredVersion
+	}
+
+	if t2.Backend != nil {
+		t.Backend = t2.Backend
+	}
+}
+
 // Backend is the configuration for the "backend" to use with Terraform.
 // A backend is responsible for all major behavior of Terraform's core.
 // The abstraction layer above the core (the "backend") allows for behavior