core: Correctly ensure that State() is a copy

The previous mechanism for testing state threw away the mutation made on
the state by calling State() twice - this commit corrects the test to
match the comment.

In addition, we replace the custom copying logic with the copystructure
library to simplify the code.

state/testing.go

@@ -118,9 +118,11 @@ func TestState(t *testing.T, s interface{}) {
 			t.Fatalf("bad: expected %d, got %d", serial, reader.State().Serial)
 		}
 
-		// Check that State() returns a copy
-		reader.State().Serial++
-		if reflect.DeepEqual(reader.State(), current) {
+		// Check that State() returns a copy by modifying the copy and comparing
+		// to the current state.
+		stateCopy := reader.State()
+		stateCopy.Serial++
+		if reflect.DeepEqual(stateCopy, current) {
 			t.Fatal("State() should return a copy")
 		}
 	}

terraform/state.go

@@ -458,23 +458,12 @@ func (s *State) SameLineage(other *State) bool {
 // DeepCopy performs a deep copy of the state structure and returns
 // a new structure.
 func (s *State) DeepCopy() *State {
-	if s == nil {
-		return nil
+	copy, err := copystructure.Copy(s)
+	if err != nil {
+		panic(err)
 	}
-	n := &State{
-		Version:   s.Version,
-		Lineage:   s.Lineage,
-		TFVersion: s.TFVersion,
-		Serial:    s.Serial,
-		Modules:   make([]*ModuleState, 0, len(s.Modules)),
-	}
-	for _, mod := range s.Modules {
-		n.Modules = append(n.Modules, mod.deepcopy())
-	}
-	if s.Remote != nil {
-		n.Remote = s.Remote.deepcopy()
-	}
-	return n
+
+	return copy.(*State)
 }
 
 // IncrementSerialMaybe increments the serial number of this state
@@ -1187,28 +1176,12 @@ func (r *ResourceState) init() {
 }
 
 func (r *ResourceState) deepcopy() *ResourceState {
-	if r == nil {
-		return nil
+	copy, err := copystructure.Copy(r)
+	if err != nil {
+		panic(err)
 	}
 
-	n := &ResourceState{
-		Type:         r.Type,
-		Dependencies: nil,
-		Primary:      r.Primary.DeepCopy(),
-		Provider:     r.Provider,
-	}
-	if r.Dependencies != nil {
-		n.Dependencies = make([]string, len(r.Dependencies))
-		copy(n.Dependencies, r.Dependencies)
-	}
-	if r.Deposed != nil {
-		n.Deposed = make([]*InstanceState, 0, len(r.Deposed))
-		for _, inst := range r.Deposed {
-			n.Deposed = append(n.Deposed, inst.DeepCopy())
-		}
-	}
-
-	return n
+	return copy.(*ResourceState)
 }
 
 // prune is used to remove any instances that are no longer required
@@ -1278,27 +1251,12 @@ func (i *InstanceState) init() {
 }
 
 func (i *InstanceState) DeepCopy() *InstanceState {
-	if i == nil {
-		return nil
+	copy, err := copystructure.Copy(i)
+	if err != nil {
+		panic(err)
 	}
-	n := &InstanceState{
-		ID:        i.ID,
-		Ephemeral: *i.Ephemeral.DeepCopy(),
-		Tainted:   i.Tainted,
-	}
-	if i.Attributes != nil {
-		n.Attributes = make(map[string]string, len(i.Attributes))
-		for k, v := range i.Attributes {
-			n.Attributes[k] = v
-		}
-	}
-	if i.Meta != nil {
-		n.Meta = make(map[string]string, len(i.Meta))
-		for k, v := range i.Meta {
-			n.Meta[k] = v
-		}
-	}
-	return n
+
+	return copy.(*InstanceState)
 }
 
 func (s *InstanceState) Empty() bool {
@@ -1446,17 +1404,12 @@ func (e *EphemeralState) init() {
 }
 
 func (e *EphemeralState) DeepCopy() *EphemeralState {
-	if e == nil {
-		return nil
+	copy, err := copystructure.Copy(e)
+	if err != nil {
+		panic(err)
 	}
-	n := &EphemeralState{}
-	if e.ConnInfo != nil {
-		n.ConnInfo = make(map[string]string, len(e.ConnInfo))
-		for k, v := range e.ConnInfo {
-			n.ConnInfo[k] = v
-		}
-	}
-	return n
+
+	return copy.(*EphemeralState)
 }
 
 type jsonStateVersionIdentifier struct {