terraform: error if import with collision

terraform/context_import.go

@@ -48,12 +48,12 @@ func (c *Context) Import(opts *ImportOpts) (*State, error) {
 	// Build the graph!
 	graph, err := builder.Build(RootModulePath)
 	if err != nil {
-		return nil, err
+		return c.state, err
 	}
 
 	// Walk it
 	if _, err := c.walk(graph, walkImport); err != nil {
-		return nil, err
+		return c.state, err
 	}
 
 	// Clean the state

terraform/context_import_test.go

@@ -39,6 +39,56 @@ func TestContextImport_basic(t *testing.T) {
 	}
 }
 
+func TestContextImport_collision(t *testing.T) {
+	p := testProvider("aws")
+	ctx := testContext2(t, &ContextOpts{
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+
+		State: &State{
+			Modules: []*ModuleState{
+				&ModuleState{
+					Path: []string{"root"},
+					Resources: map[string]*ResourceState{
+						"aws_instance.foo": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "bar",
+							},
+						},
+					},
+				},
+			},
+		},
+	})
+
+	p.ImportStateReturn = []*InstanceState{
+		&InstanceState{
+			ID:        "foo",
+			Ephemeral: EphemeralState{Type: "aws_instance"},
+		},
+	}
+
+	state, err := ctx.Import(&ImportOpts{
+		Targets: []*ImportTarget{
+			&ImportTarget{
+				Addr: "aws_instance.foo",
+				ID:   "bar",
+			},
+		},
+	})
+	if err == nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(state.String())
+	expected := strings.TrimSpace(testImportCollisionStr)
+	if actual != expected {
+		t.Fatalf("bad: \n%s", actual)
+	}
+}
+
 func TestContextImport_missingType(t *testing.T) {
 	p := testProvider("aws")
 	ctx := testContext2(t, &ContextOpts{
@@ -66,7 +116,7 @@ func TestContextImport_missingType(t *testing.T) {
 	}
 
 	actual := strings.TrimSpace(state.String())
-	expected := "<nil>"
+	expected := "<no state>"
 	if actual != expected {
 		t.Fatalf("bad: \n%s", actual)
 	}
@@ -367,6 +417,11 @@ aws_instance.foo:
   provider = aws
 `
 
+const testImportCollisionStr = `
+aws_instance.foo:
+  ID = bar
+`
+
 const testImportModuleStr = `
 <no state>
 module.foo:

terraform/transform_import_state.go

@@ -117,6 +117,29 @@ func (n *graphNodeImportState) DynamicExpand(ctx EvalContext) (*Graph, error) {
 		addrs[i] = &addr
 	}
 
+	// Verify that all the addresses are clear
+	state, lock := ctx.State()
+	lock.RLock()
+	defer lock.RUnlock()
+	filter := &StateFilter{State: state}
+	for _, addr := range addrs {
+		result, err := filter.Filter(addr.String())
+		if err != nil {
+			return nil, fmt.Errorf("Error verifying address %s: %s", addr, err)
+		}
+
+		// Go through the filter results and it is an error if we find
+		// a matching InstanceState, meaning that we would have a collision.
+		for _, r := range result {
+			if _, ok := r.Value.(*InstanceState); ok {
+				return nil, fmt.Errorf(
+					"Can't import %s, would collide with an existing resource.\n\n"+
+						"Please remove or rename this resource before continuing.",
+					addr)
+			}
+		}
+	}
+
 	// For each of the states, we add a node to handle the refresh/add to state.
 	// "n.states" is populated by our own EvalTree with the result of
 	// ImportState. Since DynamicExpand is always called after EvalTree, this