Adding ignore_changes lifecycle meta property

.gitignore

@@ -18,3 +18,4 @@ website/node_modules
 *.bak
 *~
 .*.swp
+.idea

config/config.go

@@ -86,6 +86,7 @@ type Resource struct {
 type ResourceLifecycle struct {
 	CreateBeforeDestroy bool     `mapstructure:"create_before_destroy"`
 	PreventDestroy      bool     `mapstructure:"prevent_destroy"`
+	IgnoreChanges       []string `mapstructure:"ignore_changes"`
 }
 
 // Provisioner is a configured provisioner step on a resource.

config/loader_test.go

@@ -440,6 +440,54 @@ func TestLoadFile_createBeforeDestroy(t *testing.T) {
 	}
 }
 
+func TestLoadFile_ignoreChanges(t *testing.T) {
+	c, err := LoadFile(filepath.Join(fixtureDir, "ignore-changes.tf"))
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if c == nil {
+		t.Fatal("config should not be nil")
+	}
+
+	actual := resourcesStr(c.Resources)
+	print(actual)
+	if actual != strings.TrimSpace(ignoreChangesResourcesStr) {
+		t.Fatalf("bad:\n%s", actual)
+	}
+
+	// Check for the flag value
+	r := c.Resources[0]
+	if r.Name != "web" && r.Type != "aws_instance" {
+		t.Fatalf("Bad: %#v", r)
+	}
+
+	// Should populate ignore changes
+	if len(r.Lifecycle.IgnoreChanges) == 0 {
+		t.Fatalf("Bad: %#v", r)
+	}
+
+	r = c.Resources[1]
+	if r.Name != "bar" && r.Type != "aws_instance" {
+		t.Fatalf("Bad: %#v", r)
+	}
+
+	// Should not populate ignore changes
+	if len(r.Lifecycle.IgnoreChanges) > 0 {
+		t.Fatalf("Bad: %#v", r)
+	}
+
+	r = c.Resources[2]
+	if r.Name != "baz" && r.Type != "aws_instance" {
+		t.Fatalf("Bad: %#v", r)
+	}
+
+	// Should not populate ignore changes
+	if len(r.Lifecycle.IgnoreChanges) > 0 {
+		t.Fatalf("Bad: %#v", r)
+	}
+}
+
 func TestLoad_preventDestroyString(t *testing.T) {
 	c, err := LoadFile(filepath.Join(fixtureDir, "prevent-destroy-string.tf"))
 	if err != nil {
@@ -676,3 +724,12 @@ aws_instance[bar] (x1)
 aws_instance[web] (x1)
   ami
 `
+
+const ignoreChangesResourcesStr = `
+aws_instance[bar] (x1)
+  ami
+aws_instance[baz] (x1)
+  ami
+aws_instance[web] (x1)
+  ami
+`

config/test-fixtures/ignore-changes.tf

@@ -0,0 +1,17 @@
+resource "aws_instance" "web" {
+    ami = "foo"
+    lifecycle {
+        ignore_changes = ["ami"]
+    }
+}
+
+resource "aws_instance" "bar" {
+    ami = "foo"
+    lifecycle {
+        ignore_changes = []
+    }
+}
+
+resource "aws_instance" "baz" {
+  ami = "foo"
+}

terraform/context_plan_test.go

@@ -1672,3 +1672,49 @@ func TestContext2Plan_varListErr(t *testing.T) {
 		t.Fatal("should error")
 	}
 }
+
+func TestContext2Plan_ignoreChanges(t *testing.T) {
+	m := testModule(t, "plan-ignore-changes")
+	p := testProvider("aws")
+	p.DiffFn = testDiffFn
+	s := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_instance.foo": &ResourceState{
+						Primary: &InstanceState{
+							ID:         "bar",
+							Attributes: map[string]string{"ami": "ami-abcd1234"},
+						},
+					},
+				},
+			},
+		},
+	}
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		Variables: map[string]string{
+			"foo": "ami-1234abcd",
+		},
+		State: s,
+	})
+
+	plan, err := ctx.Plan()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if len(plan.Diff.RootModule().Resources) < 1 {
+		t.Fatalf("bad: %#v", plan.Diff.RootModule().Resources)
+	}
+
+	actual := strings.TrimSpace(plan.String())
+	expected := strings.TrimSpace(testTerraformPlanIgnoreChangesStr)
+	if actual != expected {
+		t.Fatalf("bad:\n%s\n\nexpected\n\n%s", actual, expected)
+	}
+}

terraform/eval_ignore_changes.go

@@ -0,0 +1,32 @@
+package terraform
+import (
+	"github.com/hashicorp/terraform/config"
+	"strings"
+)
+
+// EvalIgnoreChanges is an EvalNode implementation that removes diff
+// attributes if their name matches names provided by the resource's
+// IgnoreChanges lifecycle.
+type EvalIgnoreChanges struct {
+	Resource *config.Resource
+	Diff **InstanceDiff
+}
+
+func (n *EvalIgnoreChanges) Eval(ctx EvalContext) (interface{}, error) {
+	if n.Diff == nil || *n.Diff == nil || n.Resource == nil || n.Resource.Id() == "" {
+		return nil, nil
+	}
+
+	diff := *n.Diff
+	ignoreChanges := n.Resource.Lifecycle.IgnoreChanges
+
+	for _, ignoredName := range	ignoreChanges {
+		for name := range diff.Attributes {
+			if strings.HasPrefix(name, ignoredName) {
+				delete(diff.Attributes, name)
+			}
+		}
+	}
+
+	return nil, nil
+}

terraform/terraform_test.go

@@ -1286,3 +1286,16 @@ STATE:
 
 <no state>
 `
+
+const testTerraformPlanIgnoreChangesStr = `
+DIFF:
+
+UPDATE: aws_instance.foo
+  type: "" => "aws_instance"
+
+STATE:
+
+aws_instance.foo:
+  ID = bar
+  ami = ami-abcd1234
+`

terraform/test-fixtures/plan-ignore-changes/main.tf

@@ -0,0 +1,9 @@
+variable "foo" {}
+
+resource "aws_instance" "foo" {
+  ami = "${var.foo}"
+
+  lifecycle {
+    ignore_changes = ["ami"]
+  }
+}

terraform/transform_resource.go

@@ -318,6 +318,10 @@ func (n *graphNodeExpandedResource) EvalTree() EvalNode {
 					Resource: n.Resource,
 					Diff:     &diff,
 				},
+				&EvalIgnoreChanges{
+					Resource: n.Resource,
+					Diff: &diff,
+				},
 				&EvalWriteState{
 					Name:         n.stateId(),
 					ResourceType: n.Resource.Type,

website/source/docs/configuration/resources.html.md

@@ -68,11 +68,20 @@ The `lifecycle` block allows the following keys to be set:
       destruction of a given resource. When this is set to `true`, any plan
       that includes a destroy of this resource will return an error message.
 
+  * `ignore_changes` (list of strings) - Customizes how diffs are evaluated for
+      resources, allowing individual attributes to be ignored through changes.
+      As an example, this can be used to ignore dynamic changes to the
+      resource from external resources. Other meta-parameters cannot be ignored.
+
 ~> **NOTE on create\_before\_destroy and dependencies:** Resources that utilize
 the `create_before_destroy` key can only depend on other resources that also
 include `create_before_destroy`. Referencing a resource that does not include
 `create_before_destroy` will result in a dependency graph cycle. 
 
+~> **NOTE on ignore\_changes:** Ignored attribute names can be matched by their
+name, not state ID. For example, if an `aws_route_table` has two routes defined
+and the `ignore_changes` list contains "route", both routes will be ignored.
+
 -------------
 
 Within a resource, you can optionally have a **connection block**.
@@ -191,6 +200,8 @@ where `LIFECYCLE` is:
 ```
 lifecycle {
     [create_before_destroy = true|false]
+    [prevent_destroy = true|false]
+    [ignore_changes = [ATTRIBUTE NAME, ...]]
 }
 ```