core: Don't filter ignore_changes on create

The ignore_changes diff filter was stripping out attributes on Create
but the diff was still making it down to the provider, so Create would
end up missing attributes, causing a full failure if any required
attributes were being ignored.

This should fix `ignore_changes` problems with Required attributes.

Here we also introduce a `test` provider meant as an aid to exposing
via automated tests issues involving interactions between
`helper/schema` and Terraform core.

This has been helpful so far in diagnosing `ignore_changes` problems,
and I imagine it will be helpful in other contexts as well.

We'll have to be careful to prevent the `test` provider from becoming a
dumping ground for poorly specified tests that have a clear home
elsewhere. But for bug exposure I think it's useful to have.

Refs #5627

helper/resource/testing.go

@@ -173,6 +173,22 @@ func Test(t TestT, c TestCase) {
 	}
 }
 
+// UnitTest is a helper to force the acceptance testing harness to run in the
+// normal unit test suite. This should only be used for resource that don't
+// have any external dependencies.
+func UnitTest(t TestT, c TestCase) {
+	oldEnv := os.Getenv(TestEnvVar)
+	if err := os.Setenv(TestEnvVar, "UnitTestOverride"); err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		if err := os.Setenv(TestEnvVar, oldEnv); err != nil {
+			t.Fatal(err)
+		}
+	}()
+	Test(t, c)
+}
+
 func testStep(
 	opts terraform.ContextOpts,
 	state *terraform.State,

terraform/context_apply_test.go

@@ -4084,3 +4084,44 @@ aws_instance.ifailedprovisioners: (1 tainted)
 		t.Fatalf("expected state: \n%s\ngot: \n%s", expected, actual)
 	}
 }
+
+func TestContext2Apply_ignoreChangesCreate(t *testing.T) {
+	m := testModule(t, "apply-ignore-changes-create")
+	p := testProvider("aws")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	})
+
+	if p, err := ctx.Plan(); err != nil {
+		t.Fatalf("err: %s", err)
+	} else {
+		t.Logf(p.String())
+	}
+
+	state, err := ctx.Apply()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	mod := state.RootModule()
+	if len(mod.Resources) != 1 {
+		t.Fatalf("bad: %s", state)
+	}
+
+	actual := strings.TrimSpace(state.String())
+	// Expect no changes from original state
+	expected := strings.TrimSpace(`
+aws_instance.foo:
+  ID = foo
+  required_field = set
+  type = aws_instance
+`)
+	if actual != expected {
+		t.Fatalf("bad: \n%s", actual)
+	}
+}

terraform/eval_ignore_changes.go

@@ -22,6 +22,12 @@ func (n *EvalIgnoreChanges) Eval(ctx EvalContext) (interface{}, error) {
 	diff := *n.Diff
 	ignoreChanges := n.Resource.Lifecycle.IgnoreChanges
 
+	// If we're just creating the resource, we shouldn't alter the
+	// Diff at all
+	if diff.ChangeType() == DiffCreate {
+		return nil, nil
+	}
+
 	for _, ignoredName := range ignoreChanges {
 		for name := range diff.Attributes {
 			if strings.HasPrefix(name, ignoredName) {

terraform/test-fixtures/apply-ignore-changes-create/main.tf

@@ -0,0 +1,7 @@
+resource "aws_instance" "foo" {
+  required_field = "set"
+
+  lifecycle {
+    ignore_changes = ["required_field"]
+  }
+}