core: don't error on computed value during input walk

fixes #2987

terraform/context_input_test.go

@@ -510,3 +510,62 @@ aws_instance.foo:
 		t.Fatalf("expected: \n%s\ngot: \n%s\n", expectedStr, actualStr)
 	}
 }
+
+func TestContext2Input_varPartiallyComputed(t *testing.T) {
+	input := new(MockUIInput)
+	m := testModule(t, "input-var-partially-computed")
+	p := testProvider("aws")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		Variables: map[string]string{
+			"foo": "foovalue",
+		},
+		UIInput: input,
+		State: &State{
+			Modules: []*ModuleState{
+				&ModuleState{
+					Path: rootModulePath,
+					Resources: map[string]*ResourceState{
+						"aws_instance.foo": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "i-abc123",
+								Attributes: map[string]string{
+									"id": "i-abc123",
+								},
+							},
+						},
+					},
+				},
+				&ModuleState{
+					Path: append(rootModulePath, "child"),
+					Resources: map[string]*ResourceState{
+						"aws_instance.mod": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "i-bcd345",
+								Attributes: map[string]string{
+									"id":    "i-bcd345",
+									"value": "one,i-abc123",
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+	})
+
+	if err := ctx.Input(InputModeStd); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if _, err := ctx.Plan(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+}

terraform/interpolate.go

@@ -370,7 +370,13 @@ MISSING:
 	// be unknown. Instead, we return that the value is computed so
 	// that the graph can continue to refresh other nodes. It doesn't
 	// matter because the config isn't interpolated anyways.
-	if i.Operation == walkRefresh || i.Operation == walkPlanDestroy {
+	//
+	// For a Destroy, we're also fine with computed values, since our goal is
+	// only to get destroy nodes for existing resources.
+	//
+	// For an input walk, computed values are okay to return because we're only
+	// looking for missing variables to prompt the user for.
+	if i.Operation == walkRefresh || i.Operation == walkPlanDestroy || i.Operation == walkInput {
 		return config.UnknownVariableValue, nil
 	}
 
@@ -446,7 +452,13 @@ func (i *Interpolater) computeResourceMultiVariable(
 		// be unknown. Instead, we return that the value is computed so
 		// that the graph can continue to refresh other nodes. It doesn't
 		// matter because the config isn't interpolated anyways.
-		if i.Operation == walkRefresh || i.Operation == walkPlanDestroy {
+		//
+		// For a Destroy, we're also fine with computed values, since our goal is
+		// only to get destroy nodes for existing resources.
+		//
+		// For an input walk, computed values are okay to return because we're only
+		// looking for missing variables to prompt the user for.
+		if i.Operation == walkRefresh || i.Operation == walkPlanDestroy || i.Operation == walkInput {
 			return config.UnknownVariableValue, nil
 		}
 

terraform/test-fixtures/input-var-partially-computed/child/main.tf

@@ -0,0 +1,5 @@
+variable "in" {}
+
+resource "aws_instance" "mod" {
+  value = "${var.in}"
+}

terraform/test-fixtures/input-var-partially-computed/main.tf

@@ -0,0 +1,7 @@
+resource "aws_instance" "foo" { }
+resource "aws_instance" "bar" { }
+
+module "child" {
+  source = "./child"
+  in = "one,${aws_instance.foo.id},${aws_instance.bar.id}"
+}