From 48b9614556a1a874e67e9e448bad627c3ee8d51a Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <mitchell.hashimoto@gmail.com>
Date: Sat, 10 Jan 2015 15:39:29 -0800
Subject: [PATCH] helper/schema: Exists API

---
 helper/schema/resource.go                    | 27 ++++++++
 helper/schema/resource_test.go               | 68 ++++++++++++++++++++
 website/source/docs/plugins/provider.html.md |  4 ++
 3 files changed, 99 insertions(+)

diff --git a/helper/schema/resource.go b/helper/schema/resource.go
index 7f5736bc6..f0e0515cd 100644
--- a/helper/schema/resource.go
+++ b/helper/schema/resource.go
@@ -41,10 +41,17 @@ type Resource struct {
 	// If any errors occur during each of the operation, an error should be
 	// returned. If a resource was partially updated, be careful to enable
 	// partial state mode for ResourceData and use it accordingly.
+	//
+	// Exists is a function that is called to check if a resource still
+	// exists. If this returns false, then this will affect the diff
+	// accordingly. If this function isn't set, it will not be called. It
+	// is highly recommended to set it. The *ResourceData passed to Exists
+	// should _not_ be modified.
 	Create CreateFunc
 	Read   ReadFunc
 	Update UpdateFunc
 	Delete DeleteFunc
+	Exists ExistsFunc
 }
 
 // See Resource documentation.
@@ -59,6 +66,9 @@ type UpdateFunc func(*ResourceData, interface{}) error
 // See Resource documentation.
 type DeleteFunc func(*ResourceData, interface{}) error
 
+// See Resource documentation.
+type ExistsFunc func(*ResourceData, interface{}) (bool, error)
+
 // Apply creates, updates, and/or deletes a resource.
 func (r *Resource) Apply(
 	s *terraform.InstanceState,
@@ -131,6 +141,23 @@ func (r *Resource) Validate(c *terraform.ResourceConfig) ([]string, []error) {
 func (r *Resource) Refresh(
 	s *terraform.InstanceState,
 	meta interface{}) (*terraform.InstanceState, error) {
+	if r.Exists != nil {
+		// Make a copy of data so that if it is modified it doesn't
+		// affect our Read later.
+		data, err := schemaMap(r.Schema).Data(s, nil)
+		if err != nil {
+			return s, err
+		}
+
+		exists, err := r.Exists(data, meta)
+		if err != nil {
+			return s, err
+		}
+		if !exists {
+			return nil, nil
+		}
+	}
+
 	data, err := schemaMap(r.Schema).Data(s, nil)
 	if err != nil {
 		return s, err
diff --git a/helper/schema/resource_test.go b/helper/schema/resource_test.go
index 845bd264d..150346922 100644
--- a/helper/schema/resource_test.go
+++ b/helper/schema/resource_test.go
@@ -409,3 +409,71 @@ func TestResourceRefresh_delete(t *testing.T) {
 		t.Fatalf("bad: %#v", actual)
 	}
 }
+
+func TestResourceRefresh_existsError(t *testing.T) {
+	r := &Resource{
+		Schema: map[string]*Schema{
+			"foo": &Schema{
+				Type:     TypeInt,
+				Optional: true,
+			},
+		},
+	}
+
+	r.Exists = func(*ResourceData, interface{}) (bool, error) {
+		return false, fmt.Errorf("error")
+	}
+
+	r.Read = func(d *ResourceData, m interface{}) error {
+		panic("shouldn't be called")
+	}
+
+	s := &terraform.InstanceState{
+		ID: "bar",
+		Attributes: map[string]string{
+			"foo": "12",
+		},
+	}
+
+	actual, err := r.Refresh(s, 42)
+	if err == nil {
+		t.Fatalf("should error")
+	}
+	if !reflect.DeepEqual(actual, s) {
+		t.Fatalf("bad: %#v", actual)
+	}
+}
+
+func TestResourceRefresh_noExists(t *testing.T) {
+	r := &Resource{
+		Schema: map[string]*Schema{
+			"foo": &Schema{
+				Type:     TypeInt,
+				Optional: true,
+			},
+		},
+	}
+
+	r.Exists = func(*ResourceData, interface{}) (bool, error) {
+		return false, nil
+	}
+
+	r.Read = func(d *ResourceData, m interface{}) error {
+		panic("shouldn't be called")
+	}
+
+	s := &terraform.InstanceState{
+		ID: "bar",
+		Attributes: map[string]string{
+			"foo": "12",
+		},
+	}
+
+	actual, err := r.Refresh(s, 42)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	if actual != nil {
+		t.Fatalf("should have no state")
+	}
+}
diff --git a/website/source/docs/plugins/provider.html.md b/website/source/docs/plugins/provider.html.md
index 76c4bd003..232dd8c8c 100644
--- a/website/source/docs/plugins/provider.html.md
+++ b/website/source/docs/plugins/provider.html.md
@@ -158,6 +158,10 @@ The CRUD operations in more detail, along with their contracts:
   * `Delete` - This is called to delete the resource. Terraform guarantees
       an existing ID will be set.
 
+  * `Exists` - This is called to verify a resource still exists. It is
+      called prior to `Read`, and lowers the burden of `Read` to be able
+      to assume the resource exists.
+
 ## Schemas
 
 Both providers and resources require a schema to be specified. The schema