feat(sns): enable passing PolicyDocument to TopicPolicy (aws#10559)

Adds optional `policyDocument` prop to `TopicPolicyProps` to allow passing existing policy documents.

fixes aws#7934

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-sns/README.md

@@ -131,3 +131,43 @@ codeCommitRepository.onCommit(new targets.SnsTopic(myTopic));
 
 This will result in adding a target to the event rule and will also modify the
 topic resource policy to allow CloudWatch events to publish to the topic.
+
+## Topic Policy
+
+A topic policy is automatically created when `addToResourcePolicy` is called, if
+one doesn't already exist. Using `addToResourcePolicy` is the simplest way to
+add policies, but a `TopicPolicy` can also be created manually.
+
+```ts
+const topic = new sns.Topic(stack, 'Topic');
+const topicPolicy = new sns.TopicPolicy(stack, 'TopicPolicy', {
+  topics: [topic],
+});
+
+topicPolicy.document.addStatements(new iam.PolicyStatement({
+  actions: ["sns:Subscribe"],
+  principals: [new iam.AnyPrincipal()],
+  resources: [topic.topicArn],
+}));
+```
+
+A policy document can also be passed on `TopicPolicy` construction
+
+```ts
+const topic = new sns.Topic(stack, 'Topic');
+const policyDocument = new iam.PolicyDocument({
+  assignSids: true,
+  statements: [
+    new iam.PolicyStatement({
+      actions: ["sns:Subscribe"],
+      principals: [new iam.AnyPrincipal()],
+      resources: [topic.topicArn] 
+    }),
+  ],
+});
+
+const topicPolicy = new sns.TopicPolicy(this, 'Policy', {
+  topics: [topic],
+  policyDocument,
+});
+```

packages/@aws-cdk/aws-sns/lib/policy.ts

@@ -12,6 +12,12 @@ export interface TopicPolicyProps {
    * The set of topics this policy applies to.
    */
   readonly topics: ITopic[];
+  /**
+   * IAM policy document to apply to topic(s).
+   * @default empty policy document
+   */
+  readonly policyDocument?: PolicyDocument;
+
 }
 
 /**
@@ -32,6 +38,8 @@ export class TopicPolicy extends Resource {
   constructor(scope: Construct, id: string, props: TopicPolicyProps) {
     super(scope, id);
 
+    this.document = props.policyDocument ?? this.document;
+
     new CfnTopicPolicy(this, 'Resource', {
       policyDocument: this.document,
       topics: props.topics.map(t => t.topicArn),

packages/@aws-cdk/aws-sns/test/test.sns.ts

@@ -276,6 +276,97 @@ export = {
     test.done();
   },
 
+  'TopicPolicy passed document'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const topic = new sns.Topic(stack, 'MyTopic');
+    const ps = new iam.PolicyStatement({
+      actions: ['service:statement0'],
+      principals: [new iam.ArnPrincipal('arn')],
+    });
+
+    // WHEN
+    new sns.TopicPolicy(stack, 'topicpolicy', { topics: [topic], policyDocument: new iam.PolicyDocument({ assignSids: true, statements: [ps] }) });
+
+    // THEN
+    expect(stack).toMatch({
+      'Resources': {
+        'MyTopic86869434': {
+          'Type': 'AWS::SNS::Topic',
+        },
+        'topicpolicyF8CF12FD': {
+          'Type': 'AWS::SNS::TopicPolicy',
+          'Properties': {
+            'PolicyDocument': {
+              'Statement': [
+                {
+                  'Action': 'service:statement0',
+                  'Effect': 'Allow',
+                  'Principal': { 'AWS': 'arn' },
+                  'Sid': '0',
+                },
+              ],
+              'Version': '2012-10-17',
+            },
+            'Topics': [
+              {
+                'Ref': 'MyTopic86869434',
+              },
+            ],
+          },
+        },
+      },
+    });
+
+    test.done();
+  },
+
+  'Add statements to policy'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const topic = new sns.Topic(stack, 'MyTopic');
+
+    // WHEN
+    const topicPolicy = new sns.TopicPolicy(stack, 'TopicPolicy', {
+      topics: [topic],
+    });
+    topicPolicy.document.addStatements(new iam.PolicyStatement({
+      actions: ['service:statement0'],
+      principals: [new iam.ArnPrincipal('arn')],
+    }));
+
+    // THEN
+    expect(stack).toMatch({
+      'Resources': {
+        'MyTopic86869434': {
+          'Type': 'AWS::SNS::Topic',
+        },
+        'TopicPolicyA24B096F': {
+          'Type': 'AWS::SNS::TopicPolicy',
+          'Properties': {
+            'PolicyDocument': {
+              'Statement': [
+                {
+                  'Action': 'service:statement0',
+                  'Effect': 'Allow',
+                  'Principal': { 'AWS': 'arn' },
+                  'Sid': '0',
+                },
+              ],
+              'Version': '2012-10-17',
+            },
+            'Topics': [
+              {
+                'Ref': 'MyTopic86869434',
+              },
+            ],
+          },
+        },
+      },
+    });
+    test.done();
+  },
+
   'topic resource policy includes unique SIDs'(test: Test) {
     const stack = new cdk.Stack();