Add support for CloudFormation intrinsic function for role

- based on #111
- thanks @alex-berger

Author: bweigel

README.md

@@ -626,6 +626,7 @@ $ sls invoke stepf --name <stepfunctionname> --data '{"foo":"bar"}'
 - --path or -p The path to a json file with input data to be passed to the invoked step function.
 
 ## IAM Role
+
 The IAM roles required to run Statemachine are automatically generated. It is also possible to specify ARN directly.
 
 Here's an example:
@@ -637,6 +638,28 @@ stepFunctions:
       role: arn:aws:iam::xxxxxxxx:role/yourRole
       definition:
 ```
+
+It is also possible to use the [CloudFormation intrinsic functions](https://docs.aws.amazon.com/en_en/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference.html) to reference resources from elsewhere:
+
+```yml
+stepFunctions:
+  stateMachines:
+    hello:
+      role:
+        Ref: StateMachineRole
+      definition:
+        ...
+
+resources:
+  Resources:
+    StateMachineRole:
+      Type: AWS::IAM::Role
+      Properties: 
+        ...
+```
+
+The short form of the intrinsic functions (i.e. `!Sub`, `!Ref`) is not supported at the moment.
+
 ## Tips
 ### How to specify the stateMachine ARN to environment variables
 Here is serverless.yml sample to specify the stateMachine ARN to environment variables.

lib/deploy/stepFunctions/compileStateMachines.js

@@ -2,7 +2,13 @@
 const _ = require('lodash');
 const BbPromise = require('bluebird');
 
+function isIntrinsic(obj) {
+  const isObject = typeof obj === 'object';
+  return isObject && Object.keys(obj).some((k) => k.startsWith('Fn::') || k.startsWith('Ref'));
+}
+
 module.exports = {
+  isIntrinsic,
   compileStateMachines() {
     if (this.isStateMachines()) {
       this.getAllStateMachines().forEach((stateMachineName) => {
@@ -37,15 +43,18 @@ module.exports = {
                 ' Please check the README for more info.',
               ].join('');
               throw new this.serverless.classes
-              .Error(errorMessage);
+                .Error(errorMessage);
             }
+          } else if (isIntrinsic(stateMachineObj.role)) {
+            RoleArn = stateMachineObj.role;
           } else {
             const errorMessage = [
-              `role property in stateMachine "${stateMachineName}" is not an string`,
+              `role property in stateMachine "${stateMachineName}" is neither a string`,
+              ' nor a CloudFormation intrinsic function',
               ' Please check the README for more info.',
             ].join('');
             throw new this.serverless.classes
-            .Error(errorMessage);
+              .Error(errorMessage);
           }
         } else {
           RoleArn = {
@@ -61,15 +70,14 @@ module.exports = {
           stateMachineObj);
         const stateMachineOutputLogicalId = this
           .getStateMachineOutputLogicalId(stateMachineName, stateMachineObj);
-        const stateMachineTemplate =
-          {
-            Type: 'AWS::StepFunctions::StateMachine',
-            Properties: {
-              DefinitionString,
-              RoleArn,
-            },
-            DependsOn,
-          };
+        const stateMachineTemplate = {
+          Type: 'AWS::StepFunctions::StateMachine',
+          Properties: {
+            DefinitionString,
+            RoleArn,
+          },
+          DependsOn,
+        };
 
         const newStateMachineObject = {
           [stateMachineLogicalId]: stateMachineTemplate,

lib/deploy/stepFunctions/compileStateMachines.test.js

@@ -234,6 +234,45 @@ describe('#compileStateMachines', () => {
     ).to.equal('StateMachineBeta2');
   });
 
+  it('should respect CloudFormation intrinsic functions for role property', () => {
+    serverless.service.stepFunctions = {
+      stateMachines: {
+        myStateMachine1: {
+          name: 'stateMachineWithIntrinsicRole1',
+          definition: 'definition1\n',
+          role: { 'Fn::Attr': ['RoleID', 'Arn'] },
+        },
+        myStateMachine2: {
+          name: 'stateMachineWithIntrinsicRole2',
+          definition: 'definition1\n',
+          role: { Ref: 'CloudformationId' },
+        },
+      },
+    };
+    serverlessStepFunctions.compileStateMachines();
+    expect(serverlessStepFunctions.serverless.service
+      .provider.compiledCloudFormationTemplate.Resources
+      .StateMachineWithIntrinsicRole1.Properties.RoleArn
+    ).to.deep.equal({ 'Fn::Attr': ['RoleID', 'Arn'] });
+    expect(serverlessStepFunctions.serverless.service
+      .provider.compiledCloudFormationTemplate.Resources
+      .StateMachineWithIntrinsicRole2.Properties.RoleArn
+    ).to.deep.equal({ Ref: 'CloudformationId' });
+  });
+
+  it('should throw error if role property is neither string nor intrinsic functions', () => {
+    serverless.service.stepFunctions = {
+      stateMachines: {
+        myStateMachine1: {
+          name: 'stateMachineWithIntrinsicRole',
+          definition: 'definition1\n',
+          role: { XXX: ['RoleID', 'Arn'] },
+        },
+      },
+    };
+    expect(() => serverlessStepFunctions.compileStateMachines()).to.throw(Error);
+  });
+
   it('should throw error when definition property is not given', () => {
     serverless.service.stepFunctions = {
       stateMachines: {