fix(core): unable to reference resources across multiple nested stacks

packages/@aws-cdk/aws-cloudformation/lib/nested-stack.ts

@@ -1,5 +1,7 @@
 import * as sns from '@aws-cdk/aws-sns';
-import { Aws, CfnOutput, CfnParameter, CfnResource, Construct, Duration, Fn, IResolvable, IResolveContext, Lazy, Reference, Stack, Token } from '@aws-cdk/core';
+import { Aws, CfnResource, Construct, Duration, FileAssetPackaging, Fn, IResolveContext, Stack, Token } from '@aws-cdk/core';
+import { Lazy } from 'constructs';
+import * as crypto from 'crypto';
 import { CfnStack } from './cloudformation.generated';
 
 const NESTED_STACK_SYMBOL = Symbol.for('@aws-cdk/aws-cloudformation.NestedStack');
@@ -10,7 +12,6 @@ const NESTED_STACK_SYMBOL = Symbol.for('@aws-cdk/aws-cloudformation.NestedStack'
  * @experimental
  */
 export interface NestedStackProps {
-
   /**
    * The set value pairs that represent the parameters passed to CloudFormation
    * when this nested stack is created. Each parameter has a name corresponding
@@ -82,12 +83,16 @@ export class NestedStack extends Stack {
   private readonly resource: CfnStack;
   private readonly _contextualStackId: string;
   private readonly _contextualStackName: string;
+  private _templateUrl?: string;
+  private _parentStack: Stack;
 
   constructor(scope: Construct, id: string, props: NestedStackProps = { }) {
     const parentStack = findParentStack(scope);
 
     super(scope, id, { env: { account: parentStack.account, region: parentStack.region } });
 
+    this._parentStack = parentStack;
+
     // @deprecate: remove this in v2.0 (redundent)
     const parentScope = new Construct(scope, id + '.NestedStack');
 
@@ -99,7 +104,7 @@ export class NestedStack extends Stack {
     this.parameters = props.parameters || {};
 
     this.resource = new CfnStack(parentScope, `${id}.NestedStackResource`, {
-      templateUrl: this.templateUrl,
+      templateUrl: Lazy.stringValue({ produce: () => this._templateUrl || '<unresolved>' }),
       parameters: Lazy.anyValue({ produce: () => Object.keys(this.parameters).length > 0 ? this.parameters : undefined }),
       notificationArns: props.notifications ? props.notifications.map(n => n.topicArn) : undefined,
       timeoutInMinutes: props.timeout ? props.timeout.toMinutes() : undefined,
@@ -144,62 +149,46 @@ export class NestedStack extends Stack {
   }
 
   /**
-   * Called by the base "prepare" method when a reference is found.
+   * Assign a value to one of the nested stack parameters.
+   * @param name The parameter name (ID)
+   * @param value The value to assign
    */
-  protected prepareCrossReference(sourceStack: Stack, reference: Reference): IResolvable {
-    const targetStack = Stack.of(reference.target);
-
-    // the nested stack references a resource from the parent stack:
-    // we pass it through a as a cloudformation parameter
-    if (targetStack === sourceStack.nestedStackParent) {
-      // we call "this.resolve" to ensure that tokens do not creep in (for example, if the reference display name includes tokens)
-      const paramId = this.resolve(`reference-to-${reference.target.node.uniqueId}.${reference.displayName}`);
-      let param = this.node.tryFindChild(paramId) as CfnParameter;
-      if (!param) {
-        param = new CfnParameter(this, paramId, { type: 'String' });
-        this.parameters[param.logicalId] = Token.asString(reference);
-      }
-
-      return param.value;
-    }
-
-    // parent stack references a resource from the nested stack:
-    // we output it from the nested stack and use "Fn::GetAtt" as the reference value
-    if (targetStack === this && targetStack.nestedStackParent === sourceStack) {
-      return this.getCreateOutputForReference(reference);
-    }
-
-    // sibling nested stacks (same parent):
-    // output from one and pass as parameter to the other
-    if (targetStack.nestedStackParent && targetStack.nestedStackParent === sourceStack.nestedStackParent) {
-      const outputValue = this.getCreateOutputForReference(reference);
-      return (sourceStack as NestedStack).prepareCrossReference(sourceStack, outputValue);
-    }
-
-    // nested stack references a value from some other non-nested stack:
-    // normal export/import, with dependency between the parents
-    if (sourceStack.nestedStackParent && sourceStack.nestedStackParent !== targetStack) {
-      return super.prepareCrossReference(sourceStack, reference);
-    }
+  public setParameter(name: string, value: string) {
+    this.parameters[name] = value;
+  }
 
-    // some non-nested stack (that is not the parent) references a resource inside the nested stack:
-    // we output the value and let our parent export it
-    if (!sourceStack.nestedStackParent && targetStack.nestedStackParent && targetStack.nestedStackParent !== sourceStack) {
-      const outputValue = this.getCreateOutputForReference(reference);
-      return (targetStack.nestedStackParent as NestedStack).prepareCrossReference(sourceStack, outputValue);
+  /**
+   * Defines an asset at the parent stack which represents the template of this
+   * nested stack.
+   *
+   * This private API is used by `App.prepare()` within a loop that rectifies
+   * references every time an asset is added. This is because (at the moment)
+   * assets are addressed using CloudFormation parameters.
+   *
+   * @returns `true` if a new asset was added or `false` if an asset was
+   * previously added. When this returns `true`, App will do another reference
+   * rectification cycle.
+   *
+   * @internal
+   */
+  public _prepareTemplateAsset() {
+    if (this._templateUrl) {
+      return false;
     }
 
-    throw new Error('unexpected nested stack cross reference');
-  }
+    const cfn = JSON.stringify((this as any)._toCloudFormation());
+    const templateHash = crypto.createHash('sha256').update(cfn).digest('hex');
 
-  private getCreateOutputForReference(reference: Reference) {
-    const outputId = `${reference.target.node.uniqueId}${reference.displayName}`;
-    let output = this.node.tryFindChild(outputId) as CfnOutput;
-    if (!output) {
-      output = new CfnOutput(this, outputId, { value: Token.asString(reference) });
-    }
+    const templateLocation = this._parentStack.addFileAsset({
+      packaging: FileAssetPackaging.FILE,
+      sourceHash: templateHash,
+      fileName: this.templateFile
+    });
 
-    return this.resource.getAtt(`Outputs.${output.logicalId}`);
+    // if bucketName/objectKey are cfn parameters from a stack other than the parent stack, they will
+    // be resolved as cross-stack references like any other (see "multi" tests).
+    this._templateUrl = `https://s3.${this._parentStack.region}.${this._parentStack.urlSuffix}/${templateLocation.bucketName}/${templateLocation.objectKey}`;
+    return true;
   }
 
   private contextualAttribute(innerValue: string, outerValue: string) {

packages/@aws-cdk/aws-cloudformation/test/integ.nested-stack.expected.json

@@ -158,7 +158,7 @@
               },
               "/",
               {
-                "Ref": "AssetParametersdddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccafS3BucketE3660F43"
+                "Ref": "AssetParameters4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1aS3Bucket1DDC9C52"
               },
               "/",
               {
@@ -168,7 +168,7 @@
                     "Fn::Split": [
                       "||",
                       {
-                        "Ref": "AssetParametersdddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccafS3VersionKeyFD0B0470"
+                        "Ref": "AssetParameters4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1aS3VersionKey2B4F31C1"
                       }
                     ]
                   }
@@ -181,7 +181,7 @@
                     "Fn::Split": [
                       "||",
                       {
-                        "Ref": "AssetParametersdddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccafS3VersionKeyFD0B0470"
+                        "Ref": "AssetParameters4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1aS3VersionKey2B4F31C1"
                       }
                     ]
                   }
@@ -254,29 +254,29 @@
     }
   },
   "Parameters": {
-    "AssetParameters0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22dfS3BucketB322F951": {
+    "AssetParameters4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1aS3Bucket1DDC9C52": {
       "Type": "String",
-      "Description": "S3 bucket for asset \"0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22df\""
+      "Description": "S3 bucket for asset \"4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1a\""
     },
-    "AssetParameters0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22dfS3VersionKeyAA9C5AF4": {
+    "AssetParameters4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1aS3VersionKey2B4F31C1": {
       "Type": "String",
-      "Description": "S3 key for asset version \"0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22df\""
+      "Description": "S3 key for asset version \"4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1a\""
     },
-    "AssetParameters0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22dfArtifactHash5D335705": {
+    "AssetParameters4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1aArtifactHash3AA59378": {
       "Type": "String",
-      "Description": "Artifact hash for asset \"0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22df\""
+      "Description": "Artifact hash for asset \"4ed2bec8961a74942e0627883abee066300275e2c2b03fe650d313898fe68f1a\""
     },
-    "AssetParametersdddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccafS3BucketE3660F43": {
+    "AssetParameters0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22dfS3BucketB322F951": {
       "Type": "String",
-      "Description": "S3 bucket for asset \"dddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccaf\""
+      "Description": "S3 bucket for asset \"0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22df\""
     },
-    "AssetParametersdddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccafS3VersionKeyFD0B0470": {
+    "AssetParameters0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22dfS3VersionKeyAA9C5AF4": {
       "Type": "String",
-      "Description": "S3 key for asset version \"dddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccaf\""
+      "Description": "S3 key for asset version \"0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22df\""
     },
-    "AssetParametersdddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccafArtifactHashEECD8E35": {
+    "AssetParameters0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22dfArtifactHash5D335705": {
       "Type": "String",
-      "Description": "Artifact hash for asset \"dddca70fcceefd0a4532c8eb5ad3d5da6f51d64fda9343f0b57dd664736dccaf\""
+      "Description": "Artifact hash for asset \"0d0404717d8867c09534f2cf382e8e24531ff64a968afa2efd7f071ad65a22df\""
     }
   }
 }

packages/@aws-cdk/aws-cloudformation/test/integ.nested-stacks-multi-refs.expected.json

@@ -0,0 +1,116 @@
+{
+  "Resources": {
+    "Level1ABBD39B3": {
+      "Type": "AWS::SNS::Topic"
+    },
+    "Nested1NestedStackNested1NestedStackResourceCD0AD36B": {
+      "Type": "AWS::CloudFormation::Stack",
+      "Properties": {
+        "TemplateURL": {
+          "Fn::Join": [
+            "",
+            [
+              "https://s3.",
+              {
+                "Ref": "AWS::Region"
+              },
+              ".",
+              {
+                "Ref": "AWS::URLSuffix"
+              },
+              "/",
+              {
+                "Ref": "AssetParametersad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95S3BucketDB605F9E"
+              },
+              "/",
+              {
+                "Fn::Select": [
+                  0,
+                  {
+                    "Fn::Split": [
+                      "||",
+                      {
+                        "Ref": "AssetParametersad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95S3VersionKey26685906"
+                      }
+                    ]
+                  }
+                ]
+              },
+              {
+                "Fn::Select": [
+                  1,
+                  {
+                    "Fn::Split": [
+                      "||",
+                      {
+                        "Ref": "AssetParametersad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95S3VersionKey26685906"
+                      }
+                    ]
+                  }
+                ]
+              }
+            ]
+          ]
+        },
+        "Parameters": {
+          "referencetonestedstacksmultirefsLevel19FB2466DTopicName": {
+            "Fn::GetAtt": [
+              "Level1ABBD39B3",
+              "TopicName"
+            ]
+          },
+          "referencetonestedstacksmultirefsAssetParameters495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3aS3Bucket03F0C3B1Ref": {
+            "Ref": "AssetParameters495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3aS3Bucket58724FCA"
+          },
+          "referencetonestedstacksmultirefsAssetParameters495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3aS3VersionKey5F9CF809Ref": {
+            "Ref": "AssetParameters495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3aS3VersionKey2CCE0573"
+          },
+          "referencetonestedstacksmultirefsAssetParameterscc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847S3Bucket8F1E17B9Ref": {
+            "Ref": "AssetParameterscc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847S3Bucket9A14AA6D"
+          },
+          "referencetonestedstacksmultirefsAssetParameterscc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847S3VersionKey9EEEF950Ref": {
+            "Ref": "AssetParameterscc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847S3VersionKeyF124C0D9"
+          }
+        }
+      }
+    }
+  },
+  "Parameters": {
+    "AssetParameters495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3aS3Bucket58724FCA": {
+      "Type": "String",
+      "Description": "S3 bucket for asset \"495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3a\""
+    },
+    "AssetParameters495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3aS3VersionKey2CCE0573": {
+      "Type": "String",
+      "Description": "S3 key for asset version \"495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3a\""
+    },
+    "AssetParameters495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3aArtifactHashAE1436B7": {
+      "Type": "String",
+      "Description": "Artifact hash for asset \"495a6bc36c13a0adeb3778c921d18ac4a8205f5471108fcc199a291d14855c3a\""
+    },
+    "AssetParameterscc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847S3Bucket9A14AA6D": {
+      "Type": "String",
+      "Description": "S3 bucket for asset \"cc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847\""
+    },
+    "AssetParameterscc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847S3VersionKeyF124C0D9": {
+      "Type": "String",
+      "Description": "S3 key for asset version \"cc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847\""
+    },
+    "AssetParameterscc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847ArtifactHashAF64C405": {
+      "Type": "String",
+      "Description": "Artifact hash for asset \"cc623add53df153cf6a7df1cea4dc90740d7be087472579110754a633ec90847\""
+    },
+    "AssetParametersad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95S3BucketDB605F9E": {
+      "Type": "String",
+      "Description": "S3 bucket for asset \"ad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95\""
+    },
+    "AssetParametersad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95S3VersionKey26685906": {
+      "Type": "String",
+      "Description": "S3 key for asset version \"ad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95\""
+    },
+    "AssetParametersad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95ArtifactHashAF8D54FC": {
+      "Type": "String",
+      "Description": "Artifact hash for asset \"ad23da1cfc8b3fd7916c6ffc7debacadf084765e62fab8acf0b8b0a9b0289f95\""
+    }
+  }
+}

packages/@aws-cdk/aws-cloudformation/test/integ.nested-stacks-multi-refs.ts

@@ -0,0 +1,31 @@
+import * as sns from '@aws-cdk/aws-sns';
+import { App, Fn, Stack } from '@aws-cdk/core';
+import { NestedStack } from '../lib';
+
+const app = new App();
+const top = new Stack(app, 'nested-stacks-multi-refs');
+const level1 = new sns.Topic(top, 'Level1');
+const nested1 = new NestedStack(top, 'Nested1');
+const nested2 = new NestedStack(nested1, 'Nested2');
+const nested3 = new NestedStack(nested2, 'Nested3');
+
+// WHEN
+const level2 = new sns.Topic(nested2, 'Level2ReferencesLevel1', {
+  displayName: shortName(level1.topicName)
+});
+
+new sns.Topic(nested3, 'Level3ReferencesLevel1', {
+  displayName: shortName(level1.topicName)
+});
+
+new sns.Topic(nested3, 'Level3ReferencesLevel2', {
+  displayName: shortName(level2.topicName)
+});
+
+app.synth();
+
+// topicName is too long for displayName, so just take the second part:
+// Stack1-NestedUnderStack1NestedStackNestedUnderStack1NestedStackResourceF616305B-EM64TEGA04J9-TopicInNestedUnderStack115E329C4-HEO7NLYC1AFL
+function shortName(topicName: string) {
+  return Fn.select(1, Fn.split('-', topicName));
+}