expect (from @aws-cdk/assert) fails when passed a CloudFormationStackArtifact

[cmackie]

The override of expect defined in @aws-cdk/assert fails when passed a CloudFormationStackArtifact object retrieved with the SynthUtils api. This only happens in projects which do not have a flat dependency tree, instead producing the full dependency tree (which should be supported) in node_modules. The root of the problem is that @aws-cdk/core has @aws-cdk/cx-api as a direct dependency even though it exposes classes within @aws-cdk/cx-api (the CloudFormationStackArtifact retrieved though SynthUtils is ultimately created in ConstructNode. This means that when the instanceof operator is used on a CloudFormationStackArtifact anywhere in @aws-cdk/assert, the left hand side was created by the @aws-cdk/cx-api module required by @aws-cdk/core, whereas the right hand side class comes from the @aws-cdk/cx-api module required by @aws-cdk/assert. This causes instanceof to return false.

In addition to the issue with expect (which I think is most serious), this problem causes several other methods (like SynthUtils.toCloudFormation within @aws-cdk/assert to misbehave subtly).

Reproduction Steps

While I've only been able to naturally reproduce this in my own project (which I cannot share here), this is fairly easy to reproduce with the simple project created by cdk init. Just follow these steps:

1. mkdir dep-test && cd dep-test
2. cdk init --typescript
3. Replace dep-test/test/dep-test.test.ts with the attached file.
   dep-test.test.txt

At this point, running npm run test succeeds because npm install creates a flat node_modules. To reproduce the error, we need to simulate the results of using an older version of npm, or any other build system which creates the full dependency tree.

4. mkdir -p node_modules/@aws-cdk/assert/node_modules/@aws-cdk && cp -r node_modules/@aws-cdk/cx-api "$_"
5. mkdir -p node_modules/@aws-cdk/core/node_modules/@aws-cdk && cp -r node_modules/@aws-cdk/cx-api "$_"

Now running npm run test fails, for the reason I described earlier in this report.

Error Log

TypeError: Cannot read property 'root' of undefined

       8 |     const stack = new HelloCdk.HelloCdkStack(app, 'MyTestStack');
       9 |     // THEN
    > 10 |     expectCDK(SynthUtils.synthesize(stack));
         |     ^
      11 | });

      at Function._synthesizeWithNested (node_modules/@aws-cdk/assert/lib/synth-utils.ts:54:29)
      at Object.expect (node_modules/@aws-cdk/assert/lib/expect.ts:8:90)
      at Object.<anonymous> (test/hello-cdk.test.ts:10:5)

Environment

• CLI Version : 1.22.0
• Framework Version: 1.22.0
• OS : macOS 10.14.6
• Language : TypeScript

Other

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This is 🐛 Bug Report

[eladb]

We are aware that there are issues related to how CDK is released ("hyper-modular"). The current best practice is to always use a single version line and have npm flatten the dependency graph.

You can stay tuned here for updates: aws/aws-cdk-rfcs#6