fix: automatically clean up unit test assemblies

The CDK framework creates a temporary directory in `/tmp` for every unit
test, and never cleans those up.

In this PR, register all temporary assembly directories created by doing
`new Stack()` or `new App()` without an `outdir`, and delete them when
the Node process exits.

This will only affect unit tests: if the `outdir` property is set
explicitly, or the CDK App is being synthesized by the CLI (and
`$CDK_OUTDIR` is set), the assembly directory will not be cleaned.

Author: rix0rrr

packages/aws-cdk-lib/core/lib/stack.ts

@@ -439,7 +439,6 @@ export class Stack extends Construct implements ITaggable {
     // as the parent because apps implement much of the synthesis logic.
     scope = scope ?? new App({
       autoSynth: false,
-      outdir: FileSystem.mkdtemp('cdk-test-app-'),
     });
 
     // "Default" is a "hidden id" from a `node.uniqueId` perspective

packages/aws-cdk-lib/cx-api/lib/cloud-assembly.ts

@@ -476,7 +476,14 @@ function ignore(_x: any) {
  * Turn the given optional output directory into a fixed output directory
  */
 function determineOutputDirectory(outdir?: string) {
-  return outdir ?? fs.mkdtempSync(path.join(fs.realpathSync(os.tmpdir()), 'cdk.out'));
+  if (outdir) {
+    return outdir;
+  }
+
+  // Make a temporary directory; clean it up automatically if this is done for testing.
+  const tmpDir = fs.mkdtempSync(path.join(fs.realpathSync(os.tmpdir()), 'cdk.out'));
+  TEMPORARY_ASSEMBLY_DIRS.push(tmpDir);
+  return outdir ?? tmpDir;
 }
 
 function ensureDirSync(dir: string) {
@@ -488,3 +495,11 @@ function ensureDirSync(dir: string) {
     fs.mkdirSync(dir, { recursive: true });
   }
 }
+
+// On process exit, delete all temporary assembly directories
+const TEMPORARY_ASSEMBLY_DIRS: string[] = [];
+process.on('exit', () => {
+  for (const dir of TEMPORARY_ASSEMBLY_DIRS) {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});