chore(integtests): make installing NPM7 safe against concurrent calls (…

…aws#12642)

Do this by memoizing the Promise of the async call; each call gets the
same promise. This is fine because it is safe to await a Promise
multiple times.

Effectively the function will only get invoked once and each caller
waits for the one copy to finish.

We lose "debugging" output here, where it would write the progress
bar of NPM to the output log of the first test to install NPM7. I did
not think maintaining that was worth breaking the clear contract of the
memoize implementation (0 arguments clearly implies that there is no
argument-value based cache here).

(Failure to install will still bubble up as an exception including the shell
output)

----

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

packages/aws-cdk/test/integ/helpers/cdk.ts

@@ -3,6 +3,7 @@ import * as fs from 'fs';
 import * as os from 'os';
 import * as path from 'path';
 import { outputFromStack, AwsClients } from './aws';
+import { memoize0 } from './memoize';
 import { findYarnPackages } from './monorepo';
 import { ResourcePool } from './resource-pool';
 import { TestContext } from './test-helpers';
@@ -489,7 +490,7 @@ async function installNpmPackages(fixture: TestFixture, packages: Record<string,
   }, undefined, 2), { encoding: 'utf-8' });
 
   // Now install that `package.json` using NPM7
-  const npm7 = await installNpm7(fixture.output);
+  const npm7 = await installNpm7();
   await fixture.shell([npm7, 'install']);
 }
 
@@ -500,20 +501,14 @@ async function installNpmPackages(fixture: TestFixture, packages: Record<string,
  * - The install is cached so we don't have to install it over and over again
  *   for every test.
  */
-async function installNpm7(output?: NodeJS.WritableStream): Promise<string> {
-  if (NPM7_INSTALL_LOCATION === undefined) {
-    const installDir = path.join(os.tmpdir(), 'cdk-integ-npm7');
-    await shell(['rm', '-rf', installDir], { output });
-    await shell(['mkdir', '-p', installDir], { output });
+const installNpm7 = memoize0(async (): Promise<string> => {
+  const installDir = path.join(os.tmpdir(), 'cdk-integ-npm7');
+  await shell(['rm', '-rf', installDir]);
+  await shell(['mkdir', '-p', installDir]);
 
-    await shell(['npm', 'install',
-      '--prefix', installDir,
-      'npm@7'], { output });
+  await shell(['npm', 'install',
+    '--prefix', installDir,
+    'npm@7']);
 
-    NPM7_INSTALL_LOCATION = path.join(installDir, 'node_modules', '.bin', 'npm');
-  }
-
-  return NPM7_INSTALL_LOCATION;
-}
-
-let NPM7_INSTALL_LOCATION: string | undefined;
+  return path.join(installDir, 'node_modules', '.bin', 'npm');
+});

packages/aws-cdk/test/integ/helpers/memoize.ts

@@ -0,0 +1,14 @@
+/**
+ * Return a memoized version of an function with 0 arguments.
+ *
+ * Async-safe.
+ */
+export function memoize0<A>(fn: () => Promise<A>): () => Promise<A> {
+  let promise: Promise<A> | undefined;
+  return () => {
+    if (!promise) {
+      promise = fn();
+    }
+    return promise;
+  };
+}