fix(core): incorrect temp directory when bundling assets (aws#8469)

The `os.tmpdir()` built-in doesn't return the real path when the
returned path is a symlink.

Add a `FileSystem.tmpdir` that wraps `os.tmpdir()` in a
`fs.realpathSync()` and caches the result.

Add a `FileSystem.mkdtemp()` to create temp directories in
the system temp directory.

Fixes aws#8465


----

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

packages/@aws-cdk/core/lib/asset-staging.ts

@@ -1,6 +1,5 @@
 import * as cxapi from '@aws-cdk/cx-api';
 import * as fs from 'fs';
-import * as os from 'os';
 import * as path from 'path';
 import { AssetHashType, AssetOptions } from './assets';
 import { BUNDLING_INPUT_DIR, BUNDLING_OUTPUT_DIR, BundlingOptions } from './bundling';
@@ -137,7 +136,7 @@ export class AssetStaging extends Construct {
 
   private bundle(options: BundlingOptions): string {
     // Create temporary directory for bundling
-    const bundleDir = fs.mkdtempSync(path.resolve(path.join(os.tmpdir(), 'cdk-asset-bundle-')));
+    const bundleDir = FileSystem.mkdtemp('cdk-asset-bundle-');
 
     // Always mount input and output dir
     const volumes = [

packages/@aws-cdk/core/lib/fs/index.ts

@@ -1,4 +1,6 @@
 import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
 import { copyDirectory } from './copy';
 import { fingerprint } from './fingerprint';
 import { CopyOptions, FingerprintOptions } from './options';
@@ -43,4 +45,27 @@ export class FileSystem {
   public static isEmpty(dir: string): boolean {
     return fs.readdirSync(dir).length === 0;
   }
+
+  /**
+   * The real path of the system temp directory
+   */
+  public static get tmpdir(): string {
+    if (FileSystem._tmpdir) {
+      return FileSystem._tmpdir;
+    }
+    FileSystem._tmpdir = fs.realpathSync(os.tmpdir());
+    return FileSystem._tmpdir;
+  }
+
+  /**
+   * Creates a unique temporary directory in the **system temp directory**.
+   *
+   * @param prefix A prefix for the directory name. Six random characters
+   * will be generated and appended behind this prefix.
+   */
+  public static mkdtemp(prefix: string): string {
+    return fs.mkdtempSync(path.join(FileSystem.tmpdir, prefix));
+  }
+
+  private static _tmpdir?: string;
 }

packages/@aws-cdk/core/test/fs/test.fs.ts

@@ -0,0 +1,48 @@
+import * as fs from 'fs';
+import { Test } from 'nodeunit';
+import * as os from 'os';
+import * as path from 'path';
+import * as sinon from 'sinon';
+import { FileSystem } from '../../lib/fs';
+
+export = {
+  'tearDown'(callback: any) {
+    sinon.restore();
+    callback();
+  },
+
+  'tmpdir returns a real path and is cached'(test: Test) {
+    // Create symlink that points to /tmp
+    const symlinkTmp = path.join(__dirname, 'tmp-link');
+    fs.symlinkSync(os.tmpdir(), symlinkTmp);
+
+    // Now stub os.tmpdir() to return this link instead of /tmp
+    const tmpdirStub = sinon.stub(os, 'tmpdir').returns(symlinkTmp);
+
+    test.ok(path.isAbsolute(FileSystem.tmpdir));
+
+    const p = path.join(FileSystem.tmpdir, 'tmpdir-test.txt');
+    fs.writeFileSync(p, 'tmpdir-test');
+
+    test.equal(p, fs.realpathSync(p));
+    test.equal(fs.readFileSync(p, 'utf8'), 'tmpdir-test');
+
+    test.ok(tmpdirStub.calledOnce); // cached result
+
+    fs.unlinkSync(p);
+    fs.unlinkSync(symlinkTmp);
+
+    test.done();
+  },
+
+  'mkdtemp creates a temporary directory in the system temp'(test: Test) {
+    const tmpdir = FileSystem.mkdtemp('cdk-mkdtemp-');
+
+    test.equal(path.dirname(tmpdir), FileSystem.tmpdir);
+    test.ok(fs.existsSync(tmpdir));
+
+    fs.rmdirSync(tmpdir);
+
+    test.done();
+  },
+};