feat(core): show output during asset bundling

Show Docker outpout during asset bundling.

Closes aws#8956

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

@@ -171,6 +171,7 @@ export class AssetStaging extends Construct {
     ];
 
     try {
+      console.error(`Bundling asset ${this.node.path}...`); // tslint:disable-line no-console
       options.image._run({
         command: options.command,
         user,

packages/@aws-cdk/core/lib/bundling.ts

@@ -1,4 +1,4 @@
-import { spawnSync } from 'child_process';
+import { spawnSync, SpawnSyncOptions } from 'child_process';
 
 /**
  * Bundling options
@@ -121,7 +121,7 @@ export class BundlingDockerImage {
       ...command,
     ];
 
-    dockerExec(dockerArgs);
+    dockerExec(dockerArgs, { stdio: 'inherit' }); // show Docker output in console
   }
 }
 
@@ -222,16 +222,19 @@ function flatten(x: string[][]) {
   return Array.prototype.concat([], ...x);
 }
 
-function dockerExec(args: string[]) {
+function dockerExec(args: string[], options?: SpawnSyncOptions) {
   const prog = process.env.CDK_DOCKER ?? 'docker';
-  const proc = spawnSync(prog, args);
+  const proc = spawnSync(prog, args, options);
 
   if (proc.error) {
     throw proc.error;
   }
 
   if (proc.status !== 0) {
-    throw new Error(`[Status ${proc.status}] stdout: ${proc.stdout?.toString().trim()}\n\n\nstderr: ${proc.stderr?.toString().trim()}`);
+    if (proc.stdout || proc.stderr) {
+      throw new Error(`[Status ${proc.status}] stdout: ${proc.stdout?.toString().trim()}\n\n\nstderr: ${proc.stderr?.toString().trim()}`);
+    }
+    throw new Error(`${prog} exited with status ${proc.status}`);
   }
 
   return proc;

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

@@ -40,7 +40,7 @@ export = {
       '-w', '/working-directory',
       'alpine',
       'cool', 'command',
-    ]));
+    ], { stdio: 'inherit' }));
     test.done();
   },
 

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

@@ -108,6 +108,7 @@ export = {
     const ensureDirSyncSpy = sinon.spy(fs, 'ensureDirSync');
     const mkdtempSyncSpy = sinon.spy(fs, 'mkdtempSync');
     const chmodSyncSpy = sinon.spy(fs, 'chmodSync');
+    const consoleErrorSpy = sinon.spy(console, 'error');
 
     // WHEN
     new AssetStaging(stack, 'Asset', {
@@ -138,6 +139,9 @@ export = {
     test.ok(mkdtempSyncSpy.calledWith(sinon.match(path.join(stagingTmp, 'asset-bundle-'))));
     test.ok(chmodSyncSpy.calledWith(sinon.match(path.join(stagingTmp, 'asset-bundle-')), 0o777));
 
+    // shows a message before bundling
+    test.ok(consoleErrorSpy.calledWith('Bundling asset stack/Asset...'));
+
     test.done();
   },