fix(cli): release outdir lock when synth fails (#30874)

### Issue # (if applicable)

Closes #27864

### Reason for this change

When using cdk watch mode, a synth failure causes the CDK CLI to no longer deploy changes. The CDK CLI must be restarted to resume watch mode. The cause of the issue is that CDK CLI never releases the outdir write lock if synthing fails, so subsequent attempts to exec the user's app cannot acquire the outdir writer lock.



### Description of changes

I added a try/catch that releases the outdir writer lock & rethrows the error when a synth fails.



### Description of how you validated changes

I added a unit test. I also ran the modified cdk cli on a project of my own and simulated the failure of a synth to see whether the issue was resolved, and it is.



### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

packages/aws-cdk/lib/api/cxapp/exec.ts

@@ -58,37 +58,42 @@ export async function execProgram(aws: SdkProvider, config: Configuration): Prom
   debug('outdir:', outdir);
   env[cxapi.OUTDIR_ENV] = outdir;
 
-  // Acquire a read lock on the output directory
+  // Acquire a lock on the output directory
   const writerLock = await new RWLock(outdir).acquireWrite();
 
-  // Send version information
-  env[cxapi.CLI_ASM_VERSION_ENV] = cxschema.Manifest.version();
-  env[cxapi.CLI_VERSION_ENV] = versionNumber();
+  try {
+    // Send version information
+    env[cxapi.CLI_ASM_VERSION_ENV] = cxschema.Manifest.version();
+    env[cxapi.CLI_VERSION_ENV] = versionNumber();
 
-  debug('env:', env);
+    debug('env:', env);
 
-  const envVariableSizeLimit = os.platform() === 'win32' ? 32760 : 131072;
-  const [smallContext, overflow] = splitBySize(context, spaceAvailableForContext(env, envVariableSizeLimit));
+    const envVariableSizeLimit = os.platform() === 'win32' ? 32760 : 131072;
+    const [smallContext, overflow] = splitBySize(context, spaceAvailableForContext(env, envVariableSizeLimit));
 
-  // Store the safe part in the environment variable
-  env[cxapi.CONTEXT_ENV] = JSON.stringify(smallContext);
+    // Store the safe part in the environment variable
+    env[cxapi.CONTEXT_ENV] = JSON.stringify(smallContext);
 
-  // If there was any overflow, write it to a temporary file
-  let contextOverflowLocation;
-  if (Object.keys(overflow ?? {}).length > 0) {
-    const contextDir = await fs.mkdtemp(path.join(os.tmpdir(), 'cdk-context'));
-    contextOverflowLocation = path.join(contextDir, 'context-overflow.json');
-    fs.writeJSONSync(contextOverflowLocation, overflow);
-    env[cxapi.CONTEXT_OVERFLOW_LOCATION_ENV] = contextOverflowLocation;
-  }
+    // If there was any overflow, write it to a temporary file
+    let contextOverflowLocation;
+    if (Object.keys(overflow ?? {}).length > 0) {
+      const contextDir = await fs.mkdtemp(path.join(os.tmpdir(), 'cdk-context'));
+      contextOverflowLocation = path.join(contextDir, 'context-overflow.json');
+      fs.writeJSONSync(contextOverflowLocation, overflow);
+      env[cxapi.CONTEXT_OVERFLOW_LOCATION_ENV] = contextOverflowLocation;
+    }
 
-  await exec(commandLine.join(' '));
+    await exec(commandLine.join(' '));
 
-  const assembly = createAssembly(outdir);
+    const assembly = createAssembly(outdir);
 
-  contextOverflowCleanup(contextOverflowLocation, assembly);
+    contextOverflowCleanup(contextOverflowLocation, assembly);
 
-  return { assembly, lock: await writerLock.convertToReaderLock() };
+    return { assembly, lock: await writerLock.convertToReaderLock() };
+  } catch (e) {
+    await writerLock.release();
+    throw e;
+  }
 
   async function exec(commandAndArgs: string) {
     return new Promise<void>((ok, fail) => {

packages/aws-cdk/test/api/exec.test.ts

@@ -12,6 +12,7 @@ import { Configuration } from '../../lib/settings';
 import { testAssembly } from '../util';
 import { mockSpawn } from '../util/mock-child_process';
 import { MockSdkProvider } from '../util/mock-sdk';
+import { RWLock } from '../../lib/api/util/rwlock';
 
 let sdkProvider: MockSdkProvider;
 let config: Configuration;
@@ -234,6 +235,25 @@ test('cli does not throw when the `build` script succeeds', async () => {
   await lock.release();
 }, TEN_SECOND_TIMEOUT);
 
+test('cli releases the outdir lock when execProgram throws', async () => {
+  // GIVEN
+  config.settings.set(['app'], 'cloud-executable');
+  mockSpawn({
+    commandLine: 'fake-command',
+    exitCode: 127,
+  });
+
+  // WHEN
+  await expect(execProgram(sdkProvider, config)).rejects.toThrow();
+
+  const output = config.settings.get(['output']);
+  expect(output).toBeDefined();
+
+  // check that the lock is released
+  const lock = await new RWLock(output).acquireWrite();
+  await lock.release();
+});
+
 function writeOutputAssembly() {
   const asm = testAssembly({
     stacks: [],