chore(cli): bootstrap stack downgrading is no longer an error (#18378)

Previously, bootstrap downgrading is an error. That is, when the
bootstrap stack is currently version '9', and you're trying to run `cdk
bootstrap` with version '8', the command will currently fail.

This makes it hard to script around, and in fact this is causing us
problems in the pipeline where we unconditionally run `cdk bootstrap`
and an accidentally upgraded bootstrap stack fails all tests.

This is happening for no good reason, since the bootstrap stacks are
backwards compatible.

Turn this case from an error into a successful no-op.


----

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

Author: rix0rrr

packages/aws-cdk/lib/api/bootstrap/deploy-bootstrap.ts

@@ -3,6 +3,7 @@ import * as path from 'path';
 import * as cxschema from '@aws-cdk/cloud-assembly-schema';
 import * as cxapi from '@aws-cdk/cx-api';
 import * as fs from 'fs-extra';
+import * as logging from '../../logging';
 import { Mode, SdkProvider, ISDK } from '../aws-auth';
 import { deployStack, DeployStackResult } from '../deploy-stack';
 import { DEFAULT_TOOLKIT_STACK_NAME, ToolkitInfo } from '../toolkit-info';
@@ -65,7 +66,18 @@ export class BootstrapStack {
 
     const newVersion = bootstrapVersionFromTemplate(template);
     if (this.currentToolkitInfo.found && newVersion < this.currentToolkitInfo.version && !options.force) {
-      throw new Error(`Not downgrading existing bootstrap stack from version '${this.currentToolkitInfo.version}' to version '${newVersion}'. Use --force to force or set the '@aws-cdk/core:newStyleStackSynthesis' feature flag in cdk.json to use the latest bootstrap version.`);
+      logging.warning(`Bootstrap stack already at version '${this.currentToolkitInfo.version}'. Not downgrading it to version '${newVersion}' (use --force if you intend to downgrade)`);
+      if (newVersion === 0) {
+        // A downgrade with 0 as target version means we probably have a new-style bootstrap in the account,
+        // and an old-style bootstrap as current target, which means the user probably forgot to put this flag in.
+        logging.warning('(Did you set the \'@aws-cdk/core:newStyleStackSynthesis\' feature flag in cdk.json?)');
+      }
+
+      return {
+        noOp: true,
+        outputs: {},
+        stackArn: this.currentToolkitInfo.bootstrapStack.stackId,
+      };
     }
 
     const outdir = await fs.mkdtemp(path.join(os.tmpdir(), 'cdk-bootstrap'));

packages/aws-cdk/lib/api/deploy-stack.ts

@@ -28,7 +28,6 @@ export interface DeployStackResult {
   readonly noOp: boolean;
   readonly outputs: { [name: string]: string };
   readonly stackArn: string;
-  readonly stackArtifact: cxapi.CloudFormationStackArtifact;
 }
 
 export interface DeployStackOptions {
@@ -241,7 +240,6 @@ export async function deployStack(options: DeployStackOptions): Promise<DeploySt
       noOp: true,
       outputs: cloudFormationStack.outputs,
       stackArn: cloudFormationStack.stackId,
-      stackArtifact,
     };
   } else {
     debug(`${deployName}: deploying...`);
@@ -330,7 +328,7 @@ async function prepareAndExecuteChangeSet(
       debug('Deleting empty change set %s', changeSet.Id);
       await cfn.deleteChangeSet({ StackName: deployName, ChangeSetName: changeSetName }).promise();
     }
-    return { noOp: true, outputs: cloudFormationStack.outputs, stackArn: changeSet.StackId!, stackArtifact };
+    return { noOp: true, outputs: cloudFormationStack.outputs, stackArn: changeSet.StackId! };
   }
 
   const execute = options.execute === undefined ? true : options.execute;
@@ -367,7 +365,7 @@ async function prepareAndExecuteChangeSet(
     print('Changeset %s created and waiting in review for manual execution (--no-execute)', changeSet.Id);
   }
 
-  return { noOp: false, outputs: cloudFormationStack.outputs, stackArn: changeSet.StackId!, stackArtifact };
+  return { noOp: false, outputs: cloudFormationStack.outputs, stackArn: changeSet.StackId! };
 }
 
 /**

packages/aws-cdk/lib/api/hotswap-deployments.ts

@@ -55,7 +55,7 @@ export async function tryHotswapDeployment(
   // apply the short-circuitable changes
   await applyAllHotswappableChanges(sdk, hotswappableChanges);
 
-  return { noOp: hotswappableChanges.length === 0, stackArn: cloudFormationStack.stackId, outputs: cloudFormationStack.outputs, stackArtifact };
+  return { noOp: hotswappableChanges.length === 0, stackArn: cloudFormationStack.stackId, outputs: cloudFormationStack.outputs };
 }
 
 async function findAllHotswappableChanges(

packages/aws-cdk/test/api/bootstrap2.test.ts

@@ -172,8 +172,7 @@ describe('Bootstrapping v2', () => {
       parameters: {
         cloudFormationExecutionPolicies: ['arn:policy'],
       },
-    }))
-      .rejects.toThrow('Not downgrading existing bootstrap stack');
+    })).resolves.toEqual(expect.objectContaining({ noOp: true }));
   });
 
   test('bootstrap template has the right exports', async () => {