refactor(cli): stack/security diff no longer prints directly to a stream

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

@@ -16,7 +16,7 @@ import { AssetManifestBuilder } from './asset-manifest-builder';
 import type { Deployments } from './deployments';
 import { ToolkitError } from '../../../../@aws-cdk/tmp-toolkit-helpers/src/api';
 import type { IoHelper } from '../../../../@aws-cdk/tmp-toolkit-helpers/src/api/io/private';
-import { debug } from '../../cli/messages';
+import { debug, info } from '../../cli/messages';
 import { formatErrorMessage, deserializeStructure } from '../../util';
 import type { ICloudFormationClient, SdkProvider } from '../aws-auth';
 import { StackStatus } from '../stack-events';
@@ -330,7 +330,6 @@ export type PrepareChangeSetOptions = {
   uuid: string;
   willExecute: boolean;
   sdkProvider: SdkProvider;
-  stream: NodeJS.WritableStream;
   parameters: { [name: string]: string | undefined };
   resourcesToImport?: ResourcesToImport;
 }
@@ -413,9 +412,9 @@ async function uploadBodyParameterAndCreateChangeSet(
     const exists = (await CloudFormationStack.lookup(cfn, options.stack.stackName, false)).exists;
 
     const executionRoleArn = await env.replacePlaceholders(options.stack.cloudFormationExecutionRoleArn);
-    options.stream.write(
+    ioHelper.notify(info(
       'Hold on while we create a read-only change set to get a diff with accurate replacement information (use --no-change-set to use a less accurate but faster template-only diff)\n',
-    );
+    ));
 
     return await createChangeSet(ioHelper, {
       cfn,
@@ -431,9 +430,9 @@ async function uploadBodyParameterAndCreateChangeSet(
     });
   } catch (e: any) {
     await ioHelper.notify(debug(e));
-    options.stream.write(
+    ioHelper.notify(info(
       'Could not create a change set, will base the diff on template differences (run again with -v to see the reason)\n',
-    );
+    ));
 
     return undefined;
   }

packages/aws-cdk/lib/cli/cdk-toolkit.ts

@@ -177,7 +177,6 @@ export class CdkToolkit {
 
     const strict = !!options.strict;
     const contextLines = options.contextLines || 3;
-    const stream = options.stream || process.stderr;
     const quiet = options.quiet || false;
 
     let diffs = 0;
@@ -196,9 +195,14 @@ export class CdkToolkit {
       }
 
       const template = deserializeStructure(await fs.readFile(options.templatePath, { encoding: 'UTF-8' }));
-      diffs = options.securityOnly
-        ? numberFromBool(printSecurityDiff(template, stacks.firstStack, RequireApproval.Broadening, quiet))
-        : printStackDiff(template, stacks.firstStack, strict, contextLines, quiet, undefined, undefined, false, stream);
+
+      if (options.securityOnly) {
+        diffs = numberFromBool(printSecurityDiff(template, stacks.firstStack, RequireApproval.Broadening, quiet));
+      } else {
+        const { stackDiffCount, printableStackDiff } = printStackDiff(template, stacks.firstStack, strict, contextLines, quiet, undefined, undefined, false);
+        diffs = stackDiffCount;
+        info(printableStackDiff);
+      }
     } else {
       // Compare N stacks against deployed templates
       for (const stack of stacks.stackArtifacts) {
@@ -231,7 +235,7 @@ export class CdkToolkit {
           } catch (e: any) {
             debug(formatErrorMessage(e));
             if (!quiet) {
-              stream.write(
+              info(
                 `Checking if the stack ${stack.stackName} exists before creating the changeset has failed, will base the diff on template differences (run again with -v to see the reason)\n`,
               );
             }
@@ -247,7 +251,6 @@ export class CdkToolkit {
               sdkProvider: this.props.sdkProvider,
               parameters: Object.assign({}, parameterMap['*'], parameterMap[stack.stackName]),
               resourcesToImport,
-              stream,
             });
           } else {
             debug(
@@ -256,8 +259,8 @@ export class CdkToolkit {
           }
         }
 
-        const stackCount = options.securityOnly
-          ? numberFromBool(
+        if (options.securityOnly) {
+          diffs += numberFromBool(
             printSecurityDiff(
               currentTemplate,
               stack,
@@ -266,8 +269,9 @@ export class CdkToolkit {
               stack.displayName,
               changeSet,
             ),
-          )
-          : printStackDiff(
+          );
+        } else {
+          const { stackDiffCount, printableStackDiff } = printStackDiff(
             currentTemplate,
             stack,
             strict,
@@ -276,15 +280,15 @@ export class CdkToolkit {
             stack.displayName,
             changeSet,
             !!resourcesToImport,
-            stream,
             nestedStacks,
           );
-
-        diffs += stackCount;
+          info(printableStackDiff);
+          diffs += stackDiffCount;
+        }
       }
     }
 
-    stream.write(format('\n✨  Number of stacks with differences: %s\n', diffs));
+    info(format('\n✨  Number of stacks with differences: %s\n', diffs));
 
     return diffs && options.fail ? 1 : 0;
   }
@@ -1360,13 +1364,6 @@ export interface DiffOptions {
    */
   contextLines?: number;
 
-  /**
-   * Where to write the default
-   *
-   * @default stderr
-   */
-  stream?: NodeJS.WritableStream;
-
   /**
    * Whether to fail with exit code 1 in case of diff
    *

packages/aws-cdk/lib/cli/cli.ts

@@ -274,7 +274,6 @@ export async function exec(args: string[], synthesizer?: Synthesizer): Promise<n
           contextLines: args.contextLines,
           securityOnly: args.securityOnly,
           fail: args.fail != null ? args.fail : !enableDiffNoFail,
-          stream: args.ci ? process.stdout : undefined,
           compareAgainstProcessedTemplate: args.processed,
           quiet: args.quiet,
           changeSet: args['change-set'],

packages/aws-cdk/lib/commands/diff.ts

@@ -14,6 +14,25 @@ import * as chalk from 'chalk';
 import { ToolkitError } from '../../../@aws-cdk/tmp-toolkit-helpers/src/api';
 import { type NestedStackTemplates } from '../api/deployments';
 import { info, warning } from '../logging';
+import { Writable } from 'stream';
+
+// Custom writable stream that collects text into a string buffer
+class StringWriteStream extends Writable {
+  private buffer: string[] = [];
+
+  constructor() {
+    super();
+  }
+
+  _write(chunk: any, _encoding: string, callback: (error?: Error | null) => void): void {
+    this.buffer.push(chunk.toString());
+    callback();
+  }
+
+  toString(): string {
+    return this.buffer.join('');
+  }
+}
 
 /**
  * Pretty-prints the differences between two template states to the console.
@@ -35,10 +54,11 @@ export function printStackDiff(
   stackName?: string,
   changeSet?: DescribeChangeSetOutput,
   isImport?: boolean,
-  stream: FormatStream = process.stderr,
-  nestedStackTemplates?: { [nestedStackLogicalId: string]: NestedStackTemplates }): number {
+  nestedStackTemplates?: { [nestedStackLogicalId: string]: NestedStackTemplates }): { stackDiffCount: number, printableStackDiff: string } {
   let diff = fullDiff(oldTemplate, newTemplate.template, changeSet, isImport);
 
+  const stream = new StringWriteStream();
+
   // must output the stack name if there are differences, even if quiet
   if (stackName && (!quiet || !diff.isEmpty)) {
     stream.write(format('Stack %s\n', chalk.bold(stackName)));
@@ -66,12 +86,18 @@ export function printStackDiff(
   }
 
   let stackDiffCount = 0;
+  let printableStackDiff = '';
   if (!diff.isEmpty) {
     stackDiffCount++;
+
+    // formatDifferences updates the stream with the formatted stack diff
     formatDifferences(stream, diff, {
       ...logicalIdMapFromTemplate(oldTemplate),
       ...buildLogicalToPathMap(newTemplate),
     }, context);
+
+    // store the stream containing a formatted stack diff
+    printableStackDiff = stream.toString();
   } else if (!quiet) {
     info(chalk.green('There were no differences'));
   }
@@ -86,7 +112,7 @@ export function printStackDiff(
     const nestedStack = nestedStackTemplates[nestedStackLogicalId];
 
     (newTemplate as any)._template = nestedStack.generatedTemplate;
-    stackDiffCount += printStackDiff(
+    const { stackDiffCount: newSackDiffCount, printableStackDiff: newPrintableStackDiff } = printStackDiff(
       nestedStack.deployedTemplate,
       newTemplate,
       strict,
@@ -95,12 +121,13 @@ export function printStackDiff(
       nestedStack.physicalName ?? nestedStackLogicalId,
       undefined,
       isImport,
-      stream,
       nestedStack.nestedStackTemplates,
     );
+    stackDiffCount += newSackDiffCount;
+    printableStackDiff += newPrintableStackDiff;
   }
 
-  return stackDiffCount;
+  return { stackDiffCount, printableStackDiff };
 }
 
 export enum RequireApproval {