aws-s3-deployment: Source.data fails for empty content

[sean-xyz]

Describe the bug

if an empty string is given as the data parameter to Source.data, it yields an error.

populating the argument with a non-empty ("0", below) string succeeds:

new s3deploy.BucketDeployment(this, "some-object", {
	sources: [s3deploy.Source.data(`path/to/empty/object`, "0")],
	destinationBucket: bucket,
});

Regression Issue

• Select this option if this issue appears to be a regression.

Last Known Working CDK Library Version

0.2.211

Expected Behavior

successful deployment with a created s3 object

Current Behavior

deploy error:

TypeError: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined
    at Object.writeFileSync (node:fs:2434:5)
    at Object.bind (node_modules/aws-cdk-lib/aws-s3-deployment/lib/source.js:1:2576)
    at node_modules/aws-cdk-lib/aws-s3-deployment/lib/bucket-deployment.js:1:6564
    at Array.map (<anonymous>)
    at new BucketDeployment2 (node_modules/aws-cdk-lib/aws-s3-deployment/lib/bucket-deployment.js:1:6545)
    at new BucketDeployment2 (node_modules/aws-cdk-lib/core/lib/prop-injectable.js:1:681)
    at new BadStack (lib/stacks/bad.ts:43:3)
    at new App (bin/cdk.ts:122:21)
    at Object.<anonymous> (bin/cdk.ts:157:1)
    at Module._compile (node:internal/modules/cjs/loader:1706:14) {
  code: 'ERR_INVALID_ARG_TYPE'

Reproduction Steps

import * as cdk from "aws-cdk-lib";
import * as s3 from "aws-cdk-lib/aws-s3";
import * as s3deploy from "aws-cdk-lib/aws-s3-deployment";

export class BadStack extends cdk.Stack {
	constructor(scope: Construct, id: string, props: StackProps) {
		super(scope, id, props);

		const bucket = new s3.Bucket(this, "bucket", {
			bucketName: "my-bucket-name",
			blockPublicAccess: s3.BlockPublicAccess.BLOCK_ALL,
		});

		new s3deploy.BucketDeployment(this, "empty-object", {
			sources: [s3deploy.Source.data("path/to/empty", "")],
			destinationBucket: bucket,
		});
	}
}

Possible Solution

No response

Additional Information/Context

No response

AWS CDK Library version (aws-cdk-lib)

2.220.0

AWS CDK CLI version

2.1004.0 (build f0ad96e)

Node.js Version

v22.19.0

OS

nixos v25.11.20251019.5e2a59a

Language

TypeScript

Language Version

5.9.3

Other information

No response

[pahud]

Hi @sean-xyz,

Thank you for reporting this issue. I can reproduce this bug and it was initially introduced in v2.207.0

✅ Issue Confirmed - This is a REGRESSION

Version Testing Results:
• ✅ v2.200.0: WORKS
• ✅ v2.205.0: WORKS
• ✅ v2.206.0: WORKS (Last working version)
• ❌ v2.207.0: FAILS (Regression introduced)
• ❌ v2.210.0: FAILS
• ❌ v2.211.0: FAILS
• ❌ v2.220.0: FAILS (Current version)

Based on the analysis, it appears to be PR #34916 which fixed issue #22843 where Source.data() failed with cross-stack references. The problem was that the s3-deployment construct had its own token resolution implementation that didn't support cross-stack references.

Before PR #34916:
• Tokens were resolved immediately within the s3-deployment construct
• Non-tokens were handled directly without markers

After PR #34916:
• Tokens are passed through as-is to synthesis
• Token resolution happens during the synthesis stage
• This exposed an edge case in StringConcat.join() in the core library

Flow Diagram:

Source.data("path", "")
         ↓
    renderData("")
         ↓
    Token resolution during synthesis
         ↓
    StringConcat.join(undefined, undefined)
         ↓
    Returns: undefined (instead of "")
         ↓
    fs.writeFileSync(path, undefined)
         ↓
    TypeError: data must be string or Buffer

While PR #34916 successfully fixed cross-stack references by deferring token resolution to synthesis, it inadvertently exposed a bug in StringConcat.join() that was previously hidden. The edge case where both
parameters are undefined was not handled correctly - it should return an empty string "" but instead returns undefined.

The Fix Needed

The fix for issue #35809 should be in renderData() , we need to handle empty string case explicitly to avoid StringConcat.join(undefined, undefined).

This ensures that when both parameters are undefined (as happens with empty string content), the method returns an empty string instead of undefined, which is the correct behavior for string concatenation.

I will submit an immediate PR to get it fixed.