cloudformation package produces incorrect YAML line folding in long strings starting from 2.34.13

[rkamach-smtc]

Describe the bug

aws cloudformation package incorrectly serializes long YAML strings inside Fn::Sub
starting from version 2.34.13, dropping backslash escapes at line breaks and inserting
unexpected spaces into the rendered string.

Regression Issue

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

Expected Behavior

Long YAML double-quoted strings inside Fn::Sub are serialized with correct backslash
escapes at line breaks (2.34.12 output):

- "...\n  {{- $queryExpr\
          \ := \"\" }}\n  {{- $queryExprRaw := \"\" }}\n..."

Current Behavior

Starting from 2.34.13, backslash escapes at line breaks are dropped (2.34.16 output):

- "...\n  {{- $queryExpr
          := \"\" }}\n  {{- $queryExprRaw := \"\" }}\n..."

In YAML double-quoted strings, a \ at end of line means "ignore the newline and leading
whitespace on the next line". Without it, the newline folds to a space, inserting unexpected
spaces into the rendered string and corrupting the content.

In our case this caused AWS Managed Prometheus to reject the Alertmanager config with invalid syntax.

Reproduction Steps

1. Create dummy-cf.yaml:

AWSTemplateFormatVersion: '2010-09-09'
Parameters:
  EnvName:
    Type: String
    Default: test
Resources:
  TestResource:
    Type: AWS::SSM::Parameter
    Properties:
      Name: !Sub "${EnvName}-test"
      Type: String
      Value:
        Fn::Sub:
          - |
            template: |
              {{ define "test.message" }}
              {{- $a := index .Alerts 0 -}}
              {{- $envName := printf "%s" "${EnvName}" -}}
              {{- $queryExprClean := "" }}
              {{- $queryExprTemplated := reReplaceAll "\\]" "%5D" (
                  reReplaceAll "\\[" "%5B" (
                  reReplaceAll "=" "%3D" (
                  reReplaceAll "\\}" "%7D" (
                  reReplaceAll "\\{" "%7B" (
                  reReplaceAll ">" "%3E" (
                  reReplaceAll "<" "%3C" (
                  reReplaceAll " " "%20" (
                  reReplaceAll "\"" "%22" (
                  reReplaceAll "'" "%27" (
                    $queryExprClean
                  )))))))))) }}
              {{- $desc := printf "%s\nEnvironment: %s\nState Change Time: %s\nNext Steps:\n- Upstream URL: %s\n- %s" $descClean $envName ($a.StartsAt.Format "Mon, 02 Jan 2006 15:04:05 MST") $grafanaUrl $formattedSteps -}}
              {{ end }}
          - {}

2. Run:

docker run --rm -v $(pwd):/workspace amazon/aws-cli:2.34.12 \
  cloudformation package \
  --template-file /workspace/dummy-cf.yaml \
  --output-template-file /workspace/packaged-2.34.12.yml \
  --s3-bucket dummy-bucket

docker run --rm -v $(pwd):/workspace amazon/aws-cli:2.34.16 \
  cloudformation package \
  --template-file /workspace/dummy-cf.yaml \
  --output-template-file /workspace/packaged-2.34.16.yml \
  --s3-bucket dummy-bucket

diff packaged-2.34.12.yml packaged-2.34.16.yml

Possible Solution

The only non-API change in 2.34.13 was upgrading the bundled Python interpreter to 3.14.3.
All other changes in 2.34.13–2.34.16 are API-level and unrelated to YAML serialization, so this upgrade is the likely cause .

Additional Information/Context

No response

CLI version used

2.34.16

Environment details (OS name and version, etc.)

Reproduced using official amazon/aws-cli Docker images on macOS

[EndlessLucky]

@rkamach-smtc Can I work on this issue?

[hssyoo]

Thanks for raising this issue. In addition to upgrading the bundled interpreter to Python 3.14, AWS CLI 2.34.13 also upgraded its ruamel.yaml dependency from 0.17.21 to 0.19.1 in order to be able to build on Python 3.14. We isolated this regression to changes introduced in 0.17.23, and the changelog seems to confirm it:

[0.17.23, 2023-05-05]:

• • fix issue with emitter conservatively inserting extra backslashes in wrapped quoted strings (reported by thebenman on StackOverflow)

We're looking into a fix right now.

[rkamach-smtc]

Yes, it will be great!

@rkamach-smtc Can I work on this issue?

Yes, that would be great! Thank you for picking this up 🙏

[EndlessLucky]

@RyanFitzSimmonsAK I made a PR for this issue. Should I stop working on this issue? Who is the maintainer for this repo?

[ashovlin]

@EndlessLucky, please hold off on creating a PR for this one. It was caused by the recent internal work to upgrade to Python 3.14 with some dependency upgrades. We're working on a fix internally.

Generally, issues with contribution-ready label are open for contributors. See https://aws.github.io/aws-cli/contribution_process.html#github-issue-review-process for how we manage issues and the labels.