Invalid YAML Anchor Syntax in transform.Remarshal

Hugo Bug Report: Invalid YAML Anchor Syntax in transform.Remarshal

Summary

transform.Remarshal "yaml" generates invalid YAML syntax when marshalling data structures with repeated empty arrays. It creates self-referencing anchors (options: &options *options) which violates YAML specification and causes parse errors.

Affected Versions

• Broken: Hugo v0.155.0, v0.155.1, v0.155.2, v0.155.3, v0.156.0, v0.157.0
• Works: Hugo v0.147.7 and earlier

The bug appears to have been introduced between v0.147.7 and v0.155.0.

Impact

Any Hugo site using transform.Remarshal "yaml" to generate YAML files will produce invalid YAML that cannot be parsed by standard YAML parsers (yq, Python PyYAML, Go yaml.v3, etc.).

Description

When Hugo's transform.Remarshal "yaml" encounters repeated empty arrays in a data structure, it attempts to optimize by creating YAML anchors. However, it generates syntactically invalid YAML:

Invalid output (Hugo v0.155.0+):

slug:
  options: &options *options  # ❌ INVALID: Can't define and reference on same line
  pattern: /^[a-z0-9]+$/

Valid output (Hugo v0.147.7):

slug:
  options: []  # ✓ VALID: Simple empty array
  pattern: /^[a-z0-9]+$/

Correct anchor syntax would be:

slug:
  options: &options []  # Define anchor with value
  pattern: /^[a-z0-9]+$/

title:
  options: *options     # Reference anchor elsewhere
  pattern: /^.+$/

Minimal Reproducible Example

1. Create Hugo site

hugo new site yaml-bug-test
cd yaml-bug-test

2. Create go.mod

module example.com/yaml-bug-test

go 1.18

3. Create config.yml

title: YAML Bug Test

outputFormats:
  TestSchema:
    baseName: test-schema
    mediaType: text/yaml
    isPlainText: true

outputs:
  home:
    - TestSchema

params:
  testData:
    field1:
      options: []
      pattern: /^test1$/
    field2:
      options: []
      pattern: /^test2$/
    field3:
      options: []
      pattern: /^test3$/

4. Create layouts/index.testschema.yaml

{{ .Site.Params.testData | transform.Remarshal "yaml" -}}

5. Build and verify

hugo --gc

# Check for invalid anchors
grep "options:.*\*" public/test-schema.yaml

# Attempt to parse (will fail with v0.155.0+)
yq eval public/test-schema.yaml

Expected output (Hugo v0.147.7):

field1:
  options: []
  pattern: /^test1$/
field2:
  options: []
  pattern: /^test2$/
field3:
  options: []
  pattern: /^test3$/

Actual output (Hugo v0.155.0+):

field1:
  options: &options []
  pattern: /^test1$/
field2:
  options: *options *options    # ❌ INVALID SYNTAX
  pattern: /^test2$/
field3:
  options: *options *options    # ❌ INVALID SYNTAX
  pattern: /^test3$/

Error When Parsing

$ yq eval public/test-schema.yaml
Error: bad file 'public/test-schema.yaml': yaml: line X: could not find expected ':'

Root Cause Analysis

The bug appears to be in Hugo's YAML marshaller when it attempts to create anchors for repeated values. The logic incorrectly generates:

key: &anchor *anchor

Instead of the correct two-step process:

1. Define anchor: key: &anchor value
2. Reference anchor elsewhere: key: *anchor

Workarounds

Temporary Fix 1: Downgrade Hugo

Use Hugo v0.147.7 or earlier:

# In CI/CD workflows
hugo-version: "0.147.7"

Temporary Fix 2: Use JSON + yq

Avoid Hugo's YAML marshaller entirely:

Change template from:

{{ $data | transform.Remarshal "yaml" -}}

To:

{{ $data | jsonify -}}

Then convert JSON to YAML externally:

hugo --gc
yq eval -P public/schema.json > public/schema.yaml

Environment

• OS: macOS (ARM64), Linux (AMD64) - affects all platforms
• Hugo Installation: Binary, Docker (hugomods/hugo), all installation methods
• Go Version: Not relevant (Hugo binary issue)

Additional Context

This bug breaks any workflow that:

1. Uses transform.Remarshal "yaml" to generate YAML configuration files
2. Has data structures with repeated empty arrays or slices
3. Needs to process the generated YAML with external tools (yq, Ansible, Kubernetes, etc.)

The bug appears to be in Hugo's underlying YAML library or its usage of it. The self-referencing anchor syntax &anchor *anchor has never been valid YAML according to the YAML 1.2 specification.

Reproduction Repository

[Link to minimal reproduction repo if you create one]

Related Issues

[Search results from Hugo issues if any similar issues exist]

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This bug makes transform.Remarshal "yaml" unusable for production YAML generation in Hugo v0.155.0+