🐛 fix: Guard engine helper registration and template adapter races

[Copilot]

• Inspect the reported mustache CI lint failure and the affected file
• Make the smallest code change to resolve the mustache revive warning
• Run targeted mustache lint/test validation
• Run final validation for the updated PR changes
• Reply to the PR comment with the addressing commit

[gaby]

@copilot The amber template has a race condition during unit-tests.

[Copilot]

@copilot The amber template has a race condition during unit-tests.

Fixed in 50e1a17. The amber adapter now serializes access to the upstream package-global amber.FuncMap during Load, which removes the race between parallel amber tests. I re-ran cd /home/runner/work/template/template/amber && GOTOOLCHAIN=go1.25.0 go test ./... -race -count=1 -v and it passed.

[gaby]

@copilot race condition in django tests:

=== NAME Test_HelperOutputIsEscaped
testing.go:1617: race detected during execution of test
--- FAIL: Test_HelperOutputIsEscaped (0.00s)
=== NAME Test_XSS_DefaultAutoEscape
testing.go:1617: race detected during execution of test
--- FAIL: Test_XSS_DefaultAutoEscape (0.00s)
=== NAME Test_Layout_DoesNotTrustUserProvidedEmbed
testing.go:1617: race detected during execution of test

[Copilot]

@copilot race condition in django tests:

=== NAME Test_HelperOutputIsEscaped
...

Fixed in 2dca1a5. The django adapter now stops mutating pongo2’s global autoescape setting during Load and instead serializes render-time autoescape swaps around template execution, which removes the race across parallel tests. I re-ran cd /home/runner/work/template/template/django && GOTOOLCHAIN=go1.25.0 go test ./... -race -count=1 -v and it passed.

[gaby]

@copilot Running [/home/runner/go/bin/golangci-lint run --path-mode=abs --tests=false --timeout=5m] in [/home/runner/work/template/template/mustache] ...
Error: /home/runner/work/template/template/mustache/mustache.go:114:17: import-shadowing: The name 'path' shadows an import name (revive)
walkFn := func(path string, info os.FileInfo, err error) error {
^
1 issues:

• revive: 1

[Copilot]

@copilot Running [/home/runner/go/bin/golangci-lint run --path-mode=abs --tests=false --timeout=5m] in [/home/runner/work/template/template/mustache] ...
Error: /home/runner/work/template/templat...

Fixed in 6d5b760. I renamed the local walkFn parameter in mustache/mustache.go so it no longer shadows the imported path package, and the CI-matching lint command now passes: cd /home/runner/work/template/template/mustache && GOTOOLCHAIN=go1.25.0 go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.6.1 run --path-mode=abs --tests=false --timeout=5m.

[Copilot]

Pull request overview

This PR hardens the core template Engine and several adapters against races, and adds security-focused regression tests and documentation notes around escaping/trust boundaries across engines.

Changes:

• Initialize Engine.Funcmap on-demand in AddFunc / AddFuncMap to support zero-value Engine usage safely.
• Serialize access to upstream package-global state in adapters (Django/pongo2 autoescape; Amber compiler func map) and add Mustache partial-name sanitization + traversal tests.
• Add per-engine XSS/regression tests and document security guidance in engine READMEs.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
template.go	Initializes Funcmap lazily during helper registration under lock.
template_test.go	Adds regression tests ensuring AddFunc/AddFuncMap initialize Funcmap.
ace/security_test.go	Adds XSS/layout escaping regression tests for Ace templates.
ace/README.md	Adds Security guidance and adjusts code snippet formatting.
amber/amber.go	Serializes global amber.FuncMap usage during compilation to avoid races.
amber/security_test.go	Adds XSS/layout escaping regression tests for Amber templates.
amber/README.md	Adds Security guidance and adjusts code snippet formatting.
django/django.go	Serializes pongo2 autoescape toggling at render time to prevent cross-engine races.
django/security_test.go	Adds XSS/layout/helper escaping regression tests for Django templates.
django/README.md	Adds Security guidance for auto-escape and safe output.
handlebars/security_test.go	Adds XSS/layout/helper escaping regression tests for Handlebars templates.
handlebars/README.md	Adds Security guidance and adjusts code snippet formatting.
html/security_test.go	Adds XSS/layout + contextual escaping regression tests for html/template adapter.
html/README.md	Adds Security guidance and adjusts code snippet formatting.
jet/security_test.go	Adds XSS/layout/helper escaping regression tests for Jet templates.
jet/README.md	Adds Security guidance and adjusts code snippet formatting.
mustache/mustache.go	Adds partial name sanitization and safer path/root handling for partial loading.
mustache/security_test.go	Adds XSS/layout escaping tests plus partial traversal tests.
mustache/README.md	Adds Security guidance and adjusts code snippet formatting.
pug/security_test.go	Adds XSS/layout escaping regression tests for Pug templates.
pug/README.md	Adds Security guidance and adjusts code snippet formatting.
slim/security_test.go	Adds XSS/layout/helper escaping regression tests for Slim templates.
slim/README.md	Adds Security guidance and adjusts code snippet formatting.
README.md	Removes stray whitespace in top-level README.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.