feat: Enable cel: template expressions by chmouel · Pull Request #2363 · tektoncd/pipelines-as-code

chmouel · 2025-12-18T11:13:58Z

📝 Description of the Change

This PR adds support for evaluating CEL expressions inside templates using a cel: prefix. It’s meant to make templates more expressive without forcing everything to be precomputed as PAC params: it allows doing simple conditionals, string ops, and presence checks right where the value is used.

It also includes a small but important follow-up fix found during review: CEL evaluation used to fail when there was no webhook payload body (rawEvent == nil). In that situation, cel: expressions would silently resolve to an empty string, which broke “pac-only” expressions (anything that only depends on pac.*, headers, or files). This PR makes CEL evaluation treat a missing body as an empty object so these expressions work consistently across all execution paths.

🚀 Feature: `cel:` expressions in templates

With the cel: prefix, let us evaluate arbitrary CEL expressions with access to:

body: webhook JSON payload (when available)
headers: request headers
files: changed-files data (when available)
pac: standard PAC parameters (pac.target_branch, pac.revision, …)
plus direct convenience variables (e.g. revision, target_branch, …) mapped from PAC params

What this unlocks in practice:

Branch/event-based selection:
- {{ cel: pac.target_branch == "main" ? "prod" : "dev" }}
Safe access to optional payload fields:
- {{ cel: has(body.pull_request) ? body.pull_request.number : "" }}
Readable, localized logic without adding extra params or custom glue code.

🐛 Fix: CEL works when there’s no webhook body

Previously, CEL evaluation marshaled the event body and unmarshaled it into a map for the body namespace. When rawEvent was nil (or effectively JSON null), JSON unmarshalling failed, and the templating layer would replace cel: results with an empty string.

Now, a nil/null body is treated as an empty object for CEL (body == {}), so expressions that only depend on PAC data (or headers/files) behave as expected, even when there is no incoming webhook payload.

Example that now works reliably in PAC-only contexts:

{{ cel: pac.revision }}
{{ cel: pac.target_branch == "main" ? "production" : "staging" }}

👨🏻‍ Linked Jira

https://issues.redhat.com/browse/SRVKP-8619

🔗 Linked GitHub Issue

🚀 Type of Change

🧪 Testing Strategy

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✅ Submitter Checklist

gemini-code-assist · 2025-12-18T11:14:20Z

Summary of Changes

Hello @chmouel, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the templating capabilities by integrating Common Expression Language (CEL) expressions. This allows users to embed complex logic directly into templates, making them more powerful and flexible for various use cases like conditional environment selection or safe access to optional fields. Additionally, it addresses a critical bug to ensure CEL expressions function reliably even when no webhook body is available, improving the overall robustness of the system.

Highlights

Feature: cel: prefix for templates: Introduces support for Common Expression Language (CEL) expressions directly within templates using a cel: prefix. This allows for more dynamic and expressive conditional logic, string operations, and presence checks, making templates more powerful without precomputing everything as PAC parameters.
Improved CEL evaluation for nil webhook bodies: Fixes an issue where CEL expressions would fail or silently resolve to an empty string when no webhook payload body (rawEvent == nil) was present. This ensures consistent and reliable behavior for PAC-only expressions that depend on pac.*, headers, or files.

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Code Review

This pull request introduces a powerful new feature, the cel: prefix for template expressions, which significantly enhances the expressiveness of templates. It also commendably includes a fix to ensure CEL evaluation works correctly even without a webhook payload. The implementation is robust, backed by a comprehensive suite of unit and end-to-end tests across multiple Git providers. The documentation has also been updated to reflect these new capabilities. I have a couple of minor suggestions to further refine the code and documentation.

docs/content/docs/guide/authoringprs.md

pkg/cel/cel.go

Copilot

Pull request overview

This PR enables the use of cel: prefix in templates for evaluating arbitrary CEL expressions with access to body, headers, files, and pac namespaces. It also includes a critical bug fix that prevents CEL evaluation failures when no webhook payload body is present (nil/null), allowing PAC-only expressions to work consistently.

Key changes:

Added cel: prefix support for inline CEL expressions in templates, enabling conditional logic, string operations, and safe field access without requiring precomputed PAC parameters
Fixed CEL evaluation to treat nil/null webhook bodies as empty objects instead of failing, enabling reliable evaluation of PAC-only expressions

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/cel/cel.go	Enhanced body handling to treat nil/null values as empty objects, enabling CEL evaluation without webhook payloads
pkg/templates/templating.go	Added `cel:` prefix support for arbitrary CEL expressions with access to pac namespace via dico parameter
pkg/cel/cel_test.go	Added test for pac-only queries with nil body
pkg/templates/templating_test.go	Added comprehensive unit tests covering cel: prefix functionality including ternary expressions, pac namespace access, has() function, files access, headers access, and error handling
test/testdata/pipelinerun-cel-prefix-test.yaml	Test pipeline for Gitea demonstrating cel: prefix with various expression types
test/testdata/pipelinerun-cel-prefix-gitlab.yaml	Test pipeline for GitLab demonstrating cel: prefix with GitLab-specific expressions
test/testdata/pipelinerun-cel-prefix-github.yaml	Test pipeline for GitHub demonstrating cel: prefix with GitHub-specific expressions
test/testdata/TestGiteaParamsCelPrefix.golden	Expected output for Gitea cel: prefix test
test/testdata/TestGitlabMergeRequestCelPrefix.golden	Expected output for GitLab cel: prefix test
test/testdata/TestGithubPullRequestCelPrefix.golden	Expected output for GitHub cel: prefix test
test/gitlab_merge_request_test.go	E2E test validating cel: prefix expressions in GitLab merge request context
test/github_pullrequest_test.go	E2E test validating cel: prefix expressions in GitHub pull request context
test/gitea_params_test.go	E2E test validating cel: prefix expressions in Gitea pull request context
docs/content/docs/guide/authoringprs.md	User documentation with examples of cel: prefix usage including conditionals, safe field access, file checks, and string concatenation

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test/gitlab_merge_request_test.go

pkg/cel/cel.go

pkg/templates/templating.go

test/github_pullrequest_test.go

CEL evaluation marshaled the event body then unmarshaled into a map. When body was nil (or marshaled to JSON null), json.Unmarshal failed with "cannot unmarshal null into map[string]interface{}". This caused cel: placeholders to be replaced with an empty string, breaking pac-only expressions in contexts without a webhook payload. Treat nil/null body as an empty object for CEL evaluation and add a unit test to cover pac-only expressions with a nil body. Signed-off-by: Chmouel Boudjnah <chmouel@redhat.com> Assisted-by: GPT-5.2 (via Cursor)

Introduced the `cel:` prefix to allow evaluating complex Common Expression Language (CEL) expressions within PipelineRun templates. Enabled access to the `body`, `headers`, `files`, or `pac` namespaces to facilitate advanced logic like ternary operations, safe field access with `has()`, or collection processing. Ensured expressions return an empty string upon evaluation or syntax errors to prevent pipeline disruptions. Added comprehensive documentation, unit tests, plus integration tests for various use cases. E2E Tests was added for Gitea, GitHub and GitLab to validate the feature. Jira: https://issues.redhat.com/browse/SRVKP-8619 Co-authored-by: Claude <noreply@anthropic.com> Signed-off-by: Chmouel Boudjnah <chmouel@redhat.com>

Copilot AI review requested due to automatic review settings December 18, 2025 11:13

pipelines-as-code bot added documentation Improvements or additions to documentation e2e labels Dec 18, 2025

Copilot started reviewing on behalf of chmouel December 18, 2025 11:14 View session

gemini-code-assist bot reviewed Dec 18, 2025

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chmouel changed the title ~~Enable cel: template expressions~~ feat: Enable cel: template expressions Dec 18, 2025

Copilot AI reviewed Dec 18, 2025

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zakisk reviewed Dec 22, 2025

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chmouel force-pushed the srvkp-8619-enable-cel-expression-evaluation branch from 7b420d1 to e11e4ae Compare December 23, 2025 13:56

chmouel and others added 2 commits December 23, 2025 21:19

chmouel force-pushed the srvkp-8619-enable-cel-expression-evaluation branch from e11e4ae to 9c12c8c Compare December 23, 2025 14:19

pipelines-as-code bot added feature New feature or request fix labels Dec 23, 2025

Merge branch 'main' into srvkp-8619-enable-cel-expression-evaluation

1a052c5

chmouel mentioned this pull request Jan 14, 2026

feat(github): add glob pattern support for token scope repos #2386

Merged

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theakshaypant approved these changes Jan 15, 2026

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zakisk approved these changes Jan 16, 2026

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chmouel merged commit 751f931 into tektoncd:main Jan 16, 2026
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chmouel deleted the srvkp-8619-enable-cel-expression-evaluation branch January 16, 2026 12:38

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feat: Enable cel: template expressions#2363

feat: Enable cel: template expressions#2363
chmouel merged 3 commits intotektoncd:mainfrom
chmouel:srvkp-8619-enable-cel-expression-evaluation

chmouel commented Dec 18, 2025 •

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chmouel commented Dec 18, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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📝 Description of the Change

🚀 Feature: cel: expressions in templates

🐛 Fix: CEL works when there’s no webhook body

👨🏻‍ Linked Jira

🔗 Linked GitHub Issue

🚀 Type of Change

🧪 Testing Strategy

🤖 AI Assistance

✅ Submitter Checklist

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gemini-code-assist bot commented Dec 18, 2025

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chmouel commented Dec 18, 2025 •

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🚀 Feature: `cel:` expressions in templates