Canvas pages log a session error when cron indexes them

Problem/Motivation

#3624787 moved search indexing from on-save to cron, which removed about 200 install-time errors. It introduced a smaller one in their place: when cron indexes a Canvas page, rendering it tries to start a session and fails.

RuntimeException occurred during rendering of component c1d3d26c-982d-5656-9b36-4d09c996af08
in Page Home (9), field components: Failed to start the session because headers have already
been sent by "/var/www/html/vendor/symfony/http-foundation/Response.php" at line 388.

25 of these on a fresh install, on both bases.

What is not affected:

• Indexing completes. Drupal CMS 57/57, Varbase 11 59/59.
• Search returns results on both.
• Every page serves 200, including the welcome dashboard, with no cache rebuild.

Measured on Drupal CMS and Varbase 11, RightUp 1.0.x at 27b1d7b, 21 September 2026.

Steps to reproduce

1. Build a fresh site on drupal/cms or on drupal/varbase_project.
2. Install through the browser installer, picking the RightUp site template.
3. Read the log after the install finishes.

Proposed resolution

Not determined. Likely the render context used when indexing a canvas_page in a cron/CLI request, rather than anything in the recipe's own configuration. Worth checking whether this reproduces with Canvas pages outside RightUp before assuming it belongs here.

Remaining tasks

• ✅ File an issue
• ❌ Confirm whether this is RightUp-specific or a Canvas/Search API interaction
• ❌ Decide whether it belongs upstream
• ❌ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A