Carry a failure's stderr on the perform.hot_cell event

[flavorjones]

The HotCell (…) line each application logs from the perform.hot_cell event reads "cause":"crashed" with no diagnosis. The cell sends the worker's stderr on the wire and Failure#to_s renders it into the exception's message, but only the Retrying … line logs that message, and only when a retry follows. A failure that was discarded lost the one line that explained it, libgomp: Thread creation failed: Resource temporarily unavailable, and reading it meant querying the cell's own Loki stream. Post-mortem: card 10261975276; this is the "Log the cell's stderr and signal Rails-side" action item, card 10270096231.

Client#publish now sets event[:stderr] = failure&.stderr beside signal. The field is already bounded by Failure.sanitize on the way in, tail-kept, so a hostile cell cannot grow it past MAX_MESSAGE_BYTES. It is text a tool wrote while processing a hostile file, and the comment on publish says a subscriber should write it to a log field and interpolate it nowhere else.

The test is a crashed failure with stderr publishing an event that carries it, beside the existing cause and signal cases; the success case asserts the field is nil. Changelog under HotCell::Client / Added. VERSION is already 0.3.2.dev, so no bump.

The application half is separate: once this ships, Yabeda::HotCell.log_perform in bc3, haystack and fizzy logs signal and stderr on the HotCell (…) line.

[Copilot]

🟢 Approval recommended

The change is small, consistent with existing Failure sanitization/bounding behavior, and is covered by targeted tests and changelog documentation.

Pull request overview

This PR improves observability of HotCell failures by including the cell/worker’s captured stderr on the perform.hot_cell event payload, so subscribers can log the crash diagnosis even when a failure is discarded (i.e., not retried) and its exception message isn’t otherwise emitted.

Changes:

• Add event[:stderr] = failure&.stderr to HotCell::Client#publish alongside existing code/cause/signal/permanent fields.
• Extend classification tests to assert stderr is present for a crashed failure and nil on success.
• Document the new event field in CHANGELOG.md under HotCell::Client / Added.

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File summaries

File	Description
hotcell-client/lib/hot_cell/client.rb	Publishes stderr on perform.hot_cell events so subscribers can log crash diagnostics.
hotcell-client/test/classification_test.rb	Adds assertions covering stderr propagation for crash failures and nil on success.
CHANGELOG.md	Documents the addition of stderr on the perform.hot_cell event.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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