Workers errors: document cross-request promise cancellation#29797
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Adds a new 'Cause 3' subsection under the 'script will never generate a response' error explaining cross-request promise cancellation — a common pitfall when using promise-based lazy initialization (e.g. OnceCell or getOrInit patterns) in global scope. - Describes the exact warning message logged to the console - Explains the root cause: Promises are owned by the request context that created them; awaiting a Promise from a different request context causes cancellation when that context ends - Shows a common problematic pattern: promise-based OnceCell/getOrInit in global scope where concurrent requests await each other's Promises - Shows the correct fix: synchronous check-then-set pattern where each request initializes independently without cross-request awaiting - Notes the no_handle_cross_request_promise_resolution compatibility flag as a suppression-only workaround (not a real fix)
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Hey there, we've marked this pull request as stale because there's no recent activity on it. This label helps us identify PRs that might need updates (or to be closed out by our team if no longer relevant). |
Oxyjun
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Hey there, we've closed out this pull request because it's been stale for a while and there's been no additional action on it. If these changes are still relevant, open a new pull request (or flag to us in a GitHub issue). |
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Summary
OnceCell.getOrInit()) in global scope. Includes the exact warning message, a problematic code example, and the recommended check-then-set fix pattern.Background
The cross-request promise cancellation behavior was discovered when diagnosing 500 errors in a Rust-based Worker that used
tokio::sync::OnceCell::get_or_try_initfor lazy initialization of a certificate pool. Concurrent cold-start requests would fail because request B awaited a Promise created by request A; when A's context ended, B's continuation was cancelled. The exact runtime warning is:This scenario is not currently covered in the errors docs. The existing "Cause 1: Unresolved Promises" section covers a different case (a Promise that is never resolved), and "Cannot perform I/O on behalf of a different request" covers sharing I/O objects — but neither addresses cross-request Promise awaiting.