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fix(sync): Avoid missed long-poll wake-ups #2068

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eleboucher wants to merge 1 commit from eleboucher/continuwuity:fix/sliding-sync-reliability into main
pull from: eleboucher/continuwuity:fix/sliding-sync-reliability
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eleboucher commented 2026-07-28 12:55:14 +00:00
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Avoid missed long-poll wake-ups in /sync and simplified sliding sync.

Previously, a request collected its response before registering its wake waiter. A room update arriving in that gap could be missed, leaving the client waiting until its timeout even though new data was already available. Simplified sliding sync also returned the empty response built before the wake, requiring another client request before the update appeared.

This change subscribes to a per-user generation watch before collecting data. Wakes are retained and coalesced, so updates during collection are observed when the request reaches its wait point. Both sync endpoints re-collect once after a wake; sliding sync returns the newly collected response directly.

Sliding sync delays known-room updates and to-device acknowledgement until the response being returned is selected. A discarded first collection therefore cannot advance connection state or consume to-device events.

This is lighter than the previous database-watcher approach: each request has one per-user receiver, wake-ups coalesce, and a request performs at most two collections. There is no polling or retry loop.

Pull request checklist:

  • This pull request targets the main branch, and the branch is named something other than
    main.
  • I have written an appropriate pull request title and my description is clear.
  • I understand I am responsible for the contents of this pull request.
  • I have followed the contributing guidelines:
<!-- In order to help reviewers know what your pull request does at a glance, you should ensure that 1. Your PR title is a short, single sentence describing what you changed 2. You have described in more detail what you have changed, why you have changed it, what the intended effect is, and why you think this will be beneficial to the project. If you have made any potentially strange/questionable design choices, but didn't feel they'd benefit from code comments, please don't mention them here - after opening your pull request, go to "files changed", and click on the "+" symbol in the line number gutter, and attach comments to the lines that you think would benefit from some clarification. --> Avoid missed long-poll wake-ups in `/sync` and simplified sliding sync. Previously, a request collected its response before registering its wake waiter. A room update arriving in that gap could be missed, leaving the client waiting until its timeout even though new data was already available. Simplified sliding sync also returned the empty response built before the wake, requiring another client request before the update appeared. This change subscribes to a per-user generation watch before collecting data. Wakes are retained and coalesced, so updates during collection are observed when the request reaches its wait point. Both sync endpoints re-collect once after a wake; sliding sync returns the newly collected response directly. Sliding sync delays known-room updates and to-device acknowledgement until the response being returned is selected. A discarded first collection therefore cannot advance connection state or consume to-device events. This is lighter than the previous database-watcher approach: each request has one per-user receiver, wake-ups coalesce, and a request performs at most two collections. There is no polling or retry loop. <!-- Example: This pull request allows us to warp through time and space ten times faster than before by double-inverting the warp drive with hyperheated jump fluid, both making the drive faster and more efficient. This resolves the common issue where we have to wait more than 10 milliseconds to engage, use, and disengage the warp drive when travelling between galaxies. --> <!-- Closes: #... --> <!-- Fixes: #... --> <!-- Uncomment the above line(s) if your pull request fixes an issue or closes another pull request by superseding it. Replace `#...` with the issue/pr number, such as `#123`. --> **Pull request checklist:** <!-- You need to complete these before your PR can be considered. If you aren't sure about some, feel free to ask for clarification in #dev:continuwuity.org. --> - [x] This pull request targets the `main` branch, and the branch is named something other than `main`. - [x] I have written an appropriate pull request title and my description is clear. - [x] I understand I am responsible for the contents of this pull request. - I have followed the [contributing guidelines][c1]: - [x] My contribution follows the [code style][c2], if applicable. - [x] I ran [pre-commit checks][c1pc] before opening/drafting this pull request. - [x] I have [tested my contribution][c1t] (or proof-read it for documentation-only changes) myself, if applicable. This includes ensuring code compiles. - [x] My commit messages follow the [commit message format][c1cm] and are descriptive. <!-- Notes on these requirements: - While not required, we encourage you to sign your commits with GPG or SSH to attest the authenticity of your changes. - While we allow LLM-assisted contributions, we do not appreciate contributions that are low quality, which is typical of machine-generated contributions that have not had a lot of love and care from a human. Please do not open a PR if all you have done is asked ChatGPT to tidy up the codebase with a +-100,000 diff. - In the case of code style violations, reviewers may leave review comments/change requests indicating what the ideal change would look like. 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Your branch will be rebased on main before it is merged if it has fallen behind. - We typically only do fast-forward merges, so your entire commit log will be included. Once in main, it's difficult to get out cleanly, so put on your best dress, smile for the cameras! --> [c1]: https://forgejo.ellis.link/continuwuation/continuwuity/src/branch/main/CONTRIBUTING.md [c2]: https://forgejo.ellis.link/continuwuation/continuwuity/src/branch/main/docs/development/code_style.mdx [c1pc]: https://forgejo.ellis.link/continuwuation/continuwuity/src/branch/main/CONTRIBUTING.md#pre-commit-checks [c1t]: https://forgejo.ellis.link/continuwuation/continuwuity/src/branch/main/CONTRIBUTING.md#running-tests-locally [c1cm]: https://forgejo.ellis.link/continuwuation/continuwuity/src/branch/main/CONTRIBUTING.md#commit-messages
fix(sync): Avoid missed long-poll wake-ups
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