Only update attachSerial for non-resumed attaches (RTL15c)

[SimonWoolf]

Implements ably/specification#503 (spec 6.1.1).

attachSerial is used as the fromSerial bound for untilAttach history, which returns history contiguous with the realtime message stream. A resumed ATTACHED (resumed=true) indicates a brief disconnection that did not break message continuity, so it must not move the point up to which untilAttach history is contiguous.

Previously the SDK updated attachSerial on every ATTACHED regardless of the RESUMED flag; now it only does so when resumed is false (RTL15a → RTL15c).

Changes

• realtimechannel.ts: guard the attachSerial update on !resumed in the ATTACHED handler. channelSerial still updates unconditionally per RTL15b.
• UTS ports (test/uts/.../channel_properties.test.ts): renamed the two RTL15a tests to RTL15c, and added a new test asserting attachSerial is left unchanged by a resumed ATTACHED (while channelSerial still updates, confirming the message was processed).

attachSerial is not part of the public ably.d.ts (experimental/unstable per RTL15), so there's no public docstring to update.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Fixed channel attachment handling so resumed connections preserve the existing attachment serial.
  • Ensured channel serials continue updating correctly after resumed attachments.

• Tests

  • Added coverage for resumed and non-resumed attachment scenarios to verify serial behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a563b7f2-72a5-425c-b7db-89d3cd922982

📥 Commits

Reviewing files that changed from the base of the PR and between 004e99c and 1d9a372.

📒 Files selected for processing (2)

• src/common/lib/client/realtimechannel.ts
• test/uts/realtime/unit/channels/channel_properties.test.ts

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Walkthrough

ATTACHED handling now preserves attachSerial for resumed attaches while continuing to update channelSerial. RTL15c tests cover non-resumed updates and resumed-message behavior.

Changes

Attach serial resume handling

Layer / File(s)	Summary
Conditional attachSerial update src/common/lib/client/realtimechannel.ts	processMessage() updates attachSerial only for non-resumed ATTACHED messages.
RTL15c attach serial coverage test/uts/realtime/unit/channels/channel_properties.test.ts	Updates existing attach serial scenarios and adds coverage confirming resumed attaches update channelSerial without changing attachSerial.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• ably/ably-js#2273: Adjusts related realtime channel attach and resume handling.

Suggested reviewers: ttypic, paddybyers, sacoo7

Poem

I’m a bunny guarding serials tight,
Resumed attaches hop through the night.
channelSerial moves with cheer,
attachSerial stays right here.
RTL15c makes the behavior clear!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: attachSerial is now updated only for non-resumed attaches in RTL15c.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch only-update-attachserial-for-non-resumed-attaches

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