feat(transaction): add Transaction::start_ts and scan_unbounded

[ariesdevil]

• start_ts exposes the transaction's TSO start timestamp, matching client-go's KVTxn.StartTS.

• scan_unbounded (on Transaction and Snapshot) returns all pairs in a range by paginating internally in batches of 1024, matching client-go's KVSnapshot.
  Iter semantics: a short page means the range is exhausted, a full page advances past the last key.

Summary by CodeRabbit

• New Features
  • Added unbounded range scanning for transactions and snapshots, automatically retrieving all matching records across multiple batches.
  • Added access to a transaction’s start timestamp.
• Tests
  • Expanded coverage for the new transaction timestamp accessor and unbounded scan pagination behavior.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign marsishandsome for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

Welcome @ariesdevil!

It looks like this is your first PR to tikv/client-rust 🎉.

I'm the bot to help you request reviewers, add labels and more, See available commands.

We want to make sure your contribution gets all the attention it needs!



Thank you, and welcome to tikv/client-rust. 😃

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 7e717407-3b1f-4568-822a-e05f9a42b13a

📥 Commits

Reviewing files that changed from the base of the PR and between dd640ed and 2bfa214.

📒 Files selected for processing (2)

• src/transaction/snapshot.rs
• src/transaction/transaction.rs

🚧 Files skipped from review as they are similar to previous changes (2)

• src/transaction/snapshot.rs
• src/transaction/transaction.rs

---

📝 Walkthrough

Walkthrough

Transaction exposes its start timestamp and adds paginated unbounded range scanning. Snapshot delegates the new scan API, while tests verify timestamp retrieval, batch sizing, pagination, call counts, ordering, and values.

Changes

Transaction scanning

Layer / File(s)	Summary
Transaction scan implementation src/transaction/transaction.rs	Adds start_ts() and paginated scan_unbounded() using fixed-size scan batches and successive keys.
Snapshot surface and validation src/transaction/snapshot.rs, src/transaction/transaction.rs	Adds the Snapshot delegation wrapper and tests for timestamp access and multi-page scan results.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Snapshot
  participant Transaction
  participant PdClient
  Snapshot->>Transaction: scan_unbounded(range)
  Transaction->>PdClient: scan(start, end, batch size)
  PdClient-->>Transaction: page of KvPair results
  Transaction->>PdClient: scan(next key, end, batch size)
  PdClient-->>Transaction: final page of KvPair results
  Transaction-->>Snapshot: all KvPair results

Loading

🚥 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 changes: adding Transaction::start_ts and scan_unbounded.
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 unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/transaction/transaction.rs`:
- Around line 122-127: Update the documentation for the transaction start
timestamp accessor to describe start_ts solely as the transaction’s read
timestamp. Remove claims that writes become visible at this timestamp or that
its physical component is suitable for commit, lease, or fencing semantics;
clarify that visibility is determined by the commit timestamp.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: e1b0c858-08d6-4555-82b4-3d9b87eec3e9

📥 Commits

Reviewing files that changed from the base of the PR and between 2c0f2f3 and dd640ed.

📒 Files selected for processing (2)

• src/transaction/snapshot.rs
• src/transaction/transaction.rs