fix(raw): scan reverse ranges across regions by lance6716 · Pull Request #579 · tikv/client-rust

lance6716 · 2026-10-29T08:44:24Z

Fixes #512

Reverse raw scans selected the Region using the lower bound even though TiKV receives the upper bound as the request start for reverse scans. Cross-Region ranges therefore failed with KeyNotInRegion.

This change starts reverse scans from the Region containing the upper bound, uses PD's GetPrevRegion at Region boundaries, and continues toward lower keys while preserving the range bounds. It also adds a mock regression test for cross-Region scans and upper-bound boundary cases.

Tests:

CARGO_NET_OFFLINE=true cargo test --lib
CARGO_NET_OFFLINE=true cargo test --all --no-run
CARGO_NET_OFFLINE=true cargo clippy --all-targets --all-features -- -D warnings

LLM model: gpt-6.1-sol
Thinking level: xhigh
Client: unknown; codex-cli 0.161.0

Summary by CodeRabbit

New Features
- Reverse range scans can traverse multiple regions while respecting the requested start and end bounds.
- Reverse scans support an unbounded upper limit and can locate preceding regions when continuing across region boundaries.
- Scans can fall back to boundary-by-boundary region lookup when the usual region scan is unavailable.
Bug Fixes
- Scan results honor the requested limit and stop at the start of the requested range; a zero limit returns no results.
- Reverse scans refresh outdated region information and retry at region boundaries to help avoid missed results.

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Walkthrough

Reverse scans now traverse preceding regions at region boundaries and support unbounded upper bounds. The PD client path adds previous-region and last-region lookup. Tests cover multi-region scans, zero limits, stale boundaries, and epoch mismatch recovery.

Changes: Reverse scan traversal

Layer / File(s)	Summary
Previous-region lookup path `src/pd/client.rs`, `src/pd/cluster.rs`, `src/pd/retry.rs`, `src/region_cache.rs`, `src/mock.rs`, `src/request/mod.rs`, `src/transaction/transaction/scanner.rs`	The PD client path adds previous-region lookup through the cluster RPC, retry client, and region cache. The cache adds paginated last-region lookup and preserves a cached leader when the returned region has the same version. The mock client and test clients implement predecessor lookup.
Reverse scan traversal `src/raw/client.rs`	Reverse scans track the original range bounds and current cursor. At region boundaries, reverse scans look up the preceding region and refresh stale region boundaries. Reverse scans support unbounded upper bounds. Tests cover multi-region scans, zero limits, stale boundaries, and epoch mismatch recovery.

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant RawClient
  participant RegionCache
  participant PD
  participant RegionStore
  RawClient->>RegionCache: resolve scan region
  RegionCache->>PD: fetch preceding region when needed
  PD-->>RegionCache: return region
  RegionCache-->>RawClient: return region
  RawClient->>RegionStore: send reverse scan request
  RegionStore-->>RawClient: return keys and region boundary
  RawClient->>RegionCache: resolve preceding region at boundary

Possibly related PRs

tikv/client-rust#572: Adds a C API that invokes the RawKV reverse-scan implementation changed here.

Merge Risk: Low · up to a1415

Reverse scans remain functional on PD versions without batch-region scans, but they incur avoidable lookup requests. This bounded cost does not block merging.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 10.53% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: fixing reverse raw scans that traverse multiple regions.
Linked Issues check		Issue [`#512`] requires reverse raw scans to find keys when the range spans multiple Regions. The whole-PR diff of src/raw/client.rs was read from the merge base to the reviewed head. For reverse bounde…
Out of Scope Changes check		The changes support reverse traversal across Region boundaries, which is the requirement of [`#512`]. Supporting changes in the reviewed diff are: the reverse Region-selection and boundary-retry logic i…

Full details: Linked Issues check

Explanation

Issue [#512] requires reverse raw scans to find keys when the range spans multiple Regions. The whole-PR diff of src/raw/client.rs was read from the merge base to the reviewed head. For reverse bounded scans, scan_inner starts at the upper bound (current_key = range_end) and resolves the Region containing that key. When the cursor equals a Region start, it calls prev_region_for_key and accepts the result only when its end key equals the cursor. The request is then bounded by [max(region_start, range_start), cursor), and the next cursor is the Region start. The loop stops at range_start. This matches the PR description and the mock test expectations in the diff (for example, range 1..255 with limit 2 issues requests (255, 250,250) and (250,250, 10)). Stale predecessor results invalidate the cache and retry, up to MAX_REVERSE_SCAN_BOUNDARY_RETRIES (10). Unbounded reverse scans locate the final Region first. Limitations: the src/region_cache.rs text diff was unavailable (changed blobs missing from checkout), so get_prev_region_by_key and the PD batch-scan path are confirmed only from the change summary. The truncated portion of the test module was not read. The test commands in the PR description were not re-run.

Full details: Out of Scope Changes check

Explanation

The changes support reverse traversal across Region boundaries, which is the requirement of [#512]. Supporting changes in the reviewed diff are: the reverse Region-selection and boundary-retry logic in src/raw/client.rs; the prev_region_for_key and last_region_for_key trait methods and their stub implementations in test clients (src/request/mod.rs, src/transaction/transaction/scanner.rs); and the mock hooks in src/mock.rs. Other supporting changes are described in the summary only: PD previous-Region lookup and batch-region scan in src/pd/cluster.rs and src/pd/retry.rs, and the predecessor lookup and paginated final-Region scan in src/region_cache.rs. Unbounded reverse scans (empty upper bound) and their documentation update go beyond the stated PR description. They are not unrelated, because they use the same reverse Region-selection path to cross Region boundaries. No change is shown to be unrelated to reverse scans. The src/region_cache.rs diff could not be read, so its content relies on the summary.

Full details: Docstring Coverage

Explanation

Docstring coverage is 10.53% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 19 functions across 8 files.

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

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Signed-off-by: tianyuan <tianyuan@pingcap.cn>

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🟡 Changes recommended

Reverse scans can miss keys or fail due to mishandled unbounded bounds and stale region boundaries, while repeated PD lookups can overwrite corrected leaders and trigger retries without backoff.

3 open issues

Treat empty key as unbounded upper bound · New
Invalidate stale cached region after merge · New
Preserve corrected leader across reverse-scan retries · New

What changed in this PR

This PR addresses #512 by making bounded raw reverse scans traverse regions backward, with previous-region lookup support across the PD client and cache layers.

Changes:

Start bounded reverse scans at the upper bound and continue through previous regions.
Add previous-region lookup plumbing through PD and the region cache.
Extend mocks and add regression coverage for cross-region scans and boundary-aligned upper bounds.

File	Description
`src/region_cache.rs`	Fetch and cache previous-region metadata from PD.
`src/raw/client.rs`	Update reverse-scan traversal and add regression tests.
`src/pd/retry.rs`	Add retry support for previous-region requests.
`src/pd/cluster.rs`	Send previous-region requests to PD.
`src/pd/client.rs`	Expose previous-region lookup with key encoding support.
`src/mock.rs`	Implement previous-region lookup for tests.

_{Review effort: Balanced}

-        let (start_key, end_key) = range.clone().into_keys();
-        let mut current_key: Key = start_key;
+        let (range_start, range_end) = range.into_keys();
+        let reverse_unbounded = reverse && range_end.is_none();


+            let region = if scan_args.reverse && !scan_args.reverse_unbounded {
+                let region = self.rpc.clone().region_for_key(&current_key).await?;
+                if !current_key.is_empty() && region.start_key() == current_key {
+                    self.rpc.clone().prev_region_for_key(&current_key).await?


+            .clone()
+            .get_prev_region(key.into())
+            .await?;
+        self.add_region(region.clone()).await;


Signed-off-by: tianyuan <tianyuan@pingcap.cn>

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Review comments at @src/raw/client.rs:
- Around line 775-778: Update the reverse_unbounded handling in the scan flow so
an open-ended reverse scan without a keyspace starts at the final region and
traverses preceding regions toward the lower bound, rather than treating the
selected region’s end key as the open upper bound. Preserve the requested limit
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Inline comments:
Review comments at @src/raw/client.rs:
- Line 848: Update the unbounded scan flow around `region_for_key` to locate the
final Region without resolving every intervening Region; use a cheaper
final-Region lookup supported by the PD or region-cache API, while preserving
the scan behavior.
- Around line 920-922: Update the reverse-scan handling of region_override so it
is retained only for a known-leader correction with valid Region metadata; when
handle_region_error invalidates the Region without current_regions, do not
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Inline comments:
Review comments at @src/region_cache.rs:
- Line 109: Update the predecessor lookup filter in the region-cache
read-through path so a cached region without a leader is treated as a miss and
can be refreshed from PD; preserve the existing end-key match for regions with a
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Minor · Return immediately when limit is zero. · client.rs:776-789

src/raw/client.rs:776-789
Minor · Stability & Availability · Quick win

Return immediately when limit is zero.

scan_reverse accepts zero, but unbounded reverse scans resolve the final region before checking current_limit. This can traverse regions and return a PD error instead of the empty result returned by the base implementation.

Suggested fix

         if limit > MAX_RAW_KV_SCAN_LIMIT {
             return Err(Error::MaxScanLimitExceeded {
                 limit,
                 max_limit: MAX_RAW_KV_SCAN_LIMIT,
             });
         }
+        if limit == 0 {
+            return Ok(Vec::new());
+        }
         let backoff = DEFAULT_STORE_BACKOFF;

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Review comment at @src/raw/client.rs around lines 776 - 789:
Update scan_reverse to return an empty result immediately when limit is zero,
before resolving regions or performing scan setup, while preserving the existing
maximum-limit validation.

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Review comments at @src/raw/client.rs:
- Around line 776-789: Update scan_reverse to return an empty result immediately
when limit is zero, before resolving regions or performing scan setup, while
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Make the new predecessor-region methods required, matching the existing internal trait API and avoiding an inconsistent default error implementation. Signed-off-by: tianyuan <tianyuan@pingcap.cn>

Keep cache read-through behavior inside RegionCache and share region RPC request handling between current and predecessor lookups. Signed-off-by: tianyuan <tianyuan@pingcap.cn>

Signed-off-by: tianyuan <tianyuan@pingcap.cn>

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Minor · Bound retries for stale reverse-scan boundaries. · client.rs:878-884

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Outside diff comments:
Review comments at @src/raw/client.rs:
- Around line 878-884: Bound retries in retryable_scan and last_region_for_key
when a resolved predecessor’s end_key() does not match current_key; return an
error once the retry limit is reached. A backoff may be added between retries to
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Review comments at @src/raw/client.rs:
- Around line 925-942: Update the immediate retry path in the scan loop after
handle_region_error returns true for reverse_unbounded scans: consume the
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accounting before continuing, including when current_regions is empty. Preserve
the separate bounds in the predecessor-boundary and final-region walkers.

Review comments at @src/region_cache.rs:
- Around line 168-173: Update read_through_last_region_by_key to bound
batch_scan_regions requests using its supported limit parameter, then continue
scanning from the last returned region’s end_key until a region has an empty
end_key. Retain only the state needed to return the final region rather than
accumulating and processing every region at once.

---

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Review comments at @src/raw/client.rs:
- Around line 521-524: Update the unbounded upper-bound documentation near
`last_region_for_key` to describe the normal single PD batch scan through
`read_through_last_region_by_key` and clarify that walking Region boundaries
from the lower bound is only the fallback when PD returns `Unimplemented`.

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- Around line 288-290: Update the batch_scan_regions retry path so
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fix(raw): scan reverse ranges across regions

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fix(raw): handle reverse scan cache refreshes

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Comment thread src/raw/client.rs Resolved

fix(raw): traverse final region for unbounded reverse scans

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Comment thread src/raw/client.rs Resolved

Comment thread src/raw/client.rs Resolved

refactor(pd): reuse cached predecessor regions

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tianyuan added 2 commits October 31, 2026 20:44

refactor(pd): require predecessor lookup implementations

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Make the new predecessor-region methods required, matching the existing internal trait API and avoiding an inconsistent default error implementation. Signed-off-by: tianyuan <tianyuan@pingcap.cn>

refactor(pd): centralize predecessor region lookup

b4ff3d7

Keep cache read-through behavior inside RegionCache and share region RPC request handling between current and predecessor lookups. Signed-off-by: tianyuan <tianyuan@pingcap.cn>

lance6716 force-pushed the lance6716/fix-issue-512 branch from a54c020 to 4d01693 Compare October 31, 2026 12:50

fix(raw): reload invalidated reverse scan regions

afae5f8

Signed-off-by: tianyuan <tianyuan@pingcap.cn>

lance6716 force-pushed the lance6716/fix-issue-512 branch from 4d01693 to afae5f8 Compare October 31, 2026 12:51

coderabbitai Bot reviewed Oct 31, 2026

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fix(raw): bound reverse scan boundary retries

4daca63

Signed-off-by: tianyuan <tianyuan@pingcap.cn>

fix(raw): batch lookup final reverse scan region

46130fa

Signed-off-by: tianyuan <tianyuan@pingcap.cn>

fix(pd): preserve final region leader from cache

edfc599

Signed-off-by: tianyuan <tianyuan@pingcap.cn>

coderabbitai Bot requested changes Nov 2, 2026

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Comment thread src/raw/client.rs Resolved

Comment thread src/region_cache.rs Resolved

fix(raw): bound region lookup pages and scan retries

a1415bb

Signed-off-by: tianyuan <tianyuan@pingcap.cn>

coderabbitai Bot reviewed Nov 2, 2026

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coderabbitai Bot approved these changes Nov 2, 2026

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