fix(core): verify stored section heads to fix flaky chain indexer reorg, #19748 #20506

[gzliudan]

Proposed changes

fix ci error: https://github.com/XinFinOrg/XDPoSChain/actions/runs/34191522129/job/101950397917?pr=2545

--- FAIL: TestChainIndexerWithChildren (4.29s)
    chain_indexer_test.go:155: Canonical section count mismatch: have 79, want 78
FAIL

Ref:

• core: fix chain indexer reorg bug ethereum/go-ethereum#19748
• core: fix indexer unit test ethereum/go-ethereum#20506
• chain_indexer_test failed ethereum/go-ethereum#20497

Changes:

• newHead: on reorg, revert from section (head+1)/sectionSize since the common ancestor head itself is still valid
• updateLoop: verify the last stored section head against the canonical chain before processing and after a failed processing, rolling back stale sections so the indexer can never get stuck with an invalid stored section (the root cause of the flaky 'Canonical section count mismatch: have 79, want 78' failure in TestChainIndexerWithChildren)
• Sections(): report an up-to-date section count via verifyLastHead
• updateLoop: guard SectionHead underflow with section == 0
• chain_indexer_test: sync reorg expectation helper with the new revert semantics

Types of changes

What types of changes does your code introduce to XDC network?
Put an ✅ in the boxes that apply

• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Changes that don't change source code or tests
• docs: Documentation only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that neither fixes a bug nor adds a feature
• revert: Revert something
• style: Changes that do not affect the meaning of the code
• test: Adding missing tests or correcting existing tests

Impacted Components

Which parts of the codebase does this PR touch?
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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

Checklist

Put an ✅ in the boxes once you have confirmed below actions (or provide reasons on not doing so) that

• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Tested on a private network from the genesis block and monitored the chain operating correctly for multiple epochs.
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A

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[Copilot]

🟢 Approval recommended

The reviewed changes address the reorg issue with aligned test expectations and no unresolved findings.

Pull request overview

Fixes flaky chain indexer reorg handling by detecting and rolling back stale section metadata.

Changes:

• Preserves sections ending at the common ancestor.
• Validates stored section heads against the canonical chain.
• Updates reorg test expectations.

File summaries

File	Description
core/chain_indexer.go	Corrects reorg boundaries and safely rolls back stale sections.
core/chain_indexer_test.go	Aligns expected section rollback behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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