fix(eth/downloader): don't mistake a lagging head for a stalling peer

[gzliudan]

Proposed changes

Importing the post-pivot blocks moves the header head back to the block being inserted, so CurrentHeader can trail the headers the peer already delivered. When processHeaders reached the terminating batch inside that window, the fast sync TD check saw a head below the promised total difficulty and returned errStallingPeer, which rolled the whole chain back and dropped the only peer. This is what made TestFastSyncDisabling flaky under load: once the peer was dropped, BestPeer stayed nil and the sync loop could never recover.

processHeaders now tracks the highest header written to the light chain in this cycle (lastInserted) and uses it when it is ahead of CurrentHeader. Attack detection is unchanged: a peer that bails out of delivering the post-pivot headers never advances it, so TestHighTDStarvationAttack* still returns errStallingPeer.

The stalling-peer checks are also guarded against a missing local TD: GetTd returns nil when the TD is absent from the database (legacy XDPoS chaindata), which td.Cmp(nil) would turn into a panic. Both checks now skip the comparison and report the occurrence via a new eth/downloader/headers/missingtd counter metric, warning once per downloader lifetime.

Compatibility: no consensus, protocol, or database schema changes; this is a sync-logic fix only and needs no migration or operator coordination.

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?
Put an ✅ in the boxes that apply

• 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

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 80e3107b-d705-482e-95f4-3169c1b5970c

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

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.}

[Copilot]

Pull request overview

Fixes false errStallingPeer detection during fast sync when block imports temporarily move the current header backward.

Changes:

• Tracks the highest successfully inserted header for the current sync cycle.
• Uses that header for the terminating TD check when CurrentHeader lags.
• Adds regression tests across supported protocol versions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
eth/downloader/downloader.go	Corrects the fast/light sync stalling-peer check.
eth/downloader/downloader_test.go	Simulates header-head lag and verifies successful fast sync.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

Suppressed comments (1)

eth/downloader/downloader_test.go:1159

• This test cannot reliably prove that its own two sync cycles increment the counter while it runs in parallel: increments from the other eight cases can satisfy delta >= 2 even if this case never records an occurrence. Run these metric-delta tests serially (or use an instance-local hook/counter) so regressions are not masked.

	t.Parallel()