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FastSync Timeout Investigation

Note: This document is a historical record. The investigation found that the test is stable and passing.

Date: November 15, 2025
Issue: FastSyncSpec test timing
Status: ✅ Resolved - Test is passing consistently

Summary

Investigation found the test is stable and passing consistently: - Local runs: 3/3 successful (completed in ~1.8 seconds each) - CI run: PASSED (completed in 1.8 seconds)

The test uses proper sequencing with IO for-comprehensions, ensuring each step completes before the next. The storage layer uses synchronous commits, and the test fixture correctly shares storage instances.

Background

According to VM-007, after fixing the noEmptyAccounts configuration bug (PR #421), one test remained failing:

  • Test: FastSyncSpec - "returns Syncing with block progress once both header and body is fetched"
  • Error: TimeoutException after 30 seconds
  • Root Cause (suspected): "Parent chain weight not found for block 1" causing peer blacklisting loop
  • Classification: Pre-existing async/timing issue unrelated to EIP-161 configuration

Investigation Process

1. Test Execution

  • Local runs: 3/3 successful (completed in ~1.8 seconds each)
  • CI run #1902 (develop): PASSED (completed in 1.8 seconds)
  • Conclusion: Test is stable and passing

2. Code Analysis

How the Error Would Occur

The error would happen in this sequence:

  1. FastSync.handleBlockHeaders() processes incoming block headers
  2. For each header, it validates and looks up parent chain weight: 
    def getParentChainWeight(header: BlockHeader) =
  blockchainReader.getChainWeightByHash(header.parentHash)
    .toRight(ParentChainWeightNotFound(header))
  3. If parent not found → blacklist peer and rewind
  4. If all peers blacklisted → timeout waiting for sync progress

Test Setup

for {
  _ <- saveGenesis                    // Save block 0 with ChainWeight=1
  _ <- startSync                      // Start FastSync actor
  _ <- etcPeerManager.onPeersConnected  // Wait for peers
  _ <- etcPeerManager.pivotBlockSelected.head.compile.lastOrError  // Pivot selected
  blocksBatch <- etcPeerManager.fetchedBlocks.head.compile.lastOrError  // Blocks fetched
  status <- getSyncStatus             // Get sync status
  ...

The test properly sequences operations using Cats Effect IO, ensuring genesis is saved before sync starts.

3. Potential Race Conditions

Theoretically, a race could occur if: - Genesis save didn't commit fully before FastSync queried it - Storage initialization had ordering issues
- Peer manager sent blocks faster than genesis could be stored

However, the code uses .commit() which is synchronous, and the test uses proper sequencing with for-comprehension.

Findings

Why the Test is Passing

  1. Proper Sequencing: The test uses for-comprehension with IO, ensuring each step completes before the next
  2. Synchronous Storage: blockchainWriter.save() calls .commit() which is synchronous
  3. Shared Storage: Test fixture uses same storage instance for reader/writer
  4. No Recent Changes: No code changes to FastSync or test since PR #421

Why It Might Have Failed Before

The failure was likely: 1. Extremely rare race condition that cannot be easily reproduced 2. Already fixed inadvertently by other changes (possibly in test framework or dependencies) 3. Environmental factors in specific CI runs (CPU timing, resource contention)

Conclusion

The test is currently stable and passing. Investigation found no code issues requiring fixes.

Recommendations

  1. Close Issue: Mark as "Cannot Reproduce" with note to monitor
  2. CI Monitoring: Watch for future failures in this test
  3. If Recurs: Add logging around genesis save and parent weight lookups
  4. Future Enhancement: Consider adding explicit synchronization barrier after genesis save if failures recur

Test Execution Evidence

Local Test Run #1:
[info]   - returns Syncing with block progress once both header and body is fetched (1 second, 847 milliseconds)
[info] All tests passed.

Local Test Run #2:
[info]   - returns Syncing with block progress once both header and body is fetched (1 second, 664 milliseconds)
[info] All tests passed.

Local Test Run #3:
[info]   - returns Syncing with block progress once both header and body is fetched (1 second, 772 milliseconds)
[info] All tests passed.
  • VM-007: docs/adr/vm/VM-007-eip-161-noemptyaccounts-fix.md
  • PR #421: Fix noEmptyAccounts EVM config and update test fixtures
  • FastSync Test: src/test/scala/com/chipprbots/ethereum/blockchain/sync/FastSyncSpec.scala
  • FastSync Implementation: src/main/scala/com/chipprbots/ethereum/blockchain/sync/fast/FastSync.scala