Simplify chain switching

[jackzhhuang]

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Other information

Summary by CodeRabbit

• New Features

  • Services now react to DAG block events: on-DAG updates can update the on-chain state root and apply new DAG heads.

• Refactor

  • Centralized DAG block switching so the chain mutates its state in-place, streamlining state transitions and reducing redundant operations.

• Bug Fixes

  • Removed an extra head-switch step during block processing to prevent unnecessary state churn on startup.

• Tests

  • Updated tests to assert on the original chain object after state changes rather than on returned copies.

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📝 Walkthrough

Walkthrough

This PR centralizes DAG-driven chain switches by adding BlockChain::switch_to_block (in-place state reconstruction) and changing select_dag_state to mutate the chain (Result<()>). Services and callers were updated to call select_dag_state on a mutable main chain (added get_main_mut()), and DAG events now trigger state-root updates and optional head application.

Changes

Cohort / File(s)	Summary
Core DAG switching refactor chain/src/chain.rs	Added pub fn switch_to_block(block_id: HashValue) -> Result<()> to rebuild accumulators, VM/state DBs, epoch and status in-place. Changed pub fn select_dag_state(&mut self, header: &BlockHeader) to return Result<()> and delegate final switching to switch_to_block.
Service layer mutable access chain/service/src/chain_service.rs	Added pub fn get_main_mut(&mut self) -> &mut BlockChain. Replaced fork-and-assign logic for NewDagBlock handling with direct calls on get_main_mut() and select_dag_state.
State services subscribe to DAG events state/service/src/service.rs, vm2/service/state/src/service.rs	Subscribed ChainStateService to NewDagBlock; added handlers that extract the executed block’s multi-state root and call change_root(...) (uses state_root1 or state_root2 as appropriate).
Sync: apply new head from DAG & switch_header simplified sync/src/block_connector/write_block_chain.rs, sync/src/block_connector/block_connector_service.rs	Added apply_new_head_from_dag(executed_block: ExecutedBlock) -> Result<()>. switch_header now relies on select_dag_state side-effects and no longer captures/returns a new branch; block connector uses switch_header and, when headers match, calls apply_new_head_from_dag.
Tests updated for side-effects chain/tests/test_select_dag_state.rs	Updated tests to stop capturing a returned chain from select_dag_state; assertions now inspect the mutated test_chain instance.

Sequence Diagram(s)

sequenceDiagram
  participant DAG as "DAG / EventBus"
  participant SyncSvc as "Sync Block Connector\n(write_block_chain)"
  participant ChainReader as "ChainReaderService\n(get_main_mut)"
  participant BlockChain as "BlockChain\n(select_dag_state / switch_to_block)"
  participant ChainState as "ChainStateService"
  participant VMState as "VM / State DB"

  DAG->>SyncSvc: NewDagBlock(msg with executed_block)
  SyncSvc->>ChainReader: switch_header(executed_block.header())
  ChainReader->>BlockChain: get_main_mut().select_dag_state(header)
  Note over BlockChain: select_dag_state calls switch_to_block when needed
  BlockChain->>VMState: rebuild accumulators & state DBs (in-place)
  BlockChain-->>ChainReader: return (Result<()>)
  alt header matches current head
    ChainReader->>SyncSvc: apply_new_head_from_dag(executed_block)
    SyncSvc-->>ChainReader: result
  end
  BlockChain->>ChainState: emit/update state root
  ChainState-->>VMState: change_root(state_root)

Loading

Note: rectangles/styles use default participant styling; component interactions and in-place switching are shown.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• Block time target #4540 — Modifies DAG/state switching and NewDagBlock handling; touches select_dag_state/switching and event handlers.
• Parallelize the process of minting and executing a block #4648 — Related changes to BlockChain and DAG execution/control flow that overlap selection/switch logic.
• Pruning logic #4194 — Changes pruning-related DAG logic in chain/src/chain.rs, potentially affecting switch/pruning paths.

Suggested reviewers

• sanlee42
• welbon
• simonjiao
• nkysg

Poem

🐰 I hopped through DAGs and found a smoother track,
switch_to_block rebuilt the path, no need to hack.
get_main_mut gave me access, neat and clean,
NewDagBlock whispers, and states update unseen.
A tiny rabbit cheer for code now lean.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 45.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Simplify chain switching' accurately reflects the main objective of the changeset, which centralizes and refactors DAG switching logic across multiple files.

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✨ Finishing touches

• 📝 Generate docstrings

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve head selection side effects in switch_header

switch_header now only mutates self.main via select_dag_state and immediately updates startup info, but it no longer calls select_head/do_new_head. That removes the total-difficulty check and, more importantly, skips reorg handling (txpool updates, metrics, and NewHeadBlock/NewBranch broadcasts). When a NewDagBlock arrives (handled in BlockConnectorService), the chain will silently switch to the selected block without notifying subscribers or updating the txpool, leaving downstream components out of sync after a head change or reorg.

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