fix: reduce db read ops in chain export

[hanabi1224]

Summary of changes

(Originally part of #5867)
This PR makes some small refactoring to stream_export and unordered_stream_export to reduce db read operations.

I see ~5% perf gain in calibnet snapshot export on my laptop

Changes introduced in this pull request:

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation. All new code adheres to the team's documentation standards,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

Summary by CodeRabbit

Summary by CodeRabbit

• New Features
  • Improved block streaming performance by carrying block data alongside CIDs, reducing redundant data retrieval.
• Refactor
  • Updated internal handling of block identifiers and data to optimize processing and retrieval.
  • Enhanced block header serialization and CID computation for improved consistency.
• Bug Fixes
  • Enhanced error handling for missing block data during chain traversal.

[coderabbitai]

Walkthrough

The changes introduce a new method for serializing and computing CIDs for block headers, replacing direct CID computation with a process that returns both the CID and serialized bytes. Additionally, chain traversal logic is refactored to propagate optional block data alongside CIDs, optimizing data retrieval by using available block data when possible and minimizing unnecessary database fetches.

Changes

Cohort / File(s)	Change Summary
Block Header Serialization & CID Computation src/blocks/header.rs	Refactored RawBlockHeader to add car_block() for CBOR serialization and CID computation using Blake2b256. Updated cid() to use car_block(). Adjusted import statements.
Chain Traversal with Block Data Propagation src/ipld/util.rs	Changed Task::Emit to carry both Cid and optional block data. Updated traversal queues in ChainStream and UnorderedChainStream to handle (Cid, Option<Vec<u8>>) tuples. Refactored logic to use block data when available, reducing redundant database fetches. Extracted ipld_to_cid.

Sequence Diagram(s)

sequenceDiagram
participant Client
participant RawBlockHeader
participant Multihash
participant CID
Client->>RawBlockHeader: car_block()
RawBlockHeader->>RawBlockHeader: serialize to CBOR (Vec<u8>)
RawBlockHeader->>Multihash: blake2b256(serialized bytes)
Multihash-->>RawBlockHeader: multihash digest
RawBlockHeader->>CID: create CID(multihash, codec)
CID-->>RawBlockHeader: CID
RawBlockHeader-->>Client: (CID, serialized bytes)

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sequenceDiagram
participant ChainStream
participant Queue
participant DB
ChainStream->>Queue: pop (Cid, Option<Vec<u8>>)
alt Data present
Queue-->>ChainStream: (Cid, Some(data))
ChainStream-->>Client: emit (Cid, data)
else Data missing
Queue-->>ChainStream: (Cid, None)
ChainStream->>DB: fetch data for Cid
alt Data found
DB-->>ChainStream: data
ChainStream-->>Client: emit (Cid, data)
else Data not found
DB-->>ChainStream: error
ChainStream-->>Client: error (if fail_on_dead_links)
end
end

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Reviewing files that changed from the base of the PR and between 7bf39d8 and 42b9081.

📒 Files selected for processing (2)

• src/blocks/header.rs (2 hunks)
• src/ipld/util.rs (11 hunks)

🚧 Files skipped from review as they are similar to previous changes (2)

• src/blocks/header.rs
• src/ipld/util.rs

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Actionable comments posted: 1

📜 Review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 19c11c8 and 3108e1b.

📒 Files selected for processing (2)

• src/blocks/header.rs (2 hunks)
• src/ipld/util.rs (11 hunks)

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🔇 Additional comments (7)

src/blocks/header.rs (2)

11-22: Clean import reorganization!

The imports are well-organized and the addition of multihash utilities is appropriate for the new car_block() functionality.

---

103-110: Excellent optimization for reducing DB reads!

The car_block() method efficiently returns both the CID and serialized bytes, which aligns perfectly with the PR objective of reducing database read operations during chain export.

src/ipld/util.rs (5)

105-105: Smart optimization for Task enum!

Adding optional block data to the Emit variant enables reusing already-serialized data, avoiding redundant database lookups.

---

190-201: Well-implemented data retrieval optimization!

The logic correctly prioritizes using provided block data before falling back to database fetches, maintaining the same error handling behavior.

---

291-291: Consistent queue type update!

The queue type change to Vec<(Cid, Option<Vec<u8>>)> properly supports the optimization across both stream implementations.

---

468-491: Consistent optimization in unordered stream!

The implementation properly handles optional block data, mirroring the optimization in ChainStream while maintaining appropriate error handling.

---

559-565: Good refactoring!

Extracting the ipld_to_cid function improves code readability and eliminates duplication.

[LesnyRumcajs]

Do you have some metrics for mainnet?

[hanabi1224]

@LesnyRumcajs no, mainnet snap export is slow on my laptop. I can measure with a DO droplet if you prefer.

[LesnyRumcajs]

Yes, please measure it. Let's have at least five runs before the change and five runs after, optimally from exactly the same tipset.

[hanabi1224]

Yes, please measure it. Let's have at least five runs before the change and five runs after, optimally from exactly the same tipset.

@LesnyRumcajs Instead of using DO droplet with shared CPU, I tested on my laptop with time forest-cli snapshot export --dry-run -d 900 -t 5184000, I see ~2% perf gain on mainnet.

# PR branch
./forest_snapshot_mainnet_2025-07-29_height_5184000.forest.car.zst: 0B (0B/s)
Export completed.
real 28m58.045s
user 0m0.278s
sys 0m0.285s
./forest_snapshot_mainnet_2025-07-29_height_5184000.forest.car.zst: 0B (0B/s)
Export completed.
real 27m57.873s
user 0m0.284s
sys 0m0.222s
./forest_snapshot_mainnet_2025-07-29_height_5184000.forest.car.zst: 0B (0B/s)
Export completed.
real 29m19.508s
user 0m0.190s
sys 0m0.390s
# main branch
./forest_snapshot_mainnet_2025-07-29_height_5184000.forest.car.zst: 0B (0B/s)
Export completed.
real 29m53.178s
user 0m0.332s
sys 0m0.279s
./forest_snapshot_mainnet_2025-07-29_height_5184000.forest.car.zst: 0B (0B/s)
Export completed.
real 28m49.859s
user 0m0.321s
sys 0m0.260s
./forest_snapshot_mainnet_2025-07-29_height_5184000.forest.car.zst: 0B (0B/s)
Export completed.
real 29m13.298s
user 0m0.474s
sys 0m0.286s