PR Review Club: Add mutation check inside compact block's FillBlock

[Prabhat1308]

Bitcoin Core PR Review Club

Session Details

• Date: June 05 , 2025
• Time: IST 19:00 (UTC 13:30)
• Title: p2p:Add mutation check inside compact block's FillBlock
• PR: p2p: Add witness mutation check inside FillBlock bitcoin/bitcoin#32646

PR Summary

This PR extends mutation checks originally introduced in PR #29412, which prevented immediate processing of mutated blocks received via the legacy BLOCK message. The new change applies the same protection to compact blocks during PartiallyDownloadedBlock::FillBlock, just before returning READ_STATUS_OK.

This acts as a belt-and-suspenders safeguard against potential mutation-based attacks during block relay by honest peers. It introduces a behavior change: if a compact block decodes successfully but fails mutation checks, the peer is now disconnected and discouraged rather than the block being silently discarded.

Prerequisites

p2p

Notes and Questions

1. What is CheckBlockFn and IsBlockMutatedFn ? How are they helpful ?
2. What Does DeploymentActiveAfter function checks and how is it important for systems like Bitcoin ?
3. What does FillBlock does ?

Preparation Steps

1. Clone the Bitcoin Core repository if you haven't already:

   git clone https://github.com/bitcoin/bitcoin.git
   

2. Fetch and checkout the PR:

   cd bitcoin
   git fetch origin pull/32646
   git checkout instagibbs:2025-05-fillblock-mutated
   

3. Build and run tests (if applicable):

   cmake -B build
   cmake --build build
   

4. Review the code changes and consider the provided questions.

Joining the Session

The Jitsi link will be shared in the #announcements channel of the Bitshala Discord shortly before the session.

Looking forward to seeing you there!

[Prabhat1308]

What is CheckBlockFn and IsBlockMutatedFn ? How are they helpful ?

They are the name of important function signatures that we use . These helps in improved readability , less complex code and enables us to allow different implementations of the same function signatures where we would have had to write separate functions instead.

What Does DeploymentActiveAfter function checks and how is it important for systems like Bitcoin ?

In projects like Bitcoin , it is necessary that important events like Consensus changes are tagged and stored somewhere else to used . DeploymentActiveAfter is one such function where important consensus changes are tagged with the block number they were deployed at which is useful to check if the block we are querying was mined after the consensus was deployed or before.

What does FillBlock does ?

FillBlock fills the PartiallyDownloadedBlock with the transactions that we give which are obtained from our own mempool .