qa: Intermittent failure in feature_block.py under MSAN #36217

Is there an existing issue for this?

• I have searched the existing issues

Current behaviour

CI run in #36209 failed supriously:

https://github.com/bitcoin/bitcoin/actions/runs/34467708105/job/102840068846

It appears node0 recieved a block with a merkleroot mismatch causing a disconnect:

 node0 2026-09-10T10:53:05.402885Z [msghand] [net_processing.cpp:3830] [ProcessMessage] [net] received: block (1000000 bytes) peer=73 
 node0 2026-09-10T10:53:05.404748Z [msghand] [net_processing.cpp:5116] [ProcessMessage] [net] received block 3c326d36a415b441327bc01928c3f9b98e6c37a93500f220ccdce40c3b910ac3 peer=73 
 test  2026-09-10T10:53:05.404830Z TestFramework.p2p (DEBUG): Send message to 127.0.0.1:14552: msg_block(block=CBlock(nVersion=4 hashPrevBlock=686494e3013af3cb445e9200d56b2405c0e04ec3c1a0f346fac1b7fd10f1eb2b hashMerkleRoot=64beaeb8229780dbe40d94b742a9f411e80040824612c1aeb2dfd343b13cfd5c nTime=Thu Sep 10 11:00:14 2026 nBits=207fffff nNonce=00000000 vtx=[CTransaction(version=2 vin=[CTxIn(prevout=COutPoint(hash=0000000000000000000000000000000000000000000000000000000000000000 n=4294967295) scriptSig=02ec01 nSequence=4294967294)] vout=[CTxOut(nValue=118.74999996 scriptPubKey=2103baec77cd856657... (msg truncated) 
 node0 2026-09-10T10:53:05.404832Z [msghand] [validation.cpp:4039] [IsBlockMutated] [validation] Block mutated: bad-txnmrklroot, hashMerkleRoot mismatch 
 node0 2026-09-10T10:53:05.404900Z [msghand] [net_processing.cpp:5123] [ProcessMessage] [net] Received mutated block from peer=73 
 node0 2026-09-10T10:53:05.404965Z [msghand] [net_processing.cpp:2021] [Misbehaving] [net] Misbehaving: peer=73: mutated block 
 node0 2026-09-10T10:53:05.405460Z [msghand] [net_processing.cpp:5392] [MaybeDiscourageAndDisconnect] [net] Warning: disconnecting but not discouraging local peer 73! 
 node0 2026-09-10T10:53:05.408222Z [net] [net.cpp:567] [CloseSocketDisconnect] [net] Resetting socket for peer=73 
 node0 2026-09-10T10:53:05.408312Z [msghand] [logging/timer.h:58] [Log] [lock] ContendedLock: lock contention connman.m_nodes_mutex, net.h:1872 started 
 node0 2026-09-10T10:53:05.408422Z [msghand] [logging/timer.h:58] [Log] [lock] ContendedLock: lock contention connman.m_nodes_mutex, net.h:1872 completed (26μs) 
 test  2026-09-10T10:53:05.408693Z TestFramework.p2p (WARNING): Connection lost to 127.0.0.1:14552 due to [Errno 104] Connection reset by peer

There is no MemorySanitizer diagnostic.

Expected behaviour

Test should pass.

Steps to reproduce

I am trying to reproduce using the same seed with:

env -i HOME="$HOME" PATH="$PATH" USER="$USER" \
  FILE_ENV=./ci/test/00_setup_env_native_msan.sh \
  MAKEJOBS="-j$(nproc)" \
  DOCKER_BUILD_CACHE_ARG="${DOCKER_BUILD_CACHE_ARG:-}" \
  TEST_RUNNER_EXTRA="feature_block.py --randomseed=162348186461537714" \
  ./ci/test_run_all.sh

Will update if I manage to do so.

Relevant log output

No response

How did you obtain Bitcoin Core

Compiled from source

What version of Bitcoin Core are you using?

merge of PR 36209 and master @ 279f1726b48b16f04cd7f132e5f04ad0d4bb8d82

Operating system and version

ubuntu:26.04

Machine specifications

No response

Seems I can't reproduce this locally:

+ eval 'TEST_RUNNER_EXTRA=(feature_block.py --randomseed=162348186461537714)'
++ TEST_RUNNER_EXTRA=(feature_block.py --randomseed=162348186461537714)
+ LD_LIBRARY_PATH=/ci_container_base/depends/x86_64-pc-linux-gnu/lib
+ '/ci_container_base/ci/scratch_ ₿🧪_/build-x86_64-pc-linux-gnu/test/functional/test_runner.py' -j16 --tmpdirprefix '/ci_container_base/ci/scratch_ ₿🧪_/test_runner/' --ansi --combinedlogslen=99999999 --timeout-factor=40 feature_block.py --randomseed=162348186461537714 --quiet --failfast
Remaining jobs: [feature_block.py]

TEST             | STATUS    | DURATION

feature_block.py | ✓ Passed  | 68 s

ALL              | ✓ Passed  | 68 s (accumulated)
Runtime: 68 s