[Icedcoco]

Description

Stop the MasterService eviction worker before snapshot/restore validation in
the shared snapshot test fixture, and serialize manual snapshot writes under
snapshot_mutex_. This prevents background eviction from changing replica
metadata while the test compares the original and restored services.

Module

• Transfer Engine (mooncake-transfer-engine)
• Mooncake Store (mooncake-store)
• Reshard (mooncake-reshard)
• Mooncake EP (mooncake-ep)
• Mooncake PG (mooncake-pg)
• Integration (mooncake-integration)
• P2P Store (mooncake-p2p-store)
• Python Wheel (mooncake-wheel)
• Common (mooncake-common)
• Mooncake RL (mooncake-rl)
• CI/CD
• Docs
• Other

Type of Change

• Bug fix
• New feature
• Refactor
• Breaking change
• Documentation update
• Performance improvement
• Other

How Has This Been Tested?

Test commands:

ctest --test-dir /tmp/mooncake-build-fix \
  -R '^master_service_ssd_test_for_snapshot$' \
  --output-on-failure --repeat until-fail:2

/tmp/mooncake-build-fix/mooncake-store/tests/master_service_ssd_test_for_snapshot \
  --gtest_filter='MasterServiceSSDSnapshotTest.EvictObject'

ctest --test-dir /tmp/mooncake-build-fix \
  -R '^(master_service_test_for_snapshot|master_service_promotion_test_for_snapshot)$' \
  --output-on-failure

pre-commit run --files \
  mooncake-store/tests/ha/snapshot/master_service_test_for_snapshot_base.h

Test results:

• Unit tests pass
• Integration tests pass (not applicable)
• Manual testing done (not applicable)

The SSD snapshot target passed twice, EvictObject passed ten consecutive
runs, and the other snapshot fixtures passed. Pre-commit passed all applicable
hooks.

Checklist

• I have performed a self-review of my own code
• I have formatted my code using ./scripts/code_format.sh
• I have run pre-commit on the files changed in this PR and all hooks pass
• I have updated the documentation (not applicable)
• I have added tests to prove my changes are effective
• For changes >500 LOC: I have filed an RFC issue (not applicable)

AI Assistance Disclosure

• No AI tools were used
• AI tools were used (analysis and implementation assistance). The human
  submitter must review and defend every changed line end-to-end.

[Aionw]

LGTM