Fix race conditions in @fedify/relay follower management

[sij411]

Summary

Fixes race conditions in @fedify/relay follower management system by replacing array-based storage with individual key-value pairs for atomic operations, and improves type safety following Fedify's design patterns.

Closes #505

Problem

The previous implementation used a single key ["followers"] storing an array of follower actor IDs. This created race conditions in concurrent operations:

Race Condition Scenario 1: Concurrent Additions

1. Server A reads ["followers"] → [Alice]
2. Server B reads ["followers"] → [Alice]
3. Server A adds Bob → writes [Alice, Bob]
4. Server B adds Carol → writes [Alice, Carol]
5. Result: Bob is lost, final state is [Alice, Carol]

Race Condition Scenario 2: Concurrent Removal

1. Server A reads ["followers"] → [Alice, Bob, Carol]
2. Server B reads ["followers"] → [Alice, Bob, Carol]
3. Server A removes Bob → writes [Alice, Carol]
4. Server B removes Carol → writes [Alice, Bob]
5. Result: Carol reappears, final state is [Alice, Bob]

Both scenarios stem from the read-modify-write pattern on a shared array.

Solution

1. Individual Key-Value Storage (Commit 04aa2fd)

Changed from:

• Single key: ["followers"] → [ActorID_A, ActorID_B, ...]

To:

• Individual keys: ["follower", ActorID] → RelayFollower

This enables atomic operations:

• Addition: kv.set(["follower", actorId], data) - atomic write
• Removal: kv.delete(["follower", actorId]) - atomic delete
• Listing: kv.list(["follower"]) - iterate over all followers

2. Implementation Changes

packages/relay/src/builder.ts:

• Modified getFollowerActors() to use kv.list(["follower"]) instead of reading followers array
• Eliminates read-modify-write pattern

packages/relay/src/follow.ts:

• Simplified handleUndoFollow() to use atomic delete() only
• Removed followers array manipulation

packages/relay/src/mastodon.ts:

• Removed followers array read-modify-write in Follow handler
• Now uses only atomic set() operation

packages/relay/src/litepub.ts:

• Removed followers array read-modify-write in Accept handler
• Now updates state directly with atomic set()

Test files:

• Updated mastodon.test.ts and litepub.test.ts to remove ["followers"] array references
• Fixed variable naming to match implementation (followerId instead of followActivityId)

3. Comprehensive Test Coverage (Commit 7dbe704)

Added 9 new tests across both relay implementations:

Common tests (4 each for Mastodon and LitePub):

• Empty list handling
• Multiple follower additions
• Deletion exclusion verification
• Data integrity validation

LitePub-specific test (1):

• State filtering for pending vs accepted followers

Total: +274 lines of test coverage

4. Type Safety Improvements (Commit 7e97abc)

Replaced unsafe type casting with type predicates following Fedify's established patterns (similar to isActor()):

packages/relay/src/types.ts:

• Added isRelayFollower() type predicate function
• Provides both runtime validation and compile-time type narrowing

packages/relay/src/builder.ts:

• Replaced value as RelayFollower cast with isRelayFollower(value) guard
• Removed unused RelayFollower type import
• TypeScript now automatically narrows types after guard check

Benefits:

• Compile-time type narrowing without unsafe assertions
• Runtime validation of KV store data
• Consistent with Fedify's type-safe design philosophy
• No reliance on runtime type casting

Changes

Files modified:

• packages/relay/src/builder.ts (-16/+12 lines)
• packages/relay/src/follow.ts (-8/+2 lines)
• packages/relay/src/mastodon.ts (-8/+2 lines)
• packages/relay/src/litepub.ts (-11/+4 lines)
• packages/relay/src/types.ts (+18 lines)
• packages/relay/src/mastodon.test.ts (-7/+119 lines)
• packages/relay/src/litepub.test.ts (-18/+173 lines)

Net changes:

• First commit: -49 net lines (simplification through atomic operations)
• Second commit: +274 lines (comprehensive test coverage)
• Third commit: +17 lines (type safety improvements)

Test Plan

• All existing tests pass
• New tests verify list() functionality
• Race condition scenarios cannot occur with atomic operations
• Both Mastodon and LitePub relay protocols work correctly
• Type predicates provide compile-time type narrowing

🤖 Generated with Claude Code

[gemini-code-assist]

Summary of Changes

Hello @sij411, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses and resolves critical race conditions within the @fedify/relay follower management system. The previous implementation, which stored all follower IDs in a single array, was susceptible to data loss and inconsistencies during concurrent add or remove operations. The solution refactors the storage mechanism to use individual key-value pairs for each follower, allowing for atomic operations that guarantee data integrity and system reliability, significantly improving the robustness of follower management.

Highlights

• Race Condition Fix: Eliminated race conditions in follower management by switching from a shared array to individual key-value entries for atomic operations.
• Storage Model Refactor: Transitioned follower storage from a single "followers" array key to individual "follower, ActorID" keys, enabling atomic set and delete operations.
• Enhanced Test Coverage: Added 9 new, comprehensive tests across Mastodon and LitePub relay implementations, increasing test coverage by 274 lines to validate the new storage mechanism and prevent regressions.

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[gemini-code-assist]

Code Review

This pull request effectively resolves a critical race condition in follower management by transitioning from a shared array in the key-value store to individual keys for each follower. This change enables atomic add and remove operations, eliminating the problematic read-modify-write pattern. The implementation is clean and simplifies the logic in several places. The accompanying tests are thorough, covering the new storage strategy and ensuring the fix is robust. Overall, this is an excellent and important improvement.

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/relay/src/builder.ts	60.00%	2 Missing ⚠️
packages/relay/src/types.ts	88.88%	1 Missing ⚠️

Files with missing lines	Coverage Δ
packages/relay/src/follow.ts	80.35% <100.00%> (+0.35%)	⬆️
packages/relay/src/litepub.ts	85.36% <ø> (-0.47%)	⬇️
packages/relay/src/mastodon.ts	68.49% <ø> (-1.64%)	⬇️
packages/relay/src/mod.ts	100.00% <100.00%> (ø)
packages/relay/src/types.ts	90.00% <88.88%> (-10.00%)	⬇️
packages/relay/src/builder.ts	81.17% <60.00%> (-1.78%)	⬇️

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[sij411]

/gemini review

[gemini-code-assist]

Code Review

This is an excellent pull request that correctly identifies and fixes a critical race condition in follower management. The move from an array-based storage to individual key-value pairs is a solid architectural change that enables atomic operations and improves robustness. I appreciate the comprehensive test coverage added to validate the new approach. The introduction of the isRelayFollower type predicate is also a great improvement for type safety, making the code more reliable. I've left a few minor suggestions in the new test files to further improve type safety by using this new type guard, making the tests consistent with the type-safe philosophy of the PR.

[dahlia]

Looks good to me!