fix: register subscribers for locally-cached contracts (issue #2001)

[sanity]

Summary

Fixes the flaky test_multiple_clients_subscription test (issue #2001) by ensuring subscriptions are properly registered for locally-cached contracts.

Problem

The test exhibited 20-40% failure rate due to missed UPDATE notifications. Through systematic investigation with targeted logging, I discovered the root cause:

When a contract was already cached locally, the SUBSCRIBE operation took a fast path that:

1. ✓ Verified contract exists locally
2. ✓ Sent SubscribeResponse to client
3. ✗ Never called add_subscriber() to register the subscription

This caused UPDATE operations to find no subscribers for locally-cached contracts, silently dropping notifications.

Why It Was Flaky

• Contract on remote node: Normal flow → add_subscriber() called → works
• Contract locally cached: Fast path → add_subscriber() skipped → breaks
• Timing-dependent: Whether contract is local when subscription occurs (20-40% chance)

The Fix

File: crates/core/src/operations/subscribe.rs:91-98

Added call to add_subscriber() in the local subscription fast path, ensuring subscriptions are registered regardless of cache location:

// CRITICAL FIX for issue #2001: Register subscriber in DashMap before completing
// Without this, UPDATE operations won't find subscribers for locally-cached contracts
let subscriber = op_manager.ring.connection_manager.own_location();
if op_manager.ring.add_subscriber(key, subscriber.clone()).is_err() {
    tracing::error!(%key, tx = %id, "Failed to add local subscriber - max subscribers reached");
    // Continue anyway - client requested subscription and contract is local
} else {
    tracing::debug!(%key, tx = %id, subscriber = %subscriber.peer, "Successfully registered local subscriber");
}

Verification

Ran test 15 times - all passes (100% success rate vs. previous 60-80%)

Results: 15 passed, 0 failed out of 15 runs
Success rate: 100.0%

Impact

• Fixes intermittent test failures
• Ensures UPDATE notifications work correctly for locally-cached contracts
• No performance impact (single DashMap insertion)
• Removes need for 5-second delay workaround (also removed in this PR)

Test Changes

Also removed the 5-second delay workaround from test_multiple_clients_subscription that was added in commit cbd0849, as it's no longer needed with the proper fix.

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Closes #2001

[AI-assisted debugging and comment]

[Copilot]

Pull Request Overview

Fixes a critical bug where subscriptions for locally-cached contracts were not being registered, causing UPDATE notifications to be silently dropped. This resolves the flaky test_multiple_clients_subscription test that was failing 20-40% of the time.

Key Changes:

• Added add_subscriber() call in the local subscription fast path to register subscriptions in the DashMap
• Added informational logging to track subscription registration flow
• Removed unnecessary 5-second delay workaround from the test

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/core/src/operations/subscribe.rs	Added subscriber registration logic for locally-cached contracts to ensure UPDATE notifications are properly delivered
crates/core/tests/operations.rs	Added logging statement to track subscription completion timing

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[iduartgomez]

This looks good, me what happens if the peer acquires downstream/upstream external subcriptions later on subsequent updates?

[sanity]

@iduartgomez Great question! The fix handles this correctly. Here's what happens:

Scenario: Local subscription first, external subscriptions later

Initial state (what the fix addresses):

1. Contract is locally cached
2. Local subscription registered: add_subscriber(key, own_location()) (subscribe.rs:95)
3. Subscribers list: [self]

Later UPDATE from another peer (external subscription chain):

1. Peer receives SeekNode from PeerA (wants to subscribe)
2. Peer forwards to PeerB (downstream, has the contract)
3. Receives ReturnSub(true) from PeerB
4. Calls add_subscriber(key, PeerB.clone()) (subscribe.rs:435)
5. Forwards ReturnSub to PeerA (upstream, subscribe.rs:446-459)
6. Subscribers list: [self, PeerB]

Why this works correctly

1. add_subscriber() is idempotent (seeding.rs:116-123):

   if let Err(next_idx) = subs.value_mut().binary_search(&subscriber) {
       subs.insert(next_idx, subscriber);  // Only inserts if not found
   }

   Multiple calls with the same subscriber are safe - it checks for duplicates via binary_search.

2. Multiple subscriptions coexist:

   • The Vec can contain both the local peer and external downstream peers
   • Each represents a valid subscription path that needs UPDATE notifications

3. UPDATE broadcasts to all (update.rs:720-728):

   let subscribers = self.ring.subscribers_of(key)
       .map(|subs| subs.value().iter()
           .filter(|pk| &pk.peer != sender)
           .cloned()
           .collect())

   Updates are sent to all subscribers in the list, so both local clients and external peers receive notifications.

Edge cases handled

• Duplicate subscriber: Prevented by binary_search check
• Max subscribers: Both paths respect the MAX_SUBSCRIBERS limit and return appropriate errors
• Self-referential: Local subscriber is always own_location(), external subscribers are always other peers - no conflict

The fix ensures that when a contract is locally cached, the local subscription is registered immediately before completing the SUBSCRIBE operation. Any subsequent external subscriptions augment this list, creating a proper subscription tree where UPDATE notifications flow to all registered paths.

[AI-assisted debugging and comment]