*: fix DKG exchanger deadlock

[KaloyanTanev]

exchange and pushPsigs shared a single size-1 channel. exchange first calls StoreInternal to store this node's own partial signatures and only then enters the loop that receives collected signatures from the channel. pushPsigs is the parsigdb threshold subscriber, so it both handles peer signatures (via StoreExternal) and runs synchronously on the exchange goroutine (via StoreInternal) - and on every threshold it sends the collected map on that shared channel.

Example scenario:

1. Peers' partial signatures for validator A arrive and complete its threshold, so pushPsigs sends A's map and fills the size-1 channel.
2. Current node's StoreInternal is still running (the exchange goroutine has not reached its receive loop yet, so nothing is draining the channel).
3. That same StoreInternal completes the threshold for validator B, so pushPsigs is invoked inline on the exchange goroutine.
4. pushPsigs tries to send B's map on the already-full channel and blocks.
5. The exchange goroutine is now stuck inside StoreInternal and never reaches the receive loop that would drain the channel, so the send can never complete — a self-deadlock.
6. This node never broadcasts its aggregated result, so the other peers block waiting for it too, and the whole ceremony stalls.

I've tried to keep it as close as possible to established paradigms in the other parts of the code, as the DKG is vital part and I personally would like as small changes as possible. The fix reworks exchange to use the same await pattern as dutydb/aggsigdb: each call registers a query with its own buffered response channel, and every store resolves the queries whose signatures are fully collected.

Also added a test to mimic the faulty behaviour which now is passing.

category: bug
ticket: none

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

Fixes a self-deadlock in the DKG partial-signature exchanger by removing the shared size-1 result channel and adopting a per-call “await query” pattern similar to other workflow components, plus a regression test to ensure pushPsigs doesn’t block when no exchange call is draining results.

Changes:

• Replace the shared sigDatasChan flow with per-exchange pending queries (exchangeQuery) and a resolver (resolveQueriesUnsafe).
• Update pushPsigs to store collected signature data and resolve any satisfied pending exchange queries.
• Add a regression test that would have deadlocked under the prior shared-channel design.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dkg/exchanger.go	Reworks exchanger result delivery from a shared channel to per-call buffered response channels with query resolution.
dkg/exchanger_internal_test.go	Adds a regression test ensuring repeated pushPsigs calls don’t block without an exchange receiver.

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.55%. Comparing base (80ddd49) to head (d987b6f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
dkg/exchanger.go	85.71%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4590      +/-   ##
==========================================
+ Coverage   57.40%   57.55%   +0.15%     
==========================================
  Files         245      245              
  Lines       33560    33668     +108     
==========================================
+ Hits        19265    19379     +114     
+ Misses      11856    11845      -11     
- Partials     2439     2444       +5     

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[pinebit]

Reviewed manually + GPT, no issues observed.