fix: thread-safe access to Organization.Sessions/Groups from background threads

[btessiau]

Problem

Organization.Sessions and Organization.Groups are plain List<T> collections designed for UI-thread-only access. However, several background threads read them without synchronization:

• SaveActiveSessionsToDisk — timer callback reads Sessions to snapshot GroupId
• RunCodespaceHealthCheckAsync — background task enumerates Groups
• NotifyCodespaceConnected — health check reads Sessions to find group's first session
• ResumeCodespaceSessionsAsync — health check reads Sessions for group members
• SaveOrganizationCore — timer callback serializes entire Organization (races with UI mutations)

This causes intermittent InvalidOperationException ("Collection was modified during enumeration") and potential data corruption.

Fix

• Add _organizationLock + SnapshotSessionMetas()/SnapshotGroups() helpers that produce thread-safe copies for off-thread reads
• Replace direct Organization.Sessions reads in the 5 off-thread sites with snapshot calls
• Wrap SaveOrganizationCore serialization with the lock to prevent tearing
• Replace the fragile try/catch(InvalidOperationException) pattern in health checks with proper SnapshotGroups()

Scope

4 files, 38 additions, 11 deletions. Minimal surface area — only touches the off-thread access sites, not the 70+ UI-thread accesses.

Fixes #327

[btessiau]

PR #328 Review — Organization.Sessions/Groups Thread-Safety

CI Status: ⚠️ No checks reported (no CI configured for this branch)
Existing reviews: None

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🔴 CRITICAL — Lock design is one-sided; race is NOT actually fixed

CopilotService.cs:269-280, CopilotService.Organization.cs:180-183
Flagged by 4/5 models

_organizationLock is acquired by readers only (snapshot helpers + SaveOrganizationCore). Every mutation site — Organization.Groups.Add(), Organization.Sessions.Add(), RemoveAll(), etc. — runs on the UI thread without holding the lock. A mutex only prevents races when all competing parties acquire it. A background thread calling SnapshotGroups() can still execute Organization.Groups.ToList() concurrently with a UI-thread Organization.Groups.Add() that is resizing the internal array. Under the C# memory model this is a data race with undefined behavior. The InvalidOperationException from foreach's _version check was avoided (.ToList() uses Array.Copy, not the enumerator), but a torn read remains possible.

More critically, this PR removed the existing try-catch fallback in RunCodespaceHealthCheckAsync:

// REMOVED — was the safety net for exactly this race:
catch (InvalidOperationException) { return; }

The code is now less resilient than before.

Required fix: Either hold _organizationLock at every mutation site, or snapshot the full Organization object under the lock before serializing/reading. The simplest correct pattern is to wrap all Organization.Sessions.Add/Remove and Organization.Groups.Add/Remove calls in lock (_organizationLock) blocks.

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🟡 MODERATE — StartCodespaceHealthCheck() line 323: bare Organization.Groups.Any() on background thread

CopilotService.Codespace.cs:323
Flagged by 4/5 models

private void StartCodespaceHealthCheck()
{
    lock (_healthCheckLock)
    {
        if (!Organization.Groups.Any(g => g.IsCodespace)) return;  // ← bare read, no snapshot

StartCodespaceHealthCheck() is called from StartAndReconnectCodespaceAsync (line 854) after multiple await calls, meaning it runs on a ThreadPool continuation — not the UI thread. This is the same race the PR claims to fix in RunCodespaceHealthCheckAsync, but it was missed here. Must use SnapshotGroups().Any(...).

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🟡 MODERATE — GetClientForGroup() line 42: bare Organization.Groups.FirstOrDefault() not migrated

CopilotService.Codespace.cs:42
Flagged by 4/5 models

private CopilotClient GetClientForGroup(string? groupId)
{
    var group = Organization.Groups.FirstOrDefault(g => g.Id == groupId);  // ← bare

Called from SendPromptAsync. Once any await in the call path runs on a ThreadPool continuation, this bare enumeration races with UI mutations. Should use SnapshotGroups().

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🟢 MINOR — Lock held during full JsonSerializer.Serialize on UI thread

CopilotService.Organization.cs:180-183
Flagged by 2/5 models

SaveOrganizationCore holds _organizationLock for the entire serialization (potentially slow for large org state). Background snapshot callers block for this duration. Consider snapshotting under the lock and serializing outside it:

string json;
OrganizationState orgSnapshot;
lock (_organizationLock) { orgSnapshot = /* clone */ ; }
json = JsonSerializer.Serialize(orgSnapshot, ...);

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🟢 MINOR — No regression tests for thread-safety behavior

PolyPilot.Tests/
Flagged by 4/5 models

SnapshotSessionMetas() and SnapshotGroups() are internal and testable, but no tests verify (a) that they return independent copies of the live list, or (b) that concurrent read+write doesn't produce exceptions. The existing test infrastructure (SessionOrganizationTests.cs, SessionPersistenceTests.cs) provides a good pattern to follow.

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Test Coverage Assessment

The PR introduces a new lock discipline but adds no tests. Suggested scenarios:

1. Verify SnapshotSessionMetas() returns a copy, not a live reference (mutating the result doesn't affect the original)
2. Concurrent background snapshot + UI mutation — confirm no InvalidOperationException
3. StartCodespaceHealthCheck call from async continuation context — verify it uses snapshot after the fix

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Previous Findings Status

This is the first review of PR #328. No prior findings to track.

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Recommended Action: ⚠️ Request Changes

The lock design has a fundamental correctness issue: locking only the readers while leaving writers unlocked does not prevent the race. The original crash trigger was swapped from foreach/enumerator to Array.Copy (which rarely throws), but the underlying data race is unresolved and undefined behavior under the C# memory model. Three specific changes are needed before merge:

1. Hold _organizationLock at all Organization.Sessions/Organization.Groups mutation sites, OR switch to a lock-protected wrapper that enforces this consistently
2. Fix StartCodespaceHealthCheck():323 — use SnapshotGroups().Any(...)
3. Fix GetClientForGroup():42 — use SnapshotGroups() or confirm it's always UI-thread-only

[btessiau]

Addressed all findings in e79c0b8

🔴 CRITICAL — Lock design is one-sided → Fixed

You're absolutely right — locking only readers while leaving writers unlocked doesn't prevent the race.

Fix: Added 6 locked mutation helpers (AddSessionMeta, RemoveSessionMetasWhere, RemoveSessionMeta, AddGroup, InsertGroup, RemoveGroupsWhere) that hold _organizationLock. Migrated all ~35 mutation sites across CopilotService.cs, Organization.cs, Codespace.cs, and Providers.cs to use these helpers. Now both readers (snapshots) AND writers (mutations) acquire the same lock.

🟡 MODERATE — StartCodespaceHealthCheck():323 bare read → Fixed

Changed to SnapshotGroups().Any(g => g.IsCodespace).

🟡 MODERATE — GetClientForGroup():42 bare read → Fixed

Changed to SnapshotGroups().FirstOrDefault(...). While it's likely always on the UI thread (MAUI SynchronizationContext), using the snapshot is defensive and consistent.

🟢 MINOR — Lock held during serialization → Fixed

SaveOrganizationCore now snapshots under lock, serializes outside:

OrganizationState snapshot;
lock (_organizationLock) {
    snapshot = new OrganizationState { Groups = ...ToList(), Sessions = ...ToList(), ... };
}
json = JsonSerializer.Serialize(snapshot, ...);

🟢 MINOR — No regression tests → Fixed

Added OrganizationThreadSafetyTests.cs with 10 tests:

• Snapshot isolation (independent copies for Sessions and Groups)
• All 6 mutation helpers work correctly
• Concurrent stress tests: parallel background reader + locked writer with no exceptions

Additional fix: StartCodespaceAsync Task.Run catch block

The Organization.Groups.FirstOrDefault() at line 299 was a bare off-thread read in a Task.Run. Fixed to use SnapshotGroups() for the existence check, then marshal the actual mutation to the UI thread via InvokeOnUI().

Build: 0 errors. Tests: 2287+ passed (10 new), 4 pre-existing failures (3 CliPath + 1 flaky ChatDatabase).

[btessiau]

PR #328 Re-Review — Multi-Model Consensus Report

CI Status: ⚠️ No checks configured
Models used: claude-opus-4.6 ×2, claude-sonnet-4.6, gemini-3-pro-preview, gpt-5.3-codex
Commits reviewed: f7550ec (original) + e79c0b8 (fixes)

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Previous Findings — All FIXED ✅

#	Finding	Status
1	🔴 CRITICAL — Lock one-sided: ~35 mutation sites unlocked	✅ FIXED — 8 locked helpers added; all sites migrated
2	🟡 MODERATE — StartCodespaceHealthCheck() bare Groups.Any() on bg thread	✅ FIXED — SnapshotGroups().Any()
3	🟡 MODERATE — GetClientForGroup():42 bare Groups.FirstOrDefault()	✅ FIXED — SnapshotGroups().FirstOrDefault()
4	🟢 MINOR — SaveOrganizationCore held lock during JSON serialization	✅ FIXED — snapshot under lock, serialize outside
5	🟢 MINOR — No regression tests	✅ FIXED — 10 tests in OrganizationThreadSafetyTests.cs

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New Finding — Consensus (4/5 models)

🟡 MODERATE — CopilotService.Persistence.cs:298,304,347,361 — Bare Organization.Groups reads in RestorePreviousSessionsAsync

Four sites still enumerate Organization.Groups directly without the lock:

// line 298
foreach (var group in Organization.Groups.Where(g => g.IsCodespace))
// line 304
foreach (var g in Organization.Groups)
// line 347
Organization.Groups.Any(g => g.Id == entry.GroupId && g.IsCodespace)
// line 361
Organization.Groups.FirstOrDefault(g => g.Id == entry.GroupId)

The IsRestoring = true flag suppresses timer callbacks, but it does not gate WsBridge SyncRemoteSessions (background thread) or SDK event handlers — both of which call the locked mutation helpers (AddGroup, RemoveGroupsWhere) regardless of IsRestoring. During ReconnectAsync, a bridge reconnect or incoming SDK event can call AddGroup() concurrently with the foreach (var g in Organization.Groups) at line 304, throwing InvalidOperationException.

Fix: Take a single snapshot at the top of the restore block:

var orgGroups = SnapshotGroups();
// then use orgGroups throughout lines 298-361

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Not Meeting Consensus (noted but filtered)

• Shallow-copy mutation of SessionGroup fields (ReconnectAttempts++ on background thread without InvokeOnUI) — pre-existing, flagged by 1/5 models, out of scope for this PR.
• Lock ordering (_healthCheckLock → _organizationLock) — 3/5 models confirmed no deadlock risk; no reverse acquisition exists.
• TOCTOU Any() + AddSessionMeta() — 1/5 flagged; 1/5 explicitly confirmed these are UI-thread-only paths, not a race.

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Recommendation: ⚠️ Request changes

One MODERATE finding remains (4/5 consensus): the 4 bare Organization.Groups reads in RestorePreviousSessionsAsync. The fix is a single SnapshotGroups() call at the top of the restore block. All other previous findings are resolved.

[btessiau]

Addressed the remaining finding in 4fad726:

🟡 MODERATE — Bare Organization.Groups reads in RestorePreviousSessionsAsync → Fixed

Replaced all 4 bare reads (lines 298, 304, 347, 361) with a single var restoreGroups = SnapshotGroups() at the top of the restore block. Bridge reconnects and SDK events can fire AddGroup/RemoveGroupsWhere concurrently during restore — the snapshot ensures the foreach enumeration is isolated.

Build: 0 errors. Tests: 10/10 thread safety tests pass.

[btessiau]

🔄 PR #328 Re-Review (Round 3) — Organization Thread-Safety Locking

CI Status: ⚠️ No checks configured (no CI for this repo)
Commits reviewed: f7550ec (original) → e79c0b8 (reader+writer locking) → 4fad726 (RestorePreviousSessionsAsync)

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Previous Findings Status

#	Severity	Finding	Status
1	🔴 CRITICAL	Lock one-sided — mutations not locked	✅ FIXED
2	🟡 MODERATE	StartCodespaceHealthCheck bare Organization.Groups read	✅ FIXED
3	🟡 MODERATE	GetClientForGroup bare Organization.Groups read	✅ FIXED
4	🟢 MINOR	Lock held during JsonSerializer.Serialize	✅ FIXED
5	🟢 MINOR	No regression tests for thread-safety	✅ FIXED
6	🟡 MODERATE	RestorePreviousSessionsAsync — 4 bare Organization.Groups reads	✅ FIXED

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Fix Verification

#1 — 8 locked mutation helpers added (AddSessionMeta, RemoveSessionMetasWhere, RemoveSessionMeta, AddGroup, InsertGroup, RemoveGroupsWhere) and all direct Organization.Sessions.Add/Remove/RemoveAll and Organization.Groups.Add/Insert/Remove call sites throughout CopilotService.cs, Codespace.cs, Organization.cs, and Providers.cs have been migrated to them. Zero remaining bare mutations outside the helpers.

#2, #3 — SnapshotGroups() now used in StartCodespaceHealthCheck guard and GetClientForGroup lookup. The old fragile try { } catch (InvalidOperationException) pattern in RunCodespaceHealthCheckAsync is gone, replaced with SnapshotGroups().

#4 — SaveOrganizationCore now snapshots under lock (ToList() on both collections), then serializes outside the lock. Lock hold time is now microseconds.

#5 — OrganizationThreadSafetyTests.cs added with 10 tests: snapshot isolation, all 6 mutation helpers, and two concurrent read+write stress tests (ConcurrentSnapshotAndMutation_DoesNotThrow, ConcurrentGroupSnapshotAndMutation_DoesNotThrow).

#6 — var restoreGroups = SnapshotGroups() taken once at the top of the restore block in RestorePreviousSessionsAsync. All 4 former bare reads (lines ~298, 304, 347, 361 in Persistence.cs) now use this snapshot.

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New Findings

None. Spot-checked the remaining bare Organization.Groups/Sessions reads in the codebase:

• RetryCodespaceConnectionAsync (line 601) and StartAndReconnectCodespaceAsync (lines 824, 852) — both are UI-button handlers that run on the Blazor renderer's SynchronizationContext; bare reads are safe there.
• if (!Organization.Sessions.Any(...)) AddSessionMeta(...) patterns throughout CreateSessionAsync — the Any check and AddSessionMeta call are on the same UI-thread execution unit; no interleaving possible.
• Background task in AddStoppedCodespaceGroup — correctly uses SnapshotGroups() for its background read and InvokeOnUI(() => Organization.Groups.FirstOrDefault(...)) for its UI-thread mutation.

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Test Coverage

10 new tests cover snapshot isolation and all mutation helpers. The two stress tests exercise actual concurrent access and verify no exceptions. The only gap (pre-existing, not introduced here) is that the tests don't cover SaveOrganizationCore snapshot correctness under contention, but the lock+snapshot pattern is simple enough that the unit tests of the helpers are sufficient.

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Verdict

✅ Approve — All 6 previous findings fully resolved. Lock design is now sound (both reads and writes protected). No new issues found. The PR is still in Draft; mark ready when you want to merge.