Fix race in ExpireAfter when item is removed or updated before expiration fires (#1076)

The expiration timer callback now checks that the item still exists and
still has a pending expiration before attempting removal. Prevents
InvalidOperationException when items are concurrently removed or updated
between when the expiration was scheduled and when it fires.

Also uses |= to accumulate expiration changes correctly.

Author: dwcullop

src/DynamicData/Cache/Internal/ExpireAfter.ForSource.cs

@@ -161,11 +161,18 @@ private void OnEditingSource(ISourceUpdater<TObject, TKey> updater)
                         {
                             _expirationDueTimesByKey.Remove(proposedExpiration.Key);
 
-                            _removedItemsBuffer.Add(new(
-                                key: proposedExpiration.Key,
-                                value: updater.Lookup(proposedExpiration.Key).Value));
-
-                            updater.RemoveKey(proposedExpiration.Key);
+                            // The item may have been removed or updated by another thread between when
+                            // this expiration was scheduled and when it fired. Check that the item is
+                            // still present and still has an expiration before removing it.
+                            var lookup = updater.Lookup(proposedExpiration.Key);
+                            if (lookup.HasValue && _timeSelector.Invoke(lookup.Value) is not null)
+                            {
+                                _removedItemsBuffer.Add(new(
+                                    key: proposedExpiration.Key,
+                                    value: lookup.Value));
+
+                                updater.RemoveKey(proposedExpiration.Key);
+                            }
                         }
                     }
                     _proposedExpirationsQueue.RemoveRange(0, proposedExpirationIndex);
@@ -273,7 +280,7 @@ private void OnSourceNext(IChangeSet<TObject, TKey> changes)
                                 {
                                     if (_timeSelector.Invoke(change.Current) is { } expireAfter)
                                     {
-                                        haveExpirationsChanged = TrySetExpiration(
+                                        haveExpirationsChanged |= TrySetExpiration(
                                             key: change.Key,
                                             dueTime: now + expireAfter);
                                     }