Background
Lazy loading introduces a concurrency bug into the shared CachedTopicRepository singleton where reading a topic's children (or other properties) used to be a safe read, but now triggers an in-place merge with no synchronization, so two concurrent requests hitting the same unloaded topic can throw a InvalidOperationException or corrupt the state.
Solution
The fix is a per-topic async gate (not a global lock) that serializes loads of the same topic while leaving unrelated topics fully concurrent, with the gate safely reclaimed once a topic is fully loaded (relying on the fact these boundaries only ever progress from LoadState.NotLoaded to LoadState.Loaded).
Note: This explicitly excludes associations (i.e., relationships and references), since their LoadState is dynamic; those will be handled separately (#128).
Background
Lazy loading introduces a concurrency bug into the shared
CachedTopicRepositorysingleton where reading a topic's children (or other properties) used to be a safe read, but now triggers an in-place merge with no synchronization, so two concurrent requests hitting the same unloaded topic can throw aInvalidOperationExceptionor corrupt the state.Solution
The fix is a per-topic async gate (not a global lock) that serializes loads of the same topic while leaving unrelated topics fully concurrent, with the gate safely reclaimed once a topic is fully loaded (relying on the fact these boundaries only ever progress from
LoadState.NotLoadedtoLoadState.Loaded).SemaphoreSlimgate with double-checked-locking pattern inEnsureLoaded()Note: This explicitly excludes associations (i.e., relationships and references), since their
LoadStateis dynamic; those will be handled separately (#128).