LocalAssetService using unprotected Dictionary<TKey, TValue> causing threads to enter infinite loop

[davkean]

Our Visual Studio performance telemetry has caught an issue where LocalAssetService.HasBeenCheckedToday is entering an infinite loop on the following path. When we caught this issue, it was consuming 6 CPU cores of CPU constantly.

doki-theme-visualstudio.dll!doki_theme_visualstudio.LocalAssetService+<HasAssetChangedAsync>d__.MoveNext
doki-theme-visualstudio.dll!doki_theme_visualstudio.LocalAssetService.IsDifferentFromRemoteAsync
mscorlib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder`[System.Boolean].Start
doki-theme-visualstudio.dll!doki_theme_visualstudio.LocalAssetService+<IsDifferentFromRemoteAsync>d__.MoveNext
doki-theme-visualstudio.dll!doki_theme_visualstudio.LocalAssetService.HasBeenCheckedToday
mscorlib!System.Collections.Generic.Dictionary`[System.__Canon,System.DateTime].FindEntry

This is because LocalAssetService can end up reading and writing to the underlying Dictionary<TKey, TValue> from multiple threads at the same time. This can cause curruption of the internal data structures of the dictionary, causing it to enter infinite loop.

Fix is simple, please replace this dictionary with a ConcurrentDictionary<TKey, TValue>.