Commits on Oct 9, 2023

1. fix session saving and loading data race …

   This fixes a bug where session data could be lost if a session is loaded
   from the store while it's also being saved back to the store. Because
   sessions are loaded and stored in memory at the beginning of a request,
   it was previously possible for a session's inner value to be correctly
   altered but for that modification to be lost between the moment the
   session is saved to the store and when it's loaded. In other words, the
   data loaded would not reflect the in-progress update and instead be
   stale.
   
   To address this we could hold a lock for the duration of the request.
   However, this imposes a performance penalty we would like to avoid.
   Alternatively we can manage a registry of sessions, a la tower-cookie's
   management of `CookieJar`: with this, we load a session first from the
   service's cache of sessions and fallback to the store only when the
   session is not present.
   
   Currently sessions are managed like this via DashMap. However,
   alternatively caching facilities, such as Moka, might prove to be a
   better fit. Further, the current implementation does not bound the total
   number of cached sessions--this should be addressed in future
   updates. At present, this means memory will continue to grow, albeit
   typically at a very small rate; this is a memory leak!
   
   Because sessions are safely written to and read from this cache, we
   prevent stale reads and thus a data race which could lead to data loss
   in highly concurrent environments, especially where multiple requests
   are made using the same session cookie simultaneously.
   
   Note that `replace_if_equal` is still required to prevent data races in
   application code.

   

   maxcountryman committed Oct 9, 2023
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