Need help regarding Send and Sync

Hi, I have a library containing a data-structure, you can think of it as an in-memory database. This data-structure can be safely accessed from multiple threads and I require all types stored to be Send + Sync. Each stored type has it's own Vec-like storage. To make any operation, you first request the storage and then can access data through a guard.

Recently I had an idea to allow !Send and !Sync types but I'm not 100% it's correct.

  • Send + !Sync -> shared and unique access in one thread at a time
  • !Send + Sync -> shared access only
  • Send + !Sync -> shared and unique access in the thread it was added in
  • The data-structure have to loose Send

The guard would have the same bound as the storage.

Are these guaranties enough?

Thank you for your time :smiley:

EDIT: mutable access is also ok for Send + !Sync
EDIT2: add !Send + Sync case

Yes, the restrictions you propose appear to be safe. Additionally:

Mutable access on one thread at a time would also be safe here.


Yes, it's the same as Send + Sync so in my head it's not a limitation on !Sync but you're absolutely right.

In the meantime !Send + Sync is sitting in the corner :stuck_out_tongue:

  • for that case, you can authorize non-&mut nor owned access to the value

Be careful not to drop the !Send on any other thread too.

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@Yandros I thought Clone for example was an issue but it's not the same value in the end and swap asks for &mut, so shared references only but in multiple threads.

@jonh Oh good catch, so the data-structure has to be !Send... that's not very nice but I'll live with it.