UnsolicitedResponse isn't Send
jonhoo opened this issue · comments
As reported in #250, the UnsolicitedResponse type isn't Send, but it should be. We should fix that.
Are you sure? rustdoc (and static_assertions) do show an Auto Trait implementation for Send and Sync on imap::types::UnsolicitedResponse.
Connection<T>, Client<T> and Session<T> do all also implement Send as long as T does, however they are !Sync. However that's not because of UnsolicitedResponse but because std::mpsc::Sender and std::mpsc::Receiver are always !Sync
Huh, that's very weird indeed. In that case I don't know what was causing #250. There, @soywod claimed that mpsc::Sender<UnsolicitedResponse> and mpsc::Receiver<UnsolicitedResponse> weren't thread-safe, but they should be if the underlying type is Send (which it apparently is). @soywod can you speak more to how exactly the compiler was complaining at you?
Here the error I get:
error[E0277]: `std::sync::mpsc::Sender<UnsolicitedResponse>` cannot be shared between threads safely
--> src/backend/imap/backend.rs:404:10
|
404 | impl<'a> Backend for ImapBackend<'a> {
| ^^^^^^^ `std::sync::mpsc::Sender<UnsolicitedResponse>` cannot be shared between threads safely
|
= help: within `backend::imap::backend::ImapBackend<'a>`, the trait `Sync` is not implemented for `std::sync::mpsc::Sender<UnsolicitedResponse>`
= note: required because it appears within the type `Session<ImapSessionStream>`
When I said "not thread-safe" I meant not Sync nor Send, and indeed std::mpsc::Sender is not Sync by definition.
Note that std::sync::mpsc::Receiver isn't Sync, either. So in a std::sync::RwLock type like used by relm4 (higher level gtk lib) this causes further issues.
Fixing the sender on the other hand would be possible with https://doc.rust-lang.org/std/sync/mpsc/fn.sync_channel.html however the same error would appear for the Receiver then
Fixing the sender on the other hand would be possible with https://doc.rust-lang.org/std/sync/mpsc/fn.sync_channel.html however the same error would appear for the Receiver then
Hmm, as I understand the code the unsolicited_responses is really mostly used in a 'self-pipe' pattern, with Session always holding both the sending and receiving end and in essence using it as a FIFO queue. Especially given that right now the uses of unsolicited_responses_tx always take it as a &mut anyway, I do think it would make sense to use a VecDeque or plain Vec instead. That would also enable the multi-threaded access patterns that I think @soywod wants to work with.
Your thoughts @jonhoo? I'd be happy to write a PR for that change if you agree that it makes sense.
Ah, so the concern is actually that the client isn't Send/Sync (because it holds these senders/receivers), not that the elements of the channel aren't. That does make more sense! Yes, I think replacing it with a VecDeque makes a lot of sense, and would be happy to look at a PR!
I initiated a PR, please let me know if it resembles what you had in mind.
With the PR, I do not have anymore the cannot be shared between threads safely, but I still cannot run multiple tasks in parallel using a same Session. My intuition is that every session fn borrows self as mut &self mut which cannot be shared safely between threads without a Mutex. The PR has no impact on the initial problem.
Ah, yes, so, that's sort of fundamental to the IMAP protocol. It doesn't allow for multiple concurrent requests. So you'd need to either use a Mutex or spin up a separate Session for each client unfortunately.