Make backend objects Send + Sync for compatibility with cln-plugin

[chrisguida]

Describe the enhancement
Not sure if this is a feature request or a bug. It sounds like BDK v1.0 wants to have good async support, so it seems this would be good to get working before then? But I'll just put it in as a feature request in case not :)

I'm trying to get this CLN plugin repo to build: https://github.com/chrisguida/watchdescriptor/tree/wip/esplora_client

Currently it seems the support for async in BDK is not quite where cln-plugin (the official Rust CLN plugin framework) is expecting it to be, as it requires everything to be Send + Sync.

I discussed this with @notmandatory and he says:

my steps were going to be to 

1. look at all bdk return types from async functions called in your code; 
2. try to determine which ones are not send+sync; 
3. try to make the necessary type(s) send+sync; 
4. iterate with fields of those types until needed return types are send+sync.
once all this research/testing is done will need to put it into a PR.. 
if any needed types come from other crates then we'll need to 
figure out if/where to wrap them in a mutex or figure out if 
a PR is needed for the dependency crate (like rust-bitcoin or rust-miniscript, 
or some other dependency)

Use case
To be able to use BDK inside CLN plugins

I will take this on myself if someone wants to coach me.

[notmandatory]

@chrisguida for reference also add a comment with the error message you got, the one that mentions the missing send+sync.

[chrisguida]

Here is the error:

error: future cannot be shared between threads safely
   --> src/main.rs:55:10
    |
55  |         .rpcmethod(
    |          ^^^^^^^^^ future returned by `watchdescriptor` is not `Sync`
    |
    = help: the trait `Sync` is not implemented for `dyn std::future::Future<Output = Result<LocalUpdate<KeychainKind, ConfirmationTimeAnchor>, bdk_esplora::esplora_client::Error>> + Send`
note: future is not `Sync` as it awaits another future which is not `Sync`
   --> src/main.rs:370:18
    |
370 |       let update = client
    |  __________________^
371 | |         .scan(
372 | |             local_chain,
373 | |             keychain_spks,
...   |
377 | |             PARALLEL_REQUESTS,
378 | |         )
    | |_________^ await occurs here on type `Pin<Box<dyn std::future::Future<Output = Result<LocalUpdate<KeychainKind, ConfirmationTimeAnchor>, bdk_esplora::esplora_client::Error>> + Send>>`, which is not `Sync`
note: required by a bound in `cln_plugin::Builder::<S, I, O>::rpcmethod`
   --> /Users/steve/.cargo/git/checkouts/lightning-a9dc92ee913af919/9675463/plugins/src/lib.rs:193:47
    |
189 |     pub fn rpcmethod<C, F>(mut self, name: &str, description: &str, callback: C) -> Builder<S, I, O>
    |            --------- required by a bound in this associated function
...
193 |         F: Future<Output = Response> + Send + Sync + 'static,
    |                                               ^^^^ required by this bound in `Builder::<S, I, O>::rpcmethod`

[LLFourn]

It looks like this is a problem with rpcmethod in the lightning plugins library?

But it's weird that something would need a Future to be Sync. See discussion: https://users.rust-lang.org/t/requiring-a-sync-future-in-a-tokio-app/59784/3 So maybe the solution is to remove that bound from the library.

[chrisguida]

cc @cdecker Please see the above comment. Are you sure the cln-plugin framework needs to require Futures to be Sync?

I'm trying to determine whether this is a problem with cln-plugin or bdk (or both).

[chrisguida]

Also FYI I got this error while building this time:

error[E0599]: the function or associated item `new_from_path` exists for struct `Store<'_, LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>>`, but its trait bounds were not satisfied
   --> src/main.rs:333:47
    |
333 |     let db = Store::<bdk::wallet::ChangeSet>::new_from_path(DB_MAGIC.as_bytes(), db_path)?;
    |                                               ^^^^^^^^^^^^^ function or associated item cannot be called due to unsatisfied trait bounds
    |
   ::: /home/cguida/.cargo/git/checkouts/bdk-bbcd84aeefa047ed/8f38e96/crates/chain/src/keychain.rs:123:1
    |
123 | pub struct LocalChangeSet<K, A> {
    | ------------------------------- doesn't satisfy `_: Append`
    |
    = note: the following trait bounds were not satisfied:
            `LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>: bdk_chain::tx_data_traits::Append`

Seems unrelated to the Send + Sync issue I was hitting on previous versions of this code (and which @notmandatory hit on this version of the code. His is the Sync + Send error in the previous message above). Does this warrant a new issue, or am I just doing something dumb?

[junderw]

require Futures to be Sync?

Considering that a Future only uses Self in the context of Pin<&mut Self> in the poll method, and &mut references can not be obtained by something shared between threads (ie. an Arc of a Future has no way to get &mut Self)...

I don't think Sync should ever be needed to bind a Future.

https://users.rust-lang.org/t/future-is-not-sync-when-calling-async-fn-of-dyn-trait-obj-in-another-async-fn/83285/2

Generally, futures should not be required to be Sync. Send should be enough. Any time you get an error saying "this future is not Sync", the solution is to remove that requirement — you should not try to make the future Sync.

[junderw]

Also FYI I got this error while building this time:

Can you try Store::<'_, bdk::wallet::ChangeSet>::new_from_path(DB_MAGIC.as_bytes(), db_path)?;

[chrisguida]

@junderw same error as before:

error[E0599]: the function or associated item `new_from_path` exists for struct `Store<'_, LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>>`, but its trait bounds were not satisfied
   --> src/main.rs:333:51
    |
333 |     let db = Store::<'_, bdk::wallet::ChangeSet>::new_from_path(DB_MAGIC.as_bytes(), db_path)?;
    |                                                   ^^^^^^^^^^^^^ function or associated item cannot be called due to unsatisfied trait bounds
    |
   ::: /home/cguida/.cargo/git/checkouts/bdk-bbcd84aeefa047ed/8f38e96/crates/chain/src/keychain.rs:123:1
    |
123 | pub struct LocalChangeSet<K, A> {
    | ------------------------------- doesn't satisfy `_: Append`
    |
    = note: the following trait bounds were not satisfied:
            `LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>: bdk_chain::tx_data_traits::Append`

[LLFourn]

pinging @evanlinjin in case he an figure out how this LocalChangeSet doesn't do Append anymore.

KeychainKind has to be Ord but I would expect the error message to say if that was the problem. Where does KeychainKind come from? bdk?

[chrisguida]

Ok, I managed to fix the above error by replacing

bdk_file_store = { version = "0.2.0" }

with

bdk_file_store = { path = "../bdk/crates/file_store" }

(i have the bdk master branch stored locally there)

Now I'm back to the original Send + Sync error that this issue is about:

error: future cannot be sent between threads safely
   --> src/main.rs:56:10
    |
56  |         .rpcmethod(
    |          ^^^^^^^^^ future returned by `watchdescriptor` is not `Send`
    |
    = help: within `impl std::future::Future<Output = Result<serde_json::Value, anyhow::Error>>`, the trait `Send` is not implemented for `Rc<RefCell<&mut Wallet<Store<'_, LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>>>>>`
note: future is not `Send` as this value is used across an await
   --> src/main.rs:407:27
    |
397 |     let mut tx_builder = wallet.build_tx();
    |         -------------- has type `TxBuilder<'_, Store<'_, LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>>, BranchAndBoundCoinSelection, CreateTx>` which is not `Send`
...
407 |     client.broadcast(&tx).await?;
    |                           ^^^^^ await occurs here, with `mut tx_builder` maybe used later
...
411 | }
    | - `mut tx_builder` is later dropped here
note: required by a bound in `cln_plugin::Builder::<S, I, O>::rpcmethod`
   --> /home/cguida/.cargo/git/checkouts/lightning-a9dc92ee913af919/9675463/plugins/src/lib.rs:193:40
    |
189 |     pub fn rpcmethod<C, F>(mut self, name: &str, description: &str, callback: C) -> Builder<S, I, O>
    |            --------- required by a bound in this associated function
...
193 |         F: Future<Output = Response> + Send + Sync + 'static,
    |                                        ^^^^ required by this bound in `Builder::<S, I, O>::rpcmethod`

error: future cannot be shared between threads safely
   --> src/main.rs:56:10
    |
56  |         .rpcmethod(
    |          ^^^^^^^^^ future returned by `watchdescriptor` is not `Sync`
    |
    = help: within `impl std::future::Future<Output = Result<serde_json::Value, anyhow::Error>>`, the trait `Sync` is not implemented for `Rc<RefCell<&mut Wallet<Store<'_, LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>>>>>`
note: future is not `Sync` as this value is used across an await
   --> src/main.rs:407:27
    |
397 |     let mut tx_builder = wallet.build_tx();
    |         -------------- has type `TxBuilder<'_, Store<'_, LocalChangeSet<KeychainKind, ConfirmationTimeAnchor>>, BranchAndBoundCoinSelection, CreateTx>` which is not `Sync`
...
407 |     client.broadcast(&tx).await?;
    |                           ^^^^^ await occurs here, with `mut tx_builder` maybe used later
...
411 | }
    | - `mut tx_builder` is later dropped here
note: required by a bound in `cln_plugin::Builder::<S, I, O>::rpcmethod`
   --> /home/cguida/.cargo/git/checkouts/lightning-a9dc92ee913af919/9675463/plugins/src/lib.rs:193:47
    |
189 |     pub fn rpcmethod<C, F>(mut self, name: &str, description: &str, callback: C) -> Builder<S, I, O>
    |            --------- required by a bound in this associated function
...
193 |         F: Future<Output = Response> + Send + Sync + 'static,
    |                                               ^^^^ required by this bound in `Builder::<S, I, O>::rpcmethod`

error: future cannot be shared between threads safely
   --> src/main.rs:56:10
    |
56  |         .rpcmethod(
    |          ^^^^^^^^^ future returned by `watchdescriptor` is not `Sync`
    |
    = help: the trait `Sync` is not implemented for `dyn std::future::Future<Output = Result<LocalUpdate<KeychainKind, ConfirmationTimeAnchor>, bdk_esplora::esplora_client::Error>> + Send`
note: future is not `Sync` as it awaits another future which is not `Sync`
   --> src/main.rs:370:18
    |
370 |       let update = client
    |  __________________^
371 | |         .scan(
372 | |             local_chain,
373 | |             keychain_spks,
...   |
377 | |             PARALLEL_REQUESTS,
378 | |         )
    | |_________^ await occurs here on type `Pin<Box<dyn std::future::Future<Output = Result<LocalUpdate<KeychainKind, ConfirmationTimeAnchor>, bdk_esplora::esplora_client::Error>> + Send>>`, which is not `Sync`
note: required by a bound in `cln_plugin::Builder::<S, I, O>::rpcmethod`
   --> /home/cguida/.cargo/git/checkouts/lightning-a9dc92ee913af919/9675463/plugins/src/lib.rs:193:47
    |
189 |     pub fn rpcmethod<C, F>(mut self, name: &str, description: &str, callback: C) -> Builder<S, I, O>
    |            --------- required by a bound in this associated function
...
193 |         F: Future<Output = Response> + Send + Sync + 'static,
    |                                               ^^^^ required by this bound in `Builder::<S, I, O>::rpcmethod`

warning: `watchdescriptor` (bin "watchdescriptor") generated 16 warnings
error: could not compile `watchdescriptor` (bin "watchdescriptor") due to 3 previous errors; 16 warnings emitted

[chrisguida]

Btw the above is just a straight copy/paste of the wallet_esplora_async example-crate's main function into a cln-plugin callback function, so I would expect it to "just work".

[junderw]

Try adding drop(tx_builder); after you call finish()

[junderw]

Then try tokio::runtime::Handle::try_current()?.block_on(client.scan(...))? instead of just as-is...