liquidity: Allow setting process_events callback in c_bindings

[tnull]

To trigger message processing, we previously had the user set a callback to PeerManager::process_events via an Fn() callback. This is however not supported by c_bindings.

Here, we therefore introduce as ProcessMesssagesCallback trait that can be used via LiquidityManager::set_process_msgs_callback_fn, which is exposed in c_bindings.

(Also confirmed to work with LDK Node)

[TheBlueMatt]

What's the difference here compared to just leaving only this for Rust users? If we require ownership anyway, the caller probably doesn't have the callback in a Box to begin with, so us doing it for them even in Rust seems fine?

[tnull]

Mhh, good point. I basically followed the recipe in Future/FutureCallback. I guess there we could also consider dropping the trait version in favor of the _fn version?

[tnull]

Took the liberty to force-push with these additional changes (to avoid another review roundtrip for the squashing):

> git
diff-tree -U2 c6abf0902 e05b76af7
diff --git a/lightning-liquidity/src/manager.rs b/lightning-liquidity/src/manager.rs
index 6b6b146af..a4c130333 100644
--- a/lightning-liquidity/src/manager.rs
+++ b/lightning-liquidity/src/manager.rs
@@ -311,25 +311,11 @@ where {
        /// let process_msgs_callback = move || process_msgs_pm.process_events();
        ///
-       /// my_liquidity_manager.set_process_msgs_callback(Box::new(process_msgs_callback));
+       /// my_liquidity_manager.set_process_msgs_callback(process_msgs_callback);
        /// # }
        /// ```
        ///
        /// [`PeerManager::process_events`]: lightning::ln::peer_handler::PeerManager::process_events
-       pub fn set_process_msgs_callback(&self, callback: Box<dyn ProcessMessagesCallback>) {
-               self.pending_messages.set_process_msgs_callback(callback);
-       }
-
-       /// Allows to set a callback that will be called after new messages are pushed to the message
-       /// queue.
-       ///
-       /// C bindings don't (currently) know how to map `Box<dyn Trait>`, and while it could add the
-       /// following wrapper, doing it in the bindings is currently much more work than simply doing it
-       /// here.
-       ///
-       /// Hence we simply allow setting a callback function that will be set via
-       /// [`Self::set_process_msgs_callback`] internally.
-       #[cfg(c_bindings)]
-       pub fn set_process_msgs_callback_fn<F: 'static + ProcessMessagesCallback>(&self, callback: F) {
-               self.set_process_msgs_callback(Box::new(callback));
+       pub fn set_process_msgs_callback<F: 'static + ProcessMessagesCallback>(&self, callback: F) {
+               self.pending_messages.set_process_msgs_callback(Box::new(callback));
        }

diff --git a/lightning-liquidity/tests/common/mod.rs b/lightning-liquidity/tests/common/mod.rs
index faf4cc5a6..8b8507a9f 100644
--- a/lightning-liquidity/tests/common/mod.rs
+++ b/lightning-liquidity/tests/common/mod.rs
@@ -479,8 +479,5 @@ pub(crate) fn create_liquidity_node(
        let process_msgs_flag = Arc::clone(&check_msgs_processed);
        let process_msgs_callback = move || process_msgs_flag.store(true, Ordering::Release);
-       #[cfg(not(c_bindings))]
-       liquidity_manager.set_process_msgs_callback(Box::new(process_msgs_callback));
-       #[cfg(c_bindings)]
-       liquidity_manager.set_process_msgs_callback_fn(process_msgs_callback);
+       liquidity_manager.set_process_msgs_callback(process_msgs_callback);

        Node {

[TheBlueMatt]

Super straightforward, just gonna land this.

[TheBlueMatt]

Backported in #3536.