From a79ec19fa97b6a5ca8fd934cc53ca29f5912ad4b Mon Sep 17 00:00:00 2001
From: Svyatoslav Nikolsky <svyatonik@gmail.com>
Date: Thu, 13 Jul 2023 17:57:56 +0300
Subject: [PATCH] introduce partial bp-xcm-bridge-hub - just
 LocalXcmChannelManager (#2265)

---
 Cargo.lock                                   |  1 +
 bridges/primitives/xcm-bridge-hub/Cargo.toml |  8 ++-
 bridges/primitives/xcm-bridge-hub/src/lib.rs | 57 +++++++++++++++++++-
 3 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index 1e3fb21b49a98..357a5d77d24c6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1928,6 +1928,7 @@ name = "bp-xcm-bridge-hub"
 version = "0.2.0"
 dependencies = [
  "sp-std 14.0.0",
+ "staging-xcm",
 ]
 
 [[package]]
diff --git a/bridges/primitives/xcm-bridge-hub/Cargo.toml b/bridges/primitives/xcm-bridge-hub/Cargo.toml
index 27881bc99d1f8..288bbd9af69af 100644
--- a/bridges/primitives/xcm-bridge-hub/Cargo.toml
+++ b/bridges/primitives/xcm-bridge-hub/Cargo.toml
@@ -15,6 +15,12 @@ workspace = true
 # Substrate Dependencies
 sp-std = { path = "../../../substrate/primitives/std", default-features = false }
 
+# Polkadot Dependencies
+xcm = { package = "staging-xcm", path = "../../../polkadot/xcm", default-features = false }
+
 [features]
 default = ["std"]
-std = ["sp-std/std"]
+std = [
+	"sp-std/std",
+	"xcm/std",
+]
diff --git a/bridges/primitives/xcm-bridge-hub/src/lib.rs b/bridges/primitives/xcm-bridge-hub/src/lib.rs
index 9745011c902d2..78f913273370d 100644
--- a/bridges/primitives/xcm-bridge-hub/src/lib.rs
+++ b/bridges/primitives/xcm-bridge-hub/src/lib.rs
@@ -16,9 +16,64 @@
 
 //! Primitives of the xcm-bridge-hub pallet.
 
-#![warn(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
 
+use xcm::latest::prelude::*;
+
 /// Encoded XCM blob. We expect the bridge messages pallet to use this blob type for both inbound
 /// and outbound payloads.
 pub type XcmAsPlainPayload = sp_std::vec::Vec<u8>;
+
+/// A manager of XCM communication channels between the bridge hub and parent/sibling chains
+/// that have opened bridges at this bridge hub.
+///
+/// We use this interface to suspend and resume channels programmatically to implement backpressure
+/// mechanism for bridge queues.
+#[allow(clippy::result_unit_err)] // XCM uses `Result<(), ()>` everywhere
+pub trait LocalXcmChannelManager {
+	// TODO: https://github.com/paritytech/parity-bridges-common/issues/2255
+	// check following assumptions. They are important at least for following cases:
+	// 1) we now close the associated outbound lane when misbehavior is reported. If we'll keep
+	//    handling inbound XCM messages after the `suspend_inbound_channel`, they will be dropped
+	// 2) the sender will be able to enqueue message to othe lanes if we won't stop handling inbound
+	//    XCM immediately. He even may open additional bridges
+
+	/// Stop handling new incoming XCM messages from given bridge `owner` (parent/sibling chain).
+	///
+	/// We assume that the channel will be suspended immediately, but we don't mind if inbound
+	/// messages will keep piling up here for some time. Once this is communicated to the
+	/// `owner` chain (in any form), we expect it to stop sending messages to us and queue
+	/// messages at that `owner` chain instead.
+	///
+	/// This method will be called if we detect a misbehavior in one of bridges, owned by
+	/// the `owner`. We expect that:
+	///
+	/// - no more incoming XCM messages from the `owner` will be processed until further
+	///  `resume_inbound_channel` call;
+	///
+	/// - soon after the call, the channel will switch to the state when incoming messages are
+	///   piling up at the sending chain, not at the bridge hub.
+	///
+	/// This method shall not fail if the channel is already suspended.
+	fn suspend_inbound_channel(owner: Location) -> Result<(), ()>;
+
+	/// Start handling incoming messages from from given bridge `owner` (parent/sibling chain)
+	/// again.
+	///
+	/// This method is called when the `owner` tries to resume bridge operations after
+	/// resolving "misbehavior" issues. The channel is assumed to be suspended by the previous
+	/// `suspend_inbound_channel` call, however we don't check it anywhere.
+	///
+	/// This method shall not fail if the channel is already resumed.
+	fn resume_inbound_channel(owner: Location) -> Result<(), ()>;
+}
+
+impl LocalXcmChannelManager for () {
+	fn suspend_inbound_channel(_owner: Location) -> Result<(), ()> {
+		Ok(())
+	}
+
+	fn resume_inbound_channel(_owner: Location) -> Result<(), ()> {
+		Err(())
+	}
+}