Rework light client (#1475)

* WIP second pass over light client code for simpler API

* First pass new light client

* pub(crate) LightClientRpc::new_raw(), and fmt

* Update examples and add back a way to configure boot nodes and fetch chainspec from a URL

* Fix light client examples

* remove unused deps and tidy lightclient feature flags

* fix wasm error

* LightClientRpc can be cloned

* update light client tests

* Other small fixes

* exclude mod unless jsonrpsee

* Fix wasm-lightclient-tests

* add back docsrs bit and web+native feature flag compile error

* update book and light client example names

* fix docs

subxt/src/utils/fetch_chain_spec.rs

@@ -0,0 +1,113 @@
+// Copyright 2019-2024 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+use crate::macros::{cfg_jsonrpsee_native, cfg_jsonrpsee_web};
+use serde_json::value::RawValue;
+
+/// Possible errors encountered trying to fetch a chain spec from an RPC node.
+#[derive(thiserror::Error, Debug)]
+#[allow(missing_docs)]
+pub enum FetchChainspecError {
+    #[error("Cannot fetch chain spec: RPC error: {0}.")]
+    RpcError(String),
+    #[error("Cannot fetch chain spec: Invalid URL.")]
+    InvalidUrl,
+    #[error("Cannot fetch chain spec: Invalid URL scheme.")]
+    InvalidScheme,
+    #[error("Cannot fetch chain spec: Handshake error establishing WS connection.")]
+    HandshakeError,
+}
+
+/// Fetch a chain spec from an RPC node at the given URL.
+pub async fn fetch_chainspec_from_rpc_node(
+    url: impl AsRef<str>,
+) -> Result<Box<RawValue>, FetchChainspecError> {
+    use jsonrpsee::core::client::{ClientT, SubscriptionClientT};
+    use jsonrpsee::rpc_params;
+
+    let client = jsonrpsee_helpers::client(url.as_ref()).await?;
+
+    let result = client
+        .request("sync_state_genSyncSpec", jsonrpsee::rpc_params![true])
+        .await
+        .map_err(|err| FetchChainspecError::RpcError(err.to_string()))?;
+
+    // Subscribe to the finalized heads of the chain.
+    let mut subscription = SubscriptionClientT::subscribe::<Box<RawValue>, _>(
+        &client,
+        "chain_subscribeFinalizedHeads",
+        rpc_params![],
+        "chain_unsubscribeFinalizedHeads",
+    )
+    .await
+    .map_err(|err| FetchChainspecError::RpcError(err.to_string()))?;
+
+    // We must ensure that the finalized block of the chain is not the block included
+    // in the chainSpec.
+    // This is a temporary workaround for: https://github.com/smol-dot/smoldot/issues/1562.
+    // The first finalized block that is received might by the finalized block could be the one
+    // included in the chainSpec. Decoding the chainSpec for this purpose is too complex.
+    let _ = subscription.next().await;
+    let _ = subscription.next().await;
+
+    Ok(result)
+}
+
+cfg_jsonrpsee_native! {
+    mod jsonrpsee_helpers {
+        use super::FetchChainspecError;
+        use tokio_util::compat::Compat;
+
+        pub use jsonrpsee::{
+            client_transport::ws::{self, EitherStream, Url, WsTransportClientBuilder},
+            core::client::Client,
+        };
+
+        pub type Sender = ws::Sender<Compat<EitherStream>>;
+        pub type Receiver = ws::Receiver<Compat<EitherStream>>;
+
+        /// Build WS RPC client from URL
+        pub async fn client(url: &str) -> Result<Client, FetchChainspecError> {
+            let url = Url::parse(url).map_err(|_| FetchChainspecError::InvalidUrl)?;
+
+                if url.scheme() != "ws" && url.scheme() != "wss" {
+                    return Err(FetchChainspecError::InvalidScheme);
+                }
+
+                let (sender, receiver) = ws_transport(url).await?;
+
+                Ok(Client::builder()
+                .max_buffer_capacity_per_subscription(4096)
+                .build_with_tokio(sender, receiver))
+            }
+
+            async fn ws_transport(url: Url) -> Result<(Sender, Receiver), FetchChainspecError> {
+                WsTransportClientBuilder::default()
+                    .build(url)
+                    .await
+                    .map_err(|_| FetchChainspecError::HandshakeError)
+            }
+        }
+}
+
+cfg_jsonrpsee_web! {
+    mod jsonrpsee_helpers {
+        use super::FetchChainspecError;
+        pub use jsonrpsee::{
+            client_transport::web,
+            core::client::{Client, ClientBuilder},
+        };
+
+        /// Build web RPC client from URL
+        pub async fn client(url: &str) -> Result<Client, FetchChainspecError> {
+            let (sender, receiver) = web::connect(url)
+                .await
+                .map_err(|_| FetchChainspecError::HandshakeError)?;
+
+            Ok(ClientBuilder::default()
+                .max_buffer_capacity_per_subscription(4096)
+                .build_with_wasm(sender, receiver))
+        }
+    }
+}

subxt/src/utils/mod.rs

@@ -14,6 +14,7 @@ mod unchecked_extrinsic;
 mod wrapper_opaque;
 
 use crate::error::RpcError;
+use crate::macros::cfg_jsonrpsee;
 use crate::Error;
 use codec::{Compact, Decode, Encode};
 use derivative::Derivative;
@@ -27,6 +28,11 @@ pub use static_type::Static;
 pub use unchecked_extrinsic::UncheckedExtrinsic;
 pub use wrapper_opaque::WrapperKeepOpaque;
 
+cfg_jsonrpsee! {
+    mod fetch_chain_spec;
+    pub use fetch_chain_spec::{fetch_chainspec_from_rpc_node, FetchChainspecError};
+}
+
 // Used in codegen
 #[doc(hidden)]
 pub use primitive_types::{H160, H256, H512};