subxt: Remove tokio as dependency, use it only for testing

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

Cargo.lock

@@ -4547,6 +4547,7 @@ dependencies = [
  "tracing",
  "tracing-subscriber 0.3.18",
  "url",
+ "wasm-bindgen-futures",
 ]
 
 [[package]]

subxt/Cargo.toml

@@ -61,7 +61,7 @@ unstable-metadata = []
 
 # Activate this to expose the Light Client functionality.
 # Note that this feature is experimental and things may break or not work as expected.
-unstable-light-client = ["subxt-lightclient", "tokio-stream", "tokio"]
+unstable-light-client = ["subxt-lightclient", "tokio-stream"]
 
 [dependencies]
 async-trait = { workspace = true }
@@ -117,7 +117,6 @@ getrandom = { workspace = true, optional = true }
 
 # Included if "native" feature is enabled
 tokio-util = { workspace = true, features = ["compat"], optional = true }
-tokio = { workspace = true, features = ["macros", "time", "rt-multi-thread"], optional = true }
 
 [dev-dependencies]
 bitvec = { workspace = true }
@@ -129,6 +128,7 @@ sp-keyring = { workspace = true }
 sp-runtime = { workspace = true }
 assert_matches = { workspace = true }
 subxt-signer = { path = "../signer" }
+wasm-bindgen-futures = { workspace = true }
 # Tracing subscriber is useful for light-client examples to ensure that
 # the `bootNodes` and chain spec are configured correctly. If all is fine, then
 # the light-client wlll emit INFO logs with

subxt/src/client/light_client/mod.rs

@@ -94,27 +94,43 @@ impl RawLightClient {
         let raw_rpc = self.raw_rpc.for_chain(chain_id);
         let rpc_client = RpcClient::new(raw_rpc.clone());
 
-        use crate::backend::unstable::UnstableBackend;
-        use std::sync::Arc;
-        let (backend, mut driver) = UnstableBackend::builder().build(rpc_client);
-        let future = async move {
-            use futures::StreamExt;
-            while let Some(val) = driver.next().await {
-                if let Err(e) = val {
-                    // This is a test; bail if something does wrong and try to
-                    // ensure that the message makes it to some logs.
-                    panic!("Error driving unstable backend in tests: {e}");
+        // Production code should use the legacy backend.
+        #[cfg(not(test))]
+        async fn build_online_client<TChainConfig: Config>(
+            rpc_client: RpcClient,
+        ) -> Result<OnlineClient<TChainConfig>, crate::Error> {
+            OnlineClient::<TChainConfig>::from_rpc_client(rpc_client).await
+        }
+
+        // For testing we are only interested in using the unstable-backend with
+        // the lightclient.
+        #[cfg(test)]
+        async fn build_online_client<TChainConfig: Config>(
+            rpc_client: RpcClient,
+        ) -> Result<OnlineClient<TChainConfig>, crate::Error> {
+            use crate::backend::unstable::UnstableBackend;
+            use std::sync::Arc;
+            let (backend, mut driver) = UnstableBackend::builder().build(rpc_client);
+            let future = async move {
+                use futures::StreamExt;
+                while let Some(val) = driver.next().await {
+                    if let Err(e) = val {
+                        // This is a test; bail if something does wrong and try to
+                        // ensure that the message makes it to some logs.
+                        panic!("Error driving unstable backend in tests: {e}");
+                    }
                 }
-            }
-        };
-        // The unstable backend needs driving:
-        #[cfg(feature = "native")]
-        tokio::spawn(future);
-        #[cfg(feature = "web")]
-        wasm_bindgen_futures::spawn_local(future);
-        let client = OnlineClient::from_backend(Arc::new(backend))
-            .await
-            .map_err(|e| format!("Cannot construct OnlineClient from backend: {e}"))?;
+            };
+            // The unstable backend needs driving:
+            #[cfg(feature = "native")]
+            tokio::spawn(future);
+            #[cfg(feature = "web")]
+            wasm_bindgen_futures::spawn_local(future);
+
+            OnlineClient::from_backend(Arc::new(backend)).await
+        }
+
+        let client = build_online_client(rpc_client).await?;
 
         Ok(LightClient { client, chain_id })
     }