feat: add low-level runtime upgrade API (#657)

* feat: add low-level runtime upgrade API

* grumbles

* cargo fmt

* dont use apply_update in perform_runtime_upgrades

* fix nit

* remove metadata check

* fix doc tests

* fix final comments

* Update subxt/src/client/online_client.rs

* cargo fmt

* Update subxt/src/client/online_client.rs

subxt/src/client/mod.rs

@@ -16,6 +16,9 @@ pub use offline_client::{
     OfflineClientT,
 };
 pub use online_client::{
+    ClientRuntimeUpdater,
     OnlineClient,
     OnlineClientT,
+    RuntimeUpdaterStream,
+    UpgradeError,
 };

subxt/src/client/online_client.rs

@@ -14,6 +14,7 @@ use crate::{
         Rpc,
         RpcClientT,
         RuntimeVersion,
+        Subscription,
     },
     storage::StorageClient,
     tx::TxClient,
@@ -33,7 +34,7 @@ pub trait OnlineClientT<T: Config>: OfflineClientT<T> {
 }
 
 /// A client that can be used to perform API calls (that is, either those
-/// requiriing an [`OfflineClientT`] or those requiring an [`OnlineClientT`]).
+/// requiring an [`OfflineClientT`] or those requiring an [`OnlineClientT`]).
 #[derive(Derivative)]
 #[derivative(Clone(bound = ""))]
 pub struct OnlineClient<T: Config> {
@@ -102,7 +103,7 @@ impl<T: Config> OnlineClient<T> {
         })
     }
 
-    /// Create an object which can be used to keep the runtime uptodate
+    /// Create an object which can be used to keep the runtime up to date
     /// in a separate thread.
     ///
     /// # Example
@@ -114,10 +115,33 @@ impl<T: Config> OnlineClient<T> {
     ///
     /// let client = OnlineClient::<PolkadotConfig>::new().await.unwrap();
     ///
+    /// // high level API.
+    ///
     /// let update_task = client.subscribe_to_updates();
     /// tokio::spawn(async move {
     ///     update_task.perform_runtime_updates().await;
     /// });
+    ///
+    ///
+    /// // low level API.
+    ///
+    /// let updater = client.subscribe_to_updates();
+    /// tokio::spawn(async move {
+    ///     let mut update_stream = updater.runtime_updates().await.unwrap();
+    ///
+    ///     while let Some(Ok(update)) = update_stream.next().await {
+    ///         let version = update.runtime_version().spec_version;
+    ///
+    ///         match updater.apply_update(update) {
+    ///             Ok(()) => {
+    ///                 println!("Upgrade to version: {} successful", version)
+    ///             }
+    ///             Err(e) => {
+    ///                println!("Upgrade to version {} failed {:?}", version, e);
+    ///             }
+    ///        };
+    ///     }
+    /// });
     /// # }
     /// ```
     pub fn subscribe_to_updates(&self) -> ClientRuntimeUpdater<T> {
@@ -209,34 +233,110 @@ impl<T: Config> ClientRuntimeUpdater<T> {
         &curr.runtime_version != new
     }
 
+    fn do_update(&self, update: Update) {
+        let mut writable = self.0.inner.write();
+        writable.metadata = update.metadata;
+        writable.runtime_version = update.runtime_version;
+    }
+
+    /// Tries to apply a new update.
+    pub fn apply_update(&self, update: Update) -> Result<(), UpgradeError> {
+        if !self.is_runtime_version_different(&update.runtime_version) {
+            return Err(UpgradeError::SameVersion)
+        }
+
+        self.do_update(update);
+
+        Ok(())
+    }
+
     /// Performs runtime updates indefinitely unless encountering an error.
     ///
     /// *Note:* This will run indefinitely until it errors, so the typical usage
     /// would be to run it in a separate background task.
     pub async fn perform_runtime_updates(&self) -> Result<(), Error> {
         // Obtain an update subscription to further detect changes in the runtime version of the node.
-        let mut update_subscription = self.0.rpc.subscribe_runtime_version().await?;
+        let mut runtime_version_stream = self.runtime_updates().await?;
 
-        while let Some(new_runtime_version) = update_subscription.next().await {
-            // The Runtime Version obtained via subscription.
-            let new_runtime_version = new_runtime_version?;
+        while let Some(update) = runtime_version_stream.next().await {
+            let update = update?;
 
-            // Ignore this update if there is no difference.
-            if !self.is_runtime_version_different(&new_runtime_version) {
-                continue
-            }
-
-            // Fetch new metadata.
-            let new_metadata = self.0.rpc.metadata().await?;
-
-            // Do the update.
-            let mut writable = self.0.inner.write();
-            writable.metadata = new_metadata;
-            writable.runtime_version = new_runtime_version;
+            // This only fails if received the runtime version is the same the current runtime version
+            // which might occur because that runtime subscriptions in substrate sends out the initial
+            // value when they created and not only when runtime upgrades occurs.
+            // Thus, fine to ignore here as it strictly speaking isn't really an error
+            let _ = self.apply_update(update);
         }
 
         Ok(())
     }
+
+    /// Low-level API to get runtime updates as a stream but it's doesn't check if the
+    /// runtime version is newer or updates the runtime.
+    ///
+    /// Instead that's up to the user of this API to decide when to update and
+    /// to perform the actual updating.
+    pub async fn runtime_updates(&self) -> Result<RuntimeUpdaterStream<T>, Error> {
+        let stream = self.0.rpc().subscribe_runtime_version().await?;
+        Ok(RuntimeUpdaterStream {
+            stream,
+            client: self.0.clone(),
+        })
+    }
+}
+
+/// Stream to perform runtime upgrades.
+pub struct RuntimeUpdaterStream<T: Config> {
+    stream: Subscription<RuntimeVersion>,
+    client: OnlineClient<T>,
+}
+
+impl<T: Config> RuntimeUpdaterStream<T> {
+    /// Get the next element of the stream.
+    pub async fn next(&mut self) -> Option<Result<Update, Error>> {
+        let maybe_runtime_version = self.stream.next().await?;
+
+        let runtime_version = match maybe_runtime_version {
+            Ok(runtime_version) => runtime_version,
+            Err(err) => return Some(Err(err)),
+        };
+
+        let metadata = match self.client.rpc().metadata().await {
+            Ok(metadata) => metadata,
+            Err(err) => return Some(Err(err)),
+        };
+
+        Some(Ok(Update {
+            metadata,
+            runtime_version,
+        }))
+    }
+}
+
+/// Error that can occur during upgrade.
+#[non_exhaustive]
+#[derive(Debug, Clone)]
+pub enum UpgradeError {
+    /// The version is the same as the current version.
+    SameVersion,
+}
+
+/// Represents the state when a runtime upgrade occurred.
+pub struct Update {
+    runtime_version: RuntimeVersion,
+    metadata: Metadata,
+}
+
+impl Update {
+    /// Get the runtime version.
+    pub fn runtime_version(&self) -> &RuntimeVersion {
+        &self.runtime_version
+    }
+
+    /// Get the metadata.
+    pub fn metadata(&self) -> &Metadata {
+        &self.metadata
+    }
 }
 
 // helpers for a jsonrpsee specific OnlineClient.