rpc: Expose chainHead_unpin method for blocks

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

examples/examples/chainhead_subscription.rs

@@ -64,20 +64,9 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             .call("AccountNonceApi_account_nonce".into(), Some(&call_params))
             .await?;
         println!("[hash={:?}] call={:?}", block.hash(), call);
-    }
 
-    // Subscribe to the `chainHead_follow` method.
-    let mut follow_sub = api.rpc().subscribe_chainhead_follow(false).await?;
-
-    // Handle all subscriptions from the `chainHead_follow`.
-    while let Some(event) = follow_sub.next().await {
-        let event = event?;
-
-        println!(
-            "sub_id: {:?} event: {:?}",
-            follow_sub.subscription_id(),
-            event
-        );
+        // Unpin the block as last step.
+        block.unpin().await?;
     }
 
     Ok(())

subxt/src/blocks/block_types.rs

@@ -162,6 +162,22 @@ where
             "Failed to execute the runtime API call".into(),
         ))
     }
+
+    /// Unpin this block.
+    ///
+    /// # Note
+    ///
+    /// Call this method when you are no longer interested in making queries
+    /// against this block.
+    ///
+    /// Failing to call this method will eventually terminate the subscription.
+    pub async fn unpin(self) -> Result<(), Error> {
+        self.client
+            .rpc()
+            .chainhead_unpin(self.subscription_id, self.hash)
+            .await?;
+        Ok(())
+    }
 }
 
 /// A representation of a block.

subxt/src/rpc/rpc.rs

@@ -698,6 +698,22 @@ impl<T: Config> Rpc<T> {
         Ok(header)
     }
 
+    /// Unpin a block reported by the `chainHead_follow` subscription.
+    pub async fn chainhead_unpin(
+        &self,
+        subscription_id: String,
+        hash: T::Hash,
+    ) -> Result<(), Error> {
+        self.client
+            .request(
+                "chainHead_unstable_unpin",
+                rpc_params![subscription_id, hash],
+            )
+            .await?;
+
+        Ok(())
+    }
+
     /// Parse `chaiHead_header` response and return the block's header.
     ///
     /// # Note