fix race with chainHead single subscription ability

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

examples/examples/chainhead_subscription.rs

@@ -13,6 +13,7 @@
 use codec::Encode;
 use futures::StreamExt;
 use sp_keyring::AccountKeyring;
+use std::time::Duration;
 use subxt::{
     OnlineClient,
     PolkadotConfig,
@@ -31,8 +32,14 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let genesis = api.rpc().chainhead_genesis_hash().await?;
     println!("Genesis: {:?}", genesis);
 
-    let mut follow_sub = api.blocks().subscribe_chainhead_finalized(false).await?;
+    // TODO: spec v2 no longer uses the `pipe_from_stream`, therefore we need some
+    // time between two sequencial `chainHead_follow` subscription to propagate
+    // the `chainHead_unfollow` from the first subscription.
+    tokio::time::sleep(Duration::from_secs(5)).await;
+
+    let mut follow_sub = api.blocks().subscribe_chainhead_finalized(true).await?;
     println!("Following subscription...");
+
     // Handle all subscriptions from the `chainHead_follow`.
     while let Some(block) = follow_sub.next().await {
         let block = block?;
@@ -56,7 +63,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         let call = block
             .call("AccountNonceApi_account_nonce".into(), Some(&call_params))
             .await?;
-        // println!("[hash={:?}] call={:?}", block.hash(), call);
+        println!("[hash={:?}] call={:?}", block.hash(), call);
     }
 
     // Subscribe to the `chainHead_follow` method.

subxt/src/rpc/rpc.rs

@@ -891,8 +891,7 @@ impl<T: Config> Rpc<T> {
                 .await?;
 
             println!("Dropping subscription");
-            // Manually cancel the susbcription
+            drop(sub);
-            // {"id": 1, "method": "eth_unsubscribe", "params": ["0xcd0c3e8af590364c09d0fa6a1210faf5"]}
 
             let result: bool = self
                 .client
@@ -902,7 +901,6 @@ impl<T: Config> Rpc<T> {
                 )
                 .await?;
             println!("Subscription dropped with result={:?}", result);
-            drop(sub);
 
             (bytes, event)
         };