Add import_waiter for RpcBlockchain.

Bitcoin Core rejects importing scriptPubKeys/descriptors when it is
still rescanning the wallet. We should wait until rescan completes
before importing again.

src/blockchain/rpc.rs

@@ -46,6 +46,7 @@ use bitcoincore_rpc::json::{
     ImportMultiRequestScriptPubkey, ImportMultiRescanSince, ListTransactionResult,
     ListUnspentResultEntry, ScanningDetails,
 };
+use bitcoincore_rpc::jsonrpc::error::RpcError;
 use bitcoincore_rpc::jsonrpc::serde_json::{json, Value};
 use bitcoincore_rpc::jsonrpc::{
     self, simple_http::SimpleHttpTransport, Error as JsonRpcError, Request, Response, Transport,
@@ -274,9 +275,9 @@ macro_rules! impl_inner {
                             if io.kind() == std::io::ErrorKind::WouldBlock =>
                         {
                             let attempt = $self.attempts.fetch_add(1, Ordering::Relaxed);
-                            let delay = std::cmp::min(1000, 100 << attempt as u64);
+                            let delay = std::cmp::min(5000, 100 << attempt as u64);
 
-                            debug!(
+                            warn!(
                                 "Got a WouldBlock error at attempt {}, sleeping for {}ms",
                                 attempt, delay
                             );
@@ -543,7 +544,6 @@ impl<'a, D: BatchDatabase> DbState<'a, D> {
             scripts_iter,
             self.prog,
         )?;
-        // await_wallet_scan(client, self.params.poll_rate_sec, self.prog)?;
 
         // update import_params
         import_params.external_start_index = self.ext_spks.len();
@@ -874,6 +874,8 @@ where
     Ok(())
 }
 
+/// Import descriptors/scriptPubKeys in a pagenated manner, and wait for rescan to complete (if 
+/// import is successful or errored with code -4).
 fn pagenated_import<'a, S>(
     client: &Client,
     use_desc: bool,
@@ -886,6 +888,8 @@ fn pagenated_import<'a, S>(
 where
     S: Iterator<Item = &'a Script> + Clone,
 {
+    use bitcoincore_rpc::Error as CoreError;
+
     (0_usize..)
         .map(|page_index| {
             scripts_iter
@@ -897,12 +901,26 @@ where
         })
         .take_while(|scripts| !scripts.is_empty())
         .try_for_each(|scripts| {
-            if use_desc {
-                import_descriptors(client, start_epoch, scripts.iter())?;
+            let result = if use_desc {
+                import_descriptors(client, start_epoch, scripts.iter())
             } else {
-                import_multi(client, start_epoch, scripts.iter())?;
+                import_multi(client, start_epoch, scripts.iter())
+            };
+
+            // On successful import, or unsuccessful with code -4, we wait until rescan is complete
+            // and return. Any other error will be returned immediately.
+            match result {
+                Err(Error::Rpc(CoreError::JsonRpc(JsonRpcError::Rpc(RpcError {
+                    code: -4,
+                    message,
+                    ..
+                })))) => {
+                    warn!("import return with code -4: \"{}\", waiting...", message);
+                    Ok(())
+                }
+                res => res,
             }
-            await_wallet_scan(client, poll_rate_sec, progress)
+            .and_then(|_| await_wallet_scan(client, poll_rate_sec, progress))
         })
 }