From b324eea4f0d215f30d60e4e34d6a969ad937fdc4 Mon Sep 17 00:00:00 2001
From: James Kay <james.kay@linera.io>
Date: Wed, 8 Oct 2025 20:03:54 +0100
Subject: [PATCH] `linera_core::client`: batch downloading of missing blobs
 (#4755)

Currently, when we synchronize a chain, even though we receive the
certificates in a batch to `process_certificates`, we handle them one by
one at the local node level, and if a certificate is missing blobs we
stop, download the blobs for that certificate, then retry, making the
download of the blobs sequential. This makes startup time for the client
linear in the number of certificates-with-blobs present in its initial
chains (notably, the admin chain).

We already have an ahead-of-time indicator of which blobs will be
required by the certificates, so there's no need to download them one at
a time. If we don't have some blobs marked as required by the
certificate batch, try to download them (concurrently) before proceeding
to process the batch. ~Thereafter, `BlobsNotFound` when processing the
batch is a hard error.~ `required_blob_ids()` is conservative, so we
still need to download blobs and retry if we get a `BlobsNotFound` error
thereafter.

CI.

- These changes should be backported to the latest `testnet` branch,
then
    - be released in a new SDK.
---
 linera-core/src/client/mod.rs | 71 ++++++++++++++++++++++++-----------
 1 file changed, 50 insertions(+), 21 deletions(-)

diff --git a/linera-core/src/client/mod.rs b/linera-core/src/client/mod.rs
index 936ac080326f..55e0dcd7ca1e 100644
--- a/linera-core/src/client/mod.rs
+++ b/linera-core/src/client/mod.rs
@@ -339,7 +339,7 @@ impl<Env: Environment> Client<Env> {
             }
         };
         for certificate in certificates {
-            last_info = Some(self.handle_certificate(Box::new(certificate)).await?.info);
+            last_info = Some(self.handle_certificate(certificate).await?.info);
         }
         // Now download the rest in batches from the remote node.
         while next_height < stop {
@@ -361,6 +361,24 @@ impl<Env: Environment> Client<Env> {
         Ok(last_info)
     }
 
+    async fn download_blobs(
+        &self,
+        remote_node: &RemoteNode<impl ValidatorNode>,
+        blob_ids: impl IntoIterator<Item = BlobId>,
+    ) -> Result<(), ChainClientError> {
+        self.local_node
+            .store_blobs(
+                &futures::stream::iter(blob_ids.into_iter().map(|blob_id| async move {
+                    remote_node.try_download_blob(blob_id).await.unwrap()
+                }))
+                .buffer_unordered(self.options.max_joined_tasks)
+                .collect::<Vec<_>>()
+                .await,
+            )
+            .await
+            .map_err(Into::into)
+    }
+
     /// Tries to process all the certificates, requesting any missing blobs from the given node.
     /// Returns the chain info of the last successfully processed certificate.
     #[instrument(level = "trace", skip_all)]
@@ -370,34 +388,47 @@ impl<Env: Environment> Client<Env> {
         certificates: Vec<ConfirmedBlockCertificate>,
     ) -> Result<Option<Box<ChainInfo>>, ChainClientError> {
         let mut info = None;
-        for certificate in certificates {
-            let certificate = Box::new(certificate);
-            let mut result = self.handle_certificate(certificate.clone()).await;
+        let required_blob_ids: Vec<_> = certificates
+            .iter()
+            .flat_map(|certificate| certificate.value().required_blob_ids())
+            .collect();
 
-            if let Err(LocalNodeError::BlobsNotFound(blob_ids)) = &result {
-                let blobs =
-                    futures::stream::iter(blob_ids.iter().copied().map(|blob_id| async move {
-                        remote_node.try_download_blob(blob_id).await.unwrap()
-                    }))
-                    .buffer_unordered(self.options.max_joined_tasks)
-                    .collect::<Vec<_>>()
-                    .await;
-                self.local_node.store_blobs(&blobs).await?;
-                result = self.handle_certificate(certificate.clone()).await;
+        match self
+            .local_node
+            .read_blob_states_from_storage(&required_blob_ids)
+            .await
+        {
+            Err(LocalNodeError::BlobsNotFound(blob_ids)) => {
+                self.download_blobs(remote_node, blob_ids).await?;
             }
+            x => {
+                x?;
+            }
+        }
 
-            info = Some(result?.info);
+        for certificate in certificates {
+            info = Some(
+                match self.handle_certificate(certificate.clone()).await {
+                    Err(LocalNodeError::BlobsNotFound(blob_ids)) => {
+                        self.download_blobs(remote_node, blob_ids).await?;
+                        self.handle_certificate(certificate).await?
+                    }
+                    x => x?,
+                }
+                .info,
+            );
         }
+
         // Done with all certificates.
         Ok(info)
     }
 
     async fn handle_certificate<T: ProcessableCertificate>(
         &self,
-        certificate: Box<GenericCertificate<T>>,
+        certificate: GenericCertificate<T>,
     ) -> Result<ChainInfoResponse, LocalNodeError> {
         self.local_node
-            .handle_certificate(*certificate, &self.notifier)
+            .handle_certificate(certificate, &self.notifier)
             .await
     }
 
@@ -489,7 +520,7 @@ impl<Env: Environment> Client<Env> {
         &self,
         certificate: Box<GenericCertificate<T>>,
     ) -> Result<(), LocalNodeError> {
-        let info = self.handle_certificate(certificate).await?.info;
+        let info = self.handle_certificate(*certificate).await?.info;
         self.update_from_info(&info);
         Ok(())
     }
@@ -707,8 +738,6 @@ impl<Env: Environment> Client<Env> {
         mode: ReceiveCertificateMode,
         nodes: Option<Vec<RemoteNode<Env::ValidatorNode>>>,
     ) -> Result<(), ChainClientError> {
-        let certificate = Box::new(certificate);
-
         // Verify the certificate before doing any expensive networking.
         let (max_epoch, committees) = self.admin_committees().await?;
         if let ReceiveCertificateMode::NeedsCheck = mode {
@@ -1145,7 +1174,7 @@ impl<Env: Environment> Client<Env> {
         };
 
         if let Some(timeout) = remote_info.manager.timeout {
-            self.handle_certificate(Box::new(*timeout)).await?;
+            self.handle_certificate(*timeout).await?;
         }
         let mut proposals = Vec::new();
         if let Some(proposal) = remote_info.manager.requested_signed_proposal {