Return an error if a pending blob is not needed by any pending block. (…

…#3244)

## Motivation

We currently don't detect if the blob in `HandlePendingBlob` was needed
at all.
(See [this
discussion](#3204 (comment)))

## Proposal

Return an error if no pending block needed it.

## Test Plan

CI should catch regressions.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- Discussion:
#3204 (comment)
- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

linera-chain/src/pending_blobs.rs

@@ -44,14 +44,16 @@ where
         Ok(self.pending_blobs.get(blob_id).await?.flatten())
     }
 
-    pub async fn maybe_insert(&mut self, blob: &Blob) -> Result<(), ViewError> {
+    /// Inserts the blob. Returns whether the blob was required by the pending block.
+    pub async fn maybe_insert(&mut self, blob: &Blob) -> Result<bool, ViewError> {
         let blob_id = blob.id();
-        if let Some(maybe_blob) = self.pending_blobs.get_mut(&blob_id).await? {
-            if maybe_blob.is_none() {
-                *maybe_blob = Some(blob.clone());
-            }
+        let Some(maybe_blob) = self.pending_blobs.get_mut(&blob_id).await? else {
+            return Ok(false);
+        };
+        if maybe_blob.is_none() {
+            *maybe_blob = Some(blob.clone());
         }
-        Ok(())
+        Ok(true)
     }
 
     pub async fn update(

linera-core/src/chain_worker/state/attempted_changes.rs

@@ -628,7 +628,8 @@ where
         &mut self,
         blob: Blob,
     ) -> Result<ChainInfoResponse, WorkerError> {
-        self.state
+        let mut was_expected = self
+            .state
             .chain
             .pending_validated_blobs
             .maybe_insert(&blob)
@@ -650,8 +651,9 @@ where
                     WorkerError::TooManyPublishedBlobs(policy.maximum_published_blobs)
                 );
             }
-            pending_blobs.maybe_insert(&blob).await?;
+            was_expected = was_expected || pending_blobs.maybe_insert(&blob).await?;
         }
+        ensure!(was_expected, WorkerError::UnexpectedBlob);
         self.save().await?;
         Ok(ChainInfoResponse::new(
             &self.state.chain,

linera-core/src/unit_tests/client_tests.rs

@@ -1387,9 +1387,13 @@ where
     };
     {
         let node3 = builder.node(3);
+        let content1 = blob1.into_content();
+        assert_matches!(node3.handle_pending_blob(chain_id2, content1.clone()).await,
+            Err(NodeError::WorkerError { error })
+            if error.contains("Blob was not required by any pending block")
+        );
         let result = node3.handle_validated_certificate(validated.clone()).await;
         assert_matches!(result, Err(NodeError::BlobsNotFound(_)));
-        let content1 = blob1.into_content();
         node3.handle_pending_blob(chain_id2, content1).await?;
         let response = node3
             .handle_validated_certificate(validated.clone())

linera-core/src/worker.rs

@@ -221,6 +221,8 @@ pub enum WorkerError {
     JoinError,
     #[error("Blob exceeds size limit")]
     BlobTooLarge,
+    #[error("Blob was not required by any pending block")]
+    UnexpectedBlob,
     #[error("Bytecode exceeds size limit")]
     BytecodeTooLarge,
     #[error("Number of published blobs per block must not exceed {0}")]