fix(fast-inbox): reconstruct each block's own L1-to-L2 message root from the blob (A-1384)

Post-flip the checkpoint blob carries a per-block L1-to-L2 message tree root, since any block within a
checkpoint can insert messages. The archiver reconstruction still read the root once from the checkpoint's
first block and applied it to every block, a stale pre-flip per-checkpoint assumption. Follower nodes
(prover / sync-only) that rebuild a non-first message-inserting block via retrievedToPublishedCheckpoint
therefore got the first block's root, so the world-state synchronizer inserted the correct message,
recomputed the real root, and mismatched the reconstructed header (block state does not match world state),
forking the node from the sequencer on the first message-consuming block. Reconstruct each block's tree
root from its own blob data instead.

Author: spalladino

yarn-project/archiver/src/l1/data_retrieval.test.ts

@@ -79,6 +79,19 @@ describe('data_retrieval', () => {
       expect(reconstructedBlock1.body.txEffects.map(tx => tx.txHash.toString())).not.toEqual(
         reconstructedBlock3.body.txEffects.map(tx => tx.txHash.toString()),
       );
+
+      // Each block's L1-to-L2 message tree root must be reconstructed from its own blob data, not the
+      // checkpoint's first block. Post-flip (AZIP-22 streaming inbox) any block can insert messages, so
+      // intra-checkpoint blocks carry distinct roots; using the first block's root forks follower nodes.
+      expect(reconstructedBlock1.header.state.l1ToL2MessageTree.root.toString()).toEqual(
+        block1BlobData.l1ToL2MessageRoot.toString(),
+      );
+      expect(reconstructedBlock2.header.state.l1ToL2MessageTree.root.toString()).toEqual(
+        block2BlobData.l1ToL2MessageRoot.toString(),
+      );
+      expect(reconstructedBlock3.header.state.l1ToL2MessageTree.root.toString()).toEqual(
+        block3BlobData.l1ToL2MessageRoot.toString(),
+      );
     });
 
     it('handles single-block checkpoint', async () => {

yarn-project/archiver/src/l1/data_retrieval.ts

@@ -83,16 +83,22 @@ export async function retrievedToPublishedCheckpoint({
     .slice(1)
     .concat([archiveRoot]);
 
-  // An error will be thrown from `decodeCheckpointBlobDataFromBlobs` if it can't read a field for the
-  // `l1ToL2MessageRoot` of the first block. So below we can safely assume it exists:
-  const l1toL2MessageTreeRoot = blocksBlobData[0].l1ToL2MessageRoot!;
-
   const spongeBlob = SpongeBlob.init();
   const l2Blocks: L2Block[] = [];
   for (let i = 0; i < blocksBlobData.length; i++) {
     const blockBlobData = blocksBlobData[i];
-    const { blockEndMarker, blockEndStateField, lastArchiveRoot, noteHashRoot, nullifierRoot, publicDataRoot } =
-      blockBlobData;
+    // Post-flip (AZIP-22 streaming inbox) the blob carries a per-block L1-to-L2 message tree root: any block
+    // within a checkpoint can insert messages, so reconstruction must use each block's own root rather than
+    // the checkpoint's first block.
+    const {
+      blockEndMarker,
+      blockEndStateField,
+      lastArchiveRoot,
+      noteHashRoot,
+      nullifierRoot,
+      publicDataRoot,
+      l1ToL2MessageRoot,
+    } = blockBlobData;
 
     const l2BlockNumber = blockEndMarker.blockNumber;
 
@@ -109,7 +115,7 @@ export async function retrievedToPublishedCheckpoint({
 
     const state = StateReference.from({
       l1ToL2MessageTree: new AppendOnlyTreeSnapshot(
-        l1toL2MessageTreeRoot,
+        l1ToL2MessageRoot,
         blockEndStateField.l1ToL2MessageNextAvailableLeafIndex,
       ),
       partial: PartialStateReference.from({