From 875c09840b13bbf4c45679727cfbe6bf83e04d29 Mon Sep 17 00:00:00 2001
From: Hector Sanjuan <code@hector.link>
Date: Sat, 22 Apr 2023 00:16:26 +0200
Subject: [PATCH] chainstore: Fix raw blocks getting scanned for links during
 snapshots (#10684)

We have to save raw blocks to the snapshot, but we should not be scanning them
for additional links as if they were CBOR blocks.

This cleans the logic a bit (we were checking that the parent was a CBOR block
before queueing up the children, but then scanning the children... it was weird).

Additionally, more verbose logging is added for the next time ScanForLinks
fails (currently very little info was given).

Our ScanForLinks callback should only enqueue CBOR for further processing.
---
 chain/store/snapshot.go | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/chain/store/snapshot.go b/chain/store/snapshot.go
index 66d23675e..307bd356c 100644
--- a/chain/store/snapshot.go
+++ b/chain/store/snapshot.go
@@ -371,6 +371,9 @@ func (s *walkScheduler) enqueueIfNew(task walkTask) {
 		//log.Infow("ignored", "cid", todo.c.String())
 		return
 	}
+
+	// This lets through RAW and CBOR blocks, the only two types that we
+	// end up writing to the exported CAR.
 	if task.c.Prefix().Codec != cid.Raw && task.c.Prefix().Codec != cid.DagCBOR {
 		//log.Infow("ignored", "cid", todo.c.String())
 		return
@@ -442,10 +445,19 @@ func (s *walkScheduler) processTask(t walkTask, workerN int) error {
 		b: blk,
 	}
 
+	// We exported the ipld block. If it wasn't a CBOR block, there's nothing
+	// else to do and we can bail out early as it won't have any links
+	// etc.
+	if t.c.Prefix().Codec != cid.DagCBOR || t.c.Prefix().MhType == mh.IDENTITY {
+		return nil
+	}
+
+	rawData := blk.RawData()
+
 	// extract relevant dags to walk from the block
 	if t.taskType == blockTask {
 		var b types.BlockHeader
-		if err := b.UnmarshalCBOR(bytes.NewBuffer(blk.RawData())); err != nil {
+		if err := b.UnmarshalCBOR(bytes.NewBuffer(rawData)); err != nil {
 			return xerrors.Errorf("unmarshalling block header (cid=%s): %w", blk, err)
 		}
 		if b.Height%1_000 == 0 {
@@ -521,11 +533,7 @@ func (s *walkScheduler) processTask(t walkTask, workerN int) error {
 	}
 
 	// Not a chain-block: we scan for CIDs in the raw block-data
-	return cbg.ScanForLinks(bytes.NewReader(blk.RawData()), func(c cid.Cid) {
-		if t.c.Prefix().Codec != cid.DagCBOR || t.c.Prefix().MhType == mh.IDENTITY {
-			return
-		}
-
+	err = cbg.ScanForLinks(bytes.NewReader(rawData), func(c cid.Cid) {
 		s.enqueueIfNew(walkTask{
 			c:                c,
 			taskType:         dagTask,
@@ -534,6 +542,13 @@ func (s *walkScheduler) processTask(t walkTask, workerN int) error {
 			epoch:            t.epoch,
 		})
 	})
+
+	if err != nil {
+		return xerrors.Errorf(
+			"ScanForLinks(%s). Task: %s. Block: %s (%s). Epoch: %d. Err: %w",
+			t.c, t.taskType, t.topLevelTaskType, t.blockCid, t.epoch, err)
+	}
+	return nil
 }
 
 func (cs *ChainStore) ExportRange(