core: re-enable the legacy snapshot after sync (#35163)

core/blockchain.go

@@ -1194,10 +1194,13 @@ func (bc *BlockChain) SnapSyncComplete(hash common.Hash, isSnapV2 bool) error {
 		return fmt.Errorf("non existent state [%x..]", root[:4])
 	}
 
-	// The legacy snapshot tree needs to be wiped and rebuilt from the trie
-	// after a snap/1 sync.
-	if !isSnapV2 && bc.snaps != nil {
-		bc.snaps.Rebuild(root)
+	// The legacy snapshot tree (hash scheme only) was persistently disabled
+	// before the sync, re-enables it explicitly.
+	//
+	// For snap/2 the downloaded flat state is already complete and root-verified,
+	// so the background generation is unnecessary.
+	if bc.snaps != nil {
+		bc.snaps.Rebuild(root, !isSnapV2)
 	}
 
 	// If all checks out, manually set the head block.

core/blockchain_snapshot_test.go

@@ -29,6 +29,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/consensus"
 	"github.com/ethereum/go-ethereum/consensus/ethash"
 	"github.com/ethereum/go-ethereum/core/rawdb"
@@ -721,3 +722,40 @@ func TestRecoverSnapshotFromWipingCrash(t *testing.T) {
 		test.teardown()
 	}
 }
+
+// TestSnapSyncCompleteRebuildsSnapshot verifies that completing a snap sync
+// re-enables the legacy snapshot tree on the hash scheme for both syncer
+// versions: SnapSyncStart persistently disables the tree, and only the
+// rebuild on completion clears the marker again.
+func TestSnapSyncCompleteRebuildsSnapshot(t *testing.T) {
+	for _, isSnapV2 := range []bool{false, true} {
+		_, _, chain, err := newCanonical(ethash.NewFaker(), 8, true, rawdb.HashScheme)
+		if err != nil {
+			t.Fatalf("failed to create chain: %v", err)
+		}
+		if err := chain.SnapSyncStart(); err != nil {
+			t.Fatalf("failed to start snap sync: %v", err)
+		}
+		if !rawdb.ReadSnapshotDisabled(chain.db) {
+			t.Fatal("snapshot should be disabled during snap sync")
+		}
+		head := chain.CurrentBlock()
+		if err := chain.SnapSyncComplete(head.Hash(), isSnapV2); err != nil {
+			t.Fatalf("failed to complete snap sync (v2=%v): %v", isSnapV2, err)
+		}
+		if rawdb.ReadSnapshotDisabled(chain.db) {
+			t.Fatalf("snapshot should be re-enabled after snap sync completion (v2=%v)", isSnapV2)
+		}
+		// snap/2 adopts the flat state without regeneration, so the snapshot
+		// must be immediately usable; snap/1 schedules a background rebuild
+		// instead (which may or may not have finished, no assertion there).
+		if isSnapV2 {
+			it, err := chain.snaps.AccountIterator(head.Root, common.Hash{})
+			if err != nil {
+				t.Fatalf("adopted snapshot not immediately usable: %v", err)
+			}
+			it.Release()
+		}
+		chain.Stop()
+	}
+}

core/state/snapshot/snapshot.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"sync"
 
+	"github.com/VictoriaMetrics/fastcache"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/rawdb"
 	"github.com/ethereum/go-ethereum/core/types"
@@ -213,7 +214,7 @@ func New(config Config, diskdb ethdb.KeyValueStore, triedb *triedb.Database, roo
 	if err != nil {
 		log.Warn("Failed to load snapshot", "err", err)
 		if !config.NoBuild {
-			snap.Rebuild(root)
+			snap.Rebuild(root, true)
 			return snap, nil
 		}
 		return nil, err // Bail out the error, don't rebuild automatically.
@@ -683,10 +684,12 @@ func (t *Tree) Journal(root common.Hash) (common.Hash, error) {
 	return base, nil
 }
 
-// Rebuild wipes all available snapshot data from the persistent database and
-// discard all caches and diff layers. Afterwards, it starts a new snapshot
-// generator with the given root hash.
-func (t *Tree) Rebuild(root common.Hash) {
+// Rebuild discards all caches and diff layers and re-enables the snapshot
+// feature. With generate set, it starts a new snapshot generator with the
+// given root hash, wiping and regenerating the persistent flat state in the
+// background. Without it, the on-disk flat state is adopted as the fully
+// generated disk layer directly.
+func (t *Tree) Rebuild(root common.Hash, generate bool) {
 	t.lock.Lock()
 	defer t.lock.Unlock()
 
@@ -715,6 +718,26 @@ func (t *Tree) Rebuild(root common.Hash) {
 			panic(fmt.Sprintf("unknown layer type: %T", layer))
 		}
 	}
+	// Adopt the existing flat state as the generated disk layer if
+	// regeneration was not requested.
+	if !generate {
+		batch := t.diskdb.NewBatch()
+		rawdb.WriteSnapshotRoot(batch, root)
+		journalProgress(batch, nil, nil)
+		if err := batch.Write(); err != nil {
+			log.Crit("Failed to write snapshot completion marker", "err", err)
+		}
+		log.Info("Adopted state snapshot", "root", root)
+		t.layers = map[common.Hash]snapshot{
+			root: &diskLayer{
+				diskdb: t.diskdb,
+				triedb: t.triedb,
+				cache:  fastcache.New(t.config.CacheSize * 1024 * 1024),
+				root:   root,
+			},
+		}
+		return
+	}
 	// Start generating a new snapshot from scratch on a background thread. The
 	// generator will run a wiper first if there's not one running right now.
 	log.Info("Rebuilding state snapshot")