core: nuke legacy snapshot supporting (#22663)

core/blockchain.go

@@ -207,9 +207,8 @@ type BlockChain struct {
 	processor  Processor // Block transaction processor interface
 	vmConfig   vm.Config
 
-	shouldPreserve     func(*types.Block) bool        // Function used to determine whether should preserve the given block.
-	terminateInsert    func(common.Hash, uint64) bool // Testing hook used to terminate ancient receipt chain insertion.
-	writeLegacyJournal bool                           // Testing flag used to flush the snapshot journal in legacy format.
+	shouldPreserve  func(*types.Block) bool        // Function used to determine whether should preserve the given block.
+	terminateInsert func(common.Hash, uint64) bool // Testing hook used to terminate ancient receipt chain insertion.
 }
 
 // NewBlockChain returns a fully initialised block chain using information
@@ -1002,14 +1001,8 @@ func (bc *BlockChain) Stop() {
 	var snapBase common.Hash
 	if bc.snaps != nil {
 		var err error
-		if bc.writeLegacyJournal {
-			if snapBase, err = bc.snaps.LegacyJournal(bc.CurrentBlock().Root()); err != nil {
-				log.Error("Failed to journal state snapshot", "err", err)
-			}
-		} else {
-			if snapBase, err = bc.snaps.Journal(bc.CurrentBlock().Root()); err != nil {
-				log.Error("Failed to journal state snapshot", "err", err)
-			}
+		if snapBase, err = bc.snaps.Journal(bc.CurrentBlock().Root()); err != nil {
+			log.Error("Failed to journal state snapshot", "err", err)
 		}
 	}
 	// Ensure the state of a recent block is also stored to disk before exiting.

core/blockchain_snapshot_test.go

@@ -39,7 +39,6 @@ import (
 
 // snapshotTestBasic wraps the common testing fields in the snapshot tests.
 type snapshotTestBasic struct {
-	legacy        bool   // Wether write the snapshot journal in legacy format
 	chainBlocks   int    // Number of blocks to generate for the canonical chain
 	snapshotBlock uint64 // Block number of the relevant snapshot disk layer
 	commitBlock   uint64 // Block number for which to commit the state to disk
@@ -104,19 +103,13 @@ func (basic *snapshotTestBasic) prepare(t *testing.T) (*BlockChain, []*types.Blo
 			chain.stateCache.TrieDB().Commit(blocks[point-1].Root(), true, nil)
 		}
 		if basic.snapshotBlock > 0 && basic.snapshotBlock == point {
-			if basic.legacy {
-				// Here we commit the snapshot disk root to simulate
-				// committing the legacy snapshot.
-				rawdb.WriteSnapshotRoot(db, blocks[point-1].Root())
-			} else {
-				// Flushing the entire snap tree into the disk, the
-				// relavant (a) snapshot root and (b) snapshot generator
-				// will be persisted atomically.
-				chain.snaps.Cap(blocks[point-1].Root(), 0)
-				diskRoot, blockRoot := chain.snaps.DiskRoot(), blocks[point-1].Root()
-				if !bytes.Equal(diskRoot.Bytes(), blockRoot.Bytes()) {
-					t.Fatalf("Failed to flush disk layer change, want %x, got %x", blockRoot, diskRoot)
-				}
+			// Flushing the entire snap tree into the disk, the
+			// relavant (a) snapshot root and (b) snapshot generator
+			// will be persisted atomically.
+			chain.snaps.Cap(blocks[point-1].Root(), 0)
+			diskRoot, blockRoot := chain.snaps.DiskRoot(), blocks[point-1].Root()
+			if !bytes.Equal(diskRoot.Bytes(), blockRoot.Bytes()) {
+				t.Fatalf("Failed to flush disk layer change, want %x, got %x", blockRoot, diskRoot)
 			}
 		}
 	}
@@ -129,12 +122,6 @@ func (basic *snapshotTestBasic) prepare(t *testing.T) (*BlockChain, []*types.Blo
 	basic.db = db
 	basic.gendb = gendb
 	basic.engine = engine
-
-	// Ugly hack, notify the chain to flush the journal in legacy format
-	// if it's requested.
-	if basic.legacy {
-		chain.writeLegacyJournal = true
-	}
 	return chain, blocks
 }
 
@@ -484,46 +471,6 @@ func TestRestartWithNewSnapshot(t *testing.T) {
 	// Expected snapshot disk  : G
 	test := &snapshotTest{
 		snapshotTestBasic{
-			legacy:             false,
-			chainBlocks:        8,
-			snapshotBlock:      0,
-			commitBlock:        0,
-			expCanonicalBlocks: 8,
-			expHeadHeader:      8,
-			expHeadFastBlock:   8,
-			expHeadBlock:       8,
-			expSnapshotBottom:  0, // Initial disk layer built from genesis
-		},
-	}
-	test.test(t)
-	test.teardown()
-}
-
-// Tests a Geth restart with valid but "legacy" snapshot. Before the shutdown,
-// all snapshot journal will be persisted correctly. In this case no snapshot
-// recovery is required.
-func TestRestartWithLegacySnapshot(t *testing.T) {
-	// Chain:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8 (HEAD)
-	//
-	// Commit:   G
-	// Snapshot: G
-	//
-	// SetHead(0)
-	//
-	// ------------------------------
-	//
-	// Expected in leveldb:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8
-	//
-	// Expected head header    : C8
-	// Expected head fast block: C8
-	// Expected head block     : C8
-	// Expected snapshot disk  : G
-	t.Skip("Legacy format testing is not supported")
-	test := &snapshotTest{
-		snapshotTestBasic{
-			legacy:             true,
 			chainBlocks:        8,
 			snapshotBlock:      0,
 			commitBlock:        0,
@@ -563,7 +510,6 @@ func TestNoCommitCrashWithNewSnapshot(t *testing.T) {
 	// Expected snapshot disk  : C4
 	test := &crashSnapshotTest{
 		snapshotTestBasic{
-			legacy:             false,
 			chainBlocks:        8,
 			snapshotBlock:      4,
 			commitBlock:        0,
@@ -603,7 +549,6 @@ func TestLowCommitCrashWithNewSnapshot(t *testing.T) {
 	// Expected snapshot disk  : C4
 	test := &crashSnapshotTest{
 		snapshotTestBasic{
-			legacy:             false,
 			chainBlocks:        8,
 			snapshotBlock:      4,
 			commitBlock:        2,
@@ -643,7 +588,6 @@ func TestHighCommitCrashWithNewSnapshot(t *testing.T) {
 	// Expected snapshot disk  : C4
 	test := &crashSnapshotTest{
 		snapshotTestBasic{
-			legacy:             false,
 			chainBlocks:        8,
 			snapshotBlock:      4,
 			commitBlock:        6,
@@ -658,131 +602,6 @@ func TestHighCommitCrashWithNewSnapshot(t *testing.T) {
 	test.teardown()
 }
 
-// Tests a Geth was crashed and restarts with a broken and "legacy format"
-// snapshot. In this case the entire legacy snapshot should be discared
-// and rebuild from the new chain head. The new head here refers to the
-// genesis because there is no committed point.
-func TestNoCommitCrashWithLegacySnapshot(t *testing.T) {
-	// Chain:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8 (HEAD)
-	//
-	// Commit:   G
-	// Snapshot: G, C4
-	//
-	// CRASH
-	//
-	// ------------------------------
-	//
-	// Expected in leveldb:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8
-	//
-	// Expected head header    : C8
-	// Expected head fast block: C8
-	// Expected head block     : G
-	// Expected snapshot disk  : G
-	t.Skip("Legacy format testing is not supported")
-	test := &crashSnapshotTest{
-		snapshotTestBasic{
-			legacy:             true,
-			chainBlocks:        8,
-			snapshotBlock:      4,
-			commitBlock:        0,
-			expCanonicalBlocks: 8,
-			expHeadHeader:      8,
-			expHeadFastBlock:   8,
-			expHeadBlock:       0,
-			expSnapshotBottom:  0, // Rebuilt snapshot from the latest HEAD(genesis)
-		},
-	}
-	test.test(t)
-	test.teardown()
-}
-
-// Tests a Geth was crashed and restarts with a broken and "legacy format"
-// snapshot. In this case the entire legacy snapshot should be discared
-// and rebuild from the new chain head. The new head here refers to the
-// block-2 because it's committed into the disk.
-func TestLowCommitCrashWithLegacySnapshot(t *testing.T) {
-	// Chain:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8 (HEAD)
-	//
-	// Commit:   G, C2
-	// Snapshot: G, C4
-	//
-	// CRASH
-	//
-	// ------------------------------
-	//
-	// Expected in leveldb:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8
-	//
-	// Expected head header    : C8
-	// Expected head fast block: C8
-	// Expected head block     : C2
-	// Expected snapshot disk  : C2
-	t.Skip("Legacy format testing is not supported")
-	test := &crashSnapshotTest{
-		snapshotTestBasic{
-			legacy:             true,
-			chainBlocks:        8,
-			snapshotBlock:      4,
-			commitBlock:        2,
-			expCanonicalBlocks: 8,
-			expHeadHeader:      8,
-			expHeadFastBlock:   8,
-			expHeadBlock:       2,
-			expSnapshotBottom:  2, // Rebuilt snapshot from the latest HEAD
-		},
-	}
-	test.test(t)
-	test.teardown()
-}
-
-// Tests a Geth was crashed and restarts with a broken and "legacy format"
-// snapshot. In this case the entire legacy snapshot should be discared
-// and rebuild from the new chain head.
-//
-// The new head here refers to the the genesis, the reason is:
-//   - the state of block-6 is committed into the disk
-//   - the legacy disk layer of block-4 is committed into the disk
-//   - the head is rewound the genesis in order to find an available
-//     state lower than disk layer
-func TestHighCommitCrashWithLegacySnapshot(t *testing.T) {
-	// Chain:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8 (HEAD)
-	//
-	// Commit:   G, C6
-	// Snapshot: G, C4
-	//
-	// CRASH
-	//
-	// ------------------------------
-	//
-	// Expected in leveldb:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8
-	//
-	// Expected head header    : C8
-	// Expected head fast block: C8
-	// Expected head block     : G
-	// Expected snapshot disk  : G
-	t.Skip("Legacy format testing is not supported")
-	test := &crashSnapshotTest{
-		snapshotTestBasic{
-			legacy:             true,
-			chainBlocks:        8,
-			snapshotBlock:      4,
-			commitBlock:        6,
-			expCanonicalBlocks: 8,
-			expHeadHeader:      8,
-			expHeadFastBlock:   8,
-			expHeadBlock:       0,
-			expSnapshotBottom:  0, // Rebuilt snapshot from the latest HEAD(genesis)
-		},
-	}
-	test.test(t)
-	test.teardown()
-}
-
 // Tests a Geth was running with snapshot enabled. Then restarts without
 // enabling snapshot and after that re-enable the snapshot again. In this
 // case the snapshot should be rebuilt with latest chain head.
@@ -806,47 +625,6 @@ func TestGappedNewSnapshot(t *testing.T) {
 	// Expected snapshot disk  : C10
 	test := &gappedSnapshotTest{
 		snapshotTestBasic: snapshotTestBasic{
-			legacy:             false,
-			chainBlocks:        8,
-			snapshotBlock:      0,
-			commitBlock:        0,
-			expCanonicalBlocks: 10,
-			expHeadHeader:      10,
-			expHeadFastBlock:   10,
-			expHeadBlock:       10,
-			expSnapshotBottom:  10, // Rebuilt snapshot from the latest HEAD
-		},
-		gapped: 2,
-	}
-	test.test(t)
-	test.teardown()
-}
-
-// Tests a Geth was running with leagcy snapshot enabled. Then restarts
-// without enabling snapshot and after that re-enable the snapshot again.
-// In this case the snapshot should be rebuilt with latest chain head.
-func TestGappedLegacySnapshot(t *testing.T) {
-	// Chain:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8 (HEAD)
-	//
-	// Commit:   G
-	// Snapshot: G
-	//
-	// SetHead(0)
-	//
-	// ------------------------------
-	//
-	// Expected in leveldb:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8->C9->C10
-	//
-	// Expected head header    : C10
-	// Expected head fast block: C10
-	// Expected head block     : C10
-	// Expected snapshot disk  : C10
-	t.Skip("Legacy format testing is not supported")
-	test := &gappedSnapshotTest{
-		snapshotTestBasic: snapshotTestBasic{
-			legacy:             true,
 			chainBlocks:        8,
 			snapshotBlock:      0,
 			commitBlock:        0,
@@ -885,7 +663,6 @@ func TestSetHeadWithNewSnapshot(t *testing.T) {
 	// Expected snapshot disk  : G
 	test := &setHeadSnapshotTest{
 		snapshotTestBasic: snapshotTestBasic{
-			legacy:             false,
 			chainBlocks:        8,
 			snapshotBlock:      0,
 			commitBlock:        0,
@@ -901,88 +678,6 @@ func TestSetHeadWithNewSnapshot(t *testing.T) {
 	test.teardown()
 }
 
-// Tests the Geth was running with snapshot(legacy-format) enabled and resetHead
-// is applied. In this case the head is rewound to the target(with state available).
-// After that the chain is restarted and the original disk layer is kept.
-func TestSetHeadWithLegacySnapshot(t *testing.T) {
-	// Chain:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8 (HEAD)
-	//
-	// Commit:   G
-	// Snapshot: G
-	//
-	// SetHead(4)
-	//
-	// ------------------------------
-	//
-	// Expected in leveldb:
-	//   G->C1->C2->C3->C4
-	//
-	// Expected head header    : C4
-	// Expected head fast block: C4
-	// Expected head block     : C4
-	// Expected snapshot disk  : G
-	t.Skip("Legacy format testing is not supported")
-	test := &setHeadSnapshotTest{
-		snapshotTestBasic: snapshotTestBasic{
-			legacy:             true,
-			chainBlocks:        8,
-			snapshotBlock:      0,
-			commitBlock:        0,
-			expCanonicalBlocks: 4,
-			expHeadHeader:      4,
-			expHeadFastBlock:   4,
-			expHeadBlock:       4,
-			expSnapshotBottom:  0, // The initial disk layer is built from the genesis
-		},
-		setHead: 4,
-	}
-	test.test(t)
-	test.teardown()
-}
-
-// Tests the Geth was running with snapshot(legacy-format) enabled and upgrades
-// the disk layer journal(journal generator) to latest format. After that the Geth
-// is restarted from a crash. In this case Geth will find the new-format disk layer
-// journal but with legacy-format diff journal(the new-format is never committed),
-// and the invalid diff journal is expected to be dropped.
-func TestRecoverSnapshotFromCrashWithLegacyDiffJournal(t *testing.T) {
-	// Chain:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8 (HEAD)
-	//
-	// Commit:   G
-	// Snapshot: G
-	//
-	// SetHead(0)
-	//
-	// ------------------------------
-	//
-	// Expected in leveldb:
-	//   G->C1->C2->C3->C4->C5->C6->C7->C8->C9->C10
-	//
-	// Expected head header    : C10
-	// Expected head fast block: C10
-	// Expected head block     : C8
-	// Expected snapshot disk  : C10
-	t.Skip("Legacy format testing is not supported")
-	test := &restartCrashSnapshotTest{
-		snapshotTestBasic: snapshotTestBasic{
-			legacy:             true,
-			chainBlocks:        8,
-			snapshotBlock:      0,
-			commitBlock:        0,
-			expCanonicalBlocks: 10,
-			expHeadHeader:      10,
-			expHeadFastBlock:   10,
-			expHeadBlock:       8,  // The persisted state in the first running
-			expSnapshotBottom:  10, // The persisted disk layer in the second running
-		},
-		newBlocks: 2,
-	}
-	test.test(t)
-	test.teardown()
-}
-
 // Tests the Geth was running with a complete snapshot and then imports a few
 // more new blocks on top without enabling the snapshot. After the restart,
 // crash happens. Check everything is ok after the restart.
@@ -1006,7 +701,6 @@ func TestRecoverSnapshotFromWipingCrash(t *testing.T) {
 	// Expected snapshot disk  : C10
 	test := &wipeCrashSnapshotTest{
 		snapshotTestBasic: snapshotTestBasic{
-			legacy:             false,
 			chainBlocks:        8,
 			snapshotBlock:      4,
 			commitBlock:        0,

core/state/snapshot/journal.go

@@ -66,30 +66,6 @@ type journalStorage struct {
 	Vals [][]byte
 }
 
-// loadAndParseLegacyJournal tries to parse the snapshot journal in legacy format.
-func loadAndParseLegacyJournal(db ethdb.KeyValueStore, base *diskLayer) (snapshot, journalGenerator, error) {
-	// Retrieve the journal, for legacy journal it must exist since even for
-	// 0 layer it stores whether we've already generated the snapshot or are
-	// in progress only.
-	journal := rawdb.ReadSnapshotJournal(db)
-	if len(journal) == 0 {
-		return nil, journalGenerator{}, errors.New("missing or corrupted snapshot journal")
-	}
-	r := rlp.NewStream(bytes.NewReader(journal), 0)
-
-	// Read the snapshot generation progress for the disk layer
-	var generator journalGenerator
-	if err := r.Decode(&generator); err != nil {
-		return nil, journalGenerator{}, fmt.Errorf("failed to load snapshot progress marker: %v", err)
-	}
-	// Load all the snapshot diffs from the journal
-	snapshot, err := loadDiffLayer(base, r)
-	if err != nil {
-		return nil, generator, err
-	}
-	return snapshot, generator, nil
-}
-
 // loadAndParseJournal tries to parse the snapshot journal in latest format.
 func loadAndParseJournal(db ethdb.KeyValueStore, base *diskLayer) (snapshot, journalGenerator, error) {
 	// Retrieve the disk layer generator. It must exist, no matter the
@@ -163,14 +139,9 @@ func loadSnapshot(diskdb ethdb.KeyValueStore, triedb *trie.Database, cache int,
 		cache:  fastcache.New(cache * 1024 * 1024),
 		root:   baseRoot,
 	}
-	var legacy bool
 	snapshot, generator, err := loadAndParseJournal(diskdb, base)
 	if err != nil {
 		log.Warn("Failed to load new-format journal", "error", err)
-		snapshot, generator, err = loadAndParseLegacyJournal(diskdb, base)
-		legacy = true
-	}
-	if err != nil {
 		return nil, err
 	}
 	// Entire snapshot journal loaded, sanity check the head. If the loaded
@@ -185,7 +156,7 @@ func loadSnapshot(diskdb ethdb.KeyValueStore, triedb *trie.Database, cache int,
 		// If it's legacy snapshot, or it's new-format snapshot but
 		// it's not in recovery mode, returns the error here for
 		// rebuilding the entire snapshot forcibly.
-		if legacy || !recovery {
+		if !recovery {
 			return nil, fmt.Errorf("head doesn't match snapshot: have %#x, want %#x", head, root)
 		}
 		// It's in snapshot recovery, the assumption is held that
@@ -346,94 +317,3 @@ func (dl *diffLayer) Journal(buffer *bytes.Buffer) (common.Hash, error) {
 	log.Debug("Journalled diff layer", "root", dl.root, "parent", dl.parent.Root())
 	return base, nil
 }
-
-// LegacyJournal writes the persistent layer generator stats into a buffer
-// to be stored in the database as the snapshot journal.
-//
-// Note it's the legacy version which is only used in testing right now.
-func (dl *diskLayer) LegacyJournal(buffer *bytes.Buffer) (common.Hash, error) {
-	// If the snapshot is currently being generated, abort it
-	var stats *generatorStats
-	if dl.genAbort != nil {
-		abort := make(chan *generatorStats)
-		dl.genAbort <- abort
-
-		if stats = <-abort; stats != nil {
-			stats.Log("Journalling in-progress snapshot", dl.root, dl.genMarker)
-		}
-	}
-	// Ensure the layer didn't get stale
-	dl.lock.RLock()
-	defer dl.lock.RUnlock()
-
-	if dl.stale {
-		return common.Hash{}, ErrSnapshotStale
-	}
-	// Write out the generator marker
-	entry := journalGenerator{
-		Done:   dl.genMarker == nil,
-		Marker: dl.genMarker,
-	}
-	if stats != nil {
-		entry.Accounts = stats.accounts
-		entry.Slots = stats.slots
-		entry.Storage = uint64(stats.storage)
-	}
-	log.Debug("Legacy journalled disk layer", "root", dl.root)
-	if err := rlp.Encode(buffer, entry); err != nil {
-		return common.Hash{}, err
-	}
-	return dl.root, nil
-}
-
-// Journal writes the memory layer contents into a buffer to be stored in the
-// database as the snapshot journal.
-//
-// Note it's the legacy version which is only used in testing right now.
-func (dl *diffLayer) LegacyJournal(buffer *bytes.Buffer) (common.Hash, error) {
-	// Journal the parent first
-	base, err := dl.parent.LegacyJournal(buffer)
-	if err != nil {
-		return common.Hash{}, err
-	}
-	// Ensure the layer didn't get stale
-	dl.lock.RLock()
-	defer dl.lock.RUnlock()
-
-	if dl.Stale() {
-		return common.Hash{}, ErrSnapshotStale
-	}
-	// Everything below was journalled, persist this layer too
-	if err := rlp.Encode(buffer, dl.root); err != nil {
-		return common.Hash{}, err
-	}
-	destructs := make([]journalDestruct, 0, len(dl.destructSet))
-	for hash := range dl.destructSet {
-		destructs = append(destructs, journalDestruct{Hash: hash})
-	}
-	if err := rlp.Encode(buffer, destructs); err != nil {
-		return common.Hash{}, err
-	}
-	accounts := make([]journalAccount, 0, len(dl.accountData))
-	for hash, blob := range dl.accountData {
-		accounts = append(accounts, journalAccount{Hash: hash, Blob: blob})
-	}
-	if err := rlp.Encode(buffer, accounts); err != nil {
-		return common.Hash{}, err
-	}
-	storage := make([]journalStorage, 0, len(dl.storageData))
-	for hash, slots := range dl.storageData {
-		keys := make([]common.Hash, 0, len(slots))
-		vals := make([][]byte, 0, len(slots))
-		for key, val := range slots {
-			keys = append(keys, key)
-			vals = append(vals, val)
-		}
-		storage = append(storage, journalStorage{Hash: hash, Keys: keys, Vals: vals})
-	}
-	if err := rlp.Encode(buffer, storage); err != nil {
-		return common.Hash{}, err
-	}
-	log.Debug("Legacy journalled diff layer", "root", dl.root, "parent", dl.parent.Root())
-	return base, nil
-}

core/state/snapshot/snapshot.go

@@ -137,10 +137,6 @@ type snapshot interface {
 	// flattening everything down (bad for reorgs).
 	Journal(buffer *bytes.Buffer) (common.Hash, error)
 
-	// LegacyJournal is basically identical to Journal. it's the legacy version for
-	// flushing legacy journal. Now the only purpose of this function is for testing.
-	LegacyJournal(buffer *bytes.Buffer) (common.Hash, error)
-
 	// Stale return whether this layer has become stale (was flattened across) or
 	// if it's still live.
 	Stale() bool
@@ -622,29 +618,6 @@ func (t *Tree) Journal(root common.Hash) (common.Hash, error) {
 	return base, nil
 }
 
-// LegacyJournal is basically identical to Journal. it's the legacy
-// version for flushing legacy journal. Now the only purpose of this
-// function is for testing.
-func (t *Tree) LegacyJournal(root common.Hash) (common.Hash, error) {
-	// Retrieve the head snapshot to journal from var snap snapshot
-	snap := t.Snapshot(root)
-	if snap == nil {
-		return common.Hash{}, fmt.Errorf("snapshot [%#x] missing", root)
-	}
-	// Run the journaling
-	t.lock.Lock()
-	defer t.lock.Unlock()
-
-	journal := new(bytes.Buffer)
-	base, err := snap.(snapshot).LegacyJournal(journal)
-	if err != nil {
-		return common.Hash{}, err
-	}
-	// Store the journal into the database and return
-	rawdb.WriteSnapshotJournal(t.diskdb, journal.Bytes())
-	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.