core/state/snapshot: detect and clean up dangling storage snapshot in generation (#24811)

* core/state/snapshot: check dangling storages when generating snapshot

* core/state/snapshot: polish

* core/state/snapshot: wipe the last part of the dangling storages

* core/state/snapshot: fix and add tests

* core/state/snapshot: fix comment

* README: remove mentions of fast sync (#24656)

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>

* core, cmd: expose dangling storage detector for wider usage

* core/state/snapshot: rename variable

* core, ethdb: use global iterators for snapshot generation

* core/state/snapshot: polish

* cmd, core/state/snapshot: polish

* core/state/snapshot: polish

* Update core/state/snapshot/generate.go

Co-authored-by: Martin Holst Swende <martin@swende.se>

* ethdb: extend db test suite and fix memorydb iterator

* ethdb/dbtest: rollback changes

* ethdb/memorydb: simplify iteration

* core/state/snapshot: update dangling counter

* core/state/snapshot: release iterators

* core/state/snapshot: update metrics

* core/state/snapshot: update time metrics

* metrics/influxdb: temp solution to present counter meaningfully, remove it

* add debug log, revert later

* core/state/snapshot: fix iterator panic

* all: customized snapshot iterator for backward iteration

* core, ethdb: polish

* core/state/snapshot: remove debug log

* core/state/snapshot: address comments from peter

* core/state/snapshot: reopen the iterator at the next position

* ethdb, core/state/snapshot: address comment from peter

* core/state/snapshot: reopen exhausted iterators

Co-authored-by: Tbnoapi <63448616+nuoomnoy02@users.noreply.github.com>
Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>
Co-authored-by: Martin Holst Swende <martin@swende.se>

ethdb/memorydb/memorydb.go

@@ -169,6 +169,7 @@ func (db *Database) NewIterator(prefix []byte, start []byte) ethdb.Iterator {
 		values = append(values, db.db[key])
 	}
 	return &iterator{
+		index:  -1,
 		keys:   keys,
 		values: values,
 	}
@@ -279,7 +280,7 @@ func (b *batch) Replay(w ethdb.KeyValueWriter) error {
 // value store. Internally it is a deep copy of the entire iterated state,
 // sorted by keys.
 type iterator struct {
-	inited bool
+	index  int
 	keys   []string
 	values [][]byte
 }
@@ -287,17 +288,12 @@ type iterator struct {
 // Next moves the iterator to the next key/value pair. It returns whether the
 // iterator is exhausted.
 func (it *iterator) Next() bool {
-	// If the iterator was not yet initialized, do it now
-	if !it.inited {
-		it.inited = true
-		return len(it.keys) > 0
+	// Short circuit if iterator is already exhausted in the forward direction.
+	if it.index >= len(it.keys) {
+		return false
 	}
-	// Iterator already initialize, advance it
-	if len(it.keys) > 0 {
-		it.keys = it.keys[1:]
-		it.values = it.values[1:]
-	}
-	return len(it.keys) > 0
+	it.index += 1
+	return it.index < len(it.keys)
 }
 
 // Error returns any accumulated error. Exhausting all the key/value pairs
@@ -310,26 +306,28 @@ func (it *iterator) Error() error {
 // should not modify the contents of the returned slice, and its contents may
 // change on the next call to Next.
 func (it *iterator) Key() []byte {
-	if len(it.keys) > 0 {
-		return []byte(it.keys[0])
+	// Short circuit if iterator is not in a valid position
+	if it.index < 0 || it.index >= len(it.keys) {
+		return nil
 	}
-	return nil
+	return []byte(it.keys[it.index])
 }
 
 // Value returns the value of the current key/value pair, or nil if done. The
 // caller should not modify the contents of the returned slice, and its contents
 // may change on the next call to Next.
 func (it *iterator) Value() []byte {
-	if len(it.values) > 0 {
-		return it.values[0]
+	// Short circuit if iterator is not in a valid position
+	if it.index < 0 || it.index >= len(it.keys) {
+		return nil
 	}
-	return nil
+	return it.values[it.index]
 }
 
 // Release releases associated resources. Release should always succeed and can
 // be called multiple times without causing error.
 func (it *iterator) Release() {
-	it.keys, it.values = nil, nil
+	it.index, it.keys, it.values = -1, nil, nil
 }
 
 // snapshot wraps a batch of key-value entries deep copied from the in-memory