core/rawdb: refactor db inspector for extending multiple ancient store (#25896)

This PR ports a few changes from PBSS:

- Fix the snapshot generator waiter in case the generation is not even initialized
- Refactor db inspector for ancient store

core/state/snapshot/snapshot.go

@@ -187,8 +187,9 @@ type Tree struct {
 // If the memory layers in the journal do not match the disk layer (e.g. there is
 // a gap) or the journal is missing, there are two repair cases:
 //
-//   - if the 'recovery' parameter is true, all memory diff-layers will be discarded.
-//     This case happens when the snapshot is 'ahead' of the state trie.
+//   - if the 'recovery' parameter is true, memory diff-layers and the disk-layer
+//     will all be kept. This case happens when the snapshot is 'ahead' of the
+//     state trie.
 //   - otherwise, the entire snapshot is considered invalid and will be recreated on
 //     a background thread.
 func New(config Config, diskdb ethdb.KeyValueStore, triedb *trie.Database, root common.Hash) (*Tree, error) {
@@ -199,16 +200,16 @@ func New(config Config, diskdb ethdb.KeyValueStore, triedb *trie.Database, root
 		triedb: triedb,
 		layers: make(map[common.Hash]snapshot),
 	}
-	// Create the building waiter iff the background generation is allowed
-	if !config.NoBuild && !config.AsyncBuild {
-		defer snap.waitBuild()
-	}
 	// Attempt to load a previously persisted snapshot and rebuild one if failed
 	head, disabled, err := loadSnapshot(diskdb, triedb, root, config.CacheSize, config.Recovery, config.NoBuild)
 	if disabled {
 		log.Warn("Snapshot maintenance disabled (syncing)")
 		return snap, nil
 	}
+	// Create the building waiter iff the background generation is allowed
+	if !config.NoBuild && !config.AsyncBuild {
+		defer snap.waitBuild()
+	}
 	if err != nil {
 		log.Warn("Failed to load snapshot", "err", err)
 		if !config.NoBuild {