core/rawdb, ethdb: introduce batched/atomic reads from ancients (#23566)

This PR adds a new accessor method to the freezer database. This new view offers a consistent interface, guaranteeing that all individual tables (headers, bodies etc) are all on the same number, and that this number is not changes (added/truncated) while the operation is performing.

core/rawdb/database.go

@@ -89,8 +89,8 @@ func (db *nofreezedb) Ancient(kind string, number uint64) ([]byte, error) {
 	return nil, errNotSupported
 }
 
-// ReadAncients returns an error as we don't have a backing chain freezer.
-func (db *nofreezedb) ReadAncients(kind string, start, max, maxByteSize uint64) ([][]byte, error) {
+// AncientRange returns an error as we don't have a backing chain freezer.
+func (db *nofreezedb) AncientRange(kind string, start, max, maxByteSize uint64) ([][]byte, error) {
 	return nil, errNotSupported
 }
 
@@ -119,6 +119,22 @@ func (db *nofreezedb) Sync() error {
 	return errNotSupported
 }
 
+func (db *nofreezedb) ReadAncients(fn func(reader ethdb.AncientReader) error) (err error) {
+	// Unlike other ancient-related methods, this method does not return
+	// errNotSupported when invoked.
+	// The reason for this is that the caller might want to do several things:
+	// 1. Check if something is in freezer,
+	// 2. If not, check leveldb.
+	//
+	// This will work, since the ancient-checks inside 'fn' will return errors,
+	// and the leveldb work will continue.
+	//
+	// If we instead were to return errNotSupported here, then the caller would
+	// have to explicitly check for that, having an extra clause to do the
+	// non-ancient operations.
+	return fn(db)
+}
+
 // NewDatabase creates a high level database on top of a given key-value data
 // store without a freezer moving immutable chain segments into cold storage.
 func NewDatabase(db ethdb.KeyValueStore) ethdb.Database {