core/rawdb, cmd, ethdb, eth: implement freezer tail deletion (#23954)

* core/rawdb, cmd, ethdb, eth: implement freezer tail deletion

* core/rawdb: address comments from martin and sina

* core/rawdb: fixes cornercase in tail deletion

* core/rawdb: separate metadata into a standalone file

* core/rawdb: remove unused code

* core/rawdb: add random test

* core/rawdb: polish code

* core/rawdb: fsync meta file before manipulating the index

* core/rawdb: fix typo

* core/rawdb: address comments

core/rawdb/freezer_meta.go

@@ -0,0 +1,109 @@
+// Copyright 2022 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>
+
+package rawdb
+
+import (
+	"io"
+	"os"
+
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/rlp"
+)
+
+const freezerVersion = 1 // The initial version tag of freezer table metadata
+
+// freezerTableMeta wraps all the metadata of the freezer table.
+type freezerTableMeta struct {
+	// Version is the versioning descriptor of the freezer table.
+	Version uint16
+
+	// VirtualTail indicates how many items have been marked as deleted.
+	// Its value is equal to the number of items removed from the table
+	// plus the number of items hidden in the table, so it should never
+	// be lower than the "actual tail".
+	VirtualTail uint64
+}
+
+// newMetadata initializes the metadata object with the given virtual tail.
+func newMetadata(tail uint64) *freezerTableMeta {
+	return &freezerTableMeta{
+		Version:     freezerVersion,
+		VirtualTail: tail,
+	}
+}
+
+// readMetadata reads the metadata of the freezer table from the
+// given metadata file.
+func readMetadata(file *os.File) (*freezerTableMeta, error) {
+	_, err := file.Seek(0, io.SeekStart)
+	if err != nil {
+		return nil, err
+	}
+	var meta freezerTableMeta
+	if err := rlp.Decode(file, &meta); err != nil {
+		return nil, err
+	}
+	return &meta, nil
+}
+
+// writeMetadata writes the metadata of the freezer table into the
+// given metadata file.
+func writeMetadata(file *os.File, meta *freezerTableMeta) error {
+	_, err := file.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	return rlp.Encode(file, meta)
+}
+
+// loadMetadata loads the metadata from the given metadata file.
+// Initializes the metadata file with the given "actual tail" if
+// it's empty.
+func loadMetadata(file *os.File, tail uint64) (*freezerTableMeta, error) {
+	stat, err := file.Stat()
+	if err != nil {
+		return nil, err
+	}
+	// Write the metadata with the given actual tail into metadata file
+	// if it's non-existent. There are two possible scenarios here:
+	// - the freezer table is empty
+	// - the freezer table is legacy
+	// In both cases, write the meta into the file with the actual tail
+	// as the virtual tail.
+	if stat.Size() == 0 {
+		m := newMetadata(tail)
+		if err := writeMetadata(file, m); err != nil {
+			return nil, err
+		}
+		return m, nil
+	}
+	m, err := readMetadata(file)
+	if err != nil {
+		return nil, err
+	}
+	// Update the virtual tail with the given actual tail if it's even
+	// lower than it. Theoretically it shouldn't happen at all, print
+	// a warning here.
+	if m.VirtualTail < tail {
+		log.Warn("Updated virtual tail", "have", m.VirtualTail, "now", tail)
+		m.VirtualTail = tail
+		if err := writeMetadata(file, m); err != nil {
+			return nil, err
+		}
+	}
+	return m, nil
+}