core: define and test chain rewind corner cases (#21409)

* core: define and test chain reparation cornercases

* core: write up a variety of set-head tests

* core, eth: unify chain rollbacks, handle all the cases

* core: make linter smile

* core: remove commented out legacy code

* core, eth/downloader: fix review comments

* core: revert a removed recovery mechanism

core/rawdb/database.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"sync/atomic"
 	"time"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -53,6 +54,22 @@ func (frdb *freezerdb) Close() error {
 	return nil
 }
 
+// Freeze is a helper method used for external testing to trigger and block until
+// a freeze cycle completes, without having to sleep for a minute to trigger the
+// automatic background run.
+func (frdb *freezerdb) Freeze(threshold uint64) {
+	// Set the freezer threshold to a temporary value
+	defer func(old uint64) {
+		atomic.StoreUint64(&frdb.AncientStore.(*freezer).threshold, old)
+	}(atomic.LoadUint64(&frdb.AncientStore.(*freezer).threshold))
+	atomic.StoreUint64(&frdb.AncientStore.(*freezer).threshold, threshold)
+
+	// Trigger a freeze cycle and block until it's done
+	trigger := make(chan struct{}, 1)
+	frdb.AncientStore.(*freezer).trigger <- trigger
+	<-trigger
+}
+
 // nofreezedb is a database wrapper that disables freezer data retrievals.
 type nofreezedb struct {
 	ethdb.KeyValueStore