cmd/utils: fix a startup issue on deleted chaindata but dangling ancients (#27989)

cmd/utils/flags.go

@@ -2073,10 +2073,10 @@ func MakeChainDatabase(ctx *cli.Context, stack *node.Node, readonly bool) ethdb.
 // tryMakeReadOnlyDatabase try to open the chain database in read-only mode,
 // or fallback to write mode if the database is not initialized.
 func tryMakeReadOnlyDatabase(ctx *cli.Context, stack *node.Node) ethdb.Database {
-	// If datadir doesn't exist we need to open db in write-mode
-	// so database engine can create files.
+	// If the database doesn't exist we need to open it in write-mode to allow
+	// the engine to create files.
 	readonly := true
-	if !common.FileExist(stack.ResolvePath("chaindata")) {
+	if rawdb.PreexistingDatabase(stack.ResolvePath("chaindata")) == "" {
 		readonly = false
 	}
 	return MakeChainDatabase(ctx, stack, readonly)

core/rawdb/database.go

@@ -326,10 +326,10 @@ const (
 	dbLeveldb = "leveldb"
 )
 
-// hasPreexistingDb checks the given data directory whether a database is already
+// PreexistingDatabase checks the given data directory whether a database is already
 // instantiated at that location, and if so, returns the type of database (or the
 // empty string).
-func hasPreexistingDb(path string) string {
+func PreexistingDatabase(path string) string {
 	if _, err := os.Stat(filepath.Join(path, "CURRENT")); err != nil {
 		return "" // No pre-existing db
 	}
@@ -367,7 +367,7 @@ func openKeyValueDatabase(o OpenOptions) (ethdb.Database, error) {
 	}
 	// Retrieve any pre-existing database's type and use that or the requested one
 	// as long as there's no conflict between the two types
-	existingDb := hasPreexistingDb(o.Directory)
+	existingDb := PreexistingDatabase(o.Directory)
 	if len(existingDb) != 0 && len(o.Type) != 0 && o.Type != existingDb {
 		return nil, fmt.Errorf("db.engine choice was %v but found pre-existing %v database in specified data directory", o.Type, existingDb)
 	}