review feedback: reinstate import function and dump preimages as an RLP stream

cmd/geth/chaincmd.go

@@ -121,6 +121,21 @@ last block to write. In this mode, the file will be appended
 if already existing. If the file ends with .gz, the output will
 be gzipped.`,
 	}
+	importPreimagesCommand = &cli.Command{
+		Action:    importPreimages,
+		Name:      "import-preimages",
+		Usage:     "Import the preimage database from an RLP stream",
+		ArgsUsage: "<datafile>",
+		Flags: flags.Merge([]cli.Flag{
+			utils.CacheFlag,
+			utils.SyncModeFlag,
+		}, utils.DatabaseFlags),
+		Description: `
+The import-preimages command imports hash preimages from an RLP encoded stream.
+It's deprecated, please use "geth db import" instead.
+`,
+	}
+
 	dumpCommand = &cli.Command{
 		Action:    dump,
 		Name:      "dump",
@@ -339,6 +354,29 @@ func exportChain(ctx *cli.Context) error {
 	return nil
 }
 
+// importPreimages imports preimage data from the specified file.
+// it is deprecated, and the export function has been removed, but
+// the import function is kept around for the time being so that
+// older file formats can still be imported.
+func importPreimages(ctx *cli.Context) error {
+	if ctx.Args().Len() < 1 {
+		utils.Fatalf("This command requires an argument.")
+	}
+
+	stack, _ := makeConfigNode(ctx)
+	defer stack.Close()
+
+	db := utils.MakeChainDatabase(ctx, stack, false)
+	defer db.Close()
+	start := time.Now()
+
+	if err := utils.ImportPreimages(db, ctx.Args().First()); err != nil {
+		utils.Fatalf("Import error: %v\n", err)
+	}
+	fmt.Printf("Import done in %v\n", time.Since(start))
+	return nil
+}
+
 func parseDumpConfig(ctx *cli.Context, stack *node.Node) (*state.DumpConfig, ethdb.Database, common.Hash, error) {
 	db := utils.MakeChainDatabase(ctx, stack, true)
 	defer db.Close()

cmd/utils/cmd.go

@@ -458,7 +458,11 @@ func ExportSnapshotPreimages(chaindb ethdb.Database, snaptree *snapshot.Tree, fn
 		if len(preimage) != item.Size {
 			return fmt.Errorf("invalid preimage size, have %d", len(preimage))
 		}
-		if _, err := writer.Write(preimage); err != nil {
+		rlpenc, err := rlp.EncodeToBytes(preimage)
+		if err != nil {
+			return fmt.Errorf("error encoding preimage: %w", err)
+		}
+		if _, err := writer.Write(rlpenc); err != nil {
 			return fmt.Errorf("failed to write preimage: %w", err)
 		}
 	}