implement a command to export a car

cmd/lotus-shed/export-car.go

@@ -0,0 +1,104 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/ipfs/go-blockservice"
+	"github.com/ipfs/go-cid"
+	offline "github.com/ipfs/go-ipfs-exchange-offline"
+	format "github.com/ipfs/go-ipld-format"
+	"github.com/ipfs/go-merkledag"
+	"github.com/ipld/go-car"
+	"github.com/urfave/cli/v2"
+	"golang.org/x/xerrors"
+
+	lcli "github.com/filecoin-project/lotus/cli"
+	"github.com/filecoin-project/lotus/node/repo"
+)
+
+func carWalkFunc(nd format.Node) (out []*format.Link, err error) {
+	for _, link := range nd.Links() {
+		if link.Cid.Prefix().Codec == cid.FilCommitmentSealed || link.Cid.Prefix().Codec == cid.FilCommitmentUnsealed {
+			continue
+		}
+		out = append(out, link)
+	}
+	return out, nil
+}
+
+var exportCarCmd = &cli.Command{
+	Name:        "export-car",
+	Description: "Export a car from repo (requires node to be offline)",
+	Flags: []cli.Flag{
+		&cli.StringFlag{
+			Name:  "repo",
+			Value: "~/.lotus",
+		},
+	},
+	Action: func(cctx *cli.Context) error {
+		if cctx.Args().Len() != 2 {
+			return lcli.ShowHelp(cctx, fmt.Errorf("must specify file name and object"))
+		}
+
+		outfile := cctx.Args().First()
+		var roots []cid.Cid
+		for _, arg := range cctx.Args().Tail() {
+			c, err := cid.Decode(arg)
+			if err != nil {
+				return err
+			}
+			roots = append(roots, c)
+		}
+
+		ctx := lcli.ReqContext(cctx)
+
+		r, err := repo.NewFS(cctx.String("repo"))
+		if err != nil {
+			return xerrors.Errorf("opening fs repo: %w", err)
+		}
+
+		exists, err := r.Exists()
+		if err != nil {
+			return err
+		}
+		if !exists {
+			return xerrors.Errorf("lotus repo doesn't exist")
+		}
+
+		lr, err := r.Lock(repo.FullNode)
+		if err != nil {
+			return err
+		}
+		defer lr.Close() //nolint:errcheck
+
+		fi, err := os.Create(outfile)
+		if err != nil {
+			return xerrors.Errorf("opening the output file: %w", err)
+		}
+
+		defer fi.Close() //nolint:errcheck
+
+		bs, err := lr.Blockstore(ctx, repo.UniversalBlockstore)
+		if err != nil {
+			return fmt.Errorf("failed to open blockstore: %w", err)
+		}
+
+		defer func() {
+			if c, ok := bs.(io.Closer); ok {
+				if err := c.Close(); err != nil {
+					log.Warnf("failed to close blockstore: %s", err)
+				}
+			}
+		}()
+
+		dag := merkledag.NewDAGService(blockservice.New(bs, offline.Exchange(bs)))
+		err = car.WriteCarWithWalker(ctx, dag, roots, fi, carWalkFunc)
+		if err != nil {
+			return err
+		}
+		return nil
+	},
+}

cmd/lotus-shed/main.go

@@ -43,6 +43,7 @@ func main() {
 		minerCmd,
 		mpoolStatsCmd,
 		exportChainCmd,
+		exportCarCmd,
 		consensusCmd,
 		storageStatsCmd,
 		syncCmd,