ipfsbug.go

package main

import (
	"flag"
	"fmt"
	"golang.org/x/net/context"
	"os"
	"os/user"
	"path/filepath"

	core "github.com/ipfs/go-ipfs/core"
	coreunix "github.com/ipfs/go-ipfs/core/coreunix"
	fsrepo "github.com/ipfs/go-ipfs/repo/fsrepo"
)

func main() {
	repo := flag.String("repo", "", "Path to the ipfs repo")
	hash := flag.String("hash", "", "B58 hash of the file")
	flag.Parse()

	if *hash == "" {
		fmt.Println("Need a hash.")
		return
	}

	if *repo == "" {
		user, err := user.Current()
		if err != nil {
			fmt.Println("Failed to guess user dir")
			return
		}

		*repo = filepath.Join(user.HomeDir, ".ipfs")
	}

	rp, err := fsrepo.Open(*repo)
	if err != nil {
		fmt.Println("Unable to open repo `%s`: %v", *repo, err)
		return
	}

	cfg := &core.BuildCfg{
		Repo:   rp,
		Online: false,
	}

	ctx := context.Background()
	nd, err := core.NewNode(ctx, cfg)
	if err != nil {
		fmt.Println("Creating new node failed", err)
		return
	}

	reader, err := coreunix.Cat(ctx, nd, *hash)
	if err != nil {
		fmt.Println("ipfs cat: %v", err)
		return
	}

	////////////////////////////////////////
	// END OF SETUP - TROUBLE STARTS HERE //
	////////////////////////////////////////

	// This is not necessary to trigger the bug, just
	// to show that the first call returns the correct offset
	fmt.Println(reader.Seek(0, os.SEEK_END))

	n := int64(1)  // Even 0 seems to trigger the bug.
	
	// This should be negative, but ipfs uses positive offsets for SEEK_END?
	//   https://github.com/ipfs/go-ipfs/blob/master/unixfs/io/dagreader.go#L277
	// fmt.Println(reader.Seek(-n, os.SEEK_END))
	fmt.Println(reader.Seek(n, os.SEEK_END))
	fmt.Println(reader.Read(make([]byte, n)))

	// This prints (0 EOF) for larger files (probably around 256K is the "bug border"),
	// however returns the actual size of the file for smaller files.
	fmt.Println(reader.Seek(0, os.SEEK_END))
}