func pointersToFetchForRef(ref string) ([]*lfs.WrappedPointer, error) {
	// Use SkipDeletedBlobs to avoid fetching ALL previous versions of modified files
	opts := lfs.NewScanRefsOptions()
	opts.ScanMode = lfs.ScanRefsMode
	opts.SkipDeletedBlobs = true
	return lfs.ScanRefs(ref, "", opts)
}

func uploadsBetweenRefAndRemote(ctx *uploadContext, refnames []string) {
	tracerx.Printf("Upload refs %v to remote %v", refnames, lfs.Config.CurrentRemote)

	scanOpt := lfs.NewScanRefsOptions()
	scanOpt.ScanMode = lfs.ScanLeftToRemoteMode
	scanOpt.RemoteName = lfs.Config.CurrentRemote

	if pushAll {
		scanOpt.ScanMode = lfs.ScanRefsMode
	}

	refs, err := refsByNames(refnames)
	if err != nil {
		Error(err.Error())
		Exit("Error getting local refs.")
	}

	for _, ref := range refs {
		pointers, err := lfs.ScanRefs(ref.Name, "", scanOpt)
		if err != nil {
			Panic(err, "Error scanning for Git LFS files in the %q ref", ref.Name)
		}

		upload(ctx, pointers)
	}
}

func scanAll() []*lfs.WrappedPointer {
	// converts to `git rev-list --all`
	// We only pick up objects in real commits and not the reflog
	opts := lfs.NewScanRefsOptions()
	opts.ScanMode = lfs.ScanAllMode
	opts.SkipDeletedBlobs = false

	// This could be a long process so use the chan version & report progress
	Print("Scanning for all objects ever referenced...")
	spinner := lfs.NewSpinner()
	var numObjs int64
	pointerchan, err := lfs.ScanRefsToChan("", "", opts)
	if err != nil {
		Panic(err, "Could not scan for Git LFS files")
	}

	pointers := make([]*lfs.WrappedPointer, 0)

	for p := range pointerchan {
		numObjs++
		spinner.Print(OutputWriter, fmt.Sprintf("%d objects found", numObjs))
		pointers = append(pointers, p)
	}

	spinner.Finish(OutputWriter, fmt.Sprintf("%d objects found", numObjs))
	return pointers
}

func uploadsBetweenRefs(ctx *uploadContext, left string, right string) {
	tracerx.Printf("Upload between %v and %v", left, right)

	scanOpt := lfs.NewScanRefsOptions()
	scanOpt.ScanMode = lfs.ScanRefsMode
	scanOpt.RemoteName = lfs.Config.CurrentRemote

	pointers, err := lfs.ScanRefs(left, right, scanOpt)
	if err != nil {
		Panic(err, "Error scanning for Git LFS files")
	}

	upload(ctx, pointers)
}

// Background task, must call waitg.Done() once at end
func pruneTaskGetRetainedAtRef(ref string, retainChan chan string, errorChan chan error, waitg *sync.WaitGroup) {
	defer waitg.Done()

	// Only files AT ref, recent is checked in pruneTaskGetRetainedRecentRefs
	opts := lfs.NewScanRefsOptions()
	opts.ScanMode = lfs.ScanRefsMode
	opts.SkipDeletedBlobs = true
	refchan, err := lfs.ScanRefsToChan(ref, "", opts)
	if err != nil {
		errorChan <- err
		return
	}
	for wp := range refchan {
		retainChan <- wp.Pointer.Oid
		tracerx.Printf("RETAIN: %v via ref %v", wp.Pointer.Oid, ref)
	}
}

// prePushCommand is run through Git's pre-push hook. The pre-push hook passes
// two arguments on the command line:
//
//   1. Name of the remote to which the push is being done
//   2. URL to which the push is being done
//
// The hook receives commit information on stdin in the form:
//   <local ref> <local sha1> <remote ref> <remote sha1>
//
// In the typical case, prePushCommand will get a list of git objects being
// pushed by using the following:
//
//    git rev-list --objects <local sha1> ^<remote sha1>
//
// If any of those git objects are associated with Git LFS objects, those
// objects will be pushed to the Git LFS API.
//
// In the case of pushing a new branch, the list of git objects will be all of
// the git objects in this branch.
//
// In the case of deleting a branch, no attempts to push Git LFS objects will be
// made.
func prePushCommand(cmd *cobra.Command, args []string) {
	if len(args) == 0 {
		Print("This should be run through Git's pre-push hook.  Run `git lfs update` to install it.")
		os.Exit(1)
	}

	requireGitVersion()

	// Remote is first arg
	if err := git.ValidateRemote(args[0]); err != nil {
		Exit("Invalid remote name %q", args[0])
	}

	cfg.CurrentRemote = args[0]
	ctx := newUploadContext(prePushDryRun)

	scanOpt := lfs.NewScanRefsOptions()
	scanOpt.ScanMode = lfs.ScanLeftToRemoteMode
	scanOpt.RemoteName = cfg.CurrentRemote

	// We can be passed multiple lines of refs
	scanner := bufio.NewScanner(os.Stdin)
	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())

		if len(line) == 0 {
			continue
		}

		left, right := decodeRefs(line)
		if left == prePushDeleteBranch {
			continue
		}

		pointers, err := lfs.ScanRefs(left, right, scanOpt)
		if err != nil {
			Panic(err, "Error scanning for Git LFS files")
		}

		upload(ctx, pointers)
	}
}

// Background task, must call waitg.Done() once at end
func pruneTaskGetReachableObjects(outObjectSet *lfs.StringSet, errorChan chan error, waitg *sync.WaitGroup) {
	defer waitg.Done()

	// converts to `git rev-list --all`
	// We only pick up objects in real commits and not the reflog
	opts := lfs.NewScanRefsOptions()
	opts.ScanMode = lfs.ScanAllMode
	opts.SkipDeletedBlobs = false

	pointerchan, err := lfs.ScanRefsToChan("", "", opts)
	if err != nil {
		errorChan <- fmt.Errorf("Error scanning for reachable objects: %v", err)
		return
	}

	for p := range pointerchan {
		outObjectSet.Add(p.Oid)
	}

}

func uploadsBetweenRefAndRemote(remote string, refs []string) *lfs.TransferQueue {
	tracerx.Printf("Upload refs %v to remote %v", remote, refs)

	scanOpt := lfs.NewScanRefsOptions()
	scanOpt.ScanMode = lfs.ScanLeftToRemoteMode
	scanOpt.RemoteName = remote

	if pushAll {
		if len(refs) == 0 {
			pointers := scanAll()
			Print("Pushing objects...")
			return uploadPointers(pointers)
		} else {
			scanOpt.ScanMode = lfs.ScanRefsMode
		}
	}

	// keep a unique set of pointers
	oidPointerMap := make(map[string]*lfs.WrappedPointer)

	for _, ref := range refs {
		pointers, err := lfs.ScanRefs(ref, "", scanOpt)
		if err != nil {
			Panic(err, "Error scanning for Git LFS files in the %q ref", ref)
		}

		for _, p := range pointers {
			oidPointerMap[p.Oid] = p
		}
	}

	i := 0
	pointers := make([]*lfs.WrappedPointer, len(oidPointerMap))
	for _, pointer := range oidPointerMap {
		pointers[i] = pointer
		i += 1
	}

	return uploadPointers(pointers)
}

func prePushRef(left, right string) {
	// Just use scanner here
	scanOpt := lfs.NewScanRefsOptions()
	scanOpt.ScanMode = lfs.ScanLeftToRemoteMode
	scanOpt.RemoteName = lfs.Config.CurrentRemote

	pointers, err := lfs.ScanRefs(left, right, scanOpt)
	if err != nil {
		Panic(err, "Error scanning for Git LFS files")
	}

	totalSize := int64(0)
	for _, p := range pointers {
		totalSize += p.Size
	}

	// Objects to skip because they're missing locally but on server
	var skipObjects lfs.StringSet

	if !prePushDryRun {
		// Do this as a pre-flight check since upload queue starts immediately
		skipObjects = prePushCheckForMissingObjects(pointers)
	}

	uploadQueue := lfs.NewUploadQueue(len(pointers), totalSize, prePushDryRun)

	for _, pointer := range pointers {
		if prePushDryRun {
			Print("push %s => %s", pointer.Oid, pointer.Name)
			continue
		}

		if skipObjects.Contains(pointer.Oid) {
			// object missing locally but on server, don't bother
			continue
		}

		u, err := lfs.NewUploadable(pointer.Oid, pointer.Name)
		if err != nil {
			if lfs.IsCleanPointerError(err) {
				Exit(prePushMissingErrMsg, pointer.Name, lfs.ErrorGetContext(err, "pointer").(*lfs.Pointer).Oid)
			} else if Debugging || lfs.IsFatalError(err) {
				Panic(err, err.Error())
			} else {
				Exit(err.Error())
			}
		}

		uploadQueue.Add(u)
	}

	if !prePushDryRun {
		uploadQueue.Wait()
		for _, err := range uploadQueue.Errors() {
			if Debugging || lfs.IsFatalError(err) {
				LoggedError(err, err.Error())
			} else {
				Error(err.Error())
			}
		}

		if len(uploadQueue.Errors()) > 0 {
			os.Exit(2)
		}
	}

}