You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: repack.go
    path of file: ./repos/restic/internal/repository
    
    the code of the file until where you have to start completion: package repository

import (
	"context"
	"sync"

	"github.com/restic/restic/internal/debug"
	"github.com/restic/restic/internal/errors"
	"github.com/restic/restic/internal/restic"
	"github.com/restic/restic/internal/ui/progress"

	"golang.org/x/sync/errgroup"
)

type repackBlobSet interface {
	Has(bh restic.BlobHandle) bool
	Delete(bh restic.BlobHandle)
	Len() int
}

// Repack takes a list of packs together with a list of blobs contained in
// these packs. Each pack is loaded and the blobs listed in keepBlobs is saved
// into a new pack. Returned is the list of obsolete packs which can then
// be removed.
//
// The map keepBlobs is modified by Repack, it is used to keep track of which
// blobs have been processed.
func Repack(ctx context.Context, repo restic.Repository, dstRepo restic.Repository, packs restic.IDSet, keepBlobs repackBlobSet, p *progress.Counter) (obsoletePacks restic.IDSet, err error) {
	debug.Log("repacking %d packs while keeping %d blobs", len(packs), keepBlobs.Len())

	if repo == dstRepo && dstRepo.Connections() < 2 {
		return nil, errors.New("repack step requires a backend connection limit of at least two")
	}

	wg, wgCtx := errgroup.WithContext(ctx)

	dstRepo.StartPackUploader(wgCtx, wg)
	wg.Go(func() error {
		var err error
		obsoletePacks, err = repack(wgCtx, repo, dstRepo, packs, keepBlobs, p)
		return err
	})

	if err := wg.Wait(); err != nil {
		return nil, err
	}
	return obsoletePacks, nil
}

func repack(ctx context.Context, repo restic.Repository, dstRepo restic.Repository, packs restic.IDSet, keepBlobs repackBlobSet, p *progress.Counter) (obsoletePacks restic.IDSet, err error) {
	wg, wgCtx := errgroup.WithContext(ctx)

	var keepMutex sync.Mutex
	downloadQueue := make(chan restic.PackBlobs)
	wg.Go(func() error {
		defer close(downloadQueue)
		for pbs := range repo.Index().ListP
    
func repack(ctx context.Context, repo restic.Repository, dstRepo restic.Repository, packs restic.IDSet, keepBlobs repackBlobSet, p *progress.Counter) (obsoletePacks restic.IDSet, err error) {
	wg, wgCtx := errgroup.WithContext(ctx)

	var keepMutex sync.Mutex
	downloadQueue := make(chan restic.PackBlobs)
	wg.Go(func() error {
		defer close(downloadQueue)
		for pbs := range repo.Index().ListPacks(wgCtx, packs) {
			var packBlobs []restic.Blob
			keepMutex.Lock()
			// filter out unnecessary blobs
			for _, entry := range pbs.Blobs {
				h := restic.BlobHandle{ID: entry.ID, Type: entry.Type}
				if keepBlobs.Has(h) {
					packBlobs = append(packBlobs, entry)
				}
			}
			keepMutex.Unlock()

			select {
			case downloadQueue <- restic.PackBlobs{PackID: pbs.PackID, Blobs: packBlobs}:
			case <-wgCtx.Done():
				return wgCtx.Err()
			}
		}
		return nil
	})

	worker := func() error {
		for t := range downloadQueue {
			err := repo.LoadBlobsFromPack(wgCtx, t.PackID, t.Blobs, func(blob restic.BlobHandle, buf []byte, err error) error {
				if err != nil {
					var ierr error
					// check whether we can get a valid copy somewhere else
					buf, ierr = repo.LoadBlob(wgCtx, blob.Type, blob.ID, nil)
					if ierr != nil {
						// no luck, return the original error
						return err
					}
				}

				keepMutex.Lock()
				// recheck whether some other worker was faster
				shouldKeep := keepBlobs.Has(blob)
				if shouldKeep {
					keepBlobs.Delete(blob)
				}
				keepMutex.Unlock()

				if !shouldKeep {
					return nil
				}

				// We do want to save already saved blobs!
				_, _, _, err = dstRepo.SaveBlob(wgCtx, blob.Type, buf, blob.ID, true)
				if err != nil {
					return err
				}

				debug.Log("  saved blob %v", blob.ID)
				return nil
			})
			if err != nil {
				return err
			}
			p.Add(1)
		}
		return nil
	}

	// as packs are streamed the concurrency is limited by IO
	// reduce by one to ensure that uploading is always possible
	repackWorkerCount := int(repo.Connections() - 1)
	if repo != dstRepo {
		// no need to share the upload and download connections for different repositories
		repackWorkerCount = int(repo.Connections())
	}
	for i := 0; i < repackWorkerCount; i++ {
		wg.Go(worker)
	}

	if err := wg.Wait(); err != nil {
		return nil, err
	}

	if err := dstRepo.Flush(ctx); err != nil {
		return nil, err
	}

	return packs, nil
}