Merge pull request #15489 from aaronlehmann/pull-updates

Show pull progress in terminal for inflight pull requests

graph/load.go

@@ -106,16 +106,12 @@ func (s *TagStore) recursiveLoad(address, tmpImageDir string) error {
 		}
 
 		// ensure no two downloads of the same layer happen at the same time
-		if c, err := s.poolAdd("pull", "layer:"+img.ID); err != nil {
+		if ps, found := s.poolAdd("pull", "layer:"+img.ID); found {
-			if c != nil {
+			logrus.Debugf("Image (id: %s) load is already running, waiting", img.ID)
-				logrus.Debugf("Image (id: %s) load is already running, waiting: %v", img.ID, err)
+			ps.Wait()
-				<-c
 			return nil
 		}
 
-			return err
-		}
-
 		defer s.poolRemove("pull", "layer:"+img.ID)
 
 		if img.Parent != "" {

graph/pools_test.go

@@ -3,6 +3,7 @@ package graph
 import (
 	"testing"
 
+	"github.com/docker/docker/pkg/progressreader"
 	"github.com/docker/docker/pkg/reexec"
 )
 
@@ -12,24 +13,21 @@ func init() {
 
 func TestPools(t *testing.T) {
 	s := &TagStore{
-		pullingPool: make(map[string]chan struct{}),
+		pullingPool: make(map[string]*progressreader.Broadcaster),
-		pushingPool: make(map[string]chan struct{}),
+		pushingPool: make(map[string]*progressreader.Broadcaster),
 	}
 
-	if _, err := s.poolAdd("pull", "test1"); err != nil {
+	if _, found := s.poolAdd("pull", "test1"); found {
-		t.Fatal(err)
+		t.Fatal("Expected pull test1 not to be in progress")
 	}
-	if _, err := s.poolAdd("pull", "test2"); err != nil {
+	if _, found := s.poolAdd("pull", "test2"); found {
-		t.Fatal(err)
+		t.Fatal("Expected pull test2 not to be in progress")
 	}
-	if _, err := s.poolAdd("push", "test1"); err == nil || err.Error() != "pull test1 is already in progress" {
+	if _, found := s.poolAdd("push", "test1"); !found {
-		t.Fatalf("Expected `pull test1 is already in progress`")
+		t.Fatalf("Expected pull test1 to be in progress`")
 	}
-	if _, err := s.poolAdd("pull", "test1"); err == nil || err.Error() != "pull test1 is already in progress" {
+	if _, found := s.poolAdd("pull", "test1"); !found {
-		t.Fatalf("Expected `pull test1 is already in progress`")
+		t.Fatalf("Expected pull test1 to be in progress`")
-	}
-	if _, err := s.poolAdd("wait", "test3"); err == nil || err.Error() != "Unknown pool type" {
-		t.Fatalf("Expected `Unknown pool type`")
 	}
 	if err := s.poolRemove("pull", "test2"); err != nil {
 		t.Fatal(err)
@@ -43,7 +41,4 @@ func TestPools(t *testing.T) {
 	if err := s.poolRemove("push", "test1"); err != nil {
 		t.Fatal(err)
 	}
-	if err := s.poolRemove("wait", "test3"); err == nil || err.Error() != "Unknown pool type" {
-		t.Fatalf("Expected `Unknown pool type`")
-	}
 }

graph/pull_v1.go

@@ -3,6 +3,7 @@ package graph
 import (
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"net/url"
 	"strings"
@@ -137,31 +138,29 @@ func (p *v1Puller) pullRepository(askedTag string) error {
 			}
 
 			// ensure no two downloads of the same image happen at the same time
-			if c, err := p.poolAdd("pull", "img:"+img.ID); err != nil {
+			broadcaster, found := p.poolAdd("pull", "img:"+img.ID)
-				if c != nil {
+			if found {
-					out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), "Layer already being pulled by another client. Waiting.", nil))
+				broadcaster.Add(out)
-					<-c
+				broadcaster.Wait()
 				out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), "Download complete", nil))
-				} else {
-					logrus.Debugf("Image (id: %s) pull is already running, skipping: %v", img.ID, err)
-				}
 				errors <- nil
 				return
 			}
+			broadcaster.Add(out)
 			defer p.poolRemove("pull", "img:"+img.ID)
 
 			// we need to retain it until tagging
 			p.graph.Retain(sessionID, img.ID)
 			imgIDs = append(imgIDs, img.ID)
 
-			out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Pulling image (%s) from %s", img.Tag, p.repoInfo.CanonicalName), nil))
+			broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Pulling image (%s) from %s", img.Tag, p.repoInfo.CanonicalName), nil))
 			success := false
 			var lastErr, err error
 			var isDownloaded bool
 			for _, ep := range p.repoInfo.Index.Mirrors {
 				ep += "v1/"
-				out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Pulling image (%s) from %s, mirror: %s", img.Tag, p.repoInfo.CanonicalName, ep), nil))
+				broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Pulling image (%s) from %s, mirror: %s", img.Tag, p.repoInfo.CanonicalName, ep), nil))
-				if isDownloaded, err = p.pullImage(img.ID, ep, repoData.Tokens); err != nil {
+				if isDownloaded, err = p.pullImage(broadcaster, img.ID, ep, repoData.Tokens); err != nil {
 					// Don't report errors when pulling from mirrors.
 					logrus.Debugf("Error pulling image (%s) from %s, mirror: %s, %s", img.Tag, p.repoInfo.CanonicalName, ep, err)
 					continue
@@ -172,12 +171,12 @@ func (p *v1Puller) pullRepository(askedTag string) error {
 			}
 			if !success {
 				for _, ep := range repoData.Endpoints {
-					out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Pulling image (%s) from %s, endpoint: %s", img.Tag, p.repoInfo.CanonicalName, ep), nil))
+					broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Pulling image (%s) from %s, endpoint: %s", img.Tag, p.repoInfo.CanonicalName, ep), nil))
-					if isDownloaded, err = p.pullImage(img.ID, ep, repoData.Tokens); err != nil {
+					if isDownloaded, err = p.pullImage(broadcaster, img.ID, ep, repoData.Tokens); err != nil {
 						// It's not ideal that only the last error is returned, it would be better to concatenate the errors.
 						// As the error is also given to the output stream the user will see the error.
 						lastErr = err
-						out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Error pulling image (%s) from %s, endpoint: %s, %s", img.Tag, p.repoInfo.CanonicalName, ep, err), nil))
+						broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), fmt.Sprintf("Error pulling image (%s) from %s, endpoint: %s, %s", img.Tag, p.repoInfo.CanonicalName, ep, err), nil))
 						continue
 					}
 					layersDownloaded = layersDownloaded || isDownloaded
@@ -187,11 +186,11 @@ func (p *v1Puller) pullRepository(askedTag string) error {
 			}
 			if !success {
 				err := fmt.Errorf("Error pulling image (%s) from %s, %v", img.Tag, p.repoInfo.CanonicalName, lastErr)
-				out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), err.Error(), nil))
+				broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), err.Error(), nil))
 				errors <- err
 				return
 			}
-			out.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), "Download complete", nil))
+			broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(img.ID), "Download complete", nil))
 
 			errors <- nil
 		}
@@ -226,12 +225,11 @@ func (p *v1Puller) pullRepository(askedTag string) error {
 	return nil
 }
 
-func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, error) {
+func (p *v1Puller) pullImage(out io.Writer, imgID, endpoint string, token []string) (bool, error) {
 	history, err := p.session.GetRemoteHistory(imgID, endpoint)
 	if err != nil {
 		return false, err
 	}
-	out := p.config.OutStream
 	out.Write(p.sf.FormatProgress(stringid.TruncateID(imgID), "Pulling dependent layers", nil))
 	// FIXME: Try to stream the images?
 	// FIXME: Launch the getRemoteImage() in goroutines
@@ -246,14 +244,18 @@ func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, erro
 		id := history[i]
 
 		// ensure no two downloads of the same layer happen at the same time
-		if c, err := p.poolAdd("pull", "layer:"+id); err != nil {
+		broadcaster, found := p.poolAdd("pull", "layer:"+id)
-			logrus.Debugf("Image (id: %s) pull is already running, skipping: %v", id, err)
+		if found {
-			<-c
+			logrus.Debugf("Image (id: %s) pull is already running, skipping", id)
+			broadcaster.Add(out)
+			broadcaster.Wait()
+		} else {
+			broadcaster.Add(out)
 		}
 		defer p.poolRemove("pull", "layer:"+id)
 
 		if !p.graph.Exists(id) {
-			out.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Pulling metadata", nil))
+			broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Pulling metadata", nil))
 			var (
 				imgJSON []byte
 				imgSize int64
@@ -264,7 +266,7 @@ func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, erro
 			for j := 1; j <= retries; j++ {
 				imgJSON, imgSize, err = p.session.GetRemoteImageJSON(id, endpoint)
 				if err != nil && j == retries {
-					out.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error pulling dependent layers", nil))
+					broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error pulling dependent layers", nil))
 					return layersDownloaded, err
 				} else if err != nil {
 					time.Sleep(time.Duration(j) * 500 * time.Millisecond)
@@ -273,7 +275,7 @@ func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, erro
 				img, err = image.NewImgJSON(imgJSON)
 				layersDownloaded = true
 				if err != nil && j == retries {
-					out.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error pulling dependent layers", nil))
+					broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error pulling dependent layers", nil))
 					return layersDownloaded, fmt.Errorf("Failed to parse json: %s", err)
 				} else if err != nil {
 					time.Sleep(time.Duration(j) * 500 * time.Millisecond)
@@ -289,7 +291,7 @@ func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, erro
 				if j > 1 {
 					status = fmt.Sprintf("Pulling fs layer [retries: %d]", j)
 				}
-				out.Write(p.sf.FormatProgress(stringid.TruncateID(id), status, nil))
+				broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(id), status, nil))
 				layer, err := p.session.GetRemoteImageLayer(img.ID, endpoint, imgSize)
 				if uerr, ok := err.(*url.Error); ok {
 					err = uerr.Err
@@ -298,7 +300,7 @@ func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, erro
 					time.Sleep(time.Duration(j) * 500 * time.Millisecond)
 					continue
 				} else if err != nil {
-					out.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error pulling dependent layers", nil))
+					broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error pulling dependent layers", nil))
 					return layersDownloaded, err
 				}
 				layersDownloaded = true
@@ -307,7 +309,7 @@ func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, erro
 				err = p.graph.Register(img,
 					progressreader.New(progressreader.Config{
 						In:        layer,
-						Out:       out,
+						Out:       broadcaster,
 						Formatter: p.sf,
 						Size:      imgSize,
 						NewLines:  false,
@@ -318,14 +320,14 @@ func (p *v1Puller) pullImage(imgID, endpoint string, token []string) (bool, erro
 					time.Sleep(time.Duration(j) * 500 * time.Millisecond)
 					continue
 				} else if err != nil {
-					out.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error downloading dependent layers", nil))
+					broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Error downloading dependent layers", nil))
 					return layersDownloaded, err
 				} else {
 					break
 				}
 			}
 		}
-		out.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Download complete", nil))
+		broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(id), "Download complete", nil))
 	}
 	return layersDownloaded, nil
 }

graph/pull_v2.go

@@ -73,30 +73,28 @@ func (p *v2Puller) pullV2Repository(tag string) (err error) {
 
 	}
 
-	c, err := p.poolAdd("pull", taggedName)
+	broadcaster, found := p.poolAdd("pull", taggedName)
-	if err != nil {
+	if found {
-		if c != nil {
 		// Another pull of the same repository is already taking place; just wait for it to finish
-			p.config.OutStream.Write(p.sf.FormatStatus("", "Repository %s already being pulled by another client. Waiting.", p.repoInfo.CanonicalName))
+		broadcaster.Add(p.config.OutStream)
-			<-c
+		broadcaster.Wait()
 		return nil
 	}
-		return err
-	}
 	defer p.poolRemove("pull", taggedName)
+	broadcaster.Add(p.config.OutStream)
 
 	var layersDownloaded bool
 	for _, tag := range tags {
 		// pulledNew is true if either new layers were downloaded OR if existing images were newly tagged
 		// TODO(tiborvass): should we change the name of `layersDownload`? What about message in WriteStatus?
-		pulledNew, err := p.pullV2Tag(tag, taggedName)
+		pulledNew, err := p.pullV2Tag(broadcaster, tag, taggedName)
 		if err != nil {
 			return err
 		}
 		layersDownloaded = layersDownloaded || pulledNew
 	}
 
-	writeStatus(taggedName, p.config.OutStream, p.sf, layersDownloaded)
+	writeStatus(taggedName, broadcaster, p.sf, layersDownloaded)
 
 	return nil
 }
@@ -121,18 +119,16 @@ func (p *v2Puller) download(di *downloadInfo) {
 
 	out := di.out
 
-	if c, err := p.poolAdd("pull", "img:"+di.img.ID); err != nil {
+	broadcaster, found := p.poolAdd("pull", "img:"+di.img.ID)
-		if c != nil {
+	if found {
-			out.Write(p.sf.FormatProgress(stringid.TruncateID(di.img.ID), "Layer already being pulled by another client. Waiting.", nil))
+		broadcaster.Add(out)
-			<-c
+		broadcaster.Wait()
 		out.Write(p.sf.FormatProgress(stringid.TruncateID(di.img.ID), "Download complete", nil))
-		} else {
-			logrus.Debugf("Image (id: %s) pull is already running, skipping: %v", di.img.ID, err)
-		}
 		di.err <- nil
 		return
 	}
 
+	broadcaster.Add(out)
 	defer p.poolRemove("pull", "img:"+di.img.ID)
 	tmpFile, err := ioutil.TempFile("", "GetImageBlob")
 	if err != nil {
@@ -167,7 +163,7 @@ func (p *v2Puller) download(di *downloadInfo) {
 
 	reader := progressreader.New(progressreader.Config{
 		In:        ioutil.NopCloser(io.TeeReader(layerDownload, verifier)),
-		Out:       out,
+		Out:       broadcaster,
 		Formatter: p.sf,
 		Size:      di.size,
 		NewLines:  false,
@@ -176,7 +172,7 @@ func (p *v2Puller) download(di *downloadInfo) {
 	})
 	io.Copy(tmpFile, reader)
 
-	out.Write(p.sf.FormatProgress(stringid.TruncateID(di.img.ID), "Verifying Checksum", nil))
+	broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(di.img.ID), "Verifying Checksum", nil))
 
 	if !verifier.Verified() {
 		err = fmt.Errorf("filesystem layer verification failed for digest %s", di.digest)
@@ -185,7 +181,7 @@ func (p *v2Puller) download(di *downloadInfo) {
 		return
 	}
 
-	out.Write(p.sf.FormatProgress(stringid.TruncateID(di.img.ID), "Download complete", nil))
+	broadcaster.Write(p.sf.FormatProgress(stringid.TruncateID(di.img.ID), "Download complete", nil))
 
 	logrus.Debugf("Downloaded %s to tempfile %s", di.img.ID, tmpFile.Name())
 	di.layer = layerDownload
@@ -193,9 +189,8 @@ func (p *v2Puller) download(di *downloadInfo) {
 	di.err <- nil
 }
 
-func (p *v2Puller) pullV2Tag(tag, taggedName string) (verified bool, err error) {
+func (p *v2Puller) pullV2Tag(out io.Writer, tag, taggedName string) (verified bool, err error) {
 	logrus.Debugf("Pulling tag from V2 registry: %q", tag)
-	out := p.config.OutStream
 
 	manSvc, err := p.repo.Manifests(context.Background())
 	if err != nil {

graph/push_v1.go

@@ -214,7 +214,6 @@ func (p *v1Pusher) pushImageToEndpoint(endpoint string, imageIDs []string, tags
 
 // pushRepository pushes layers that do not already exist on the registry.
 func (p *v1Pusher) pushRepository(tag string) error {
-
 	logrus.Debugf("Local repo: %s", p.localRepo)
 	p.out = ioutils.NewWriteFlusher(p.config.OutStream)
 	imgList, tags, err := p.getImageList(tag)
@@ -229,8 +228,8 @@ func (p *v1Pusher) pushRepository(tag string) error {
 		logrus.Debugf("Pushing ID: %s with Tag: %s", data.ID, data.Tag)
 	}
 
-	if _, err := p.poolAdd("push", p.repoInfo.LocalName); err != nil {
+	if _, found := p.poolAdd("push", p.repoInfo.LocalName); found {
-		return err
+		return fmt.Errorf("push or pull %s is already in progress", p.repoInfo.LocalName)
 	}
 	defer p.poolRemove("push", p.repoInfo.LocalName)
 

graph/push_v2.go

@@ -57,8 +57,8 @@ func (p *v2Pusher) getImageTags(askedTag string) ([]string, error) {
 
 func (p *v2Pusher) pushV2Repository(tag string) error {
 	localName := p.repoInfo.LocalName
-	if _, err := p.poolAdd("push", localName); err != nil {
+	if _, found := p.poolAdd("push", localName); found {
-		return err
+		return fmt.Errorf("push or pull %s is already in progress", localName)
 	}
 	defer p.poolRemove("push", localName)
 

graph/tags.go

@@ -17,6 +17,7 @@ import (
 	"github.com/docker/docker/graph/tags"
 	"github.com/docker/docker/image"
 	"github.com/docker/docker/pkg/parsers"
+	"github.com/docker/docker/pkg/progressreader"
 	"github.com/docker/docker/pkg/stringid"
 	"github.com/docker/docker/registry"
 	"github.com/docker/docker/trust"
@@ -36,8 +37,8 @@ type TagStore struct {
 	sync.Mutex
 	// FIXME: move push/pull-related fields
 	// to a helper type
-	pullingPool     map[string]chan struct{}
+	pullingPool     map[string]*progressreader.Broadcaster
-	pushingPool     map[string]chan struct{}
+	pushingPool     map[string]*progressreader.Broadcaster
 	registryService *registry.Service
 	eventsService   *events.Events
 	trustService    *trust.Store
@@ -93,8 +94,8 @@ func NewTagStore(path string, cfg *TagStoreConfig) (*TagStore, error) {
 		graph:           cfg.Graph,
 		trustKey:        cfg.Key,
 		Repositories:    make(map[string]Repository),
-		pullingPool:     make(map[string]chan struct{}),
+		pullingPool:     make(map[string]*progressreader.Broadcaster),
-		pushingPool:     make(map[string]chan struct{}),
+		pushingPool:     make(map[string]*progressreader.Broadcaster),
 		registryService: cfg.Registry,
 		eventsService:   cfg.Events,
 		trustService:    cfg.Trust,
@@ -433,27 +434,32 @@ func validateDigest(dgst string) error {
 	return nil
 }
 
-func (store *TagStore) poolAdd(kind, key string) (chan struct{}, error) {
+// poolAdd checks if a push or pull is already running, and returns
+// (broadcaster, true) if a running operation is found. Otherwise, it creates a
+// new one and returns (broadcaster, false).
+func (store *TagStore) poolAdd(kind, key string) (*progressreader.Broadcaster, bool) {
 	store.Lock()
 	defer store.Unlock()
 
-	if c, exists := store.pullingPool[key]; exists {
+	if p, exists := store.pullingPool[key]; exists {
-		return c, fmt.Errorf("pull %s is already in progress", key)
+		return p, true
 	}
-	if c, exists := store.pushingPool[key]; exists {
+	if p, exists := store.pushingPool[key]; exists {
-		return c, fmt.Errorf("push %s is already in progress", key)
+		return p, true
 	}
 
-	c := make(chan struct{})
+	broadcaster := progressreader.NewBroadcaster()
+
 	switch kind {
 	case "pull":
-		store.pullingPool[key] = c
+		store.pullingPool[key] = broadcaster
 	case "push":
-		store.pushingPool[key] = c
+		store.pushingPool[key] = broadcaster
 	default:
-		return nil, fmt.Errorf("Unknown pool type")
+		panic("Unknown pool type")
 	}
-	return c, nil
+
+	return broadcaster, false
 }
 
 func (store *TagStore) poolRemove(kind, key string) error {
@@ -461,13 +467,13 @@ func (store *TagStore) poolRemove(kind, key string) error {
 	defer store.Unlock()
 	switch kind {
 	case "pull":
-		if c, exists := store.pullingPool[key]; exists {
+		if ps, exists := store.pullingPool[key]; exists {
-			close(c)
+			ps.Close()
 			delete(store.pullingPool, key)
 		}
 	case "push":
-		if c, exists := store.pushingPool[key]; exists {
+		if ps, exists := store.pushingPool[key]; exists {
-			close(c)
+			ps.Close()
 			delete(store.pushingPool, key)
 		}
 	default:

pkg/progressreader/broadcaster.go

@@ -0,0 +1,152 @@
+package progressreader
+
+import (
+	"errors"
+	"io"
+	"sync"
+)
+
+// Broadcaster keeps track of one or more observers watching the progress
+// of an operation. For example, if multiple clients are trying to pull an
+// image, they share a Broadcaster for the download operation.
+type Broadcaster struct {
+	sync.Mutex
+	// c is a channel that observers block on, waiting for the operation
+	// to finish.
+	c chan struct{}
+	// cond is a condition variable used to wake up observers when there's
+	// new data available.
+	cond *sync.Cond
+	// history is a buffer of the progress output so far, so a new observer
+	// can catch up. The history is stored as a slice of separate byte
+	// slices, so that if the writer is a WriteFlusher, the flushes will
+	// happen in the right places.
+	history [][]byte
+	// wg is a WaitGroup used to wait for all writes to finish on Close
+	wg sync.WaitGroup
+	// isClosed is set to true when Close is called to avoid closing c
+	// multiple times.
+	isClosed bool
+}
+
+// NewBroadcaster returns a Broadcaster structure
+func NewBroadcaster() *Broadcaster {
+	b := &Broadcaster{
+		c: make(chan struct{}),
+	}
+	b.cond = sync.NewCond(b)
+	return b
+}
+
+// closed returns true if and only if the broadcaster has been closed
+func (broadcaster *Broadcaster) closed() bool {
+	select {
+	case <-broadcaster.c:
+		return true
+	default:
+		return false
+	}
+}
+
+// receiveWrites runs as a goroutine so that writes don't block the Write
+// function. It writes the new data in broadcaster.history each time there's
+// activity on the broadcaster.cond condition variable.
+func (broadcaster *Broadcaster) receiveWrites(observer io.Writer) {
+	n := 0
+
+	broadcaster.Lock()
+
+	// The condition variable wait is at the end of this loop, so that the
+	// first iteration will write the history so far.
+	for {
+		newData := broadcaster.history[n:]
+		// Make a copy of newData so we can release the lock
+		sendData := make([][]byte, len(newData), len(newData))
+		copy(sendData, newData)
+		broadcaster.Unlock()
+
+		for len(sendData) > 0 {
+			_, err := observer.Write(sendData[0])
+			if err != nil {
+				broadcaster.wg.Done()
+				return
+			}
+			n++
+			sendData = sendData[1:]
+		}
+
+		broadcaster.Lock()
+
+		// detect closure of the broadcast writer
+		if broadcaster.closed() {
+			broadcaster.Unlock()
+			broadcaster.wg.Done()
+			return
+		}
+
+		if len(broadcaster.history) == n {
+			broadcaster.cond.Wait()
+		}
+
+		// Mutex is still locked as the loop continues
+	}
+}
+
+// Write adds data to the history buffer, and also writes it to all current
+// observers.
+func (broadcaster *Broadcaster) Write(p []byte) (n int, err error) {
+	broadcaster.Lock()
+	defer broadcaster.Unlock()
+
+	// Is the broadcaster closed? If so, the write should fail.
+	if broadcaster.closed() {
+		return 0, errors.New("attempted write to closed progressreader Broadcaster")
+	}
+
+	// Add message in p to the history slice
+	newEntry := make([]byte, len(p), len(p))
+	copy(newEntry, p)
+	broadcaster.history = append(broadcaster.history, newEntry)
+
+	broadcaster.cond.Broadcast()
+
+	return len(p), nil
+}
+
+// Add adds an observer to the Broadcaster. The new observer receives the
+// data from the history buffer, and also all subsequent data.
+func (broadcaster *Broadcaster) Add(w io.Writer) error {
+	// The lock is acquired here so that Add can't race with Close
+	broadcaster.Lock()
+	defer broadcaster.Unlock()
+
+	if broadcaster.closed() {
+		return errors.New("attempted to add observer to closed progressreader Broadcaster")
+	}
+
+	broadcaster.wg.Add(1)
+	go broadcaster.receiveWrites(w)
+
+	return nil
+}
+
+// Close signals to all observers that the operation has finished.
+func (broadcaster *Broadcaster) Close() {
+	broadcaster.Lock()
+	if broadcaster.isClosed {
+		broadcaster.Unlock()
+		return
+	}
+	broadcaster.isClosed = true
+	close(broadcaster.c)
+	broadcaster.cond.Broadcast()
+	broadcaster.Unlock()
+
+	// Don't return from Close until all writers have caught up.
+	broadcaster.wg.Wait()
+}
+
+// Wait blocks until the operation is marked as completed by the Done method.
+func (broadcaster *Broadcaster) Wait() {
+	<-broadcaster.c
+}