avoid parallel pulls of the same image

To suit the needs of OpenShift CI [1], we need to avoid parallel pulls.
While we have an open PR against containers/image [2] to detect parallel
copy operations across processes, we are still far from getting this
merged; it must be split into smaller chunks and we plan to tackle in
the next few months but who knows how long this might take.

To support that in CRI-O, however, we enjoy the benefit of running a
daemon where we can easily avoid redundantly pulling the same image in
parallel as we're in the same process space and can do all
synchronisation in memory.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1785390
[2] containers/image#611

Signed-off-by: Valentin Rothberg <[email protected]>

server/image_pull.go

@@ -27,38 +27,75 @@ func (s *Server) PullImage(ctx context.Context, req *pb.PullImageRequest) (resp
 		image = img.Image
 	}
 
-	sourceCtx := *s.config.SystemContext // A shallow copy we can modify
+	pullArgs := pullArguments{image: image}
 	if req.GetAuth() != nil {
 		username := req.GetAuth().Username
 		password := req.GetAuth().Password
 		if req.GetAuth().Auth != "" {
 			username, password, err = decodeDockerAuth(req.GetAuth().Auth)
 			if err != nil {
-				log.Debugf(ctx, "error decoding authentication for image %s: %v", img, err)
+				log.Debugf(ctx, "error decoding authentication for image %s: %v", image, err)
 				return nil, err
 			}
 		}
 		// Specifying a username indicates the user intends to send authentication to the registry.
 		if username != "" {
-			sourceCtx.DockerAuthConfig = &types.DockerAuthConfig{
+			pullArgs.credentials = types.DockerAuthConfig{
 				Username: username,
 				Password: password,
 			}
 		}
 	}
 
+	// We use the server's pullCache to record which images are currently being
+	// pulled. This allows for avoiding pulling the same image in parallel.
+	// Hence, if a given image is currently being pulled, we queue into the
+	// pullOperation's waitgroup and wait for the pulling goroutine to unblock
+	// us and re-use its results.
+	pullOp, pullInProcess := s.pullOperation(&pullArgs)
+	if !pullInProcess {
+		go func() {
+			pullOp.imageRef, pullOp.err = s.pullImage(ctx, req, &pullArgs)
+			// Unblock waiting goroutines and delete the entry from the cache.
+			s.pullOperationsLock.Lock()
+			delete(s.pullOperationsInProgress, pullArgs)
+			pullOp.wg.Done()
+			s.pullOperationsLock.Unlock()
+		}()
+	}
+	// Wait for the pull operation to finish.
+	pullOp.wg.Wait()
+
+	if pullOp.err != nil {
+		return nil, pullOp.err
+	}
+
+	resp = &pb.PullImageResponse{
+		ImageRef: pullOp.imageRef,
+	}
+	return resp, nil
+}
+
+// pullImage performs the actual pull operation of PullImage. Used to separate
+// the pull implementation from the pullCache logic in PullImage and improve
+// readability and maintainability.
+func (s *Server) pullImage(ctx context.Context, req *pb.PullImageRequest, pullArgs *pullArguments) (string, error) {
+	var err error
+	sourceCtx := *s.config.SystemContext // A shallow copy we can modify
+	sourceCtx.DockerAuthConfig = &pullArgs.credentials
+
 	decryptConfig, err := getDecryptionKeys(s.config.DecryptionKeysPath)
 	if err != nil {
-		return nil, err
+		return "", err
 	}
 
 	var (
 		images []string
 		pulled string
 	)
-	images, err = s.StorageImageServer().ResolveNames(s.config.SystemContext, image)
+	images, err = s.StorageImageServer().ResolveNames(s.config.SystemContext, pullArgs.image)
 	if err != nil {
-		return nil, err
+		return "", err
 	}
 	for _, img := range images {
 		var tmpImg types.ImageCloser
@@ -159,21 +196,21 @@ func (s *Server) PullImage(ctx context.Context, req *pb.PullImageRequest) (resp
 		pulled = img
 		break
 	}
+
 	if pulled == "" && err != nil {
-		return nil, err
+		return "", err
 	}
+
 	status, err := s.StorageImageServer().ImageStatus(s.config.SystemContext, pulled)
 	if err != nil {
-		return nil, err
+		return "", err
 	}
 	imageRef := status.ID
 	if len(status.RepoDigests) > 0 {
 		imageRef = status.RepoDigests[0]
 	}
-	resp = &pb.PullImageResponse{
-		ImageRef: imageRef,
-	}
-	return resp, nil
+
+	return imageRef, nil
 }
 
 func decodeDockerAuth(s string) (user, password string, err error) {

server/server.go

@@ -16,6 +16,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/containers/image/v5/types"
 	"github.com/containers/storage/pkg/idtools"
 	"github.com/cri-o/cri-o/internal/lib"
 	"github.com/cri-o/cri-o/internal/lib/sandbox"
@@ -61,6 +62,47 @@ type Server struct {
 	defaultIDMappings *idtools.IDMappings
 
 	updateLock sync.RWMutex
+
+	// pullOperationsInProgress is used to avoid pulling the same image in parallel. Goroutines
+	// will block on the pullResult.
+	pullOperationsInProgress map[pullArguments]*pullOperation
+	// pullOperationsLock is used to synchronize pull operations.
+	pullOperationsLock sync.Mutex
+}
+
+// pullArguments are used to identify a pullOperation via an input image name and
+// possibly specified credentials.
+type pullArguments struct {
+	image       string
+	credentials types.DockerAuthConfig
+}
+
+// pullOperation is used to synchronize parallel pull operations via the
+// server's pullCache.  Goroutines can block the pullOperation's waitgroup and
+// be released once the pull operation has finished.
+type pullOperation struct {
+	// wg allows for Goroutines trying to pull the same image to wait until the
+	// currently running pull operation has finished.
+	wg sync.WaitGroup
+	// imageRef is the reference of the actually pulled image which will differ
+	// from the input if it was a short name (e.g., alpine).
+	imageRef string
+	// err is the error indicating if the pull operation has succeeded or not.
+	err error
+}
+
+// pullOperation returns a pullOperation and a bool indicating if the
+// pullOperation is already in progress.
+func (s *Server) pullOperation(pullArgs *pullArguments) (pullOp *pullOperation, inProgress bool) {
+	s.pullOperationsLock.Lock()
+	defer s.pullOperationsLock.Unlock()
+	pullOp, inProgress = s.pullOperationsInProgress[*pullArgs]
+	if !inProgress {
+		pullOp = &pullOperation{}
+		s.pullOperationsInProgress[*pullArgs] = pullOp
+		pullOp.wg.Add(1)
+	}
+	return pullOp, inProgress
 }
 
 type certConfigCache struct {
@@ -324,12 +366,13 @@ func New(
 	}
 
 	s := &Server{
-		ContainerServer:   containerServer,
-		netPlugin:         netPlugin,
-		hostportManager:   hostportManager,
-		config:            *config,
-		monitorsChan:      make(chan struct{}),
-		defaultIDMappings: idMappings,
+		ContainerServer:          containerServer,
+		netPlugin:                netPlugin,
+		hostportManager:          hostportManager,
+		config:                   *config,
+		monitorsChan:             make(chan struct{}),
+		defaultIDMappings:        idMappings,
+		pullOperationsInProgress: make(map[pullArguments]*pullOperation),
 	}
 
 	if err := configureMaxThreads(); err != nil {