Adds push/pull client functionality

These methods rely on an ObjectStore interface, which is meant to
approximate the storage behavior of the docker engine. This is very much
subject to change.

client/pull.go

@@ -0,0 +1,93 @@
+package client
+
+import (
+	"fmt"
+	"io"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+// Pull implements a client pull workflow for the image defined by the given
+// name and tag pair, using the given ObjectStore for local manifest and layer
+// storage
+func Pull(c Client, objectStore ObjectStore, name, tag string) error {
+	manifest, err := c.GetImageManifest(name, tag)
+	if err != nil {
+		return err
+	}
+	log.WithField("manifest", manifest).Info("Pulled manifest")
+
+	if len(manifest.FSLayers) != len(manifest.History) {
+		return fmt.Errorf("Length of history not equal to number of layers")
+	}
+	if len(manifest.FSLayers) == 0 {
+		return fmt.Errorf("Image has no layers")
+	}
+
+	for _, fsLayer := range manifest.FSLayers {
+		layer, err := objectStore.Layer(fsLayer.BlobSum)
+		if err != nil {
+			log.WithFields(log.Fields{
+				"error": err,
+				"layer": fsLayer,
+			}).Warn("Unable to write local layer")
+			return err
+		}
+
+		writer, err := layer.Writer()
+		if err == ErrLayerAlreadyExists {
+			log.WithField("layer", fsLayer).Info("Layer already exists")
+			continue
+		}
+		if err == ErrLayerLocked {
+			log.WithField("layer", fsLayer).Info("Layer download in progress, waiting")
+			layer.Wait()
+			continue
+		}
+		if err != nil {
+			log.WithFields(log.Fields{
+				"error": err,
+				"layer": fsLayer,
+			}).Warn("Unable to write local layer")
+			return err
+		}
+		defer writer.Close()
+
+		layerReader, length, err := c.GetImageLayer(name, fsLayer.BlobSum, 0)
+		if err != nil {
+			log.WithFields(log.Fields{
+				"error": err,
+				"layer": fsLayer,
+			}).Warn("Unable to download layer")
+			return err
+		}
+		defer layerReader.Close()
+
+		copied, err := io.Copy(writer, layerReader)
+		if err != nil {
+			log.WithFields(log.Fields{
+				"error": err,
+				"layer": fsLayer,
+			}).Warn("Unable to download layer")
+			return err
+		}
+		if copied != int64(length) {
+			log.WithFields(log.Fields{
+				"expected": length,
+				"written":  copied,
+				"layer":    fsLayer,
+			}).Warn("Wrote incorrect number of bytes for layer")
+		}
+	}
+
+	err = objectStore.WriteManifest(name, tag, manifest)
+	if err != nil {
+		log.WithFields(log.Fields{
+			"error":    err,
+			"manifest": manifest,
+		}).Warn("Unable to write image manifest")
+		return err
+	}
+
+	return nil
+}