content: refactor content store for API

After iterating on the GRPC API, the changes required for the actual
implementation are now included in the content store. The begin change
is the move to a single, atomic `Ingester.Writer` method for locking
content ingestion on a key. From this, comes several new interface
definitions.

The main benefit here is the clarification between `Status` and `Info`
that came out of the GPRC API. `Status` tells the status of a write,
whereas `Info` is for querying metadata about various blobs.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/dist/delete.go

@@ -3,9 +3,7 @@ package main
 import (
 	contextpkg "context"
 	"fmt"
-	"path/filepath"
 
-	"github.com/docker/containerd/content"
 	"github.com/docker/containerd/log"
 	digest "github.com/opencontainers/go-digest"
 	"github.com/urfave/cli"
@@ -18,30 +16,15 @@ var deleteCommand = cli.Command{
 	ArgsUsage: "[flags] [<digest>, ...]",
 	Description: `Delete one or more blobs permanently. Successfully deleted
 	blobs are printed to stdout.`,
-	Flags: []cli.Flag{
-		cli.StringFlag{
-			Name:  "root",
-			Usage: "path to content store root",
-			Value: ".content", // TODO(stevvooe): for now, just use the PWD/.content
-		},
-	},
+	Flags: []cli.Flag{},
 	Action: func(context *cli.Context) error {
 		var (
 			ctx       = contextpkg.Background()
-			root      = context.String("root")
 			args      = []string(context.Args())
 			exitError error
 		)
 
-		if !filepath.IsAbs(root) {
-			var err error
-			root, err = filepath.Abs(root)
-			if err != nil {
-				return err
-			}
-		}
-
-		cs, err := content.Open(root)
+		cs, err := resolveContentStore(context)
 		if err != nil {
 			return err
 		}