cmd/oci-cas: Add tar and zip support for file URIs

This lets us extract blobs from layout archives.  For example:

  $ cat cas-engines.json
  [
    {
      "config": {
        "protocol": "oci-cas-template-v1",
        "uri": "blobs/{algorithm}/{encoded}"
      },
      "uri": "file:///"
    }
  ]
  $ unzip -l hello-world.zip
  Archive:  hello-world.zip
    Length      Date    Time    Name
  ---------  ---------- -----   ----
          0  10-14-2017 16:34   blobs/
          0  10-14-2017 16:34   blobs/sha256/
         14  10-14-2017 16:34   blobs/sha256/c98c24b677eff44860afea6f493bbaec5bb1c4cbb209c6fc2bbb47f66ff2ad31
  ---------                     -------
         14                     3 files
  $ oci-cas --zip-file hello-world.zip sha256:c98c24b677eff44860afea6f493bbaec5bb1c4cbb209c6fc2bbb47f66ff2ad31 <cas-engines.json
  Hello, World!

The tar implementation doesn't quite work:

  $ tar -tf hello-world.tar
  ./
  ./blobs/
  ./blobs/sha256/
  ./blobs/sha256/c98c24b677eff44860afea6f493bbaec5bb1c4cbb209c6fc2bbb47f66ff2ad31
  $ ./oci-cas --tar-file hello-world.tar sha256:c98c24b677eff44860afea6f493bbaec5bb1c4cbb209c6fc2bbb47f66ff2ad31 <cas-engines.json
  FATA[0000] failed to retrieve sha256:c98c24b677eff44860afea6f493bbaec5bb1c4cbb209c6fc2bbb47f66ff2ad31

I think omeid/go-tarfs needs some love.

Signed-off-by: W. Trevor King <wking@tremily.us>

cmd/oci-cas/main.go

@@ -15,6 +15,7 @@
 package main
 
 import (
+	"archive/zip"
 	_ "crypto/sha256"
 	_ "crypto/sha512"
 	"encoding/json"
@@ -24,13 +25,16 @@ import (
 	"net/http"
 	"os"
 
+	"github.com/omeid/go-tarfs"
 	"github.com/opencontainers/go-digest"
 	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli"
 	"github.com/wking/casengine"
 	_ "github.com/wking/casengine/template"
 	"github.com/xiekeyang/oci-discovery/tools/engine"
 	"golang.org/x/net/context"
+	"golang.org/x/tools/godoc/vfs/httpfs"
+	"golang.org/x/tools/godoc/vfs/zipfs"
 )
 
 func main() {
@@ -48,7 +52,15 @@ func main() {
 		},
 		cli.StringFlag{
 			Name:  "file",
-			Usage: "Effective root for file URIs.  To allow access to your entire filesystem, use '--file /'.  More restricted values are recommended to avoid accessing sensitive information.  The default is to disable file URIs entirely; you must set this flag to enable them.",
+			Usage: "Effective root for file URIs.  To allow access to your entire filesystem, use '--file /'.  More restricted values are recommended to avoid accessing sensitive information.  The default is to disable file URIs entirely; you must set this flag (or another --*-file flag) to enable them.",
+		},
+		cli.StringFlag{
+			Name:  "tar-file",
+			Usage: "Effective root for file URIs in a tape archive file (tarball).  As an alternative to --file, use the tape archive at this path as the root of the file URI filesystem.",
+		},
+		cli.StringFlag{
+			Name:  "zip-file",
+			Usage: "Effective root for file URIs in a zip archive file.  As an alternative to --file, use the zip archive at this path as the root of the file URI filesystem.",
 		},
 	}
 
@@ -67,11 +79,52 @@ func main() {
 		ctx := context.Background()
 
 		if c.GlobalIsSet("file") {
+			if c.GlobalIsSet("tar-file") {
+				return fmt.Errorf("setting both --file and --tar-file is invalid")
+			}
+			if c.GlobalIsSet("zip-file") {
+				return fmt.Errorf("setting both --file and --zip-file is invalid")
+			}
 			path := c.GlobalString("file")
 			transport := http.NewFileTransport(http.Dir(path))
 			http.DefaultTransport.(*http.Transport).RegisterProtocol("file", transport)
 		}
 
+		if c.GlobalIsSet("tar-file") {
+			if c.GlobalIsSet("zip-file") {
+				return fmt.Errorf("setting both --tar-file and --zip-file is invalid")
+			}
+			path := c.GlobalString("tar-file")
+			reader, err := os.Open(path)
+			if err != nil {
+				return err
+			}
+			tarFS, err := tarfs.New(reader)
+			if err != nil {
+				err2 := reader.Close()
+				if err2 != nil {
+					logrus.Warn("failed to close the tar reader")
+				}
+				return err
+			}
+			err = reader.Close()
+			if err != nil {
+				return err
+			}
+			transport := http.NewFileTransport(tarFS)
+			http.DefaultTransport.(*http.Transport).RegisterProtocol("file", transport)
+		}
+
+		if c.GlobalIsSet("zip-file") {
+			path := c.GlobalString("zip-file")
+			reader, err := zip.OpenReader(path)
+			if err != nil {
+				return err
+			}
+			transport := http.NewFileTransport(httpfs.New(zipfs.New(reader, path)))
+			http.DefaultTransport.(*http.Transport).RegisterProtocol("file", transport)
+		}
+
 		var configReferences []engine.Reference
 		err = json.NewDecoder(os.Stdin).Decode(&configReferences)
 		if err != nil {