integration: add upstream validator to OCI roundtrip tests

In order to make sure that we don't create invalid OCI images that are
consistently invalid, add additional checks to ensure that both of the
generated OCI images in the round-trip test are valid according to the
upstream validator.

This commit vendors the following packages (deep breath):
* oci/image-tools@7575a09363, which requires
* oci/image-spec@v1.0.0-rc4 [revendor, but is technically an update
  because I couldn't figure out what version was vendored last time]
* oci/runtime-spec@v1.0.0-rc4
* xeipuuv/gojsonschema@6b67b3fab7
* xeipuuv/gojsonreference@e02fc20de9
* xeipuuv/gojsonpointer@e0fe6f6830
* camlistore/go4@7ce08ca145

Signed-off-by: Aleksa Sarai <asarai@suse.de>

vendor/github.com/opencontainers/image-tools/image/autodetect.go

@@ -0,0 +1,109 @@
+// Copyright 2016 The Linux Foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package image
+
+import (
+	"encoding/json"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"os"
+
+	"github.com/opencontainers/image-spec/schema"
+	"github.com/pkg/errors"
+)
+
+// supported autodetection types
+const (
+	TypeImageLayout  = "imageLayout"
+	TypeImage        = "image"
+	TypeManifest     = "manifest"
+	TypeManifestList = "manifestList"
+	TypeConfig       = "config"
+)
+
+// Autodetect detects the validation type for the given path
+// or an error if the validation type could not be resolved.
+func Autodetect(path string) (string, error) {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return "", errors.Wrapf(err, "unable to access path") // err from os.Stat includes path name
+	}
+
+	if fi.IsDir() {
+		return TypeImageLayout, nil
+	}
+
+	f, err := os.Open(path)
+	if err != nil {
+		return "", errors.Wrap(err, "unable to open file") // os.Open includes the filename
+	}
+	defer f.Close()
+
+	buf, err := ioutil.ReadAll(io.LimitReader(f, 512)) // read some initial bytes to detect content
+	if err != nil {
+		return "", errors.Wrap(err, "unable to read")
+	}
+
+	mimeType := http.DetectContentType(buf)
+
+	switch mimeType {
+	case "application/x-gzip", "application/octet-stream":
+		return TypeImage, nil
+
+	case "text/plain; charset=utf-8":
+		// might be a JSON file, will be handled below
+
+	default:
+		return "", errors.New("unknown file type")
+	}
+
+	if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+		return "", errors.Wrap(err, "unable to seek")
+	}
+
+	header := struct {
+		SchemaVersion int         `json:"schemaVersion"`
+		MediaType     string      `json:"mediaType"`
+		Config        interface{} `json:"config"`
+	}{}
+
+	if err := json.NewDecoder(f).Decode(&header); err != nil {
+		if _, errSeek := f.Seek(0, os.SEEK_SET); errSeek != nil {
+			return "", errors.Wrap(err, "unable to seek")
+		}
+
+		e := errors.Wrap(
+			schema.WrapSyntaxError(f, err),
+			"unable to parse JSON",
+		)
+
+		return "", e
+	}
+
+	switch {
+	case header.MediaType == string(schema.MediaTypeManifest):
+		return TypeManifest, nil
+
+	case header.MediaType == string(schema.MediaTypeManifestList):
+		return TypeManifestList, nil
+
+	case header.MediaType == "" && header.SchemaVersion == 0 && header.Config != nil:
+		// config files don't have mediaType/schemaVersion header
+		return TypeConfig, nil
+	}
+
+	return "", errors.New("unknown media type")
+}