Move testutil package to top-level

Since the common package no longer exists, the testutil package is being moved
up to the root. Ideally, we don't have large omnibus packages, like testutil,
but we can fix that in another refactoring round.

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

testutil/tarfile.go

@@ -0,0 +1,95 @@
+package testutil
+
+import (
+	"archive/tar"
+	"bytes"
+	"crypto/rand"
+	"fmt"
+	"io"
+	"io/ioutil"
+	mrand "math/rand"
+	"time"
+
+	"github.com/docker/docker/pkg/tarsum"
+)
+
+// CreateRandomTarFile creates a random tarfile, returning it as an
+// io.ReadSeeker along with its tarsum. An error is returned if there is a
+// problem generating valid content.
+func CreateRandomTarFile() (rs io.ReadSeeker, tarSum string, err error) {
+	nFiles := mrand.Intn(10) + 10
+	target := &bytes.Buffer{}
+	wr := tar.NewWriter(target)
+
+	// Perturb this on each iteration of the loop below.
+	header := &tar.Header{
+		Mode:       0644,
+		ModTime:    time.Now(),
+		Typeflag:   tar.TypeReg,
+		Uname:      "randocalrissian",
+		Gname:      "cloudcity",
+		AccessTime: time.Now(),
+		ChangeTime: time.Now(),
+	}
+
+	for fileNumber := 0; fileNumber < nFiles; fileNumber++ {
+		fileSize := mrand.Int63n(1<<20) + 1<<20
+
+		header.Name = fmt.Sprint(fileNumber)
+		header.Size = fileSize
+
+		if err := wr.WriteHeader(header); err != nil {
+			return nil, "", err
+		}
+
+		randomData := make([]byte, fileSize)
+
+		// Fill up the buffer with some random data.
+		n, err := rand.Read(randomData)
+
+		if n != len(randomData) {
+			return nil, "", fmt.Errorf("short read creating random reader: %v bytes != %v bytes", n, len(randomData))
+		}
+
+		if err != nil {
+			return nil, "", err
+		}
+
+		nn, err := io.Copy(wr, bytes.NewReader(randomData))
+		if nn != fileSize {
+			return nil, "", fmt.Errorf("short copy writing random file to tar")
+		}
+
+		if err != nil {
+			return nil, "", err
+		}
+
+		if err := wr.Flush(); err != nil {
+			return nil, "", err
+		}
+	}
+
+	if err := wr.Close(); err != nil {
+		return nil, "", err
+	}
+
+	reader := bytes.NewReader(target.Bytes())
+
+	// A tar builder that supports tarsum inline calculation would be awesome
+	// here.
+	ts, err := tarsum.NewTarSum(reader, true, tarsum.Version1)
+	if err != nil {
+		return nil, "", err
+	}
+
+	nn, err := io.Copy(ioutil.Discard, ts)
+	if nn != int64(len(target.Bytes())) {
+		return nil, "", fmt.Errorf("short copy when getting tarsum of random layer: %v != %v", nn, len(target.Bytes()))
+	}
+
+	if err != nil {
+		return nil, "", err
+	}
+
+	return bytes.NewReader(target.Bytes()), ts.Sum(nil), nil
+}