Move IsArchive and HeaderSize to build/context

Previously these were in docker/docker/pkg/archive, but unused

Signed-off-by: Daniel Nephin <dnephin@docker.com>

cli/command/image/build/context.go

@@ -28,6 +28,8 @@ import (
 const (
 	// DefaultDockerfileName is the Default filename with Docker commands, read by docker build
 	DefaultDockerfileName string = "Dockerfile"
+	// archiveHeaderSize is the number of bytes in an archive header
+	archiveHeaderSize = 512
 )
 
 // ValidateContextDirectory checks if all the contents of the directory
@@ -84,12 +86,12 @@ func ValidateContextDirectory(srcPath string, excludes []string) error {
 func GetContextFromReader(r io.ReadCloser, dockerfileName string) (out io.ReadCloser, relDockerfile string, err error) {
 	buf := bufio.NewReader(r)
 
-	magic, err := buf.Peek(archive.HeaderSize)
+	magic, err := buf.Peek(archiveHeaderSize)
 	if err != nil && err != io.EOF {
 		return nil, "", errors.Errorf("failed to peek context header from STDIN: %v", err)
 	}
 
-	if archive.IsArchive(magic) {
+	if IsArchive(magic) {
 		return ioutils.NewReadCloserWrapper(buf, func() error { return r.Close() }), dockerfileName, nil
 	}
 
@@ -133,6 +135,18 @@ func GetContextFromReader(r io.ReadCloser, dockerfileName string) (out io.ReadCl
 
 }
 
+// IsArchive checks for the magic bytes of a tar or any supported compression
+// algorithm.
+func IsArchive(header []byte) bool {
+	compression := archive.DetectCompression(header)
+	if compression != archive.Uncompressed {
+		return true
+	}
+	r := tar.NewReader(bytes.NewBuffer(header))
+	_, err := r.Next()
+	return err == nil
+}
+
 // GetContextFromGitURL uses a Git URL as context for a `docker build`. The
 // git repo is cloned into a temporary directory used as the context directory.
 // Returns the absolute path to the temporary context directory, the relative

cli/command/image/build/context_test.go

@@ -266,3 +266,35 @@ func chdir(t *testing.T, dir string) func() {
 	require.NoError(t, os.Chdir(dir))
 	return func() { require.NoError(t, os.Chdir(workingDirectory)) }
 }
+
+func TestIsArchive(t *testing.T) {
+	var testcases = []struct {
+		doc      string
+		header   []byte
+		expected bool
+	}{
+		{
+			doc:      "nil is not a valid header",
+			header:   nil,
+			expected: false,
+		},
+		{
+			doc:      "invalid header bytes",
+			header:   []byte{0x00, 0x01, 0x02},
+			expected: false,
+		},
+		{
+			doc:      "header for bzip2 archive",
+			header:   []byte{0x42, 0x5A, 0x68},
+			expected: true,
+		},
+		{
+			doc:      "header for 7zip archive is not supported",
+			header:   []byte{0x50, 0x4b, 0x03, 0x04},
+			expected: false,
+		},
+	}
+	for _, testcase := range testcases {
+		assert.Equal(t, testcase.expected, IsArchive(testcase.header), testcase.doc)
+	}
+}