pkg/archive:CopyTo(): fix for long dest filename

As reported in docker/for-linux/issues/484, since Docker 18.06
docker cp with a destination file name fails with the following error:

> archive/tar: cannot encode header: Format specifies USTAR; and USTAR cannot encode Name="a_very_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx_long_filename_that_is_101_characters"

The problem is caused by changes in Go 1.10 archive/tar, which
mis-guesses the tar stream format as USTAR (rather than PAX),
which, in turn, leads to inability to specify file names
longer than 100 characters.

This tar stream is sent by TarWithOptions() (which, since we switched to
Go 1.10, explicitly sets format=PAX for every file, see FileInfoHeader(),
and before Go 1.10 it was PAX by default). Unfortunately, the receiving
side, RebaseArchiveEntries(), which calls tar.Next(), mistakenly guesses
header format as USTAR, which leads to the above error.

The fix is easy: set the format to PAX in RebaseArchiveEntries()
where we read the tar stream and change the file name.

A unit test is added to prevent future regressions.

NOTE this code is not used by dockerd, but rather but docker cli
(also possibly other clients), so this needs to be re-vendored
to cli in order to take effect.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

pkg/archive/copy.go

@@ -336,6 +336,14 @@ func RebaseArchiveEntries(srcContent io.Reader, oldBase, newBase string) io.Read
 				return
 			}
 
+			// srcContent tar stream, as served by TarWithOptions(), is
+			// definitely in PAX format, but tar.Next() mistakenly guesses it
+			// as USTAR, which creates a problem: if the newBase is >100
+			// characters long, WriteHeader() returns an error like
+			// "archive/tar: cannot encode header: Format specifies USTAR; and USTAR cannot encode Name=...".
+			//
+			// To fix, set the format to PAX here. See docker/for-linux issue #484.
+			hdr.Format = tar.FormatPAX
 			hdr.Name = strings.Replace(hdr.Name, oldBase, newBase, 1)
 			if hdr.Typeflag == tar.TypeLink {
 				hdr.Linkname = strings.Replace(hdr.Linkname, oldBase, newBase, 1)

pkg/archive/copy_unix_test.go

@@ -257,6 +257,30 @@ func TestCopyErrDstNotDir(t *testing.T) {
 	}
 }
 
+// Test to check if CopyTo works with a long (>100 characters) destination file name.
+// This is a regression (see https://github.com/docker/for-linux/issues/484).
+func TestCopyLongDstFilename(t *testing.T) {
+	const longName = "a_very_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx_long_filename_that_is_101_characters"
+	tmpDirA, tmpDirB := getTestTempDirs(t)
+	defer removeAllPaths(tmpDirA, tmpDirB)
+
+	// Load A with some sample files and directories.
+	createSampleDir(t, tmpDirA)
+
+	srcInfo := CopyInfo{Path: filepath.Join(tmpDirA, "file1"), Exists: true, IsDir: false}
+
+	content, err := TarResource(srcInfo)
+	if err != nil {
+		t.Fatalf("unexpected error %T: %s", err, err)
+	}
+	defer content.Close()
+
+	err = CopyTo(content, srcInfo, filepath.Join(tmpDirB, longName))
+	if err != nil {
+		t.Fatalf("unexpected error %T: %s", err, err)
+	}
+}
+
 // Possibilities are reduced to the remaining 10 cases:
 //
 //  case | srcIsDir | onlyDirContents | dstExists | dstIsDir | dstTrSep | action