Graphdriver: fix "device" mode not being detected if "character-device" bit is set

Due to a bug in Golang (github.com/golang#27640), the "character device"
bit was omitted when checking file-modes with `os.ModeType`.

This bug was resolved in Go 1.12, but as a result, graphdrivers
would no longer recognize "device" files, causing pulling of
images that have a file with this filemode to fail;

    failed to register layer:
    unknown file type for /var/lib/docker/vfs/dir/.../dev/console

The current code checked for an exact match of Modes to be set. The
`os.ModeCharDevice` and `os.ModeDevice` bits will always be set in
tandem, however, because the code was only looking for an exact
match, this detection broke now that `os.ModeCharDevice` was added.

This patch changes the code to be more defensive, and instead
check if the `os.ModeDevice` bit is set (either with, or without
the `os.ModeCharDevice` bit).

In addition, some information was added to the error-message if
no type was matched, to assist debugging in case additional types
are added in future.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/copy/copy.go

@@ -152,8 +152,8 @@ func DirCopy(srcDir, dstDir string, copyMode Mode, copyXattrs bool) error {
 
 		isHardlink := false
 
-		switch f.Mode() & os.ModeType {
-		case 0: // Regular file
+		switch mode := f.Mode(); {
+		case mode.IsRegular():
 			id := fileID{dev: stat.Dev, ino: stat.Ino}
 			if copyMode == Hardlink {
 				isHardlink = true
@@ -171,12 +171,12 @@ func DirCopy(srcDir, dstDir string, copyMode Mode, copyXattrs bool) error {
 				copiedFiles[id] = dstPath
 			}
 
-		case os.ModeDir:
+		case mode.IsDir():
 			if err := os.Mkdir(dstPath, f.Mode()); err != nil && !os.IsExist(err) {
 				return err
 			}
 
-		case os.ModeSymlink:
+		case mode&os.ModeSymlink != 0:
 			link, err := os.Readlink(srcPath)
 			if err != nil {
 				return err
@@ -186,14 +186,14 @@ func DirCopy(srcDir, dstDir string, copyMode Mode, copyXattrs bool) error {
 				return err
 			}
 
-		case os.ModeNamedPipe:
+		case mode&os.ModeNamedPipe != 0:
 			fallthrough
-		case os.ModeSocket:
+		case mode&os.ModeSocket != 0:
 			if err := unix.Mkfifo(dstPath, stat.Mode); err != nil {
 				return err
 			}
 
-		case os.ModeDevice:
+		case mode&os.ModeDevice != 0:
 			if rsystem.RunningInUserNS() {
 				// cannot create a device if running in user namespace
 				return nil
@@ -203,7 +203,7 @@ func DirCopy(srcDir, dstDir string, copyMode Mode, copyXattrs bool) error {
 			}
 
 		default:
-			return fmt.Errorf("unknown file type for %s", srcPath)
+			return fmt.Errorf("unknown file type (%d / %s) for %s", f.Mode(), f.Mode().String(), srcPath)
 		}
 
 		// Everything below is copying metadata from src to dst. All this metadata