pkg/system: return even richer xattr errors

The names of extended attributes are not completely freeform. Attributes
are namespaced, and the kernel enforces (among other things) that only
attributes whose names are prefixed with a valid namespace are
permitted. The name of the attribute therefore needs to be known in
order to diagnose issues with lsetxattr. Include the name of the
extended attribute in the errors returned from the Lsetxattr and
Lgetxattr so users and us can more easily troubleshoot xattr-related
issues. Include the name in a separate rich-error field to provide code
handling the error enough information to determine whether or not the
failure can be ignored.

Signed-off-by: Cory Snider <csnider@mirantis.com>

builder/remotecontext/archive.go

@@ -9,6 +9,7 @@ import (
 	"github.com/docker/docker/pkg/archive"
 	"github.com/docker/docker/pkg/chrootarchive"
 	"github.com/docker/docker/pkg/longpath"
+	"github.com/docker/docker/pkg/system"
 	"github.com/docker/docker/pkg/tarsum"
 	"github.com/moby/sys/symlink"
 	"github.com/pkg/errors"
@@ -24,9 +25,11 @@ func (c *archiveContext) Close() error {
 }
 
 func convertPathError(err error, cleanpath string) error {
-	if err, ok := err.(*os.PathError); ok {
+	switch err := err.(type) {
+	case *os.PathError:
+		err.Path = cleanpath
+	case *system.XattrError:
 		err.Path = cleanpath
-		return err
 	}
 	return err
 }

pkg/system/xattrs.go

@@ -0,0 +1,18 @@
+package system // import "github.com/docker/docker/pkg/system"
+
+type XattrError struct {
+	Op   string
+	Attr string
+	Path string
+	Err  error
+}
+
+func (e *XattrError) Error() string { return e.Op + " " + e.Attr + " " + e.Path + ": " + e.Err.Error() }
+
+func (e *XattrError) Unwrap() error { return e.Err }
+
+// Timeout reports whether this error represents a timeout.
+func (e *XattrError) Timeout() bool {
+	t, ok := e.Err.(interface{ Timeout() bool })
+	return ok && t.Timeout()
+}

pkg/system/xattrs_linux.go

@@ -1,8 +1,6 @@
 package system // import "github.com/docker/docker/pkg/system"
 
 import (
-	"io/fs"
-
 	"golang.org/x/sys/unix"
 )
 
@@ -10,8 +8,8 @@ import (
 // and associated with the given path in the file system.
 // It will returns a nil slice and nil error if the xattr is not set.
 func Lgetxattr(path string, attr string) ([]byte, error) {
-	pathErr := func(err error) ([]byte, error) {
-		return nil, &fs.PathError{Op: "lgetxattr", Path: path, Err: err}
+	sysErr := func(err error) ([]byte, error) {
+		return nil, &XattrError{Op: "lgetxattr", Attr: attr, Path: path, Err: err}
 	}
 
 	// Start with a 128 length byte array
@@ -22,7 +20,7 @@ func Lgetxattr(path string, attr string) ([]byte, error) {
 		// Buffer too small, use zero-sized buffer to get the actual size
 		sz, errno = unix.Lgetxattr(path, attr, []byte{})
 		if errno != nil {
-			return pathErr(errno)
+			return sysErr(errno)
 		}
 		dest = make([]byte, sz)
 		sz, errno = unix.Lgetxattr(path, attr, dest)
@@ -32,7 +30,7 @@ func Lgetxattr(path string, attr string) ([]byte, error) {
 	case errno == unix.ENODATA:
 		return nil, nil
 	case errno != nil:
-		return pathErr(errno)
+		return sysErr(errno)
 	}
 
 	return dest[:sz], nil
@@ -43,7 +41,7 @@ func Lgetxattr(path string, attr string) ([]byte, error) {
 func Lsetxattr(path string, attr string, data []byte, flags int) error {
 	err := unix.Lsetxattr(path, attr, data, flags)
 	if err != nil {
-		return &fs.PathError{Op: "lsetxattr", Path: path, Err: err}
+		return &XattrError{Op: "lsetxattr", Attr: attr, Path: path, Err: err}
 	}
 	return nil
 }