pkg/mount: wrap mount/umount errors

The errors returned from Mount and Unmount functions are raw
syscall.Errno errors (like EPERM or EINVAL), which provides
no context about what has happened and why.

Similar to os.PathError type, introduce mount.Error type
with some context. The error messages will now look like this:

> mount /tmp/mount-tests/source:/tmp/mount-tests/target, flags: 0x1001: operation not permitted

or

> mount tmpfs:/tmp/mount-test-source-516297835: operation not permitted

Before this patch, it was just

> operation not permitted

[v2: add Cause()]
[v3: rename MountError to Error, document Cause()]
[v4: fixes; audited all users]
[v5: make Error type private; changes after @cpuguy83 reviews]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 65331369617e89ce54cc9be080dba70f3a883d1c)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Upstream-commit: 7f1c6bf5a745c8faeba695d3556dff4c4ff5f473
Component: engine

components/engine/daemon/graphdriver/btrfs/btrfs.go

@@ -178,7 +178,7 @@ func (d *Driver) Cleanup() error {
 	}
 
 	if umountErr != nil {
-		return errors.Wrapf(umountErr, "error unmounting %s", d.home)
+		return umountErr
 	}
 
 	return nil

components/engine/daemon/graphdriver/devmapper/deviceset.go

@@ -1200,7 +1200,7 @@ func (devices *DeviceSet) growFS(info *devInfo) error {
 	options = joinMountOptions(options, devices.mountOptions)
 
 	if err := mount.Mount(info.DevName(), fsMountPoint, devices.BaseDeviceFilesystem, options); err != nil {
-		return fmt.Errorf("Error mounting '%s' on '%s' (fstype='%s' options='%s'): %s\n%v", info.DevName(), fsMountPoint, devices.BaseDeviceFilesystem, options, err, string(dmesg.Dmesg(256)))
+		return errors.Wrapf(err, "Failed to mount; dmesg: %s", string(dmesg.Dmesg(256)))
 	}
 
 	defer unix.Unmount(fsMountPoint, unix.MNT_DETACH)
@@ -2381,7 +2381,7 @@ func (devices *DeviceSet) MountDevice(hash, path, mountLabel string) error {
 	options = joinMountOptions(options, label.FormatMountLabel("", mountLabel))
 
 	if err := mount.Mount(info.DevName(), path, fstype, options); err != nil {
-		return fmt.Errorf("devmapper: Error mounting '%s' on '%s' (fstype='%s' options='%s'): %s\n%v", info.DevName(), path, fstype, options, err, string(dmesg.Dmesg(256)))
+		return errors.Wrapf(err, "Failed to mount; dmesg: %s", string(dmesg.Dmesg(256)))
 	}
 
 	if fstype == "xfs" && devices.xfsNospaceRetries != "" {

components/engine/daemon/graphdriver/devmapper/driver.go

@@ -16,7 +16,6 @@ import (
 	"github.com/docker/docker/pkg/locker"
 	"github.com/docker/docker/pkg/mount"
 	"github.com/docker/go-units"
-	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
@@ -129,11 +128,7 @@ func (d *Driver) Cleanup() error {
 		return err
 	}
 
-	if umountErr != nil {
-		return errors.Wrapf(umountErr, "error unmounting %s", d.home)
-	}
-
-	return nil
+	return umountErr
 }
 
 // CreateReadWrite creates a layer that is writable for use as a container

components/engine/daemon/graphdriver/zfs/zfs.go

@@ -19,6 +19,7 @@ import (
 	"github.com/docker/docker/pkg/parsers"
 	"github.com/mistifyio/go-zfs"
 	"github.com/opencontainers/selinux/go-selinux/label"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
@@ -390,7 +391,7 @@ func (d *Driver) Get(id, mountLabel string) (_ containerfs.ContainerFS, retErr e
 	}
 
 	if err := mount.Mount(filesystem, mountpoint, "zfs", options); err != nil {
-		return nil, fmt.Errorf("error creating zfs mount of %s to %s: %v", filesystem, mountpoint, err)
+		return nil, errors.Wrap(err, "error creating zfs mount")
 	}
 
 	// this could be our first mount after creation of the filesystem, and the root dir may still have root