Move mount to core/mount

Signed-off-by: Derek McGowan <derek@mcg.dev>

core/mount/mount.go

@@ -0,0 +1,176 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package mount
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"github.com/containerd/containerd/v2/api/types"
+	"github.com/containerd/continuity/fs"
+)
+
+// Mount is the lingua franca of containerd. A mount represents a
+// serialized mount syscall. Components either emit or consume mounts.
+type Mount struct {
+	// Type specifies the host-specific of the mount.
+	Type string
+	// Source specifies where to mount from. Depending on the host system, this
+	// can be a source path or device.
+	Source string
+	// Target specifies an optional subdirectory as a mountpoint. It assumes that
+	// the subdirectory exists in a parent mount.
+	Target string
+	// Options contains zero or more fstab-style mount options. Typically,
+	// these are platform specific.
+	Options []string
+}
+
+// All mounts all the provided mounts to the provided target. If submounts are
+// present, it assumes that parent mounts come before child mounts.
+func All(mounts []Mount, target string) error {
+	for _, m := range mounts {
+		if err := m.Mount(target); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// UnmountMounts unmounts all the mounts under a target in the reverse order of
+// the mounts array provided.
+func UnmountMounts(mounts []Mount, target string, flags int) error {
+	for i := len(mounts) - 1; i >= 0; i-- {
+		mountpoint, err := fs.RootPath(target, mounts[i].Target)
+		if err != nil {
+			return err
+		}
+
+		if err := UnmountAll(mountpoint, flags); err != nil {
+			if i == len(mounts)-1 { // last mount
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// CanonicalizePath makes path absolute and resolves symlinks in it.
+// Path must exist.
+func CanonicalizePath(path string) (string, error) {
+	// Abs also does Clean, so we do not need to call it separately
+	path, err := filepath.Abs(path)
+	if err != nil {
+		return "", err
+	}
+
+	return filepath.EvalSymlinks(path)
+}
+
+// ReadOnly returns a boolean value indicating whether this mount has the "ro"
+// option set.
+func (m *Mount) ReadOnly() bool {
+	for _, option := range m.Options {
+		if option == "ro" {
+			return true
+		}
+	}
+	return false
+}
+
+// Mount to the provided target path.
+func (m *Mount) Mount(target string) error {
+	target, err := fs.RootPath(target, m.Target)
+	if err != nil {
+		return fmt.Errorf("failed to join path %q with root %q: %w", m.Target, target, err)
+	}
+	return m.mount(target)
+}
+
+// readonlyMounts modifies the received mount options
+// to make them readonly
+func readonlyMounts(mounts []Mount) []Mount {
+	for i, m := range mounts {
+		if m.Type == "overlay" {
+			mounts[i].Options = readonlyOverlay(m.Options)
+			continue
+		}
+		opts := make([]string, 0, len(m.Options))
+		for _, opt := range m.Options {
+			if opt != "rw" && opt != "ro" { // skip `ro` too so we don't append it twice
+				opts = append(opts, opt)
+			}
+		}
+		opts = append(opts, "ro")
+		mounts[i].Options = opts
+	}
+	return mounts
+}
+
+// readonlyOverlay takes mount options for overlay mounts and makes them readonly by
+// removing workdir and upperdir (and appending the upperdir layer to lowerdir) - see:
+// https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html#multiple-lower-layers
+func readonlyOverlay(opt []string) []string {
+	out := make([]string, 0, len(opt))
+	upper := ""
+	for _, o := range opt {
+		if strings.HasPrefix(o, "upperdir=") {
+			upper = strings.TrimPrefix(o, "upperdir=")
+		} else if !strings.HasPrefix(o, "workdir=") {
+			out = append(out, o)
+		}
+	}
+	if upper != "" {
+		for i, o := range out {
+			if strings.HasPrefix(o, "lowerdir=") {
+				out[i] = "lowerdir=" + upper + ":" + strings.TrimPrefix(o, "lowerdir=")
+			}
+		}
+	}
+	return out
+}
+
+// ToProto converts from [Mount] to the containerd
+// APIs protobuf definition of a Mount.
+func ToProto(mounts []Mount) []*types.Mount {
+	apiMounts := make([]*types.Mount, len(mounts))
+	for i, m := range mounts {
+		apiMounts[i] = &types.Mount{
+			Type:    m.Type,
+			Source:  m.Source,
+			Target:  m.Target,
+			Options: m.Options,
+		}
+	}
+	return apiMounts
+}
+
+// FromProto converts from the protobuf definition [types.Mount] to
+// [Mount].
+func FromProto(mm []*types.Mount) []Mount {
+	mounts := make([]Mount, len(mm))
+	for i, m := range mm {
+		mounts[i] = Mount{
+			Type:    m.Type,
+			Source:  m.Source,
+			Target:  m.Target,
+			Options: m.Options,
+		}
+	}
+	return mounts
+}