Add devicemapper GC for persistent rootfs

Upon GC at the Virtlet startup, the virtual block devices that are
created by Virtlet are recognized by name and the sector 0 of underlying
block device (Virtlet magic header). If they're not related to any
active domain, they're removed (note that this doesn't mean the data on
the device is affected in any way).

pkg/blockdev/blockdev.go

@@ -0,0 +1,244 @@
+/*
+Copyright 2018 Mirantis
+
+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 blockdev
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"github.com/Mirantis/virtlet/pkg/utils"
+	"github.com/golang/glog"
+)
+
+const (
+	// The required prefix for the virtual block devices created by Virtlet.
+	VirtletLogicalDevicePrefix   = "virtlet-dm-"
+	virtletRootfsMagic           = 0x263dbe52ba576702
+	virtletRootfsMetadataVersion = 1
+	sectorSize                   = 512
+	devnameUeventVar             = "DEVNAME="
+)
+
+type virtletRootfsHeader struct {
+	Magic           uint64
+	MetadataVersion uint16
+	ImageHash       [sha256.Size]byte
+}
+
+// LogicalDeviceHandler makes it possible to store metadata in the
+// first sector of a block device, making the rest of the device
+// available as another logical device managed by the device mapper.
+type LogicalDeviceHandler struct {
+	commander utils.Commander
+	devPath   string
+	sysfsPath string
+}
+
+// NewLogicalDeviceHandler creates a new LogicalDeviceHandler using
+// the specified commander and paths that should be used in place of
+// /dev and /sys directories (empty string to use /dev and /sys,
+// respectively)
+func NewLogicalDeviceHandler(commander utils.Commander, devPath, sysfsPath string) *LogicalDeviceHandler {
+	if devPath == "" {
+		devPath = "/dev"
+	}
+	if sysfsPath == "" {
+		sysfsPath = "/sys"
+	}
+	return &LogicalDeviceHandler{commander, devPath, sysfsPath}
+}
+
+// EnsureDevHeaderMatches returns true if the specified block device
+// has proper Virtlet header that matches the specified image hash
+func (ldh *LogicalDeviceHandler) EnsureDevHeaderMatches(devPath string, imageHash [sha256.Size]byte) (bool, error) {
+	f, err := os.OpenFile(devPath, os.O_RDWR|os.O_SYNC, 0)
+	if err != nil {
+		return false, fmt.Errorf("open %q: %v", devPath, err)
+	}
+	defer func() {
+		if f != nil {
+			f.Close()
+		}
+	}()
+
+	var hdr virtletRootfsHeader
+	if err := binary.Read(f, binary.BigEndian, &hdr); err != nil {
+		return false, fmt.Errorf("reading rootfs header: %v", err)
+	}
+
+	headerMatch := true
+	switch {
+	case hdr.Magic != virtletRootfsMagic || hdr.ImageHash != imageHash:
+		headerMatch = false
+		if _, err := f.Seek(0, os.SEEK_SET); err != nil {
+			return false, fmt.Errorf("seek: %v", err)
+		}
+		if err := binary.Write(f, binary.BigEndian, virtletRootfsHeader{
+			Magic:           virtletRootfsMagic,
+			MetadataVersion: virtletRootfsMetadataVersion,
+			ImageHash:       imageHash,
+		}); err != nil {
+			return false, fmt.Errorf("writing rootfs header: %v", err)
+		}
+	case hdr.MetadataVersion != virtletRootfsMetadataVersion:
+		// NOTE: we should handle earlier metadata versions
+		// after we introduce new ones. But we can't handle
+		// future metadata versions and any non-matching
+		// metadata versions are future ones currently, so we
+		// don't want to lose any data here.
+		return false, fmt.Errorf("unsupported virtlet root device metadata version %v", hdr.MetadataVersion)
+	}
+
+	if err := f.Close(); err != nil {
+		return false, fmt.Errorf("error closing rootfs device: %v", err)
+	}
+	f = nil
+	return headerMatch, nil
+}
+
+// blockDevSizeInSectors returns the size of the block device in sectors
+func (ldh *LogicalDeviceHandler) blockDevSizeInSectors(devPath string) (uint64, error) {
+	// NOTE: this is also doable via ioctl but this way it's
+	// shorter (no need for fake non-linux impl, extra interface,
+	// extra fake impl for it). Some links that may help if we
+	// decide to use the ioctl later on:
+	// https://github.com/karelzak/util-linux/blob/master/disk-utils/blockdev.c
+	// https://github.com/aicodix/smr/blob/24aa589f378827a69a07d220f114c169693dacec/smr.go#L29
+	out, err := ldh.commander.Command("blockdev", "--getsz", devPath).Run(nil)
+	if err != nil {
+		return 0, err
+	}
+	nSectors, err := strconv.ParseUint(strings.TrimSpace(string(out)), 10, 64)
+	if err != nil {
+		return 0, fmt.Errorf("bad size value returned by blockdev: %q: %v", out, err)
+	}
+	return nSectors, nil
+}
+
+// Map maps the device sectors starting from 1 to a new virtual block
+// device. dmName specifies the name of the new device.
+func (ldh *LogicalDeviceHandler) Map(devPath, dmName string, imageSize uint64) error {
+	if !strings.HasPrefix(dmName, VirtletLogicalDevicePrefix) {
+		return fmt.Errorf("bad logical device name %q: must have prefix %q", dmName, VirtletLogicalDevicePrefix)
+	}
+
+	nSectors, err := ldh.blockDevSizeInSectors(devPath)
+	if err != nil {
+		return err
+	}
+
+	// sector 0 is reserved for the Virtlet metadata
+	minSectors := (imageSize+sectorSize-1)/sectorSize + 1
+	if nSectors < minSectors {
+		return fmt.Errorf("block device too small for the image: need at least %d bytes (%d sectors) but got %d bytes (%d sectors)",
+			minSectors*sectorSize,
+			minSectors,
+			nSectors*sectorSize,
+			nSectors)
+	}
+
+	hostPath, err := filepath.EvalSymlinks(devPath)
+	if err != nil {
+		return err
+	}
+
+	dmTable := fmt.Sprintf("0 %d linear %s 1\n", nSectors-1, hostPath)
+	_, err = ldh.commander.Command("dmsetup", "create", dmName).Run([]byte(dmTable))
+	return err
+}
+
+// Unmap unmaps the virtual block device
+func (ldh *LogicalDeviceHandler) Unmap(dmName string) error {
+	_, err := ldh.commander.Command("dmsetup", "remove", dmName).Run(nil)
+	return err
+}
+
+// ListVirtletLogicalDevices returns a list of logical devices managed
+// by Virtlet
+func (ldh *LogicalDeviceHandler) ListVirtletLogicalDevices() ([]string, error) {
+	table, err := ldh.commander.Command("dmsetup", "table").Run(nil)
+	if err != nil {
+		return nil, fmt.Errorf("dmsetup table: %v", err)
+	}
+	var r []string
+	for _, l := range strings.Split(string(table), "\n") {
+		if l == "" {
+			continue
+		}
+		fields := strings.Fields(l)
+		if len(fields) != 6 || fields[3] != "linear" {
+			continue
+		}
+		virtDevName := fields[0]
+		if strings.HasSuffix(virtDevName, ":") {
+			virtDevName = virtDevName[:len(virtDevName)-1]
+		}
+
+		devID := fields[4]
+		ueventFile := filepath.Join(ldh.sysfsPath, "dev/block", devID, "uevent")
+		ueventContent, err := ioutil.ReadFile(ueventFile)
+		if err != nil {
+			glog.Warningf("error reading %q: %v", ueventFile, err)
+			continue
+		}
+		devName := ""
+		for _, ul := range strings.Split(string(ueventContent), "\n") {
+			ul = strings.TrimSpace(ul)
+			if strings.HasPrefix(ul, devnameUeventVar) {
+				devName = ul[len(devnameUeventVar):]
+				break
+			}
+		}
+		if devName == "" {
+			glog.Warningf("bad uevent file %q: no DEVNAME", ueventFile)
+			continue
+		}
+
+		isVbd, err := ldh.deviceHasVirtletHeader(devName)
+		if err != nil {
+			glog.Warningf("checking device file %q: %v", devName, err)
+			continue
+		}
+
+		if isVbd {
+			r = append(r, virtDevName)
+		}
+	}
+
+	return r, nil
+}
+
+func (ldh *LogicalDeviceHandler) deviceHasVirtletHeader(devName string) (bool, error) {
+	f, err := os.Open(filepath.Join(ldh.devPath, devName))
+	if err != nil {
+		return false, err
+	}
+	defer f.Close()
+
+	var hdr virtletRootfsHeader
+	if err := binary.Read(f, binary.BigEndian, &hdr); err != nil {
+		return false, err
+	}
+
+	return hdr.Magic == virtletRootfsMagic, nil
+}