CP-11399: Go Linux guest agent for XenServer

This is the reviewed and tested Go guest agent for XenServer Linux guests.

Go guest agent is a static linked binary without any dependency (e.g. Bash or Python execution environment) with below benefits:
1. Cross platform, Go version works well with all kinds Linux distributions (i386 and x86_64) with the porting ability to arm, FreeBSD, Darwin OS etc.
2. Standalone binary, works well with some restricted environment for example, CoreOS and Boot2Docker Linux
3. Easy to maintain and structured design, with Golang's nature

Change history:
1: Refined Rob Robert's comments.
2: Add unit test for xenstoreclient and refact folder structure.
3: Refined codes according to Robert's comments
4: To run 32bit xe-guest-agent in Linux 64bit OS(eg, CoreOS):
we need
  4.1 - Switch to ip/ifconfig CLI tool instead of net package
  4.2 - Switch to log package instead of syslog package

Signed-off-by: phus lu <phus.lu@citrix.com>

2 files changed, 493 insertions, 0 deletions

diff --git a/xenstoreclient/xenstore.go b/xenstoreclient/xenstore.go
new file mode 100644
index 0000000..0d3e2bb
--- /dev/null
+++ b/xenstoreclient/xenstore.go
@@ -0,0 +1,433 @@
+package xenstoreclient
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+type Permission int
+
+const (
+	PERM_NONE Permission = iota
+	PERM_READ
+	PERM_WRITE
+	PERM_READWRITE
+)
+
+type Operation uint32
+
+const (
+	XS_READ              Operation = 2
+	XS_GET_PERMS         Operation = 3
+	XS_WATCH             Operation = 4
+	XS_UNWATCH           Operation = 5
+	XS_TRANSACTION_START Operation = 6
+	XS_TRANSACTION_END   Operation = 7
+	XS_WRITE             Operation = 11
+	XS_MKDIR             Operation = 12
+	XS_RM                Operation = 13
+	XS_SET_PERMS         Operation = 14
+	XS_WATCH_EVENT       Operation = 15
+	XS_ERROR             Operation = 16
+)
+
+type Packet struct {
+	OpCode Operation
+	Req    uint32
+	TxID   uint32
+	Length uint32
+	Value  []byte
+}
+
+type Event struct {
+	Token string
+	Data  []byte
+}
+
+type XenStoreClient interface {
+	Close() error
+	DO(packet *Packet) (*Packet, error)
+	Read(path string) (string, error)
+	Mkdir(path string) error
+	Rm(path string) error
+	Write(path string, value string) error
+	GetPermission(path string) (map[int]Permission, error)
+	Watch(path string) (<-chan Event, error)
+	StopWatch() error
+}
+
+func ReadPacket(r io.Reader) (packet *Packet, err error) {
+
+	packet = &Packet{}
+
+	err = binary.Read(r, binary.LittleEndian, &packet.OpCode)
+	if err != nil {
+		return nil, err
+	}
+	err = binary.Read(r, binary.LittleEndian, &packet.Req)
+	if err != nil {
+		return nil, err
+	}
+	err = binary.Read(r, binary.LittleEndian, &packet.TxID)
+	if err != nil {
+		return nil, err
+	}
+	err = binary.Read(r, binary.LittleEndian, &packet.Length)
+	if err != nil {
+		return nil, err
+	}
+
+	if packet.Length > 0 {
+		packet.Value = make([]byte, packet.Length)
+		_, err = io.ReadFull(r, packet.Value)
+		if err != nil {
+			return nil, err
+		}
+		if packet.OpCode == XS_ERROR {
+			return nil, errors.New(strings.Split(string(packet.Value), "\x00")[0])
+		}
+	}
+
+	return packet, nil
+}
+
+func (p *Packet) Write(w io.Writer) (err error) {
+	var bw *bufio.Writer
+
+	if w1, ok := w.(*bufio.Writer); ok {
+		bw = w1
+	} else {
+		bw = bufio.NewWriter(w)
+	}
+	defer bw.Flush()
+
+	err = binary.Write(bw, binary.LittleEndian, p.OpCode)
+	if err != nil {
+		return err
+	}
+	err = binary.Write(bw, binary.LittleEndian, p.Req)
+	if err != nil {
+		return err
+	}
+	err = binary.Write(bw, binary.LittleEndian, p.TxID)
+	if err != nil {
+		return err
+	}
+	err = binary.Write(bw, binary.LittleEndian, p.Length)
+	if err != nil {
+		return err
+	}
+	if p.Length > 0 {
+		_, err = bw.Write(p.Value)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+type XenStore struct {
+	tx               uint32
+	xbFile           io.ReadWriteCloser
+	xbFileReader     *bufio.Reader
+	muWatch          *sync.Mutex
+	onceWatch        *sync.Once
+	watchQueues      map[string]chan Event
+	watchStopChan    chan struct{}
+	watchStoppedChan chan struct{}
+	nonWatchQueue    chan []byte
+}
+
+func NewXenstore(tx uint32) (XenStoreClient, error) {
+	devPath, err := getDevPath()
+	if err != nil {
+		return nil, err
+	}
+
+	xbFile, err := os.OpenFile(devPath, os.O_RDWR, 0666)
+	if err != nil {
+		return nil, err
+	}
+	return newXenstore(tx, xbFile)
+}
+
+func newXenstore(tx uint32, rwc io.ReadWriteCloser) (XenStoreClient, error) {
+	return &XenStore{
+		tx:               tx,
+		xbFile:           rwc,
+		xbFileReader:     bufio.NewReader(rwc),
+		watchQueues:      make(map[string]chan Event, 0),
+		nonWatchQueue:    nil,
+		watchStopChan:    make(chan struct{}, 1),
+		watchStoppedChan: make(chan struct{}, 1),
+		onceWatch:        &sync.Once{},
+		muWatch:          &sync.Mutex{},
+	}, nil
+}
+
+func (xs *XenStore) Close() error {
+	return xs.xbFile.Close()
+}
+
+func (xs *XenStore) DO(req *Packet) (resp *Packet, err error) {
+	err = req.Write(xs.xbFile)
+	if err != nil {
+		return nil, err
+	}
+
+	var r io.Reader
+	if xs.nonWatchQueue != nil {
+		data := <-xs.nonWatchQueue
+		r = bytes.NewReader(data)
+	} else {
+		r = xs.xbFileReader
+	}
+
+	resp, err = ReadPacket(r)
+	return resp, err
+}
+
+func (xs *XenStore) Read(path string) (string, error) {
+	v := []byte(path + "\x00")
+	req := &Packet{
+		OpCode: XS_READ,
+		Req:    0,
+		TxID:   xs.tx,
+		Length: uint32(len(v)),
+		Value:  v,
+	}
+	resp, err := xs.DO(req)
+	if err != nil {
+		return "", err
+	}
+	return string(resp.Value), nil
+}
+
+func (xs *XenStore) Mkdir(path string) error {
+	v := []byte(path + "\x00")
+	req := &Packet{
+		OpCode: XS_WRITE,
+		Req:    0,
+		TxID:   xs.tx,
+		Length: uint32(len(v)),
+		Value:  v,
+	}
+	_, err := xs.DO(req)
+	return err
+}
+
+func (xs *XenStore) Rm(path string) error {
+	v := []byte(path + "\x00")
+	req := &Packet{
+		OpCode: XS_RM,
+		Req:    0,
+		TxID:   xs.tx,
+		Length: uint32(len(v)),
+		Value:  v,
+	}
+	_, err := xs.DO(req)
+	return err
+}
+
+func (xs *XenStore) Write(path string, value string) error {
+	v := []byte(path + "\x00" + value)
+	req := &Packet{
+		OpCode: XS_WRITE,
+		Req:    0,
+		TxID:   xs.tx,
+		Length: uint32(len(v)),
+		Value:  v,
+	}
+	_, err := xs.DO(req)
+	return err
+}
+
+func (xs *XenStore) GetPermission(path string) (map[int]Permission, error) {
+	perm := make(map[int]Permission, 0)
+
+	v := []byte(path + "\x00")
+	req := &Packet{
+		OpCode: XS_GET_PERMS,
+		Req:    0,
+		TxID:   xs.tx,
+		Length: uint32(len(v)),
+		Value:  v,
+	}
+	resp, err := xs.DO(req)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, e := range strings.Split(string(resp.Value[:len(resp.Value)-1]), "\x00") {
+		k, err := strconv.Atoi(e[1:])
+		if err != nil {
+			return nil, err
+		}
+		var p Permission
+		switch e[0] {
+		case 'n':
+			p = PERM_NONE
+		case 'r':
+			p = PERM_READ
+		case 'w':
+			p = PERM_WRITE
+		case 'b':
+			p = PERM_READWRITE
+		}
+		perm[k] = p
+	}
+	return perm, nil
+}
+
+func (xs *XenStore) Watch(path string) (<-chan Event, error) {
+	watcher := func() {
+
+		type XSData struct {
+			*Packet
+			Error error
+		}
+
+		xsDataChan := make(chan XSData, 100)
+		go func(r io.Reader, out chan<- XSData) {
+			for {
+				p, err := ReadPacket(r)
+				out <- XSData{Packet: p, Error: err}
+			}
+		}(xs.xbFileReader, xsDataChan)
+
+		xs.nonWatchQueue = make(chan []byte, 100)
+		for {
+			select {
+			case <-xs.watchStopChan:
+				fmt.Printf("watch receive stop signal, quit.")
+				xs.watchStopChan <- struct{}{}
+				return
+			case xsdata := <-xsDataChan:
+				if xsdata.Error != nil {
+					fmt.Printf("watch receive error: %#v", xsdata.Error)
+					return
+				}
+				switch xsdata.Packet.OpCode {
+				case XS_WATCH_EVENT:
+					parts := strings.SplitN(string(xsdata.Value), "\x00", 2)
+					path := parts[0]
+					token := parts[1]
+					data := []byte(parts[2])
+					if c, ok := xs.watchQueues[path]; ok {
+						c <- Event{token, data}
+					}
+				default:
+					var b bytes.Buffer
+					xsdata.Packet.Write(&b)
+					xs.nonWatchQueue <- b.Bytes()
+				}
+			}
+		}
+	}
+	xs.onceWatch.Do(watcher)
+	xs.muWatch.Lock()
+	defer xs.muWatch.Unlock()
+	if _, ok := xs.watchQueues[path]; !ok {
+		xs.watchQueues[path] = make(chan Event, 100)
+	}
+	return xs.watchQueues[path], nil
+}
+
+func (xs *XenStore) StopWatch() error {
+	xs.watchStopChan <- struct{}{}
+	<-xs.watchStoppedChan
+	xs.nonWatchQueue = nil
+	return nil
+}
+
+type CachedXenStore struct {
+	xs         XenStoreClient
+	writeCache map[string]string
+	lastCommit map[string]time.Time
+}
+
+func NewCachedXenstore(tx uint32) (XenStoreClient, error) {
+	xs, err := NewXenstore(tx)
+	if err != nil {
+		return nil, err
+	}
+	return &CachedXenStore{
+		xs:         xs,
+		writeCache: make(map[string]string, 0),
+		lastCommit: make(map[string]time.Time, 0),
+	}, nil
+}
+
+func (xs *CachedXenStore) Write(path string, value string) error {
+	if v, ok := xs.writeCache[path]; ok && v == value {
+		if t, ok := xs.lastCommit[path]; ok && t.After(time.Now().Add(-2*time.Minute)) {
+			return nil
+		}
+	}
+	err := xs.xs.Write(path, value)
+	if err != nil {
+		xs.writeCache[path] = value
+		xs.lastCommit[path] = time.Now()
+	}
+	return err
+}
+
+func (xs *CachedXenStore) Close() error {
+	return xs.xs.Close()
+}
+
+func (xs *CachedXenStore) DO(req *Packet) (resp *Packet, err error) {
+	return xs.xs.DO(req)
+}
+
+func (xs *CachedXenStore) Read(path string) (string, error) {
+	return xs.xs.Read(path)
+}
+
+func (xs *CachedXenStore) Mkdir(path string) error {
+	return xs.xs.Mkdir(path)
+}
+
+func (xs *CachedXenStore) Rm(path string) error {
+	return xs.xs.Rm(path)
+}
+
+func (xs *CachedXenStore) GetPermission(path string) (map[int]Permission, error) {
+	return xs.xs.GetPermission(path)
+}
+
+func (xs *CachedXenStore) Watch(path string) (<-chan Event, error) {
+	return xs.xs.Watch(path)
+}
+
+func (xs *CachedXenStore) StopWatch() error {
+	return xs.xs.StopWatch()
+}
+
+func (xs *CachedXenStore) Clear() {
+	xs.writeCache = make(map[string]string, 0)
+	xs.lastCommit = make(map[string]time.Time, 0)
+}
+
+func getDevPath() (devPath string, err error) {
+	devPaths := []string{
+		"/proc/xen/xenbus",
+		"/dev/xen/xenbus",
+		"/kern/xen/xenbus",
+	}
+	for _, devPath = range devPaths {
+		if _, err = os.Stat(devPath); err == nil {
+			return devPath, err
+		}
+	}
+	return "", fmt.Errorf("Cannot locate xenbus dev path in %v", devPaths)
+}
diff --git a/xenstoreclient/xenstore_test.go b/xenstoreclient/xenstore_test.go
new file mode 100644
index 0000000..1259447
--- /dev/null
+++ b/xenstoreclient/xenstore_test.go
@@ -0,0 +1,60 @@
+package xenstoreclient
+
+import (
+	"bytes"
+	"testing"
+)
+
+type mockFile struct {
+	t *testing.T
+}
+
+func (f *mockFile) Read(b []byte) (n int, err error) {
+	value := "i am value"
+	p := &Packet{
+		OpCode: XS_READ,
+		Req:    0,
+		TxID:   0,
+		Length: uint32(len(value)),
+		Value:  []byte(value),
+	}
+	var buf bytes.Buffer
+	if err = p.Write(&buf); err != nil {
+		return 0, err
+	}
+	copy(b, buf.Bytes())
+	n = buf.Len()
+	f.t.Logf("Read %d bytes", n)
+	return n, nil
+}
+
+func (f *mockFile) Write(b []byte) (n int, err error) {
+	buf := bytes.NewBuffer(b)
+	if _, err := ReadPacket(buf); err != nil {
+		return 0, err
+	}
+	n = len(b)
+	f.t.Logf("Write %d bytes", n)
+	return n, nil
+}
+
+func (f *mockFile) Close() error {
+	f.t.Logf("Close()")
+	return nil
+}
+
+func TestXenStore(t *testing.T) {
+	xs, err := newXenstore(0, &mockFile{t})
+	if err != nil {
+		t.Errorf("newXenstore error: %#v\n", err)
+	}
+	defer xs.Close()
+
+	if _, err := xs.Read("foo"); err != nil {
+		t.Errorf("xs.Read error: %#v\n", err)
+	}
+
+	if err := xs.Write("foo", "bar"); err != nil {
+		t.Errorf("xs.Read error: %#v\n", err)
+	}
+}