labix.org/v2/mgo/bson 包虽然是MongoDB的golang实现，其中产生唯一ID的算法是独立的，不依赖MongoDB， 提炼这部分的代码如下：

package main

import (
    "crypto/md5"
    "crypto/rand"
    "encoding/binary"
    "encoding/hex"
    "fmt"
    "io"
    "os"
    "sync/atomic"
    "time"
)

// objectIdCounter is atomically incremented when generating a new ObjectId
// using NewObjectId() function. It‘s used as a counter part of an id.
var objectIdCounter uint32 = 0

// machineId stores machine id generated once and used in subsequent calls
// to NewObjectId function.
var machineId = readMachineId()

// ObjectId is a unique ID identifying a BSON value. It must be exactly 12 bytes
// long. MongoDB objects by default have such a property set in their "_id"
// property.
//
// http://www.mongodb.org/display/DOCS/Object+IDs
type ObjectId string

func main() {
    objID := NewObjectId()
    fmt.Println(objID)
    fmt.Println(objID.Hex())
}

// readMachineId generates machine id and puts it into the machineId global
// variable. If this function fails to get the hostname, it will cause
// a runtime error.
func readMachineId() []byte {
    var sum [3]byte
    id := sum[:]
    hostname, err1 := os.Hostname()
    if err1 != nil {
        _, err2 := io.ReadFull(rand.Reader, id)
        if err2 != nil {
            panic(fmt.Errorf("cannot get hostname: %v; %v", err1, err2))
        }
        return id
    }
    hw := md5.New()
    hw.Write([]byte(hostname))
    copy(id, hw.Sum(nil))
    fmt.Println("readMachineId:" + string(id))
    return id
}

// NewObjectId returns a new unique ObjectId.
// 4byte 时间，
// 3byte 机器ID
// 2byte pid
// 3byte 自增ID
func NewObjectId() ObjectId {
    var b [12]byte
    // Timestamp, 4 bytes, big endian
    binary.BigEndian.PutUint32(b[:], uint32(time.Now().Unix()))
    // Machine, first 3 bytes of md5(hostname)
    b[4] = machineId[0]
    b[5] = machineId[1]
    b[6] = machineId[2]
    // Pid, 2 bytes, specs don‘t specify endianness, but we use big endian.
    pid := os.Getpid()
    b[7] = byte(pid >> 8)
    b[8] = byte(pid)
    // Increment, 3 bytes, big endian
    i := atomic.AddUint32(&objectIdCounter, 1)
    b[9] = byte(i >> 16)
    b[10] = byte(i >> 8)
    b[11] = byte(i)
    return ObjectId(b[:])
}

// Hex returns a hex representation of the ObjectId.
// 返回16进制对应的字符串
func (id ObjectId) Hex() string {
    return hex.EncodeToString([]byte(id))
}

执行结果截图：

参考资料：

mgo说明文档
http://godoc.org/labix.org/v2/mgo/bson

Golang binary包——byte数组如何转int？
http://blog.cyeam.com/hash/2014/07/29/go_bytearraytoint/

golang产生guid