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moby--moby/runtime.go

882 lines
25 KiB
Go

package docker
import (
_ "code.google.com/p/gosqlite/sqlite3" // registers sqlite
"container/list"
"database/sql"
"fmt"
"github.com/dotcloud/docker/archive"
"github.com/dotcloud/docker/graphdb"
"github.com/dotcloud/docker/graphdriver"
"github.com/dotcloud/docker/graphdriver/aufs"
_ "github.com/dotcloud/docker/graphdriver/devmapper"
_ "github.com/dotcloud/docker/graphdriver/vfs"
"github.com/dotcloud/docker/utils"
"io"
"io/ioutil"
"log"
"os"
"os/exec"
"path"
"sort"
"strings"
"sync"
"time"
)
// Set the max depth to the aufs restriction
const MaxImageDepth = 42
var defaultDns = []string{"8.8.8.8", "8.8.4.4"}
type Capabilities struct {
MemoryLimit bool
SwapLimit bool
IPv4ForwardingDisabled bool
AppArmor bool
}
type Runtime struct {
repository string
sysInitPath string
containers *list.List
networkManager *NetworkManager
graph *Graph
repositories *TagStore
idIndex *utils.TruncIndex
capabilities *Capabilities
volumes *Graph
srv *Server
config *DaemonConfig
containerGraph *graphdb.Database
driver graphdriver.Driver
}
// List returns an array of all containers registered in the runtime.
func (runtime *Runtime) List() []*Container {
containers := new(History)
for e := runtime.containers.Front(); e != nil; e = e.Next() {
containers.Add(e.Value.(*Container))
}
return *containers
}
func (runtime *Runtime) getContainerElement(id string) *list.Element {
for e := runtime.containers.Front(); e != nil; e = e.Next() {
container := e.Value.(*Container)
if container.ID == id {
return e
}
}
return nil
}
// Get looks for a container by the specified ID or name, and returns it.
// If the container is not found, or if an error occurs, nil is returned.
func (runtime *Runtime) Get(name string) *Container {
if c, _ := runtime.GetByName(name); c != nil {
return c
}
id, err := runtime.idIndex.Get(name)
if err != nil {
return nil
}
e := runtime.getContainerElement(id)
if e == nil {
return nil
}
return e.Value.(*Container)
}
// Exists returns a true if a container of the specified ID or name exists,
// false otherwise.
func (runtime *Runtime) Exists(id string) bool {
return runtime.Get(id) != nil
}
func (runtime *Runtime) containerRoot(id string) string {
return path.Join(runtime.repository, id)
}
// Load reads the contents of a container from disk
// This is typically done at startup.
func (runtime *Runtime) load(id string) (*Container, error) {
container := &Container{root: runtime.containerRoot(id)}
if err := container.FromDisk(); err != nil {
return nil, err
}
if container.ID != id {
return container, fmt.Errorf("Container %s is stored at %s", container.ID, id)
}
if container.State.IsRunning() {
container.State.SetGhost(true)
}
return container, nil
}
// Register makes a container object usable by the runtime as <container.ID>
func (runtime *Runtime) Register(container *Container) error {
if container.runtime != nil || runtime.Exists(container.ID) {
return fmt.Errorf("Container is already loaded")
}
if err := validateID(container.ID); err != nil {
return err
}
if err := runtime.ensureName(container); err != nil {
return err
}
// Get the root filesystem from the driver
rootfs, err := runtime.driver.Get(container.ID)
if err != nil {
return fmt.Errorf("Error getting container filesystem %s from driver %s: %s", container.ID, runtime.driver, err)
}
container.rootfs = rootfs
container.runtime = runtime
// Attach to stdout and stderr
container.stderr = utils.NewWriteBroadcaster()
container.stdout = utils.NewWriteBroadcaster()
// Attach to stdin
if container.Config.OpenStdin {
container.stdin, container.stdinPipe = io.Pipe()
} else {
container.stdinPipe = utils.NopWriteCloser(ioutil.Discard) // Silently drop stdin
}
// done
runtime.containers.PushBack(container)
runtime.idIndex.Add(container.ID)
// FIXME: if the container is supposed to be running but is not, auto restart it?
// if so, then we need to restart monitor and init a new lock
// If the container is supposed to be running, make sure of it
if container.State.IsRunning() {
output, err := exec.Command("lxc-info", "-n", container.ID).CombinedOutput()
if err != nil {
return err
}
if !strings.Contains(string(output), "RUNNING") {
utils.Debugf("Container %s was supposed to be running but is not.", container.ID)
if runtime.config.AutoRestart {
utils.Debugf("Restarting")
container.State.SetGhost(false)
container.State.SetStopped(0)
if err := container.Start(); err != nil {
return err
}
} else {
utils.Debugf("Marking as stopped")
container.State.SetStopped(-127)
if err := container.ToDisk(); err != nil {
return err
}
}
} else {
utils.Debugf("Reconnecting to container %v", container.ID)
if err := container.allocateNetwork(); err != nil {
return err
}
container.waitLock = make(chan struct{})
go container.monitor()
}
}
return nil
}
func (runtime *Runtime) ensureName(container *Container) error {
if container.Name == "" {
name, err := generateRandomName(runtime)
if err != nil {
name = utils.TruncateID(container.ID)
}
container.Name = name
if err := container.ToDisk(); err != nil {
utils.Debugf("Error saving container name %s", err)
}
if !runtime.containerGraph.Exists(name) {
if _, err := runtime.containerGraph.Set(name, container.ID); err != nil {
utils.Debugf("Setting default id - %s", err)
}
}
}
return nil
}
func (runtime *Runtime) LogToDisk(src *utils.WriteBroadcaster, dst, stream string) error {
log, err := os.OpenFile(dst, os.O_RDWR|os.O_APPEND|os.O_CREATE, 0600)
if err != nil {
return err
}
src.AddWriter(log, stream)
return nil
}
// Destroy unregisters a container from the runtime and cleanly removes its contents from the filesystem.
func (runtime *Runtime) Destroy(container *Container) error {
if container == nil {
return fmt.Errorf("The given container is <nil>")
}
element := runtime.getContainerElement(container.ID)
if element == nil {
return fmt.Errorf("Container %v not found - maybe it was already destroyed?", container.ID)
}
if err := container.Stop(3); err != nil {
return err
}
if err := runtime.driver.Remove(container.ID); err != nil {
return fmt.Errorf("Driver %s failed to remove root filesystem %s: %s", runtime.driver, container.ID, err)
}
if _, err := runtime.containerGraph.Purge(container.ID); err != nil {
utils.Debugf("Unable to remove container from link graph: %s", err)
}
// Deregister the container before removing its directory, to avoid race conditions
runtime.idIndex.Delete(container.ID)
runtime.containers.Remove(element)
if err := os.RemoveAll(container.root); err != nil {
return fmt.Errorf("Unable to remove filesystem for %v: %v", container.ID, err)
}
return nil
}
func (runtime *Runtime) restore() error {
wheel := "-\\|/"
if os.Getenv("DEBUG") == "" && os.Getenv("TEST") == "" {
fmt.Printf("Loading containers: ")
}
dir, err := ioutil.ReadDir(runtime.repository)
if err != nil {
return err
}
containers := make(map[string]*Container)
currentDriver := runtime.driver.String()
for i, v := range dir {
id := v.Name()
container, err := runtime.load(id)
if i%21 == 0 && os.Getenv("DEBUG") == "" && os.Getenv("TEST") == "" {
fmt.Printf("\b%c", wheel[i%4])
}
if err != nil {
utils.Errorf("Failed to load container %v: %v", id, err)
continue
}
// Ignore the container if it does not support the current driver being used by the graph
if container.Driver == "" && currentDriver == "aufs" || container.Driver == currentDriver {
utils.Debugf("Loaded container %v", container.ID)
containers[container.ID] = container
} else {
utils.Debugf("Cannot load container %s because it was created with another graph driver.", container.ID)
}
}
register := func(container *Container) {
if err := runtime.Register(container); err != nil {
utils.Debugf("Failed to register container %s: %s", container.ID, err)
}
}
if entities := runtime.containerGraph.List("/", -1); entities != nil {
for _, p := range entities.Paths() {
e := entities[p]
if container, ok := containers[e.ID()]; ok {
register(container)
delete(containers, e.ID())
}
}
}
// Any containers that are left over do not exist in the graph
for _, container := range containers {
// Try to set the default name for a container if it exists prior to links
container.Name, err = generateRandomName(runtime)
if err != nil {
container.Name = utils.TruncateID(container.ID)
}
if _, err := runtime.containerGraph.Set(container.Name, container.ID); err != nil {
utils.Debugf("Setting default id - %s", err)
}
register(container)
}
if os.Getenv("DEBUG") == "" && os.Getenv("TEST") == "" {
fmt.Printf("\bdone.\n")
}
return nil
}
// FIXME: comment please!
func (runtime *Runtime) UpdateCapabilities(quiet bool) {
if cgroupMemoryMountpoint, err := utils.FindCgroupMountpoint("memory"); err != nil {
if !quiet {
log.Printf("WARNING: %s\n", err)
}
} else {
_, err1 := ioutil.ReadFile(path.Join(cgroupMemoryMountpoint, "memory.limit_in_bytes"))
_, err2 := ioutil.ReadFile(path.Join(cgroupMemoryMountpoint, "memory.soft_limit_in_bytes"))
runtime.capabilities.MemoryLimit = err1 == nil && err2 == nil
if !runtime.capabilities.MemoryLimit && !quiet {
log.Printf("WARNING: Your kernel does not support cgroup memory limit.")
}
_, err = ioutil.ReadFile(path.Join(cgroupMemoryMountpoint, "memory.memsw.limit_in_bytes"))
runtime.capabilities.SwapLimit = err == nil
if !runtime.capabilities.SwapLimit && !quiet {
log.Printf("WARNING: Your kernel does not support cgroup swap limit.")
}
}
content, err3 := ioutil.ReadFile("/proc/sys/net/ipv4/ip_forward")
runtime.capabilities.IPv4ForwardingDisabled = err3 != nil || len(content) == 0 || content[0] != '1'
if runtime.capabilities.IPv4ForwardingDisabled && !quiet {
log.Printf("WARNING: IPv4 forwarding is disabled.")
}
// Check if AppArmor seems to be enabled on this system.
if _, err := os.Stat("/sys/kernel/security/apparmor"); os.IsNotExist(err) {
utils.Debugf("/sys/kernel/security/apparmor not found; assuming AppArmor is not enabled.")
runtime.capabilities.AppArmor = false
} else {
utils.Debugf("/sys/kernel/security/apparmor found; assuming AppArmor is enabled.")
runtime.capabilities.AppArmor = true
}
}
// Create creates a new container from the given configuration with a given name.
func (runtime *Runtime) Create(config *Config, name string) (*Container, []string, error) {
// Lookup image
img, err := runtime.repositories.LookupImage(config.Image)
if err != nil {
return nil, nil, err
}
// We add 2 layers to the depth because the container's rw and
// init layer add to the restriction
depth, err := img.Depth()
if err != nil {
return nil, nil, err
}
if depth+2 >= MaxImageDepth {
return nil, nil, fmt.Errorf("Cannot create container with more than %d parents", MaxImageDepth)
}
checkDeprecatedExpose := func(config *Config) bool {
if config != nil {
if config.PortSpecs != nil {
for _, p := range config.PortSpecs {
if strings.Contains(p, ":") {
return true
}
}
}
}
return false
}
warnings := []string{}
if checkDeprecatedExpose(img.Config) || checkDeprecatedExpose(config) {
warnings = append(warnings, "The mapping to public ports on your host has been deprecated. Use -p to publish the ports.")
}
if img.Config != nil {
if err := MergeConfig(config, img.Config); err != nil {
return nil, nil, err
}
}
if len(config.Entrypoint) != 0 && config.Cmd == nil {
config.Cmd = []string{}
} else if config.Cmd == nil || len(config.Cmd) == 0 {
return nil, nil, fmt.Errorf("No command specified")
}
// Generate id
id := GenerateID()
if name == "" {
name, err = generateRandomName(runtime)
if err != nil {
name = utils.TruncateID(id)
}
}
if name[0] != '/' {
name = "/" + name
}
// Set the enitity in the graph using the default name specified
if _, err := runtime.containerGraph.Set(name, id); err != nil {
if strings.HasSuffix(err.Error(), "name are not unique") {
conflictingContainer, _ := runtime.GetByName(name)
nameAsKnownByUser := strings.TrimPrefix(name, "/")
return nil, nil, fmt.Errorf("Conflict, The name %s is already assigned to %s. You have to delete (or rename) that container to be able to assign %s to a container again.", nameAsKnownByUser, utils.TruncateID(conflictingContainer.ID), nameAsKnownByUser)
}
return nil, nil, err
}
// Generate default hostname
// FIXME: the lxc template no longer needs to set a default hostname
if config.Hostname == "" {
config.Hostname = id[:12]
}
var args []string
var entrypoint string
if len(config.Entrypoint) != 0 {
entrypoint = config.Entrypoint[0]
args = append(config.Entrypoint[1:], config.Cmd...)
} else {
entrypoint = config.Cmd[0]
args = config.Cmd[1:]
}
container := &Container{
// FIXME: we should generate the ID here instead of receiving it as an argument
ID: id,
Created: time.Now().UTC(),
Path: entrypoint,
Args: args, //FIXME: de-duplicate from config
Config: config,
hostConfig: &HostConfig{},
Image: img.ID, // Always use the resolved image id
NetworkSettings: &NetworkSettings{},
// FIXME: do we need to store this in the container?
SysInitPath: runtime.sysInitPath,
Name: name,
Driver: runtime.driver.String(),
}
container.root = runtime.containerRoot(container.ID)
// Step 1: create the container directory.
// This doubles as a barrier to avoid race conditions.
if err := os.Mkdir(container.root, 0700); err != nil {
return nil, nil, err
}
initID := fmt.Sprintf("%s-init", container.ID)
if err := runtime.driver.Create(initID, img.ID); err != nil {
return nil, nil, err
}
initPath, err := runtime.driver.Get(initID)
if err != nil {
return nil, nil, err
}
if err := setupInitLayer(initPath); err != nil {
return nil, nil, err
}
if err := runtime.driver.Create(container.ID, initID); err != nil {
return nil, nil, err
}
resolvConf, err := utils.GetResolvConf()
if err != nil {
return nil, nil, err
}
if len(config.Dns) == 0 && len(runtime.config.Dns) == 0 && utils.CheckLocalDns(resolvConf) {
//"WARNING: Docker detected local DNS server on resolv.conf. Using default external servers: %v", defaultDns
runtime.config.Dns = defaultDns
}
// If custom dns exists, then create a resolv.conf for the container
if len(config.Dns) > 0 || len(runtime.config.Dns) > 0 {
var dns []string
if len(config.Dns) > 0 {
dns = config.Dns
} else {
dns = runtime.config.Dns
}
container.ResolvConfPath = path.Join(container.root, "resolv.conf")
f, err := os.Create(container.ResolvConfPath)
if err != nil {
return nil, nil, err
}
defer f.Close()
for _, dns := range dns {
if _, err := f.Write([]byte("nameserver " + dns + "\n")); err != nil {
return nil, nil, err
}
}
} else {
container.ResolvConfPath = "/etc/resolv.conf"
}
// Step 2: save the container json
if err := container.ToDisk(); err != nil {
return nil, nil, err
}
// Step 3: register the container
if err := runtime.Register(container); err != nil {
return nil, nil, err
}
return container, warnings, nil
}
// Commit creates a new filesystem image from the current state of a container.
// The image can optionally be tagged into a repository
func (runtime *Runtime) Commit(container *Container, repository, tag, comment, author string, config *Config) (*Image, error) {
// FIXME: freeze the container before copying it to avoid data corruption?
// FIXME: this shouldn't be in commands.
if err := container.EnsureMounted(); err != nil {
return nil, err
}
rwTar, err := container.ExportRw()
if err != nil {
return nil, err
}
// Create a new image from the container's base layers + a new layer from container changes
img, err := runtime.graph.Create(rwTar, container, comment, author, config)
if err != nil {
return nil, err
}
// Register the image if needed
if repository != "" {
if err := runtime.repositories.Set(repository, tag, img.ID, true); err != nil {
return img, err
}
}
return img, nil
}
// FIXME: this is deprecated by the getFullName *function*
func (runtime *Runtime) getFullName(name string) (string, error) {
return getFullName(name)
}
func getFullName(name string) (string, error) {
if name == "" {
return "", fmt.Errorf("Container name cannot be empty")
}
if name[0] != '/' {
name = "/" + name
}
return name, nil
}
func (runtime *Runtime) GetByName(name string) (*Container, error) {
fullName, err := runtime.getFullName(name)
if err != nil {
return nil, err
}
entity := runtime.containerGraph.Get(fullName)
if entity == nil {
return nil, fmt.Errorf("Could not find entity for %s", name)
}
e := runtime.getContainerElement(entity.ID())
if e == nil {
return nil, fmt.Errorf("Could not find container for entity id %s", entity.ID())
}
return e.Value.(*Container), nil
}
func (runtime *Runtime) Children(name string) (map[string]*Container, error) {
name, err := runtime.getFullName(name)
if err != nil {
return nil, err
}
children := make(map[string]*Container)
err = runtime.containerGraph.Walk(name, func(p string, e *graphdb.Entity) error {
c := runtime.Get(e.ID())
if c == nil {
return fmt.Errorf("Could not get container for name %s and id %s", e.ID(), p)
}
children[p] = c
return nil
}, 0)
if err != nil {
return nil, err
}
return children, nil
}
func (runtime *Runtime) RegisterLink(parent, child *Container, alias string) error {
fullName := path.Join(parent.Name, alias)
if !runtime.containerGraph.Exists(fullName) {
_, err := runtime.containerGraph.Set(fullName, child.ID)
return err
}
return nil
}
// FIXME: harmonize with NewGraph()
func NewRuntime(config *DaemonConfig) (*Runtime, error) {
runtime, err := NewRuntimeFromDirectory(config)
if err != nil {
return nil, err
}
runtime.UpdateCapabilities(false)
return runtime, nil
}
func NewRuntimeFromDirectory(config *DaemonConfig) (*Runtime, error) {
// Set the default driver
graphdriver.DefaultDriver = config.GraphDriver
// Load storage driver
driver, err := graphdriver.New(config.Root)
if err != nil {
return nil, err
}
utils.Debugf("Using graph driver %s", driver)
runtimeRepo := path.Join(config.Root, "containers")
if err := os.MkdirAll(runtimeRepo, 0700); err != nil && !os.IsExist(err) {
return nil, err
}
if ad, ok := driver.(*aufs.Driver); ok {
if err := ad.Migrate(config.Root, setupInitLayer); err != nil {
return nil, err
}
}
if err := linkLxcStart(config.Root); err != nil {
return nil, err
}
g, err := NewGraph(path.Join(config.Root, "graph"), driver)
if err != nil {
return nil, err
}
// We don't want to use a complex driver like aufs or devmapper
// for volumes, just a plain filesystem
volumesDriver, err := graphdriver.GetDriver("vfs", config.Root)
if err != nil {
return nil, err
}
volumes, err := NewGraph(path.Join(config.Root, "volumes"), volumesDriver)
if err != nil {
return nil, err
}
repositories, err := NewTagStore(path.Join(config.Root, "repositories-"+driver.String()), g)
if err != nil {
return nil, fmt.Errorf("Couldn't create Tag store: %s", err)
}
if config.BridgeIface == "" {
config.BridgeIface = DefaultNetworkBridge
}
netManager, err := newNetworkManager(config)
if err != nil {
return nil, err
}
graphdbPath := path.Join(config.Root, "linkgraph.db")
initDatabase := false
if _, err := os.Stat(graphdbPath); err != nil {
if os.IsNotExist(err) {
initDatabase = true
} else {
return nil, err
}
}
conn, err := sql.Open("sqlite3", graphdbPath)
if err != nil {
return nil, err
}
graph, err := graphdb.NewDatabase(conn, initDatabase)
if err != nil {
return nil, err
}
localCopy := path.Join(config.Root, "init", fmt.Sprintf("dockerinit-%s", VERSION))
sysInitPath := utils.DockerInitPath(localCopy)
if sysInitPath == "" {
return nil, fmt.Errorf("Could not locate dockerinit: This usually means docker was built incorrectly. See http://docs.docker.io/en/latest/contributing/devenvironment for official build instructions.")
}
if !utils.IAMSTATIC {
if err := os.Mkdir(path.Join(config.Root, fmt.Sprintf("init")), 0700); err != nil && !os.IsExist(err) {
return nil, err
}
if _, err := utils.CopyFile(sysInitPath, localCopy); err != nil {
return nil, err
}
sysInitPath = localCopy
if err := os.Chmod(sysInitPath, 0700); err != nil {
return nil, err
}
}
runtime := &Runtime{
repository: runtimeRepo,
containers: list.New(),
networkManager: netManager,
graph: g,
repositories: repositories,
idIndex: utils.NewTruncIndex(),
capabilities: &Capabilities{},
volumes: volumes,
config: config,
containerGraph: graph,
driver: driver,
sysInitPath: sysInitPath,
}
if err := runtime.restore(); err != nil {
return nil, err
}
return runtime, nil
}
func (runtime *Runtime) Close() error {
errorsStrings := []string{}
if err := runtime.networkManager.Close(); err != nil {
utils.Errorf("runtime.networkManager.Close(): %s", err.Error())
errorsStrings = append(errorsStrings, err.Error())
}
if err := runtime.driver.Cleanup(); err != nil {
utils.Errorf("runtime.driver.Cleanup(): %s", err.Error())
errorsStrings = append(errorsStrings, err.Error())
}
if err := runtime.containerGraph.Close(); err != nil {
utils.Errorf("runtime.containerGraph.Close(): %s", err.Error())
errorsStrings = append(errorsStrings, err.Error())
}
if len(errorsStrings) > 0 {
return fmt.Errorf("%s", strings.Join(errorsStrings, ", "))
}
return nil
}
func (runtime *Runtime) Mount(container *Container) error {
dir, err := runtime.driver.Get(container.ID)
if err != nil {
return fmt.Errorf("Error getting container %s from driver %s: %s", container.ID, runtime.driver, err)
}
if container.rootfs == "" {
container.rootfs = dir
} else if container.rootfs != dir {
return fmt.Errorf("Error: driver %s is returning inconsistent paths for container %s ('%s' then '%s')",
runtime.driver, container.ID, container.rootfs, dir)
}
return nil
}
func (runtime *Runtime) Unmount(container *Container) error {
// FIXME: Unmount is deprecated because drivers are responsible for mounting
// and unmounting when necessary. Use driver.Remove() instead.
return nil
}
func (runtime *Runtime) Changes(container *Container) ([]archive.Change, error) {
if differ, ok := runtime.driver.(graphdriver.Differ); ok {
return differ.Changes(container.ID)
}
cDir, err := runtime.driver.Get(container.ID)
if err != nil {
return nil, fmt.Errorf("Error getting container rootfs %s from driver %s: %s", container.ID, container.runtime.driver, err)
}
initDir, err := runtime.driver.Get(container.ID + "-init")
if err != nil {
return nil, fmt.Errorf("Error getting container init rootfs %s from driver %s: %s", container.ID, container.runtime.driver, err)
}
return archive.ChangesDirs(cDir, initDir)
}
func (runtime *Runtime) Diff(container *Container) (archive.Archive, error) {
if differ, ok := runtime.driver.(graphdriver.Differ); ok {
return differ.Diff(container.ID)
}
changes, err := runtime.Changes(container)
if err != nil {
return nil, err
}
cDir, err := runtime.driver.Get(container.ID)
if err != nil {
return nil, fmt.Errorf("Error getting container rootfs %s from driver %s: %s", container.ID, container.runtime.driver, err)
}
return archive.ExportChanges(cDir, changes)
}
// Nuke kills all containers then removes all content
// from the content root, including images, volumes and
// container filesystems.
// Again: this will remove your entire docker runtime!
func (runtime *Runtime) Nuke() error {
var wg sync.WaitGroup
for _, container := range runtime.List() {
wg.Add(1)
go func(c *Container) {
c.Kill()
wg.Done()
}(container)
}
wg.Wait()
runtime.Close()
return os.RemoveAll(runtime.config.Root)
}
func linkLxcStart(root string) error {
sourcePath, err := exec.LookPath("lxc-start")
if err != nil {
return err
}
targetPath := path.Join(root, "lxc-start-unconfined")
if _, err := os.Stat(targetPath); err != nil && !os.IsNotExist(err) {
return err
} else if err == nil {
if err := os.Remove(targetPath); err != nil {
return err
}
}
return os.Symlink(sourcePath, targetPath)
}
// FIXME: this is a convenience function for integration tests
// which need direct access to runtime.graph.
// Once the tests switch to using engine and jobs, this method
// can go away.
func (runtime *Runtime) Graph() *Graph {
return runtime.graph
}
// History is a convenience type for storing a list of containers,
// ordered by creation date.
type History []*Container
func (history *History) Len() int {
return len(*history)
}
func (history *History) Less(i, j int) bool {
containers := *history
return containers[j].When().Before(containers[i].When())
}
func (history *History) Swap(i, j int) {
containers := *history
tmp := containers[i]
containers[i] = containers[j]
containers[j] = tmp
}
func (history *History) Add(container *Container) {
*history = append(*history, container)
sort.Sort(history)
}