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// Package gracenet provides a family of Listen functions that either open a
// fresh connection or provide an inherited connection from when the process
// was started. The behave like their counterparts in the net package, but
// transparently provide support for graceful restarts without dropping
// connections. This is provided in a systemd socket activation compatible form
// to allow using socket activation.
//
// BUG: Doesn't handle closing of listeners.
package gracenet
import (
"fmt"
"net"
"os"
"os/exec"
"strconv"
"strings"
"sync"
)
const (
// Used to indicate a graceful restart in the new process.
envCountKey = "LISTEN_FDS"
envCountKeyPrefix = envCountKey + "="
)
// In order to keep the working directory the same as when we started we record
// it at startup.
var originalWD, _ = os.Getwd()
// Net provides the family of Listen functions and maintains the associated
// state. Typically you will have only once instance of Net per application.
type Net struct {
inherited []net.Listener
active []net.Listener
mutex sync.Mutex
inheritOnce sync.Once
// used in tests to override the default behavior of starting from fd 3.
fdStart int
}
func (n *Net) inherit() error {
var retErr error
n.inheritOnce.Do(func() {
n.mutex.Lock()
defer n.mutex.Unlock()
countStr := os.Getenv(envCountKey)
if countStr == "" {
return
}
count, err := strconv.Atoi(countStr)
if err != nil {
retErr = fmt.Errorf("found invalid count value: %s=%s", envCountKey, countStr)
return
}
// In tests this may be overridden.
fdStart := n.fdStart
if fdStart == 0 {
// In normal operations if we are inheriting, the listeners will begin at
// fd 3.
fdStart = 3
}
for i := fdStart; i < fdStart+count; i++ {
file := os.NewFile(uintptr(i), "listener")
l, err := net.FileListener(file)
if err != nil {
file.Close()
retErr = fmt.Errorf("error inheriting socket fd %d: %s", i, err)
return
}
if err := file.Close(); err != nil {
retErr = fmt.Errorf("error closing inherited socket fd %d: %s", i, err)
return
}
n.inherited = append(n.inherited, l)
}
})
return retErr
}
// Listen announces on the local network address laddr. The network net must be
// a stream-oriented network: "tcp", "tcp4", "tcp6", "unix" or "unixpacket". It
// returns an inherited net.Listener for the matching network and address, or
// creates a new one using net.Listen.
func (n *Net) Listen(nett, laddr string) (net.Listener, error) {
switch nett {
default:
return nil, net.UnknownNetworkError(nett)
case "tcp", "tcp4", "tcp6":
addr, err := net.ResolveTCPAddr(nett, laddr)
if err != nil {
return nil, err
}
return n.ListenTCP(nett, addr)
case "unix", "unixpacket", "invalid_unix_net_for_test":
addr, err := net.ResolveUnixAddr(nett, laddr)
if err != nil {
return nil, err
}
return n.ListenUnix(nett, addr)
}
}
// ListenTCP announces on the local network address laddr. The network net must
// be: "tcp", "tcp4" or "tcp6". It returns an inherited net.Listener for the
// matching network and address, or creates a new one using net.ListenTCP.
func (n *Net) ListenTCP(nett string, laddr *net.TCPAddr) (*net.TCPListener, error) {
if err := n.inherit(); err != nil {
return nil, err
}
n.mutex.Lock()
defer n.mutex.Unlock()
// look for an inherited listener
for i, l := range n.inherited {
if l == nil { // we nil used inherited listeners
continue
}
if isSameAddr(l.Addr(), laddr) {
n.inherited[i] = nil
n.active = append(n.active, l)
return l.(*net.TCPListener), nil
}
}
// make a fresh listener
l, err := net.ListenTCP(nett, laddr)
if err != nil {
return nil, err
}
n.active = append(n.active, l)
return l, nil
}
// ListenUnix announces on the local network address laddr. The network net
// must be a: "unix" or "unixpacket". It returns an inherited net.Listener for
// the matching network and address, or creates a new one using net.ListenUnix.
func (n *Net) ListenUnix(nett string, laddr *net.UnixAddr) (*net.UnixListener, error) {
if err := n.inherit(); err != nil {
return nil, err
}
n.mutex.Lock()
defer n.mutex.Unlock()
// look for an inherited listener
for i, l := range n.inherited {
if l == nil { // we nil used inherited listeners
continue
}
if isSameAddr(l.Addr(), laddr) {
n.inherited[i] = nil
n.active = append(n.active, l)
return l.(*net.UnixListener), nil
}
}
// make a fresh listener
l, err := net.ListenUnix(nett, laddr)
if err != nil {
return nil, err
}
n.active = append(n.active, l)
return l, nil
}
// activeListeners returns a snapshot copy of the active listeners.
func (n *Net) activeListeners() ([]net.Listener, error) {
n.mutex.Lock()
defer n.mutex.Unlock()
ls := make([]net.Listener, len(n.active))
copy(ls, n.active)
return ls, nil
}
func isSameAddr(a1, a2 net.Addr) bool {
if a1.Network() != a2.Network() {
return false
}
a1s := a1.String()
a2s := a2.String()
if a1s == a2s {
return true
}
// This allows for ipv6 vs ipv4 local addresses to compare as equal. This
// scenario is common when listening on localhost.
const ipv6prefix = "[::]"
a1s = strings.TrimPrefix(a1s, ipv6prefix)
a2s = strings.TrimPrefix(a2s, ipv6prefix)
const ipv4prefix = "0.0.0.0"
a1s = strings.TrimPrefix(a1s, ipv4prefix)
a2s = strings.TrimPrefix(a2s, ipv4prefix)
return a1s == a2s
}
// StartProcess starts a new process passing it the active listeners. It
// doesn't fork, but starts a new process using the same environment and
// arguments as when it was originally started. This allows for a newly
// deployed binary to be started. It returns the pid of the newly started
// process when successful.
func (n *Net) StartProcess() (int, error) {
listeners, err := n.activeListeners()
if err != nil {
return 0, err
}
// Extract the fds from the listeners.
files := make([]*os.File, len(listeners))
for i, l := range listeners {
files[i], err = l.(filer).File()
if err != nil {
return 0, err
}
defer files[i].Close()
}
// Use the original binary location. This works with symlinks such that if
// the file it points to has been changed we will use the updated symlink.
argv0, err := exec.LookPath(os.Args[0])
if err != nil {
return 0, err
}
// Pass on the environment and replace the old count key with the new one.
var env []string
for _, v := range os.Environ() {
if !strings.HasPrefix(v, envCountKeyPrefix) {
env = append(env, v)
}
}
env = append(env, fmt.Sprintf("%s%d", envCountKeyPrefix, len(listeners)))
allFiles := append([]*os.File{os.Stdin, os.Stdout, os.Stderr}, files...)
process, err := os.StartProcess(argv0, os.Args, &os.ProcAttr{
Dir: originalWD,
Env: env,
Files: allFiles,
})
if err != nil {
return 0, err
}
return process.Pid, nil
}
type filer interface {
File() (*os.File, error)
}
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