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