Several improvements and bug fixes related to graceful reloads

Added a -grace flag to customize graceful shutdown period, fixed bugs related to closing file descriptors (and dup'ed fds), improved healthcheck signaling to parent, fixed a race condition with the graceful listener, etc. These improvements mainly provide better support for frequent reloading or unusual use cases of Start and Stop after a Restart (POSIX systems). This forum thread was valuable help in debugging: https://forum.golangbridge.org/t/bind-address-already-in-use-even-after-listener-closed/1510?u=matt

caddy/caddy.go

@@ -29,6 +29,7 @@ import (
 	"path"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/mholt/caddy/caddy/letsencrypt"
 	"github.com/mholt/caddy/server"
@@ -43,11 +44,14 @@ var (
 	// If true, initialization will not show any informative output.
 	Quiet bool
 
-	// HTTP2 indicates whether HTTP2 is enabled or not
+	// HTTP2 indicates whether HTTP2 is enabled or not.
 	HTTP2 bool // TODO: temporary flag until http2 is standard
 
-	// PidFile is the path to the pidfile to create
+	// PidFile is the path to the pidfile to create.
 	PidFile string
+
+	// GracefulTimeout is the maximum duration of a graceful shutdown.
+	GracefulTimeout time.Duration
 )
 
 var (
@@ -91,17 +95,20 @@ const (
 )
 
 // Start starts Caddy with the given Caddyfile. If cdyfile
-// is nil or the process is forked from a parent as part of
+// is nil, the LoadCaddyfile function will be called to get
-// a graceful restart, Caddy will check to see if Caddyfile
+// one.
-// was piped from stdin and use that. It blocks until all the
-// servers are listening.
 //
-// If this process is a fork and no Caddyfile was piped in,
+// This function blocks until all the servers are listening.
-// an error will be returned (the Restart() function does this
+//
-// for you automatically). If this process is NOT a fork and
+// Note (POSIX): If Start is called in the child process of a
-// cdyfile is nil, a default configuration will be assumed.
+// restart more than once within the duration of the graceful
-// In any case, an error is returned if Caddy could not be
+// cutoff (i.e. the child process called Start a first time,
-// started.
+// then called Stop, then Start again within the first 5 seconds
+// or however long GracefulTimeout is) and the Caddyfiles have
+// at least one listener address in common, the second Start
+// may fail with "address already in use" as there's no
+// guarantee that the parent process has relinquished the
+// address before the grace period ends.
 func Start(cdyfile Input) (err error) {
 	// If we return with no errors, we must do two things: tell the
 	// parent that we succeeded and write to the pidfile.
@@ -148,17 +155,6 @@ func Start(cdyfile Input) (err error) {
 	}
 	startedBefore = true
 
-	// Close remaining file descriptors we may have inherited that we don't need
-	if IsRestart() {
-		for _, fdIndex := range loadedGob.ListenerFds {
-			file := os.NewFile(fdIndex, "")
-			fln, err := net.FileListener(file)
-			if err == nil {
-				fln.Close()
-			}
-		}
-	}
-
 	// Show initialization output
 	if !Quiet && !IsRestart() {
 		var checkedFdLimit bool
@@ -192,7 +188,7 @@ func startServers(groupings bindingGroup) error {
 	errChan := make(chan error, len(groupings)) // must be buffered to allow Serve functions below to return if stopped later
 
 	for _, group := range groupings {
-		s, err := server.New(group.BindAddr.String(), group.Configs)
+		s, err := server.New(group.BindAddr.String(), group.Configs, GracefulTimeout)
 		if err != nil {
 			return err
 		}
@@ -215,7 +211,8 @@ func startServers(groupings bindingGroup) error {
 					return errors.New("listener for " + s.Addr + " was not a ListenerFile")
 				}
 
-				delete(loadedGob.ListenerFds, s.Addr) // mark it as used
+				file.Close()
+				delete(loadedGob.ListenerFds, s.Addr)
 			}
 		}
 
@@ -252,6 +249,14 @@ func startServers(groupings bindingGroup) error {
 		serversMu.Unlock()
 	}
 
+	// Close the remaining (unused) file descriptors to free up resources
+	if IsRestart() {
+		for key, fdIndex := range loadedGob.ListenerFds {
+			os.NewFile(fdIndex, "").Close()
+			delete(loadedGob.ListenerFds, key)
+		}
+	}
+
 	// Wait for all servers to finish starting
 	startupWg.Wait()
 
@@ -276,7 +281,9 @@ func Stop() error {
 
 	serversMu.Lock()
 	for _, s := range servers {
-		s.Stop() // TODO: error checking/reporting?
+		if err := s.Stop(); err != nil {
+			log.Printf("[ERROR] Stopping %s: %v", s.Addr, err)
+		}
 	}
 	servers = []*server.Server{} // don't reuse servers
 	serversMu.Unlock()
@@ -289,12 +296,13 @@ func Wait() {
 	wg.Wait()
 }
 
-// LoadCaddyfile loads a Caddyfile in a way that prioritizes
+// LoadCaddyfile loads a Caddyfile, prioritizing a Caddyfile
-// reading from stdin pipe; otherwise it calls loader to load
+// piped from stdin as part of a restart (only happens on first call
-// the Caddyfile. If loader does not return a Caddyfile, the
+// to LoadCaddyfile). If it is not a restart, this function tries
-// default one will be returned. Thus, if there are no other
+// calling the user's loader function, and if that returns nil, then
-// errors, this function always returns at least the default
+// this function resorts to the default configuration. Thus, if there
-// Caddyfile (not the previously-used Caddyfile).
+// are no other errors, this function always returns at least the
+// default Caddyfile.
 func LoadCaddyfile(loader func() (Input, error)) (cdyfile Input, err error) {
 	// If we are a fork, finishing the restart is highest priority;
 	// piped input is required in this case.

caddy/helpers.go

@@ -10,6 +10,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"sync"
 
 	"github.com/mholt/caddy/caddy/letsencrypt"
 )
@@ -44,7 +45,9 @@ func checkFdlimit() {
 // If this process is not a restart, this function does nothing.
 // Calling this function once this process has successfully initialized
 // is vital so that the parent process can unblock and kill itself.
+// This function is idempotent; it executes at most once per process.
 func signalSuccessToParent() {
+	signalParentOnce.Do(func() {
 		if IsRestart() {
 			ppipe := os.NewFile(3, "")               // parent is reading from pipe at index 3
 			_, err := ppipe.Write([]byte("success")) // we must send some bytes to the parent
@@ -53,8 +56,17 @@ func signalSuccessToParent() {
 			}
 			ppipe.Close()
 		}
+	})
 }
 
+// signalParentOnce is used to make sure that the parent is only
+// signaled once; doing so more than once breaks whatever socket is
+// at fd 4 (the reason for this is still unclear - to reproduce,
+// call Stop() and Start() in succession at least once after a
+// restart, then try loading first host of Caddyfile in the browser).
+// Do not use this directly - call signalSuccessToParent instead.
+var signalParentOnce sync.Once
+
 // caddyfileGob maps bind address to index of the file descriptor
 // in the Files array passed to the child process. It also contains
 // the caddyfile contents. Used only during graceful restarts.

caddy/restart.go

@@ -84,6 +84,11 @@ func Restart(newCaddyfile Input) error {
 		return err
 	}
 
+	// Immediately close our dup'ed fds and the write end of our signal pipe
+	for _, f := range extraFiles {
+		f.Close()
+	}
+
 	// Feed Caddyfile to the child
 	err = gob.NewEncoder(wpipe).Encode(cdyfileGob)
 	if err != nil {
@@ -92,7 +97,6 @@ func Restart(newCaddyfile Input) error {
 	wpipe.Close()
 
 	// Determine whether child startup succeeded
-	sigwpipe.Close() // close our copy of the write end of the pipe -- TODO: why?
 	answer, readErr := ioutil.ReadAll(sigrpipe)
 	if answer == nil || len(answer) == 0 {
 		cmdErr := cmd.Wait() // get exit status
@@ -103,6 +107,6 @@ func Restart(newCaddyfile Input) error {
 		return errIncompleteRestart
 	}
 
-	// Looks like child was successful; we can exit gracefully.
+	// Looks like child is successful; we can exit gracefully.
 	return Stop()
 }

dist/CHANGES.txt

@@ -10,8 +10,11 @@ CHANGES
 - New -ca flag to customize ACME CA server URL
 - New -revoke flag to revoke a certificate
 - New -log flag to enable process log
+- New -pidfile flag to enable writing pidfile
+- New -grace flag to customize the graceful shutdown timeout
 - browse: Render filenames with multiple whitespace properly
 - markdown: Include Last-Modified header in response
+- markdown: Render tables, strikethrough, and fenced code blocks
 - startup, shutdown: Better Windows support
 - templates: Bug fix for .Host when port is absent
 - templates: Include Last-Modified header in response

main.go

@@ -10,6 +10,7 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/mholt/caddy/caddy"
 	"github.com/mholt/caddy/caddy/letsencrypt"
@@ -34,6 +35,7 @@ func init() {
 	flag.StringVar(&conf, "conf", "", "Configuration file to use (default="+caddy.DefaultConfigFile+")")
 	flag.StringVar(&cpu, "cpu", "100%", "CPU cap")
 	flag.StringVar(&letsencrypt.DefaultEmail, "email", "", "Default Let's Encrypt account email address")
+	flag.DurationVar(&caddy.GracefulTimeout, "grace", 5*time.Second, "Maximum duration of graceful shutdown")
 	flag.StringVar(&caddy.Host, "host", caddy.DefaultHost, "Default host")
 	flag.BoolVar(&caddy.HTTP2, "http2", true, "HTTP/2 support") // TODO: temporary flag until http2 merged into std lib
 	flag.StringVar(&logfile, "log", "", "Process log file")

server/graceful.go

@@ -2,7 +2,6 @@ package server
 
 import (
 	"net"
-	"os"
 	"sync"
 	"syscall"
 )
@@ -13,7 +12,9 @@ func newGracefulListener(l ListenerFile, wg *sync.WaitGroup) *gracefulListener {
 	gl := &gracefulListener{ListenerFile: l, stop: make(chan error), httpWg: wg}
 	go func() {
 		<-gl.stop
+		gl.Lock()
 		gl.stopped = true
+		gl.Unlock()
 		gl.stop <- gl.ListenerFile.Close()
 	}()
 	return gl
@@ -26,10 +27,11 @@ type gracefulListener struct {
 	ListenerFile
 	stop       chan error
 	stopped    bool
+	sync.Mutex                 // protects the stopped flag
 	httpWg     *sync.WaitGroup // pointer to the host's wg used for counting connections
 }
 
-// Accept accepts a connection. This type wraps
+// Accept accepts a connection.
 func (gl *gracefulListener) Accept() (c net.Conn, err error) {
 	c, err = gl.ListenerFile.Accept()
 	if err != nil {
@@ -42,18 +44,16 @@ func (gl *gracefulListener) Accept() (c net.Conn, err error) {
 
 // Close immediately closes the listener.
 func (gl *gracefulListener) Close() error {
+	gl.Lock()
 	if gl.stopped {
+		gl.Unlock()
 		return syscall.EINVAL
 	}
+	gl.Unlock()
 	gl.stop <- nil
 	return <-gl.stop
 }
 
-// File implements ListenerFile; it gets the file of the listening socket.
-func (gl *gracefulListener) File() (*os.File, error) {
-	return gl.ListenerFile.File()
-}
-
 // gracefulConn represents a connection on a
 // gracefulListener so that we can keep track
 // of the number of connections, thus facilitating

server/server.go

@@ -8,7 +8,6 @@ import (
 	"crypto/x509"
 	"fmt"
 	"io/ioutil"
-	"log"
 	"net"
 	"net/http"
 	"os"
@@ -33,6 +32,7 @@ type Server struct {
 	listenerMu  sync.Mutex             // protects listener
 	httpWg      sync.WaitGroup         // used to wait on outstanding connections
 	startChan   chan struct{}          // used to block until server is finished starting
+	connTimeout time.Duration          // the maximum duration of a graceful shutdown
 }
 
 // ListenerFile represents a listener.
@@ -42,14 +42,17 @@ type ListenerFile interface {
 }
 
 // New creates a new Server which will bind to addr and serve
-// the sites/hosts configured in configs. This function does
+// the sites/hosts configured in configs. Its listener will
-// not start serving.
+// gracefully close when the server is stopped which will take
+// no longer than gracefulTimeout.
+//
+// This function does not start serving.
 //
 // Do not re-use a server (start, stop, then start again). We
 // could probably add more locking to make this possible, but
 // as it stands, you should dispose of a server after stopping it.
 // The behavior of serving with a spent server is undefined.
-func New(addr string, configs []Config) (*Server, error) {
+func New(addr string, configs []Config, gracefulTimeout time.Duration) (*Server, error) {
 	var tls bool
 	if len(configs) > 0 {
 		tls = configs[0].TLS.Enabled
@@ -66,6 +69,7 @@ func New(addr string, configs []Config) (*Server, error) {
 		tls:         tls,
 		vhosts:      make(map[string]virtualHost),
 		startChan:   make(chan struct{}),
+		connTimeout: gracefulTimeout,
 	}
 	s.Handler = s // this is weird, but whatever
 
@@ -241,7 +245,7 @@ func serveTLSWithSNI(s *Server, ln net.Listener, tlsConfigs []TLSConfig) error {
 // connections to close (up to a max timeout of a few
 // seconds); on Windows it will close the listener
 // immediately.
-func (s *Server) Stop() error {
+func (s *Server) Stop() (err error) {
 	s.Server.SetKeepAlivesEnabled(false)
 
 	if runtime.GOOS != "windows" {
@@ -256,20 +260,19 @@ func (s *Server) Stop() error {
 		// Wait for remaining connections to finish or
 		// force them all to close after timeout
 		select {
-		case <-time.After(5 * time.Second): // TODO: make configurable
+		case <-time.After(s.connTimeout):
 		case <-done:
 		}
 	}
 
 	// Close the listener now; this stops the server without delay
 	s.listenerMu.Lock()
-	err := s.listener.Close()
+	if s.listener != nil {
-	s.listenerMu.Unlock()
+		err = s.listener.Close()
-	if err != nil {
-		log.Printf("[ERROR] Closing listener for %s: %v", s.Addr, err)
 	}
+	s.listenerMu.Unlock()
 
-	return err
+	return
 }
 
 // WaitUntilStarted blocks until the server s is started, meaning
@@ -279,12 +282,17 @@ func (s *Server) WaitUntilStarted() {
 	<-s.startChan
 }
 
-// ListenerFd gets the file descriptor of the listener.
+// ListenerFd gets a dup'ed file of the listener. If there
+// is no underlying file, the return value will be nil. It
+// is the caller's responsibility to close the file.
 func (s *Server) ListenerFd() *os.File {
 	s.listenerMu.Lock()
 	defer s.listenerMu.Unlock()
+	if s.listener != nil {
 		file, _ := s.listener.File()
 		return file
+	}
+	return nil
 }
 
 // ServeHTTP is the entry point for every request to the address that s
@@ -370,7 +378,7 @@ func setupClientAuth(tlsConfigs []TLSConfig, config *tls.Config) error {
 
 // RunFirstStartupFuncs runs all of the server's FirstStartup
 // callback functions unless one of them returns an error first.
-// It is up the caller's responsibility to call this only once and
+// It is the caller's responsibility to call this only once and
 // at the correct time. The functions here should not be executed
 // at restarts or where the user does not explicitly start a new
 // instance of the server.