add doc for idle server auto shutdown

Reviewed-on: #2

Fixes #1

README.md

@@ -1,7 +1,9 @@
-Create http server listening on unix sockets or systemd socket activated fds
+Create http server listening on unix sockets and systemd socket activated fds
 
 ## Quick Usage
 
+    go get go.balki.me/anyhttp
+
 Just replace `http.ListenAndServe` with `anyhttp.ListenAndServe`.
 
 ```diff
@@ -42,10 +44,34 @@ Examples:
 
 ### TCP port
 
-If the address is a number less than 65536, it is assumed as a port and passed as `http.ListenAndServe(":<port>",...)`
-
-Anything else is directly passed to `http.ListenAndServe` as well. Below examples should work
+If the address is a number less than 65536, it is assumed as a port and passed
+as `http.ListenAndServe(":<port>",...)` Anything else is directly passed to
+`http.ListenAndServe` as well. Below examples should work
 
     :http
     :8888
     127.0.0.1:8080
+
+## Idle server auto shutdown
+
+When using systemd socket activation, idle servers can be shut down to save on
+resources.  They will be restarted with socket activation when new request
+arrives. Quick example for the case. (Error checking skipped for brevity)
+
+```go
+addrType, httpServer, done, _ := anyhttp.Serve(addr, idle.WrapHandler(nil))
+if addrType == anyhttp.SystemdFD {
+    idle.Wait(30 * time.Minute)
+    httpServer.Shutdown(context.TODO())
+}
+<-done
+```
+
+## Documentation
+
+https://pkg.go.dev/go.balki.me/anyhttp
+
+### Related links
+
+  * https://gist.github.com/teknoraver/5ffacb8757330715bcbcc90e6d46ac74#file-unixhttpd-go
+  * https://github.com/coreos/go-systemd/tree/main/activation

anyhttp.go

@@ -10,9 +10,24 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"sync"
 	"syscall"
 )
 
+// AddressType of the address passed
+type AddressType string
+
+var (
+	// UnixSocket - address is a unix socket, e.g. unix//run/foo.sock
+	UnixSocket AddressType = "UnixSocket"
+	// SystemdFD - address is a systemd fd, e.g. sysd/fdname/myapp.socket
+	SystemdFD AddressType = "SystemdFD"
+	// TCP - address is a TCP address, e.g. :1234
+	TCP AddressType = "TCP"
+	// Unknown - address is not recognized
+	Unknown AddressType = "Unknown"
+)
+
 // UnixSocketConfig has the configuration for Unix socket
 type UnixSocketConfig struct {
 
@@ -39,6 +54,39 @@ func NewUnixSocketConfig(socketPath string) UnixSocketConfig {
 	return usc
 }
 
+type sysdEnvData struct {
+	pid        int
+	fdNames    []string
+	fdNamesStr string
+	numFds     int
+}
+
+var sysdEnvParser = struct {
+	sysdOnce sync.Once
+	data     sysdEnvData
+	err      error
+}{}
+
+func parse() (sysdEnvData, error) {
+	p := &sysdEnvParser
+	p.sysdOnce.Do(func() {
+		p.data.pid, p.err = strconv.Atoi(os.Getenv("LISTEN_PID"))
+		if p.err != nil {
+			p.err = fmt.Errorf("invalid LISTEN_PID, err: %w", p.err)
+			return
+		}
+		p.data.numFds, p.err = strconv.Atoi(os.Getenv("LISTEN_FDS"))
+		if p.err != nil {
+			p.err = fmt.Errorf("invalid LISTEN_FDS, err: %w", p.err)
+			return
+		}
+		p.data.fdNamesStr = os.Getenv("LISTEN_FDNAMES")
+		p.data.fdNames = strings.Split(p.data.fdNamesStr, ":")
+
+	})
+	return p.data, p.err
+}
+
 // SysdConfig has the configuration for the socket activated fd
 type SysdConfig struct {
 	// Integer value starting at 0. Either index or name is required
@@ -54,7 +102,7 @@ type SysdConfig struct {
 // DefaultSysdConfig has the default values for SysdConfig
 var DefaultSysdConfig = SysdConfig{
 	CheckPID: true,
-	UnsetEnv: false,
+	UnsetEnv: true,
 }
 
 // NewSysDConfigWithFDIdx creates SysdConfig with defaults and fdIdx
@@ -112,99 +160,110 @@ func (s *SysdConfig) GetListener() (net.Listener, error) {
 		defer UnsetSystemdListenVars()
 	}
 
+	envData, err := parse()
+	if err != nil {
+		return nil, err
+	}
+
 	if s.CheckPID {
-		pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
-		if err != nil {
-			return nil, err
-		}
-		if pid != os.Getpid() {
-			return nil, fmt.Errorf("fd not for you")
+		if envData.pid != os.Getpid() {
+			return nil, fmt.Errorf("unexpected PID, current:%v, LISTEN_PID: %v", os.Getpid(), envData.pid)
 		}
 	}
 
-	numFds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
-	if err != nil {
-		return nil, err
-	}
-
-	fdNames := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
-
 	if s.FDIndex != nil {
 		idx := *s.FDIndex
-		if idx < 0 || idx >= numFds {
-			return nil, fmt.Errorf("invalid fd")
+		if idx < 0 || idx >= envData.numFds {
+			return nil, fmt.Errorf("invalid fd index, expected between 0 and %v, got: %v", envData.numFds, idx)
 		}
 		fd := StartFD + idx
-		if idx < len(fdNames) {
-			return makeFdListener(fd, fdNames[idx])
+		if idx < len(envData.fdNames) {
+			return makeFdListener(fd, envData.fdNames[idx])
 		}
 		return makeFdListener(fd, fmt.Sprintf("sysdfd_%d", fd))
 	}
 
 	if s.FDName != nil {
-		for idx, name := range fdNames {
+		for idx, name := range envData.fdNames {
 			if name == *s.FDName {
 				fd := StartFD + idx
 				return makeFdListener(fd, name)
 			}
 		}
-		return nil, fmt.Errorf("fdName not found: %q", *s.FDName)
+		return nil, fmt.Errorf("fdName not found: %q, LISTEN_FDNAMES:%q", *s.FDName, envData.fdNamesStr)
 	}
 
-	return nil, fmt.Errorf("neither FDIndex nor FDName set")
+	return nil, errors.New("neither FDIndex nor FDName set")
 }
 
 // GetListener gets a unix or systemd socket listener
-func GetListener(addr string) (net.Listener, error) {
+func GetListener(addr string) (AddressType, net.Listener, error) {
 	if strings.HasPrefix(addr, "unix/") {
 		usc := NewUnixSocketConfig(strings.TrimPrefix(addr, "unix/"))
-		return usc.GetListener()
+		l, err := usc.GetListener()
+		return UnixSocket, l, err
 	}
 
 	if strings.HasPrefix(addr, "sysd/fdidx/") {
 		idx, err := strconv.Atoi(strings.TrimPrefix(addr, "sysd/fdidx/"))
 		if err != nil {
-			return nil, err
+			return Unknown, nil, fmt.Errorf("invalid fdidx, addr:%q err: %w", addr, err)
 		}
 		sysdc := NewSysDConfigWithFDIdx(idx)
-		return sysdc.GetListener()
+		l, err := sysdc.GetListener()
+		return SystemdFD, l, err
 	}
 
 	if strings.HasPrefix(addr, "sysd/fdname/") {
 		sysdc := NewSysDConfigWithFDName(strings.TrimPrefix(addr, "sysd/fdname/"))
-		return sysdc.GetListener()
+		l, err := sysdc.GetListener()
+		return SystemdFD, l, err
 	}
 
-	return nil, nil
+	if port, err := strconv.Atoi(addr); err == nil {
+		if port > 0 && port < 65536 {
+			addr = fmt.Sprintf(":%v", port)
+		} else {
+			return Unknown, nil, fmt.Errorf("invalid port: %v", port)
+		}
+	}
+
+	if addr == "" {
+		addr = ":http"
+	}
+
+	l, err := net.Listen("tcp", addr)
+	return TCP, l, err
+}
+
+// Serve creates and serve a http server.
+func Serve(addr string, h http.Handler) (AddressType, *http.Server, <-chan error, error) {
+	addrType, listener, err := GetListener(addr)
+	if err != nil {
+		return addrType, nil, nil, err
+	}
+	srv := &http.Server{Handler: h}
+	done := make(chan error)
+	go func() {
+		done <- srv.Serve(listener)
+		close(done)
+	}()
+	return addrType, srv, done, nil
 }
 
 // ListenAndServe is the drop-in replacement for `http.ListenAndServe`.
 // Supports unix and systemd sockets in addition
 func ListenAndServe(addr string, h http.Handler) error {
-
-	listener, err := GetListener(addr)
+	_, _, done, err := Serve(addr, h)
 	if err != nil {
 		return err
 	}
-
-	if listener != nil {
-		return http.Serve(listener, h)
-	}
-
-	if port, err := strconv.Atoi(addr); err == nil {
-		if port > 0 && port < 65536 {
-
-			return http.ListenAndServe(fmt.Sprintf(":%v", port), h)
-		}
-		return fmt.Errorf("invalid port: %v", port)
-	}
-
-	return http.ListenAndServe(addr, h)
+	return <-done
 }
 
 // UnsetSystemdListenVars unsets the LISTEN* environment variables so they are not passed to any child processes
 func UnsetSystemdListenVars() {
-	os.Unsetenv("LISTEN_PID")
-	os.Unsetenv("LISTEN_FDS")
-	os.Unsetenv("LISTEN_FDNAMES")
+	_ = os.Unsetenv("LISTEN_PID")
+	_ = os.Unsetenv("LISTEN_FDS")
+	_ = os.Unsetenv("LISTEN_FDNAMES")
 }

examples/idle_server_shutdown/main.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"context"
+	"net/http"
+	"time"
+
+	"go.balki.me/anyhttp/idle"
+)
+
+func main() {
+	idler := idle.CreateIdler(10 * time.Second)
+
+	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		idler.Tick()
+		w.Write([]byte("Hey there!\n"))
+	})
+
+	http.HandleFunc("/job", func(w http.ResponseWriter, r *http.Request) {
+		go func() {
+			idler.Enter()
+			defer idler.Exit()
+
+			time.Sleep(15 * time.Second)
+		}()
+		w.Write([]byte("Job scheduled\n"))
+	})
+
+	server := http.Server{
+		Addr: ":8888",
+	}
+	go server.ListenAndServe()
+	idler.Wait()
+	server.Shutdown(context.TODO())
+}

idle/idle.go

@@ -0,0 +1,127 @@
+// Package idle helps to gracefully shutdown idle (typically http) servers
+package idle
+
+import (
+	"fmt"
+	"net/http"
+	"sync/atomic"
+	"time"
+)
+
+var (
+	// For simple servers without backgroud jobs, global singleton for simpler API
+	// Enter/Exit worn't work for global idler as Enter may be called before Wait, use CreateIdler in those cases
+	gIdler atomic.Pointer[idler]
+)
+
+// Wait waits till the server is idle and returns. i.e. no Ticks in last <timeout> duration
+func Wait(timeout time.Duration) error {
+	i := CreateIdler(timeout).(*idler)
+	ok := gIdler.CompareAndSwap(nil, i)
+	if !ok {
+		return fmt.Errorf("idler already waiting")
+	}
+	i.Wait()
+	return nil
+}
+
+// Tick records the current time. This will make the server not idle until next Tick or timeout
+func Tick() {
+	i := gIdler.Load()
+	if i != nil {
+		i.Tick()
+	}
+}
+
+// WrapHandler calls Tick() before processing passing request to http.Handler
+func WrapHandler(h http.Handler) http.Handler {
+	if h == nil {
+		h = http.DefaultServeMux
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		Tick()
+		h.ServeHTTP(w, r)
+	})
+}
+
+// WrapIdlerHandler calls idler.Tick() before processing passing request to http.Handler
+func WrapIdlerHandler(i Idler, h http.Handler) http.Handler {
+	if h == nil {
+		h = http.DefaultServeMux
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		i.Tick()
+		h.ServeHTTP(w, r)
+	})
+}
+
+// Idler helps manage idle servers
+type Idler interface {
+	// Tick records the current time. This will make the server not idle until next Tick or timeout
+	Tick()
+
+	// Wait waits till the server is idle and returns. i.e. no Ticks in last <timeout> duration
+	Wait()
+
+	// For long running background jobs, use Enter to record start time. Wait will not return while there are active jobs running
+	Enter()
+
+	// Exit records end of a background job
+	Exit()
+
+	// Get the channel to wait yourself
+	Chan() <-chan struct{}
+}
+
+type idler struct {
+	lastTick atomic.Pointer[time.Time]
+	c        chan struct{}
+	active   atomic.Int64
+}
+
+func (i *idler) Enter() {
+	i.active.Add(1)
+}
+
+func (i *idler) Exit() {
+	i.Tick()
+	i.active.Add(-1)
+}
+
+// CreateIdler creates an Idler with given timeout
+func CreateIdler(timeout time.Duration) Idler {
+	i := &idler{}
+	i.c = make(chan struct{})
+	i.Tick()
+	go func() {
+		for {
+			if i.active.Load() != 0 {
+				time.Sleep(timeout)
+				continue
+			}
+			t := *i.lastTick.Load()
+			now := time.Now()
+			dur := t.Add(timeout).Sub(now)
+			if dur == dur.Abs() {
+				time.Sleep(dur)
+				continue
+			}
+			break
+		}
+		close(i.c)
+	}()
+	return i
+}
+
+func (i *idler) Tick() {
+	now := time.Now()
+	i.lastTick.Store(&now)
+}
+
+func (i *idler) Wait() {
+	<-i.c
+}
+
+func (i *idler) Chan() <-chan struct{} {
+	return i.c
+}

idle/idle_test.go

@@ -0,0 +1,34 @@
+package idle
+
+import (
+	"testing"
+	"time"
+)
+
+func TestIdlerChan(_ *testing.T) {
+	i := CreateIdler(10 * time.Millisecond)
+	<-i.Chan()
+}
+
+func TestGlobalIdler(t *testing.T) {
+	err := Wait(10 * time.Millisecond)
+	if err != nil {
+		t.Fatalf("idle.Wait failed, %v", err)
+	}
+	err = Wait(10 * time.Millisecond)
+	if err == nil {
+		t.Fatal("idle.Wait should fail when called second time")
+	}
+}
+
+func TestIdlerEnterExit(t *testing.T) {
+	i := CreateIdler(10 * time.Millisecond).(*idler)
+	i.Enter()
+	if i.active.Load() != 1 {
+		t.FailNow()
+	}
+	i.Exit()
+	if i.active.Load() != 0 {
+		t.FailNow()
+	}
+}