Pull request 1899: nextapi-pidfile-webaddr

Squashed commit of the following:

commit 73b97b638016dd3992376c2cd7d11b2e85b2c3a4
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jun 29 18:43:05 2023 +0300

    next: use maybe; sync conf

commit 99e18b8fbfad11343a1e66f746085d54be7aafea
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jun 29 18:13:13 2023 +0300

    next: add local frontend, pidfile, webaddr

internal/next/cmd/signal.go

@@ -1,29 +1,27 @@
 package cmd
 
 import (
-	"io/fs"
 	"os"
-	"time"
+	"strconv"
 
 	"github.com/AdguardTeam/AdGuardHome/internal/aghos"
 	"github.com/AdguardTeam/AdGuardHome/internal/next/agh"
+	"github.com/AdguardTeam/AdGuardHome/internal/next/configmgr"
 	"github.com/AdguardTeam/golibs/log"
+	"github.com/google/renameio/maybe"
 )
 
 // signalHandler processes incoming signals and shuts services down.
 type signalHandler struct {
+	// confMgrConf contains the configuration parameters for the configuration
+	// manager.
+	confMgrConf *configmgr.Config
+
 	// signal is the channel to which OS signals are sent.
 	signal chan os.Signal
 
-	// confFile is the path to the configuration file.
-	confFile string
-
-	// frontend is the filesystem with the frontend and other statically
-	// compiled files.
-	frontend fs.FS
-
-	// start is the time at which AdGuard Home has been started.
-	start time.Time
+	// pidFile is the path to the file where to store the PID, if any.
+	pidFile string
 
 	// services are the services that are shut down before application exiting.
 	services []agh.Service
@@ -33,6 +31,8 @@ type signalHandler struct {
 func (h *signalHandler) handle() {
 	defer log.OnPanic("signalHandler.handle")
 
+	h.writePID()
+
 	for sig := range h.signal {
 		log.Info("sighdlr: received signal %q", sig)
 
@@ -40,6 +40,8 @@ func (h *signalHandler) handle() {
 			h.reconfigure()
 		} else if aghos.IsShutdownSignal(sig) {
 			status := h.shutdown()
+			h.removePID()
+
 			log.Info("sighdlr: exiting with status %d", status)
 
 			os.Exit(status)
@@ -62,7 +64,7 @@ func (h *signalHandler) reconfigure() {
 	// reconfigured without the full shutdown, and the error handling is
 	// currently not the best.
 
-	confMgr, err := newConfigMgr(h.confFile, h.frontend, h.start)
+	confMgr, err := newConfigMgr(h.confMgrConf)
 	check(err)
 
 	web := confMgr.Web()
@@ -83,8 +85,9 @@ func (h *signalHandler) reconfigure() {
 
 // Exit status constants.
 const (
-	statusSuccess = 0
-	statusError   = 1
+	statusSuccess       = 0
+	statusError         = 1
+	statusArgumentError = 2
 )
 
 // shutdown gracefully shuts down all services.
@@ -108,17 +111,15 @@ func (h *signalHandler) shutdown() (status int) {
 
 // newSignalHandler returns a new signalHandler that shuts down svcs.
 func newSignalHandler(
-	confFile string,
-	frontend fs.FS,
-	start time.Time,
+	confMgrConf *configmgr.Config,
+	pidFile string,
 	svcs ...agh.Service,
 ) (h *signalHandler) {
 	h = &signalHandler{
-		signal:   make(chan os.Signal, 1),
-		confFile: confFile,
-		frontend: frontend,
-		start:    start,
-		services: svcs,
+		confMgrConf: confMgrConf,
+		signal:      make(chan os.Signal, 1),
+		pidFile:     pidFile,
+		services:    svcs,
 	}
 
 	aghos.NotifyShutdownSignal(h.signal)
@@ -126,3 +127,41 @@ func newSignalHandler(
 
 	return h
 }
+
+// writePID writes the PID to the file, if needed.  Any errors are reported to
+// log.
+func (h *signalHandler) writePID() {
+	if h.pidFile == "" {
+		return
+	}
+
+	// Use 8, since most PIDs will fit.
+	data := make([]byte, 0, 8)
+	data = strconv.AppendInt(data, int64(os.Getpid()), 10)
+	data = append(data, '\n')
+
+	err := maybe.WriteFile(h.pidFile, data, 0o644)
+	if err != nil {
+		log.Error("sighdlr: writing pidfile: %s", err)
+
+		return
+	}
+
+	log.Debug("sighdlr: wrote pid to %q", h.pidFile)
+}
+
+// removePID removes the PID file, if any.
+func (h *signalHandler) removePID() {
+	if h.pidFile == "" {
+		return
+	}
+
+	err := os.Remove(h.pidFile)
+	if err != nil {
+		log.Error("sighdlr: removing pidfile: %s", err)
+
+		return
+	}
+
+	log.Debug("sighdlr: removed pid at %q", h.pidFile)
+}