Write pidfile only if server starts successfully

Whether the original parent process or a child process as part of a restart, the pidfile will not be written/changed until that process has started successfully. It is written every time caddy.Start() succeeds (may be reundant, but that's probably okay).

caddy/caddy.go

@@ -23,6 +23,7 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"net"
 	"os"
 	"path"
@@ -44,6 +45,9 @@ var (
 
 	// 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 string
 )
 
 var (
@@ -72,8 +76,8 @@ var (
 	// variable is not safe for concurrent access.
 	loadedGob caddyfileGob
 
-	// startedBefore should be set to true if caddy has been
-	// started at least once.
+	// startedBefore should be set to true if caddy has been started
+	// at least once (does not indicate whether currently running).
 	startedBefore bool
 )
 
@@ -99,9 +103,18 @@ const (
 // In any case, an error is returned if Caddy could not be
 // started.
 func Start(cdyfile Input) (err error) {
-	defer func() { signalParent(err == nil) }()
-
-	// TODO: What if already started -- is that an error?
+	// When we return, tell the parent whether we started
+	// successfully, and if so, write the pidfile (if enabled)
+	defer func() {
+		success := err == nil
+		signalParent(success)
+		if success && PidFile != "" {
+			err := writePidFile()
+			if err != nil {
+				log.Printf("[ERROR] Could not write pidfile: %v", err)
+			}
+		}
+	}()
 
 	// Input must never be nil; try to load something
 	if cdyfile == nil {

caddy/helpers.go

@@ -3,6 +3,7 @@ package caddy
 import (
 	"bytes"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"os/exec"
 	"runtime"
@@ -67,6 +68,12 @@ func IsRestart() bool {
 	return os.Getenv("CADDY_RESTART") == "true"
 }
 
+// writePidFile writes the process ID to the file at PidFile, if specified.
+func writePidFile() error {
+	pid := []byte(strconv.Itoa(os.Getpid()) + "\n")
+	return ioutil.WriteFile(PidFile, pid, 0644)
+}
+
 // CaddyfileInput represents a Caddyfile as input
 // and is simply a convenient way to implement
 // the Input interface.

main.go

@@ -19,7 +19,6 @@ var (
 	conf    string
 	cpu     string
 	logfile string
-	pidfile string
 	revoke  string
 	version bool
 )
@@ -38,7 +37,7 @@ func init() {
 	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")
-	flag.StringVar(&pidfile, "pidfile", "", "Path to write pid file")
+	flag.StringVar(&caddy.PidFile, "pidfile", "", "Path to write pid file")
 	flag.StringVar(&caddy.Port, "port", caddy.DefaultPort, "Default port")
 	flag.BoolVar(&caddy.Quiet, "quiet", false, "Quiet mode (no initialization output)")
 	flag.StringVar(&revoke, "revoke", "", "Hostname for which to revoke the certificate")
@@ -68,13 +67,6 @@ func main() {
 		log.SetOutput(file)
 	}
 
-	if pidfile != "" {
-		pid := []byte(strconv.Itoa(os.Getpid()) + "\n")
-		err := ioutil.WriteFile(pidfile, pid, 0644)
-		if err != nil {
-			log.Fatal(err)
-		}
-	}
 	if revoke != "" {
 		err := letsencrypt.Revoke(revoke)
 		if err != nil {