New -log flag to direct log output to desired place

Log file can also be stdout or stderr. Log output is disabled by default now, which makes it more feasible to add more log statements to trace program flow in debugging situations.

caddy/caddy.go

@@ -23,7 +23,6 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
-	"log"
 	"net"
 	"os"
 	"path"
@@ -183,25 +182,25 @@ func startServers(groupings Group) error {
 	for _, group := range groupings {
 		s, err := server.New(group.BindAddr.String(), group.Configs)
 		if err != nil {
-			log.Fatal(err)
+			return err
 		}
 		s.HTTP2 = HTTP2 // TODO: This setting is temporary
 
 		var ln server.ListenerFile
 		if IsRestart() {
 			// Look up this server's listener in the map of inherited file descriptors;
-			// if we don't have one, we must make a new one.
+			// if we don't have one, we must make a new one (later).
 			if fdIndex, ok := loadedGob.ListenerFds[s.Addr]; ok {
 				file := os.NewFile(fdIndex, "")
 
 				fln, err := net.FileListener(file)
 				if err != nil {
-					log.Fatal(err)
+					return err
 				}
 
 				ln, ok = fln.(server.ListenerFile)
 				if !ok {
-					log.Fatal("listener was not a ListenerFile")
+					return errors.New("listener for " + s.Addr + " was not a ListenerFile")
 				}
 
 				delete(loadedGob.ListenerFds, s.Addr) // mark it as used

main.go

@@ -20,6 +20,7 @@ var (
 	cpu     string
 	version bool
 	revoke  string
+	logfile string
 )
 
 const (
@@ -43,14 +44,31 @@ func init() {
 	flag.BoolVar(&letsencrypt.Agreed, "agree", false, "Agree to Let's Encrypt Subscriber Agreement")
 	flag.StringVar(&letsencrypt.DefaultEmail, "email", "", "Default Let's Encrypt account email address")
 	flag.StringVar(&revoke, "revoke", "", "Hostname for which to revoke the certificate")
+	flag.StringVar(&logfile, "log", "", "Process log file")
 }
 
 func main() {
-	flag.Parse()
+	flag.Parse() // called here in main() to allow other packages to set flags in their inits
 
 	caddy.AppName = appName
 	caddy.AppVersion = appVersion
 
+	// set up process log before anything bad happens
+	switch logfile {
+	case "stdout":
+		log.SetOutput(os.Stdout)
+	case "stderr":
+		log.SetOutput(os.Stderr)
+	case "":
+		log.SetOutput(ioutil.Discard)
+	default:
+		file, err := os.Create(logfile)
+		if err != nil {
+			log.Fatalf("Error opening log file: %v", err)
+		}
+		log.SetOutput(file)
+	}
+
 	if version {
 		fmt.Printf("%s %s\n", caddy.AppName, caddy.AppVersion)
 		os.Exit(0)
@@ -67,13 +85,13 @@ func main() {
 	// Set CPU cap
 	err := setCPU(cpu)
 	if err != nil {
-		log.Fatal(err)
+		mustLogFatal(err)
 	}
 
 	// Get Caddyfile input
 	caddyfile, err := caddy.LoadCaddyfile(loadCaddyfile)
 	if err != nil {
-		log.Fatal(err)
+		mustLogFatal(err)
 	}
 
 	// Start your engines
@@ -82,7 +100,7 @@ func main() {
 		if caddy.IsRestart() {
 			log.Println("error starting servers:", err)
 		} else {
-			log.Fatal(err)
+			mustLogFatal(err)
 		}
 	}
 
@@ -90,6 +108,14 @@ func main() {
 	caddy.Wait()
 }
 
+// mustLogFatal just wraps log.Fatal() in a way that ensures the
+// output is always printed to stderr so the user can see it,
+// even if the process log was not enabled.
+func mustLogFatal(args ...interface{}) {
+	log.SetOutput(os.Stderr)
+	log.Fatal(args...)
+}
+
 func loadCaddyfile() (caddy.Input, error) {
 	// -conf flag
 	if conf != "" {