adding a MainErr function to nsqlookupd, for package importers to get…

… error info

fixes #1003

nsqlookupd/nsqlookupd.go

@@ -1,6 +1,7 @@
 package nsqlookupd
 
 import (
+	"fmt"
 	"log"
 	"net"
 	"os"
@@ -42,13 +43,23 @@ func New(opts *Options) *NSQLookupd {
 	return n
 }
 
+// Main runs the service and calls os.Exit(1) if an error occurs.
 func (l *NSQLookupd) Main() {
+	err := l.MainErr()
+	if err != nil {
+		l.logf(LOG_FATAL, "%v", err)
+		os.Exit(1)
+	}
+}
+
+// MainErr returns an error immediately if there was a problem starting up, but
+// otherwise blocks indefinitely until Exit is called.
+func (l *NSQLookupd) MainErr() error {
 	ctx := &Context{l}
 
 	tcpListener, err := net.Listen("tcp", l.opts.TCPAddress)
 	if err != nil {
-		l.logf(LOG_FATAL, "listen (%s) failed - %s", l.opts.TCPAddress, err)
-		os.Exit(1)
+		return fmt.Errorf("listen (%s) failed - %s", l.opts.TCPAddress, err)
 	}
 	l.Lock()
 	l.tcpListener = tcpListener
@@ -60,8 +71,7 @@ func (l *NSQLookupd) Main() {
 
 	httpListener, err := net.Listen("tcp", l.opts.HTTPAddress)
 	if err != nil {
-		l.logf(LOG_FATAL, "listen (%s) failed - %s", l.opts.HTTPAddress, err)
-		os.Exit(1)
+		return fmt.Errorf("listen (%s) failed - %s", l.opts.TCPAddress, err)
 	}
 	l.Lock()
 	l.httpListener = httpListener
@@ -70,6 +80,8 @@ func (l *NSQLookupd) Main() {
 	l.waitGroup.Wrap(func() {
 		http_api.Serve(httpListener, httpServer, "HTTP", l.logf)
 	})
+
+	return nil
 }
 
 func (l *NSQLookupd) RealTCPAddr() *net.TCPAddr {