Add default Prometheus port

We have a port assigned, so might as well default to it.

Fixes: #74

main.go

@@ -28,7 +28,15 @@ func main() {
 	quiet := flag.Bool("quiet", false, "do not log informational messages (takes precedence over debug)")
 	json := flag.Bool("json", false, "enable JSON logging")
 	test := flag.Bool("test", false, "test crontab (does not run jobs)")
-	prometheusListen := flag.String("prometheus-listen-address", "", "give a valid ip:port address to expose Prometheus metrics at /metrics")
+	prometheusListen := flag.String(
+		"prometheus-listen-address",
+		"",
+		fmt.Sprintf(
+			"give a valid ip[:port] address to expose Prometheus metrics at /metrics (port defaults to %s), "+
+				"use 0.0.0.0 for all network interfaces.",
+			prometheus_metrics.DefaultPort,
+		),
+	)
 	splitLogs := flag.Bool("split-logs", false, "split log output into stdout/stderr")
 	passthroughLogs := flag.Bool("passthrough-logs", false, "passthrough logs from commands, do not wrap them in Supercronic logging")
 	sentry := flag.String("sentry-dsn", "", "enable Sentry error logging, using provided DSN")

prometheus_metrics/prommetrics.go

@@ -2,6 +2,7 @@ package prometheus_metrics
 
 import (
 	"context"
+	"fmt"
 	"net"
 	"net/http"
 
@@ -11,7 +12,8 @@ import (
 )
 
 const (
-	namespace = "supercronic"
+	DefaultPort = "9746"
+	namespace   = "supercronic"
 )
 
 func genMetricName(name string) string {
@@ -99,7 +101,40 @@ func (p *PrometheusMetrics) Reset() {
 	p.CronsExecutionTimeHistogram.Reset()
 }
 
+func getAddr(listenAddr string) (string, error) {
+	if listenAddr == "" {
+		return "", fmt.Errorf("Not address provided")
+	}
+
+	// If the address is fine as-is, use it
+	_, _, err1 := net.SplitHostPort(listenAddr)
+	if err1 == nil {
+		return listenAddr, nil
+	}
+
+	// Otherwise, try to add the port
+	listenAddrWithPort := net.JoinHostPort(listenAddr, DefaultPort)
+	_, _, err2 := net.SplitHostPort(listenAddrWithPort)
+	if err2 == nil {
+		return listenAddrWithPort, nil
+	}
+
+	return "", fmt.Errorf(
+		"%s is not a valid address (%v), and neither is %s after adding the default port (%v)",
+		listenAddr,
+		err1,
+		listenAddrWithPort,
+		err2,
+	)
+
+}
+
 func InitHTTPServer(listenAddr string, shutdownContext context.Context) (func() error, error) {
+	addr, err := getAddr(listenAddr)
+	if err != nil {
+		return nil, err
+	}
+
 	promSrv := &http.Server{}
 
 	http.Handle("/metrics", promhttp.Handler())
@@ -118,7 +153,7 @@ func InitHTTPServer(listenAddr string, shutdownContext context.Context) (func()
 		return promSrv.Shutdown(shutdownContext)
 	}
 
-	listener, err := net.Listen("tcp", listenAddr)
+	listener, err := net.Listen("tcp", addr)
 	if err != nil {
 		return shutdownClosure, err
 	}

prometheus_metrics/prommetrics_test.go

@@ -0,0 +1,45 @@
+package prometheus_metrics
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetAddr(t *testing.T) {
+	addr, err := getAddr("127.0.0.1:123")
+	if assert.Nil(t, err) {
+		assert.Equal(t, "127.0.0.1:123", addr)
+	}
+
+	addr, err = getAddr("127.0.0.1")
+	if assert.Nil(t, err) {
+		assert.Equal(t, "127.0.0.1:9746", addr)
+	}
+
+	addr, err = getAddr("[127.0.0.1]")
+	if assert.Nil(t, err) {
+		assert.Equal(t, "[127.0.0.1]:9746", addr)
+	}
+
+	addr, err = getAddr("[::]:123")
+	if assert.Nil(t, err) {
+		assert.Equal(t, "[::]:123", addr)
+	}
+
+	addr, err = getAddr("::")
+	if assert.Nil(t, err) {
+		assert.Equal(t, "[::]:9746", addr)
+	}
+
+	addr, err = getAddr("0.0.0.0")
+	if assert.Nil(t, err) {
+		assert.Equal(t, "0.0.0.0:9746", addr)
+	}
+
+	_, err = getAddr("")
+	assert.NotNil(t, err)
+
+	_, err = getAddr("[::]")
+	assert.NotNil(t, err)
+}