tests: make test servers choose a random port to make more reliable

Tests have been randomly failing with messages like

    listen tcp 127.0.0.1:51778: bind: address already in use

Rework all the test servers so they choose a random free port on
startup and use that for the tests to avoid.

cmd/serve/dlna/dlna_test.go

@@ -20,11 +20,11 @@ import (
 
 var (
 	dlnaServer *server
+	testURL    string
 )
 
 const (
-	testBindAddress = "localhost:51777"
-	testURL         = "http://" + testBindAddress + "/"
+	testBindAddress = "localhost:0"
 )
 
 func startServer(t *testing.T, f fs.Fs) {
@@ -32,6 +32,7 @@ func startServer(t *testing.T, f fs.Fs) {
 	opt.ListenAddr = testBindAddress
 	dlnaServer = newServer(f, &opt)
 	assert.NoError(t, dlnaServer.Serve())
+	testURL = "http://" + dlnaServer.HTTPConn.Addr().String() + "/"
 }
 
 func TestInit(t *testing.T) {

cmd/serve/http/http_test.go

@@ -3,7 +3,6 @@ package http
 import (
 	"flag"
 	"io/ioutil"
-	"net"
 	"net/http"
 	"strings"
 	"testing"
@@ -21,11 +20,11 @@ import (
 var (
 	updateGolden = flag.Bool("updategolden", false, "update golden files for regression test")
 	httpServer   *server
+	testURL      string
 )
 
 const (
-	testBindAddress = "localhost:51777"
-	testURL         = "http://" + testBindAddress + "/"
+	testBindAddress = "localhost:0"
 )
 
 func startServer(t *testing.T, f fs.Fs) {
@@ -33,13 +32,14 @@ func startServer(t *testing.T, f fs.Fs) {
 	opt.ListenAddr = testBindAddress
 	httpServer = newServer(f, &opt)
 	assert.NoError(t, httpServer.Serve())
+	testURL = httpServer.Server.URL()
 
 	// try to connect to the test server
 	pause := time.Millisecond
 	for i := 0; i < 10; i++ {
-		conn, err := net.Dial("tcp", testBindAddress)
+		resp, err := http.Head(testURL)
 		if err == nil {
-			_ = conn.Close()
+			_ = resp.Body.Close()
 			return
 		}
 		// t.Logf("couldn't connect, sleeping for %v: %v", pause, err)

cmd/serve/restic/restic_test.go

@@ -17,8 +17,7 @@ import (
 )
 
 const (
-	testBindAddress = "localhost:51779"
-	testURL         = "http://" + testBindAddress + "/"
+	testBindAddress = "localhost:0"
 	resticSource    = "../../../../../restic/restic"
 )
 
@@ -62,7 +61,7 @@ func TestRestic(t *testing.T) {
 		}
 		cmd := exec.Command("go", args...)
 		cmd.Env = append(os.Environ(),
-			"RESTIC_TEST_REST_REPOSITORY=rest:"+testURL+path,
+			"RESTIC_TEST_REST_REPOSITORY=rest:"+w.Server.URL()+path,
 		)
 		out, err := cmd.CombinedOutput()
 		if len(out) != 0 {

cmd/serve/webdav/webdav_test.go

@@ -20,8 +20,7 @@ import (
 )
 
 const (
-	testBindAddress = "localhost:51778"
-	testURL         = "http://" + testBindAddress + "/"
+	testBindAddress = "localhost:0"
 )
 
 // check interfaces
@@ -70,7 +69,7 @@ func TestWebDav(t *testing.T) {
 	cmd := exec.Command("go", args...)
 	cmd.Env = append(os.Environ(),
 		"RCLONE_CONFIG_WEBDAVTEST_TYPE=webdav",
-		"RCLONE_CONFIG_WEBDAVTEST_URL="+testURL,
+		"RCLONE_CONFIG_WEBDAVTEST_URL="+w.Server.URL(),
 		"RCLONE_CONFIG_WEBDAVTEST_VENDOR=other",
 	)
 	out, err := cmd.CombinedOutput()

fs/rc/rcserver/rcserver_test.go

@@ -18,8 +18,7 @@ import (
 )
 
 const (
-	testBindAddress = "localhost:51781"
-	testURL         = "http://" + testBindAddress + "/"
+	testBindAddress = "localhost:0"
 	testFs          = "testdata/files"
 	remoteURL       = "[" + testFs + "]/" // initial URL path to fetch from that remote
 )
@@ -39,6 +38,7 @@ func TestRcServer(t *testing.T) {
 		rcServer.Close()
 		rcServer.Wait()
 	}()
+	testURL := rcServer.Server.URL()
 
 	// Do the simplest possible test to check the server is alive
 	// Do it a few times to wait for the server to start