Testutil falls back to random ports w/o porter (#3604)

* Testutil falls back to random ports w/o porter

This PR allows the testutil server to be used without porter.

* Adds sterner-sounding fallback comments.

test/porter/client.go

@@ -9,7 +9,28 @@ import (
 	"strings"
 )
 
-var DefaultAddr = "127.0.0.1:7965"
+var (
+	// DefaultAddr is the the default bind address of a Porter server. This acts
+	// as the fallback address if the Porter server is not specified.
+	DefaultAddr = "127.0.0.1:7965"
+)
+
+const (
+	// porterErrPrefix is the string returned when displaying a porter error
+	porterErrPrefix = `Are you running porter?
+Install with 'go install github.com/hashicorp/consul/test/porter/cmd/porter'
+Then run 'porter go test ...'`
+)
+
+// PorterExistErr is used to wrap an error that is likely from Porter not being
+// run.
+type PorterExistErr struct {
+	Wrapped error
+}
+
+func (p *PorterExistErr) Error() string {
+	return fmt.Sprintf("%s:\n%s", porterErrPrefix, p.Wrapped)
+}
 
 func RandomPorts(n int) ([]int, error) {
 	addr := os.Getenv("PORTER_ADDR")
@@ -25,7 +46,7 @@ func RandomPorts(n int) ([]int, error) {
 	resp, err := http.Get(fmt.Sprintf("http://%s/%d", addr, n))
 	if err != nil {
 		if strings.Contains(err.Error(), "connection refused") {
-			return nil, fmt.Errorf("Are you running porter?\nInstall with 'go install github.com/hashicorp/consul/test/porter/cmd/porter'\nThen run 'porter go test ...'\n%s", err)
+			return nil, &PorterExistErr{Wrapped: err}
 		}
 		return nil, err
 	}

testutil/server.go

@@ -113,7 +113,14 @@ func defaultServerConfig() *TestServerConfig {
 
 	ports, err := porter.RandomPorts(6)
 	if err != nil {
-		panic(err)
+		if _, ok := err.(*porter.PorterExistErr); ok {
+			// Fall back in the case that the testutil server is being used
+			// without porter. This should NEVER be used for Consul's own
+			// unit tests. See comments for getRandomPorts() for more details.
+			ports = getRandomPorts(6)
+		} else {
+			panic(err)
+		}
 	}
 	return &TestServerConfig{
 		NodeName:          "node-" + nodeID,
@@ -383,3 +390,22 @@ func (s *TestServer) waitForLeader() error {
 	}
 	return nil
 }
+
+// getRandomPorts returns a set of random port or panics on error. This
+// is here to support external uses of testutil which may not have porter,
+// but this has been shown not to work well with parallel tests (such as
+// Consul's unit tests). This fallback should NEVER be used for Consul's
+// own tests.
+func getRandomPorts(n int) []int {
+	ports := make([]int, n)
+	for i := 0; i < n; i++ {
+		l, err := net.Listen("tcp", ":0")
+		if err != nil {
+			panic(err)
+		}
+		l.Close()
+		ports[i] = l.Addr().(*net.TCPAddr).Port
+	}
+
+	return ports
+}