Use an error constant for "nil client" errors (#90)

* Define a constant error for nil clients & return that

This should allow tests and other code that relied on some
predictable behavior on nil client usage to still receive that
behavior: Instead of `nil`, we just return ErrNoClient now.

* Add a test case for ErrNoClient behavior

* Add four methods not covered by the test

* Incr, Decr
* Timing, TimeInMilliseconds

* Remove TestNilSafe in favor of TestNilError

By using assertNotPanics in the test case, we can assert that it
doesn't panic just as well, and it doesn't require maintaining the
same list of function calls in two tests.

statsd/statsd.go

@@ -67,6 +67,16 @@ const (
 	entityIDTagName = "dd.internal.entity_id"
 )
 
+type noClientErr string
+
+// ErrNoClient is returned if statsd reporting methods are invoked on
+// a nil client.
+const ErrNoClient = noClientErr("statsd client is nil")
+
+func (e noClientErr) Error() string {
+	return string(e)
+}
+
 /*
 Stat suffixes
 */
@@ -220,7 +230,7 @@ func (c *Client) format(name string, value interface{}, suffix []byte, tags []st
 // SetWriteTimeout allows the user to set a custom UDS write timeout. Not supported for UDP.
 func (c *Client) SetWriteTimeout(d time.Duration) error {
 	if c == nil {
-		return fmt.Errorf("Client is nil")
+		return ErrNoClient
 	}
 	return c.writer.SetWriteTimeout(d)
 }
@@ -307,7 +317,7 @@ func copyAndResetBuffer(buf *bytes.Buffer) []byte {
 // Flush forces a flush of the pending commands in the buffer
 func (c *Client) Flush() error {
 	if c == nil {
-		return fmt.Errorf("Client is nil")
+		return ErrNoClient
 	}
 	c.Lock()
 	defer c.Unlock()
@@ -361,7 +371,7 @@ func (c *Client) sendMsg(msg []byte) error {
 // send handles sampling and sends the message over UDP. It also adds global namespace prefixes and tags.
 func (c *Client) send(name string, value interface{}, suffix []byte, tags []string, rate float64) error {
 	if c == nil {
-		return fmt.Errorf("Client is nil")
+		return ErrNoClient
 	}
 	if rate < 1 && rand.Float64() > rate {
 		return nil
@@ -419,7 +429,7 @@ func (c *Client) TimeInMilliseconds(name string, value float64, tags []string, r
 // Event sends the provided Event.
 func (c *Client) Event(e *Event) error {
 	if c == nil {
-		return fmt.Errorf("Client is nil")
+		return ErrNoClient
 	}
 	stat, err := e.Encode(c.Tags...)
 	if err != nil {
@@ -437,7 +447,7 @@ func (c *Client) SimpleEvent(title, text string) error {
 // ServiceCheck sends the provided ServiceCheck.
 func (c *Client) ServiceCheck(sc *ServiceCheck) error {
 	if c == nil {
-		return fmt.Errorf("Client is nil")
+		return ErrNoClient
 	}
 	stat, err := sc.Encode(c.Tags...)
 	if err != nil {
@@ -455,7 +465,7 @@ func (c *Client) SimpleServiceCheck(name string, status ServiceCheckStatus) erro
 // Close the client connection.
 func (c *Client) Close() error {
 	if c == nil {
-		return fmt.Errorf("Client is nil")
+		return ErrNoClient
 	}
 	select {
 	case c.stop <- struct{}{}:

statsd/statsd_test.go

@@ -537,23 +537,34 @@ func TestSendMsgUDP(t *testing.T) {
 	}
 }
 
-func TestNilSafe(t *testing.T) {
+func TestNilError(t *testing.T) {
 	var c *Client
-	assertNotPanics(t, func() { c.SetWriteTimeout(0) })
-	assertNotPanics(t, func() { c.Flush() })
-	assertNotPanics(t, func() { c.Close() })
-	assertNotPanics(t, func() { c.Count("", 0, nil, 1) })
-	assertNotPanics(t, func() { c.Histogram("", 0, nil, 1) })
-	assertNotPanics(t, func() { c.Distribution("", 0, nil, 1) })
-	assertNotPanics(t, func() { c.Gauge("", 0, nil, 1) })
-	assertNotPanics(t, func() { c.Set("", "", nil, 1) })
-	assertNotPanics(t, func() {
-		c.send("", "", []byte(""), nil, 1)
-	})
-	assertNotPanics(t, func() { c.Event(NewEvent("", "")) })
-	assertNotPanics(t, func() { c.SimpleEvent("", "") })
-	assertNotPanics(t, func() { c.ServiceCheck(NewServiceCheck("", Ok)) })
-	assertNotPanics(t, func() { c.SimpleServiceCheck("", Ok) })
+	tests := []func() error{
+		func() error { return c.SetWriteTimeout(0) },
+		func() error { return c.Flush() },
+		func() error { return c.Close() },
+		func() error { return c.Count("", 0, nil, 1) },
+		func() error { return c.Incr("", nil, 1) },
+		func() error { return c.Decr("", nil, 1) },
+		func() error { return c.Histogram("", 0, nil, 1) },
+		func() error { return c.Distribution("", 0, nil, 1) },
+		func() error { return c.Gauge("", 0, nil, 1) },
+		func() error { return c.Set("", "", nil, 1) },
+		func() error { return c.Timing("", time.Second, nil, 1) },
+		func() error { return c.TimeInMilliseconds("", 1, nil, 1) },
+		func() error { return c.send("", "", []byte(""), nil, 1) },
+		func() error { return c.Event(NewEvent("", "")) },
+		func() error { return c.SimpleEvent("", "") },
+		func() error { return c.ServiceCheck(NewServiceCheck("", Ok)) },
+		func() error { return c.SimpleServiceCheck("", Ok) },
+	}
+	for i, f := range tests {
+		var err error
+		assertNotPanics(t, func() { err = f() })
+		if err != ErrNoClient {
+			t.Errorf("Test case %d: expected ErrNoClient, got %#v", i, err)
+		}
+	}
 }
 
 func TestEvents(t *testing.T) {