[chore][receiver/statsd] Refactor of hardcoded tests (#28896)

**Description:** 
This is the first step to adding UDS support to the StatsD receiver.

As I started to develop it, I saw that all tests were hardcoded to UDP
and using networking and there was no possibility to add a socket
communication.

PR started to get huge so I split it into two, one to refactor tests and
another one that adds UDS support:
 * Removed all unused references.
* Made a `Transport` helper that allows centralizing all supported
protocols and constants in its package.
* Removed all hardcoded UDP protocols and generalized testing so new
protocols are easy to add.

If you need a rationale about why the changes are like this, this is the
next PR I am going to submit after this one is merged:
kilokang#2

That is the PR that is going to add UDS support properly.

**Link to tracking Issue:**
 - #21385

**Previous closed PR:**
 - #24832

receiver/statsdreceiver/config_test.go

@@ -93,6 +93,7 @@ func TestValidate(t *testing.T) {
 		noObjectNameErr                = "must specify object id for all TimerHistogramMappings"
 		statsdTypeNotSupportErr        = "statsd_type is not a supported mapping for histogram and timing metrics: %s"
 		observerTypeNotSupportErr      = "observer_type is not supported for histogram and timing metrics: %s"
+		invalidHistogramErr            = "histogram configuration requires observer_type: histogram"
 	)
 
 	tests := []test{
@@ -160,7 +161,7 @@ func TestValidate(t *testing.T) {
 					},
 				},
 			},
-			expectedErr: "histogram configuration requires observer_type: histogram",
+			expectedErr: invalidHistogramErr,
 		},
 		{
 			name: "negativeAggregationInterval",
@@ -170,7 +171,7 @@ func TestValidate(t *testing.T) {
 					{StatsdType: "timing", ObserverType: "gauge"},
 				},
 			},
-			expectedErr: "aggregation_interval must be a positive duration",
+			expectedErr: negativeAggregationIntervalErr,
 		},
 	}
 

receiver/statsdreceiver/internal/transport/client/client.go

@@ -7,90 +7,72 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"strings"
 )
 
 // StatsD defines the properties of a StatsD connection.
 type StatsD struct {
-	Host string
-	Port int
-	Conn io.Writer
+	transport string
+	address   string
+	conn      io.Writer
 }
 
-// Transport is an enum to select the type of transport.
-type Transport int
-
-const (
-	// TCP Transport
-	TCP Transport = iota
-	// UDP Transport
-	UDP
-)
-
 // NewStatsD creates a new StatsD instance to support the need for testing
 // the statsdreceiver package and is not intended/tested to be used in production.
-func NewStatsD(transport Transport, host string, port int) (*StatsD, error) {
+func NewStatsD(transport string, address string) (*StatsD, error) {
 	statsd := &StatsD{
-		Host: host,
-		Port: port,
+		transport: transport,
+		address:   address,
 	}
-	err := statsd.connect(transport)
+
+	err := statsd.connect()
 	if err != nil {
 		return nil, err
 	}
 
 	return statsd, nil
 }
 
-// connect populates the StatsD.Conn
-func (s *StatsD) connect(transport Transport) error {
-	if cl, ok := s.Conn.(io.Closer); ok {
-		err := cl.Close()
+// connect populates the StatsD.conn
+func (s *StatsD) connect() error {
+	switch s.transport {
+	case "udp":
+		udpAddr, err := net.ResolveUDPAddr(s.transport, s.address)
 		if err != nil {
 			return err
 		}
-	}
-
-	address := fmt.Sprintf("%s:%d", s.Host, s.Port)
-
-	var err error
-	switch transport {
-	case TCP:
-		s.Conn, err = net.Dial("tcp", address)
-		if err != nil {
-			return err
-		}
-	case UDP:
-		var udpAddr *net.UDPAddr
-		udpAddr, err = net.ResolveUDPAddr("udp", address)
+		s.conn, err = net.DialUDP(s.transport, nil, udpAddr)
 		if err != nil {
 			return err
 		}
-		s.Conn, err = net.DialUDP("udp", nil, udpAddr)
+	case "tcp":
+		var err error
+		s.conn, err = net.Dial(s.transport, s.address)
 		if err != nil {
 			return err
 		}
 	default:
-		return fmt.Errorf("unknown transport: %d", transport)
+		return fmt.Errorf("unknown/unsupported transport: %s", s.transport)
 	}
 
-	return err
+	return nil
 }
 
-// Disconnect closes the StatsD.Conn.
+// Disconnect closes the StatsD.conn.
 func (s *StatsD) Disconnect() error {
 	var err error
-	if cl, ok := s.Conn.(io.Closer); ok {
+	if cl, ok := s.conn.(io.Closer); ok {
 		err = cl.Close()
 	}
-	s.Conn = nil
+	s.conn = nil
 	return err
 }
 
 // SendMetric sends the input metric to the StatsD connection.
 func (s *StatsD) SendMetric(metric Metric) error {
-	_, err := fmt.Fprint(s.Conn, metric.String())
+	_, err := io.Copy(s.conn, strings.NewReader(metric.String()))
 	if err != nil {
-		return err
+		return fmt.Errorf("send metric on test client: %w", err)
 	}
 	return nil
 }

receiver/statsdreceiver/internal/transport/server.go

@@ -8,8 +8,6 @@ import (
 	"net"
 
 	"go.opentelemetry.io/collector/consumer"
-
-	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/statsdreceiver/internal/protocol"
 )
 
 var errNilListenAndServeParameters = errors.New("no parameter of ListenAndServe can be nil")
@@ -21,7 +19,6 @@ type Server interface {
 	// on the specific transport, and prepares the message to be processed by
 	// the Parser and passed to the next consumer.
 	ListenAndServe(
-		p protocol.Parser,
 		mc consumer.Metrics,
 		r Reporter,
 		transferChan chan<- Metric,

receiver/statsdreceiver/internal/transport/server_test.go

@@ -4,9 +4,9 @@
 package transport
 
 import (
+	"io"
 	"net"
 	"runtime"
-	"strconv"
 	"sync"
 	"testing"
 	"time"
@@ -16,65 +16,43 @@ import (
 	"go.opentelemetry.io/collector/consumer/consumertest"
 
 	"github.com/open-telemetry/opentelemetry-collector-contrib/internal/common/testutil"
-	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/statsdreceiver/internal/protocol"
 	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/statsdreceiver/internal/transport/client"
 )
 
 func Test_Server_ListenAndServe(t *testing.T) {
 	tests := []struct {
-		name          string
-		transport     string
-		buildServerFn func(addr string) (Server, error)
-		buildClientFn func(host string, port int) (*client.StatsD, error)
+		name              string
+		transport         Transport
+		buildServerFn     func(transport Transport, addr string) (Server, error)
+		getFreeEndpointFn func(t testing.TB, transport string) string
+		buildClientFn     func(transport string, address string) (*client.StatsD, error)
 	}{
 		{
-			name:          "udp",
-			transport:     "udp",
-			buildServerFn: NewUDPServer,
-			buildClientFn: func(host string, port int) (*client.StatsD, error) {
-				return client.NewStatsD(client.UDP, host, port)
-			},
+			name:              "udp",
+			transport:         UDP,
+			getFreeEndpointFn: testutil.GetAvailableLocalNetworkAddress,
+			buildServerFn:     NewUDPServer,
+			buildClientFn:     client.NewStatsD,
 		},
 		{
-			name:          "tcp",
-			transport:     "tcp",
-			buildServerFn: NewTCPServer,
-			buildClientFn: func(host string, port int) (*client.StatsD, error) {
-				return client.NewStatsD(client.TCP, host, port)
-			},
+			name:              "tcp",
+			transport:         TCP,
+			getFreeEndpointFn: testutil.GetAvailableLocalNetworkAddress,
+			buildServerFn:     NewTCPServer,
+			buildClientFn:     client.NewStatsD,
 		},
 	}
+
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			addr := testutil.GetAvailableLocalNetworkAddress(t, tt.transport)
-
-			if tt.transport == "udp" {
-				// Endpoint should be free.
-				ln0, err := net.ListenPacket("udp", addr)
-				require.NoError(t, err)
-				require.NotNil(t, ln0)
-
-				// Ensure that the endpoint wasn't something like ":0" by checking that a second listener will fail.
-				ln1, err := net.ListenPacket("udp", addr)
-				require.Error(t, err)
-				require.Nil(t, ln1)
+			addr := tt.getFreeEndpointFn(t, tt.name)
+			testFreeEndpoint(t, tt.name, addr)
 
-				// Unbind the local address so the mock UDP service can use it
-				err = ln0.Close()
-				require.NoError(t, err)
-			}
-
-			srv, err := tt.buildServerFn(addr)
+			srv, err := tt.buildServerFn(tt.transport, addr)
 			require.NoError(t, err)
 			require.NotNil(t, srv)
 
-			host, portStr, err := net.SplitHostPort(addr)
-			require.NoError(t, err)
-			port, err := strconv.Atoi(portStr)
-			require.NoError(t, err)
-
 			mc := new(consumertest.MetricsSink)
-			p := &protocol.StatsDParser{}
 			require.NoError(t, err)
 			mr := NewMockReporter(1)
 			transferChan := make(chan Metric, 10)
@@ -83,12 +61,12 @@ func Test_Server_ListenAndServe(t *testing.T) {
 			wgListenAndServe.Add(1)
 			go func() {
 				defer wgListenAndServe.Done()
-				assert.Error(t, srv.ListenAndServe(p, mc, mr, transferChan))
+				assert.Error(t, srv.ListenAndServe(mc, mr, transferChan))
 			}()
 
 			runtime.Gosched()
 
-			gc, err := tt.buildClientFn(host, port)
+			gc, err := tt.buildClientFn(tt.transport.String(), addr)
 			require.NoError(t, err)
 			require.NotNil(t, gc)
 			err = gc.SendMetric(client.Metric{
@@ -115,3 +93,36 @@ func Test_Server_ListenAndServe(t *testing.T) {
 		})
 	}
 }
+
+func testFreeEndpoint(t *testing.T, transport string, address string) {
+	t.Helper()
+
+	var ln0, ln1 io.Closer
+	var err0, err1 error
+
+	trans := NewTransport(transport)
+	require.NotEqual(t, trans, Transport(""))
+
+	if trans.IsPacketTransport() {
+		// Endpoint should be free.
+		ln0, err0 = net.ListenPacket(transport, address)
+		ln1, err1 = net.ListenPacket(transport, address)
+	}
+
+	if trans.IsStreamTransport() {
+		// Endpoint should be free.
+		ln0, err0 = net.Listen(transport, address)
+		ln1, err1 = net.Listen(transport, address)
+	}
+
+	// Endpoint should be free.
+	require.NoError(t, err0)
+	require.NotNil(t, ln0)
+
+	// Ensure that the endpoint wasn't something like ":0" by checking that a second listener will fail.
+	require.Error(t, err1)
+	require.Nil(t, ln1)
+
+	// Unbind the local address so the mock UDP service can use it
+	require.NoError(t, ln0.Close())
+}

receiver/statsdreceiver/internal/transport/tcp_server.go

@@ -6,43 +6,50 @@ package transport // import "github.com/open-telemetry/opentelemetry-collector-c
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"net"
 	"strings"
 	"sync"
 
 	"go.opentelemetry.io/collector/consumer"
-
-	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/statsdreceiver/internal/protocol"
 )
 
 var errTCPServerDone = errors.New("server stopped")
 
 type tcpServer struct {
-	listener net.Listener
-	reporter Reporter
-	wg       sync.WaitGroup
-	stopChan chan struct{}
+	listener  net.Listener
+	reporter  Reporter
+	wg        sync.WaitGroup
+	transport Transport
+	stopChan  chan struct{}
 }
 
+// Ensure that Server is implemented on TCP Server.
 var _ Server = (*tcpServer)(nil)
 
 // NewTCPServer creates a transport.Server using TCP as its transport.
-func NewTCPServer(addr string) (Server, error) {
-	l, err := net.Listen("tcp", addr)
-	if err != nil {
-		return nil, err
+func NewTCPServer(transport Transport, address string) (Server, error) {
+	var tsrv tcpServer
+	var err error
+
+	if !transport.IsStreamTransport() {
+		return nil, fmt.Errorf("NewTCPServer with %s: %w", transport.String(), ErrUnsupportedStreamTransport)
 	}
 
-	t := tcpServer{
-		listener: l,
-		stopChan: make(chan struct{}),
+	tsrv.transport = transport
+	tsrv.listener, err = net.Listen(transport.String(), address)
+	if err != nil {
+		return nil, fmt.Errorf("starting to listen %s socket: %w", transport.String(), err)
 	}
-	return &t, nil
+
+	tsrv.stopChan = make(chan struct{})
+	return &tsrv, nil
 }
 
-func (t *tcpServer) ListenAndServe(parser protocol.Parser, nextConsumer consumer.Metrics, reporter Reporter, transferChan chan<- Metric) error {
-	if parser == nil || nextConsumer == nil || reporter == nil {
+// ListenAndServe starts the server ready to receive metrics.
+func (t *tcpServer) ListenAndServe(nextConsumer consumer.Metrics, reporter Reporter, transferChan chan<- Metric) error {
+	if nextConsumer == nil || reporter == nil {
 		return errNilListenAndServeParameters
 	}
 
@@ -71,6 +78,7 @@ LOOP:
 	return errTCPServerDone
 }
 
+// handleConn is helper that parses the buffer and split it line by line to be parsed upstream.
 func (t *tcpServer) handleConn(c net.Conn, transferChan chan<- Metric) {
 	payload := make([]byte, 4096)
 	var remainder []byte
@@ -98,6 +106,7 @@ func (t *tcpServer) handleConn(c net.Conn, transferChan chan<- Metric) {
 	}
 }
 
+// Close closes the server.
 func (t *tcpServer) Close() error {
 	close(t.stopChan)
 	t.wg.Wait()