logtail: run HTTP tests in-memory with memnet + synctest

TestEncodeAndUploadMessages waited on the default 2s FlushDelay,
making the logtail package the slowest non-integration test in
the tree (~2s real time). Switch the shared harness from an
httptest.Server-on-loopback to a memnet.Listener-backed *http.Server
and run the tests inside synctest.Test, so fake time advances the
flush timer instantly.

Drops the net/http/httptest dependency from these tests. Combined
with the TestMain non-localhost dial guard added in the previous
commit, no test in this package can accidentally reach the real
log.tailscale.com server. Whole package now runs in ~7ms.

Updates tailscale/corp#28679

Change-Id: Ie0e7a6a79641384ed0eecb99d767e17cda8bb944
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/logtail_test.go

@@ -11,7 +11,6 @@ import (
 	"io"
 	"net"
 	"net/http"
-	"net/http/httptest"
 	"os"
 	"strings"
 	"sync"
@@ -20,6 +19,7 @@ import (
 	"time"
 
 	"github.com/go-json-experiment/json/jsontext"
+	"tailscale.com/net/memnet"
 	"tailscale.com/tstest"
 	"tailscale.com/tstime"
 	"tailscale.com/util/eventbus/eventbustest"
@@ -30,6 +30,7 @@ import (
 // test in this package. Config.BaseURL defaults to https://log.tailscale.com
 // and Config.HTTPC defaults to http.DefaultClient, so a test that forgets to
 // override either can otherwise silently hit the real logtail server.
+// Tests that need an HTTP server should use memnet (see newTestLogtailServer).
 func TestMain(m *testing.M) {
 	tr := http.DefaultTransport.(*http.Transport)
 	orig := tr.DialContext
@@ -38,25 +39,19 @@ func TestMain(m *testing.M) {
 		if err == nil && (host == "127.0.0.1" || host == "::1" || host == "localhost") {
 			return orig(ctx, network, addr)
 		}
-		return nil, fmt.Errorf("logtail tests: refusing to dial non-localhost address %q; use httptest.Server or a custom Config.HTTPC", addr)
+		return nil, fmt.Errorf("logtail tests: refusing to dial non-localhost address %q; use memnet or a custom Config.HTTPC", addr)
 	}
 	os.Exit(m.Run())
 }
 
-func TestFastShutdown(t *testing.T) {
+func TestFastShutdown(t *testing.T) { synctest.Test(t, synctestFastShutdown) }
+
+func synctestFastShutdown(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()
 
-	testServ := httptest.NewServer(http.HandlerFunc(
-		func(w http.ResponseWriter, r *http.Request) {}))
-	defer testServ.Close()
-
-	logger := NewLogger(Config{
-		BaseURL: testServ.URL,
-		Bus:     eventbustest.NewBus(t),
-	}, t.Logf)
-	err := logger.Shutdown(ctx)
-	if err != nil {
+	_, logger := newTestLogtailServer(t)
+	if err := logger.Shutdown(ctx); err != nil {
 		t.Error(err)
 	}
 }
@@ -65,49 +60,60 @@ func TestFastShutdown(t *testing.T) {
 const logLines = 3
 
 type LogtailTestServer struct {
-	srv      *httptest.Server // Log server
 	uploaded chan []byte
 }
 
-func NewLogtailTestHarness(t *testing.T) (*LogtailTestServer, *Logger) {
-	ts := LogtailTestServer{}
+// newTestLogtailServer wires up an in-memory HTTP server (via memnet) and a
+// *Logger whose HTTPC dials it. Lives inside the caller's synctest bubble so
+// the default FlushDelay and any other fake timers advance automatically.
+func newTestLogtailServer(t *testing.T) (*LogtailTestServer, *Logger) {
+	ts := &LogtailTestServer{
+		// max channel backlog = 1 "started" + #logLines x "log line" + 1 "closed"
+		uploaded: make(chan []byte, 2+logLines),
+	}
 
-	// max channel backlog = 1 "started" + #logLines x "log line" + 1 "closed"
-	ts.uploaded = make(chan []byte, 2+logLines)
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Error("failed to read HTTP request")
+		}
+		ts.uploaded <- body
+	})
 
-	ts.srv = httptest.NewServer(http.HandlerFunc(
-		func(w http.ResponseWriter, r *http.Request) {
-			body, err := io.ReadAll(r.Body)
-			if err != nil {
-				t.Error("failed to read HTTP request")
-			}
-			ts.uploaded <- body
-		}))
-
-	t.Cleanup(ts.srv.Close)
+	ln := memnet.Listen("logtail-test:0")
+	httpsrv := &http.Server{Handler: handler}
+	go httpsrv.Serve(ln)
+	t.Cleanup(func() {
+		httpsrv.Close()
+		ln.Close()
+	})
 
 	logger := NewLogger(Config{
-		BaseURL: ts.srv.URL,
+		BaseURL: "http://" + ln.Addr().String(),
 		Bus:     eventbustest.NewBus(t),
+		HTTPC: &http.Client{
+			Transport: &http.Transport{DialContext: ln.Dial},
+		},
 	}, t.Logf)
 
-	// There is always an initial "logtail started" message
+	// There is always an initial "logtail started" message.
 	body := <-ts.uploaded
 	if !strings.Contains(string(body), "started") {
 		t.Errorf("unknown start logging statement: %q", string(body))
 	}
-
-	return &ts, logger
+	return ts, logger
 }
 
-func TestDrainPendingMessages(t *testing.T) {
-	ts, logger := NewLogtailTestHarness(t)
+func TestDrainPendingMessages(t *testing.T) { synctest.Test(t, synctestDrainPendingMessages) }
+
+func synctestDrainPendingMessages(t *testing.T) {
+	ts, logger := newTestLogtailServer(t)
 
 	for range logLines {
 		logger.Write([]byte("log line"))
 	}
 
-	// all of the "log line" messages usually arrive at once, but poll if needed.
+	// All the "log line" messages usually arrive at once, but poll if needed.
 	var body strings.Builder
 	for i := 0; i <= logLines; i++ {
 		body.WriteString(string(<-ts.uploaded))
@@ -115,17 +121,17 @@ func TestDrainPendingMessages(t *testing.T) {
 		if count == logLines {
 			break
 		}
-		// if we never find count == logLines, the test will eventually time out.
 	}
 
-	err := logger.Shutdown(context.Background())
-	if err != nil {
+	if err := logger.Shutdown(context.Background()); err != nil {
 		t.Error(err)
 	}
 }
 
-func TestEncodeAndUploadMessages(t *testing.T) {
-	ts, logger := NewLogtailTestHarness(t)
+func TestEncodeAndUploadMessages(t *testing.T) { synctest.Test(t, synctestEncodeAndUploadMessages) }
+
+func synctestEncodeAndUploadMessages(t *testing.T) {
+	ts, logger := newTestLogtailServer(t)
 
 	tests := []struct {
 		name string
@@ -166,8 +172,7 @@ func TestEncodeAndUploadMessages(t *testing.T) {
 		}
 	}
 
-	err := logger.Shutdown(context.Background())
-	if err != nil {
+	if err := logger.Shutdown(context.Background()); err != nil {
 		t.Error(err)
 	}
 }