control/controlbase: deflake, speed up TestConnMemoryOverhead

This had gotten flaky with Go 1.26.

Use synctest + AllocsPerRun to make it fast and deterministic.

Updates #18682

Change-Id: If673d6ecd8c1177f59c1b9c0f3fca42309375dff
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlbase/conn_test.go

@@ -11,12 +11,11 @@ import (
 	"fmt"
 	"io"
 	"net"
-	"runtime"
 	"strings"
 	"sync"
 	"testing"
 	"testing/iotest"
-	"time"
+	"testing/synctest"
 
 	chp "golang.org/x/crypto/chacha20poly1305"
 	"golang.org/x/net/nettest"
@@ -226,79 +225,31 @@ func TestConnStd(t *testing.T) {
 	})
 }
 
-// tests that the idle memory overhead of a Conn blocked in a read is
-// reasonable (under 2K). It was previously over 8KB with two 4KB
-// buffers for rx/tx. This make sure we don't regress. Hopefully it
-// doesn't turn into a flaky test. If so, const max can be adjusted,
-// or it can be deleted or reworked.
+// tests that the memory overhead of a Conn blocked in a read is
+// reasonable. It was previously over 8KB with two 4KB buffers for
+// rx/tx. This makes sure we don't regress.
 func TestConnMemoryOverhead(t *testing.T) {
-	num := 1000
-	if testing.Short() {
-		num = 100
-	}
-	ng0 := runtime.NumGoroutine()
-
-	runtime.GC()
-	var ms0 runtime.MemStats
-	runtime.ReadMemStats(&ms0)
-
-	var closers []io.Closer
-	closeAll := func() {
-		for _, c := range closers {
-			c.Close()
+	synctest.Test(t, func(t *testing.T) {
+		// AllocsPerRun runs the function once for warmup (filling
+		// allocator slab caches, etc.) and then measures over the
+		// remaining runs, returning the average allocation count.
+		allocs := testing.AllocsPerRun(100, func() {
+			client, server := pair(t)
+			go func() {
+				var buf [1]byte
+				client.Read(buf[:])
+			}()
+			synctest.Wait()
+			client.Close()
+			server.Close()
+			synctest.Wait()
+		})
+		t.Logf("allocs per blocked-conn pair: %v", allocs)
+		const max = 400
+		if allocs > max {
+			t.Errorf("allocs per blocked-conn pair = %v, want <= %v", allocs, max)
 		}
-		closers = nil
-	}
-	defer closeAll()
-
-	for range num {
-		client, server := pair(t)
-		closers = append(closers, client, server)
-		go func() {
-			var buf [1]byte
-			client.Read(buf[:])
-		}()
-	}
-
-	t0 := time.Now()
-	deadline := t0.Add(3 * time.Second)
-	var ngo int
-	for time.Now().Before(deadline) {
-		runtime.GC()
-		ngo = runtime.NumGoroutine()
-		if ngo >= num {
-			break
-		}
-		time.Sleep(10 * time.Millisecond)
-	}
-	if ngo < num {
-		t.Fatalf("only %v goroutines; expected %v+", ngo, num)
-	}
-	runtime.GC()
-	var ms runtime.MemStats
-	runtime.ReadMemStats(&ms)
-	growthTotal := int64(ms.HeapAlloc) - int64(ms0.HeapAlloc)
-	growthEach := float64(growthTotal) / float64(num)
-	t.Logf("Alloced %v bytes, %.2f B/each", growthTotal, growthEach)
-	const max = 2048
-	if growthEach > max {
-		t.Errorf("allocated more than expected; want max %v bytes/each", max)
-	}
-
-	closeAll()
-
-	// And make sure our goroutines go away too.
-	deadline = time.Now().Add(3 * time.Second)
-	for time.Now().Before(deadline) {
-		ngo = runtime.NumGoroutine()
-		if ngo < ng0+num/10 {
-			break
-		}
-		time.Sleep(10 * time.Millisecond)
-	}
-	if ngo >= ng0+num/10 {
-		t.Errorf("goroutines didn't go back down; started at %v, now %v", ng0, ngo)
-	}
+	})
 }
 
 type readSink struct {