wgengine/magicsock: add more docs, checks to Test32bitAlignment

Per recent chat with @raggi about all this, I went and looked at this
test again.

Updates #cleanup

Change-Id: Icb7d87b1ed2cebf481ee4e358a3aa603e63fb8a4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -1460,7 +1460,20 @@ func TestDiscoStringLogRace(t *testing.T) {
 	wg.Wait()
 }
 
+// Test32bitAlignment verifies that that the 64-bit atomic mono.Time fields are
+// 64-bit aligned, so that StoreAtomic and LoadAtomic won't panic on 32-bit
+// platforms.
+//
+// For normal Go atomic types (sync/atomic.Int64, etc), the Go compiler
+// guarantees 64-bit alignment on 32-bit platforms with an unexported magic
+// embedded struct field. We can't make mono.Time use that easily. We could change
+// mono.Time to be type Time struct { atomic.Int64 }, but that's pretty invasive.
+// Instead, we just have this test to keep us safe on 32-bit platforms.
 func Test32bitAlignment(t *testing.T) {
+	if rt := reflect.TypeFor[mono.Time](); rt.Kind() != reflect.Int64 {
+		t.Fatalf("mono.Time is not a 64-bit integer type anymore; this test may be irrelevant now or out of date")
+	}
+
 	de := endpoint{
 		c: &Conn{},
 	}
@@ -1468,9 +1481,13 @@ func Test32bitAlignment(t *testing.T) {
 	if off := unsafe.Offsetof(de.lastRecvWG); off%8 != 0 {
 		t.Fatalf("endpoint.lastRecvWG is not 8-byte aligned")
 	}
+	if off := unsafe.Offsetof(de.lastRecvUDPAny); off%8 != 0 {
+		t.Fatalf("endpoint.lastRecvUDPAny is not 8-byte aligned")
+	}
 
-	de.noteRecvActivity(epAddr{}, mono.Now()) // verify this doesn't panic on 32-bit
+	// Verify these don't panic.
-	de.noteRecvActivity(epAddr{}, mono.Now()) // second call should be throttled
+	de.lastRecvWG.StoreAtomic(mono.Now())
+	de.lastRecvUDPAny.StoreAtomic(mono.Now())
 }
 
 // newTestConn returns a new Conn.