catch spurious UDP sendmsg errors in multiplex integration test (#4451)

* catch spurious UDP sendmsg errors in multiplex integration test

* platform-dependent isPermissionError

integrationtests/self/self_suite_linux_test.go

@@ -0,0 +1,21 @@
+//go:build linux
+
+package self_test
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// The first sendmsg call on a new UDP socket sometimes errors on Linux.
+// It's not clear why this happens.
+// See https://github.com/golang/go/issues/63322.
+func isPermissionError(err error) bool {
+	var serr *os.SyscallError
+	if errors.As(err, &serr) {
+		return serr.Syscall == "sendmsg" && serr.Err == unix.EPERM
+	}
+	return false
+}