tests: Deflake socket_inet_loopback_isolated_test

This test case creates one connection, then it closes one end of it. After that
it waits for a linger timeout and tries to create the second connection with
the same client source port. The test is flaky if the second half of the fist
connection isn't closed before creating the second connection.

PiperOrigin-RevId: 715924851

runsc/boot/network.go

@@ -295,7 +295,11 @@ func (n *Network) CreateLinksAndRoutes(args *CreateLinksAndRoutesArgs, _ *struct
 		nicID := n.Stack.NextNICID()
 		nicids[link.Name] = nicID
 
-		linkEP := ethernet.New(loopback.New())
+		var linkEP stack.LinkEndpoint
+		linkEP = ethernet.New(loopback.New())
+		if args.LogPackets {
+			linkEP = sniffer.New(linkEP)
+		}
 
 		log.Infof("Enabling loopback interface %q with id %d on addresses %+v", link.Name, nicID, link.Addresses)
 		opts := stack.NICOptions{

test/syscalls/linux/socket_inet_loopback_isolated.cc

@@ -210,6 +210,13 @@ TEST_P(SocketInetLoopbackIsolatedTest, TCPFinWait2Test) {
   ASSERT_THAT(bind(conn_fd2.get(), AsSockAddr(&conn_bound_addr), conn_addrlen),
               SyscallSucceeds());
 
+  // Close the `accepted` end otherwise connect can return ECONNREFUSED.
+  constexpr int kTCPLingerTimeout0 = 0;
+  EXPECT_THAT(setsockopt(accepted.get(), IPPROTO_TCP, TCP_LINGER2,
+                         &kTCPLingerTimeout0, sizeof(kTCPLingerTimeout0)),
+              SyscallSucceedsWithValue(0));
+  shutdown(accepted.get(), SHUT_WR);
+
   ASSERT_THAT(
       RetryEINTR(connect)(conn_fd2.get(), AsSockAddr(&conn_addr), conn_addrlen),
       SyscallSucceeds());