tests/e2e: concurrent UDP endpoints must share a reverse tunnel intact

Covers gost#911 at the wire level: four UDP source sockets send through one
RUDP reverse tunnel bound on a relay server, and every datagram must come
back byte-identical to its own endpoint.

A frame torn by an interleaved writer shows up twice — as a corrupted
payload, and as the torn frame leaving half a header in the stream, which the
far end reads as "unexpected EOF" and answers by rebuilding the whole tunnel.
So the suite asserts both: no endpoint sees a corrupted datagram, and no
endpoint sees a receive gap long enough to be a teardown and rebind.

Verified against a binary built before the fix (4 endpoints, 35% loss, 1.4s
gaps, suite fails) and after it (0% loss, no corruption, 0.2s gaps, passes).

tests/e2e/rudpbind_test.go

@@ -0,0 +1,147 @@
+package e2e
+
+import (
+	"context"
+	"io"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/suite"
+	"github.com/testcontainers/testcontainers-go"
+)
+
+// RUDPBindSuite reproduces go-gost/gost#911: concurrent UDP source sockets
+// sharing ONE reverse UDP tunnel over a relay server.
+//
+// Topology (all three gost processes plus a UDP echo server):
+//
+//	UDP source sockets (N) -> client udp service -> relay server
+//	  -> reverse-bound UDP port -> host rudp listener (ONE shared stream)
+//	  -> udp-echo
+//
+// A datagram frame is written to that shared stream as three separate Write
+// calls (RSV/FRAG, SOCKS5 address, payload). Before the fix, with N endpoints
+// writing concurrently, frames interleaved between those calls: replies came
+// back corrupted, and a torn frame left half a header in the stream, which the
+// far end read as "unexpected EOF" and answered by tearing down and rebinding
+// the whole reverse tunnel every second.
+//
+// The host is the side that binds the tunnel, so its log is where a reset
+// shows up as "unexpected EOF, retrying in 1s" followed by a rebind.
+type RUDPBindSuite struct {
+	suite.Suite
+	ctx     context.Context
+	udpC    testcontainers.Container
+	serverC testcontainers.Container
+	hostC   testcontainers.Container
+	clientC testcontainers.Container
+}
+
+func (s *RUDPBindSuite) SetupSuite() {
+	s.ctx = context.Background()
+
+	udpC, err := RunUDPEchoContainer(s.ctx, SharedNetworkName)
+	s.Require().NoError(err)
+	s.udpC = udpC
+
+	serverC, err := RunGostContainerWithOptions(s.ctx, SharedNetworkName,
+		"testdata/rudpbind/server.yaml", []string{"gost-server"}, []string{"8430/tcp"})
+	s.Require().NoError(err)
+	s.serverC = serverC
+
+	// The host's rudp listener opens no local port, so it exposes a dummy TCP
+	// port (service-1) purely for the readiness wait.
+	hostC, err := RunGostContainerWithOptions(s.ctx, SharedNetworkName,
+		"testdata/rudpbind/host.yaml", []string{"gost-host"}, []string{"8431/tcp"})
+	s.Require().NoError(err)
+	s.hostC = hostC
+
+	clientC, err := RunGostContainerWithFiles(s.ctx, SharedNetworkName,
+		"testdata/rudpbind/client.yaml",
+		[]testcontainers.ContainerFile{
+			{HostFilePath: "scripts/udp_rudp_concurrent.py", ContainerFilePath: "/scripts/udp_rudp_concurrent.py", FileMode: 0644},
+		})
+	s.Require().NoError(err)
+	s.clientC = clientC
+
+	// The reverse tunnel binds remotely on first traffic, not at startup.
+	up := assert.Eventually(s.T(), func() bool {
+		out := s.sendConcurrent(1, 1)
+		if !strings.Contains(out, "PASS") {
+			s.T().Logf("waiting for tunnel, sender said:\n%s", out)
+		}
+		return out != "" && strings.Contains(out, "PASS")
+	}, 30*time.Second, time.Second, "reverse UDP tunnel never came up")
+	if !up {
+		DumpLogs(s.T(), s.ctx, "host logs", s.hostC)
+		DumpLogs(s.T(), s.ctx, "server logs", s.serverC)
+		DumpLogs(s.T(), s.ctx, "client logs", s.clientC)
+		s.FailNow("reverse UDP tunnel never came up")
+	}
+}
+
+func (s *RUDPBindSuite) TearDownSuite() {
+	for _, c := range []testcontainers.Container{s.clientC, s.hostC, s.serverC, s.udpC} {
+		if c != nil {
+			c.Terminate(s.ctx)
+		}
+	}
+}
+
+// sendConcurrent runs the concurrent sender inside the client container and
+// returns its combined stdout+stderr.
+func (s *RUDPBindSuite) sendConcurrent(endpoints, seconds int) string {
+	cmd := []string{
+		"python3", "/scripts/udp_rudp_concurrent.py",
+		// The client service listens in this same container, so loopback is the
+		// correct target — no network alias needed for the sender.
+		"127.0.0.1", "8432",
+		strconv.Itoa(endpoints), strconv.Itoa(seconds),
+	}
+	_, out, err := s.clientC.Exec(s.ctx, cmd)
+	if err != nil {
+		return ""
+	}
+	var sb strings.Builder
+	buf := make([]byte, 4096)
+	for {
+		n, err := out.Read(buf)
+		sb.Write(buf[:n])
+		if err != nil {
+			break
+		}
+	}
+	return sb.String()
+}
+
+// TestConcurrentEndpointsShareTunnel is the regression assertion: several UDP
+// source sockets sharing one reverse tunnel must each get their own datagrams
+// back unchanged, and the tunnel must not be rebuilt underneath them.
+func (s *RUDPBindSuite) TestConcurrentEndpointsShareTunnel() {
+	out := s.sendConcurrent(4, 8)
+	s.T().Logf("concurrent send:\n%s", out)
+
+	if strings.Contains(out, "FAIL") {
+		DumpLogs(s.T(), s.ctx, "host logs", s.hostC)
+		DumpLogs(s.T(), s.ctx, "server logs", s.serverC)
+	}
+	s.Require().Contains(out, "PASS", "concurrent endpoints over one reverse tunnel failed:\n%s", out)
+
+	// Belt and braces: the host log is where the reset appears directly.
+	hostLogs, err := s.hostC.Logs(s.ctx)
+	if err == nil {
+		body, readErr := io.ReadAll(hostLogs)
+		hostLogs.Close()
+		if readErr == nil {
+			s.Require().NotContains(string(body), "unexpected EOF",
+				"reverse tunnel was torn down while endpoints were sending")
+		}
+	}
+}
+
+func TestRUDPBindSuite(t *testing.T) {
+	suite.Run(t, new(RUDPBindSuite))
+}