server/group: refactor with shared abstractions and fix concurrency issues (#5222)

* server/group: refactor group package with shared abstractions and fix concurrency issues

Extract common patterns into reusable components:
- groupRegistry[G]: generic concurrent map for group lifecycle management
- baseGroup: shared plumbing for listener-based groups (TCP, HTTPS, TCPMux)
- Listener: unified virtual listener replacing 3 identical implementations

Fix concurrency issues:
- Stale-pointer race: isCurrent check + errGroupStale + controller retry loops
- Worker generation safety: pass realLn and acceptCh as params instead of reading mutable fields
- Connection leak: close conn on worker panic recovery path
- ABBA deadlock in HTTP UnRegister: consistent lock ordering (group.mu -> registry.mu)
- Round-robin overflow in HTTPGroup: use unsigned modulo

Add unit tests (17 tests) for registry, listener, and baseGroup.
Add TCPMux group load balancing e2e test.

* server/group: replace tautological assertion with require.NotPanics

* server/group: remove blank line between doc comment and type declaration

server/group/base_test.go

@@ -0,0 +1,169 @@
+package group
+
+import (
+	"net"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// fakeLn is a controllable net.Listener for tests.
+type fakeLn struct {
+	connCh chan net.Conn
+	closed chan struct{}
+	once   sync.Once
+}
+
+func newFakeLn() *fakeLn {
+	return &fakeLn{
+		connCh: make(chan net.Conn, 8),
+		closed: make(chan struct{}),
+	}
+}
+
+func (f *fakeLn) Accept() (net.Conn, error) {
+	select {
+	case c := <-f.connCh:
+		return c, nil
+	case <-f.closed:
+		return nil, net.ErrClosed
+	}
+}
+
+func (f *fakeLn) Close() error {
+	f.once.Do(func() { close(f.closed) })
+	return nil
+}
+
+func (f *fakeLn) Addr() net.Addr { return fakeAddr("127.0.0.1:9999") }
+
+func (f *fakeLn) inject(c net.Conn) {
+	select {
+	case f.connCh <- c:
+	case <-f.closed:
+	}
+}
+
+func TestBaseGroup_WorkerFanOut(t *testing.T) {
+	fl := newFakeLn()
+	var bg baseGroup
+	bg.initBase("g", "key", fl, func() {})
+
+	go bg.worker(fl, bg.acceptCh)
+
+	c1, c2 := net.Pipe()
+	defer c2.Close()
+	fl.inject(c1)
+
+	select {
+	case got := <-bg.acceptCh:
+		assert.Equal(t, c1, got)
+		got.Close()
+	case <-time.After(time.Second):
+		t.Fatal("timed out waiting for connection on acceptCh")
+	}
+
+	fl.Close()
+}
+
+func TestBaseGroup_WorkerStopsOnListenerClose(t *testing.T) {
+	fl := newFakeLn()
+	var bg baseGroup
+	bg.initBase("g", "key", fl, func() {})
+
+	done := make(chan struct{})
+	go func() {
+		bg.worker(fl, bg.acceptCh)
+		close(done)
+	}()
+
+	fl.Close()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("worker did not stop after listener close")
+	}
+}
+
+func TestBaseGroup_WorkerClosesConnOnClosedChannel(t *testing.T) {
+	fl := newFakeLn()
+	var bg baseGroup
+	bg.initBase("g", "key", fl, func() {})
+
+	// Close acceptCh before worker sends.
+	close(bg.acceptCh)
+
+	done := make(chan struct{})
+	go func() {
+		bg.worker(fl, bg.acceptCh)
+		close(done)
+	}()
+
+	c1, c2 := net.Pipe()
+	defer c2.Close()
+	fl.inject(c1)
+
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("worker did not stop after panic recovery")
+	}
+
+	// c1 should have been closed by worker's panic recovery path.
+	buf := make([]byte, 1)
+	_, err := c1.Read(buf)
+	assert.Error(t, err, "connection should be closed by worker")
+}
+
+func TestBaseGroup_CloseLastListenerTriggersCleanup(t *testing.T) {
+	fl := newFakeLn()
+	var bg baseGroup
+	cleanupCalled := 0
+	bg.initBase("g", "key", fl, func() { cleanupCalled++ })
+
+	bg.mu.Lock()
+	ln1 := bg.newListener(fl.Addr())
+	ln2 := bg.newListener(fl.Addr())
+	bg.mu.Unlock()
+
+	go bg.worker(fl, bg.acceptCh)
+
+	ln1.Close()
+	assert.Equal(t, 0, cleanupCalled, "cleanup should not run while listeners remain")
+
+	ln2.Close()
+	assert.Equal(t, 1, cleanupCalled, "cleanup should run after last listener closed")
+}
+
+func TestBaseGroup_CloseOneOfTwoListeners(t *testing.T) {
+	fl := newFakeLn()
+	var bg baseGroup
+	cleanupCalled := 0
+	bg.initBase("g", "key", fl, func() { cleanupCalled++ })
+
+	bg.mu.Lock()
+	ln1 := bg.newListener(fl.Addr())
+	ln2 := bg.newListener(fl.Addr())
+	bg.mu.Unlock()
+
+	go bg.worker(fl, bg.acceptCh)
+
+	ln1.Close()
+	assert.Equal(t, 0, cleanupCalled)
+
+	// ln2 should still receive connections.
+	c1, c2 := net.Pipe()
+	defer c2.Close()
+	fl.inject(c1)
+
+	got, err := ln2.Accept()
+	require.NoError(t, err)
+	assert.Equal(t, c1, got)
+	got.Close()
+
+	ln2.Close()
+	assert.Equal(t, 1, cleanupCalled)
+}