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/http.go

@@ -9,53 +9,42 @@ import (
 	"github.com/fatedier/frp/pkg/util/vhost"
 )
 
+// HTTPGroupController manages HTTP groups that use round-robin
+// callback routing (fundamentally different from listener-based groups).
 type HTTPGroupController struct {
-	// groups indexed by group name
-	groups map[string]*HTTPGroup
-
-	// register createConn for each group to vhostRouter.
-	// createConn will get a connection from one proxy of the group
+	groupRegistry[*HTTPGroup]
 	vhostRouter *vhost.Routers
-
-	mu sync.Mutex
 }
 
 func NewHTTPGroupController(vhostRouter *vhost.Routers) *HTTPGroupController {
 	return &HTTPGroupController{
-		groups:      make(map[string]*HTTPGroup),
-		vhostRouter: vhostRouter,
+		groupRegistry: newGroupRegistry[*HTTPGroup](),
+		vhostRouter:   vhostRouter,
 	}
 }
 
 func (ctl *HTTPGroupController) Register(
 	proxyName, group, groupKey string,
 	routeConfig vhost.RouteConfig,
-) (err error) {
-	indexKey := group
-	ctl.mu.Lock()
-	g, ok := ctl.groups[indexKey]
-	if !ok {
-		g = NewHTTPGroup(ctl)
-		ctl.groups[indexKey] = g
+) error {
+	for {
+		g := ctl.getOrCreate(group, func() *HTTPGroup {
+			return NewHTTPGroup(ctl)
+		})
+		err := g.Register(proxyName, group, groupKey, routeConfig)
+		if err == errGroupStale {
+			continue
+		}
+		return err
 	}
-	ctl.mu.Unlock()
-
-	return g.Register(proxyName, group, groupKey, routeConfig)
 }
 
 func (ctl *HTTPGroupController) UnRegister(proxyName, group string, _ vhost.RouteConfig) {
-	indexKey := group
-	ctl.mu.Lock()
-	defer ctl.mu.Unlock()
-	g, ok := ctl.groups[indexKey]
+	g, ok := ctl.get(group)
 	if !ok {
 		return
 	}
-
-	isEmpty := g.UnRegister(proxyName)
-	if isEmpty {
-		delete(ctl.groups, indexKey)
-	}
+	g.UnRegister(proxyName)
 }
 
 type HTTPGroup struct {
@@ -87,6 +76,9 @@ func (g *HTTPGroup) Register(
 ) (err error) {
 	g.mu.Lock()
 	defer g.mu.Unlock()
+	if !g.ctl.isCurrent(group, func(cur *HTTPGroup) bool { return cur == g }) {
+		return errGroupStale
+	}
 	if len(g.createFuncs) == 0 {
 		// the first proxy in this group
 		tmp := routeConfig // copy object
@@ -123,7 +115,7 @@ func (g *HTTPGroup) Register(
 	return nil
 }
 
-func (g *HTTPGroup) UnRegister(proxyName string) (isEmpty bool) {
+func (g *HTTPGroup) UnRegister(proxyName string) {
 	g.mu.Lock()
 	defer g.mu.Unlock()
 	delete(g.createFuncs, proxyName)
@@ -135,10 +127,11 @@ func (g *HTTPGroup) UnRegister(proxyName string) (isEmpty bool) {
 	}
 
 	if len(g.createFuncs) == 0 {
-		isEmpty = true
 		g.ctl.vhostRouter.Del(g.domain, g.location, g.routeByHTTPUser)
+		g.ctl.removeIf(g.group, func(cur *HTTPGroup) bool {
+			return cur == g
+		})
 	}
-	return
 }
 
 func (g *HTTPGroup) createConn(remoteAddr string) (net.Conn, error) {
@@ -151,7 +144,7 @@ func (g *HTTPGroup) createConn(remoteAddr string) (net.Conn, error) {
 	location := g.location
 	routeByHTTPUser := g.routeByHTTPUser
 	if len(g.pxyNames) > 0 {
-		name := g.pxyNames[int(newIndex)%len(g.pxyNames)]
+		name := g.pxyNames[newIndex%uint64(len(g.pxyNames))]
 		f = g.createFuncs[name]
 	}
 	g.mu.RUnlock()
@@ -174,7 +167,7 @@ func (g *HTTPGroup) chooseEndpoint() (string, error) {
 	location := g.location
 	routeByHTTPUser := g.routeByHTTPUser
 	if len(g.pxyNames) > 0 {
-		name = g.pxyNames[int(newIndex)%len(g.pxyNames)]
+		name = g.pxyNames[newIndex%uint64(len(g.pxyNames))]
 	}
 	g.mu.RUnlock()