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.go

@@ -0,0 +1,77 @@
+package group
+
+import (
+	"net"
+	"sync"
+
+	gerr "github.com/fatedier/golib/errors"
+)
+
+// baseGroup contains the shared plumbing for listener-based groups
+// (TCP, HTTPS, TCPMux). Each concrete group embeds this and provides
+// its own Listen method with protocol-specific validation.
+type baseGroup struct {
+	group    string
+	groupKey string
+
+	acceptCh  chan net.Conn
+	realLn    net.Listener
+	lns       []*Listener
+	mu        sync.Mutex
+	cleanupFn func()
+}
+
+// initBase resets the baseGroup for a fresh listen cycle.
+// Must be called under mu when len(lns) == 0.
+func (bg *baseGroup) initBase(group, groupKey string, realLn net.Listener, cleanupFn func()) {
+	bg.group = group
+	bg.groupKey = groupKey
+	bg.realLn = realLn
+	bg.acceptCh = make(chan net.Conn)
+	bg.cleanupFn = cleanupFn
+}
+
+// worker reads from the real listener and fans out to acceptCh.
+// The parameters are captured at creation time so that the worker is
+// bound to a specific listen cycle and cannot observe a later initBase.
+func (bg *baseGroup) worker(realLn net.Listener, acceptCh chan<- net.Conn) {
+	for {
+		c, err := realLn.Accept()
+		if err != nil {
+			return
+		}
+		err = gerr.PanicToError(func() {
+			acceptCh <- c
+		})
+		if err != nil {
+			c.Close()
+			return
+		}
+	}
+}
+
+// newListener creates a new Listener wired to this baseGroup.
+// Must be called under mu.
+func (bg *baseGroup) newListener(addr net.Addr) *Listener {
+	ln := newListener(bg.acceptCh, addr, bg.closeListener)
+	bg.lns = append(bg.lns, ln)
+	return ln
+}
+
+// closeListener removes ln from the list. When the last listener is removed,
+// it closes acceptCh, closes the real listener, and calls cleanupFn.
+func (bg *baseGroup) closeListener(ln *Listener) {
+	bg.mu.Lock()
+	defer bg.mu.Unlock()
+	for i, l := range bg.lns {
+		if l == ln {
+			bg.lns = append(bg.lns[:i], bg.lns[i+1:]...)
+			break
+		}
+	}
+	if len(bg.lns) == 0 {
+		close(bg.acceptCh)
+		bg.realLn.Close()
+		bg.cleanupFn()
+	}
+}