Settings > Sources: turn syslog reception on/off, choose UDP/TCP

- SyslogServer opens and closes the UDP/TCP listeners at runtime from the
  configuration saved in /data/syslog.json; a busy port no longer stops
  Logstream, the error is shown in Settings instead.
- Sources tab: syslog section with an on/off switch, UDP and TCP labels,
  the live listening state and the published port (SYSLOG_PORT, passed as
  SYSLOG_PUBLIC_PORT for display; the mapping stays in docker-compose).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

syslogserver.go

@@ -0,0 +1,145 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"log"
+	"net"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+// syslogConfig is saved in /data/syslog.json and edited in Settings > Sources.
+type syslogConfig struct {
+	Enabled bool `json:"enabled"`
+	UDP     bool `json:"udp"`
+	TCP     bool `json:"tcp"`
+}
+
+// SyslogServer owns the UDP and TCP listeners and opens or closes them when
+// the configuration changes, without restarting Logstream.
+type SyslogServer struct {
+	parent     context.Context
+	addr       string // listening address inside the container, e.g. ":5514"
+	publicPort string // port published on the host by docker-compose (display only)
+	sink       func(*Entry)
+	cfgPath    string
+
+	mu     sync.Mutex
+	cfg    syslogConfig
+	cancel context.CancelFunc
+	pc     net.PacketConn
+	ln     net.Listener
+	errUDP string
+	errTCP string
+}
+
+func NewSyslogServer(ctx context.Context, addr, publicPort, dataDir string, sink func(*Entry)) *SyslogServer {
+	s := &SyslogServer{
+		parent:     ctx,
+		addr:       addr,
+		publicPort: publicPort,
+		sink:       sink,
+		cfgPath:    filepath.Join(dataDir, "syslog.json"),
+		cfg:        syslogConfig{Enabled: true, UDP: true, TCP: true},
+	}
+	if b, err := os.ReadFile(s.cfgPath); err == nil {
+		var cfg syslogConfig
+		if err := json.Unmarshal(b, &cfg); err == nil {
+			s.cfg = cfg
+		}
+	}
+	s.apply()
+	go func() {
+		<-ctx.Done()
+		s.mu.Lock()
+		s.closeLocked()
+		s.mu.Unlock()
+	}()
+	return s
+}
+
+// closeLocked stops the listeners synchronously, so that the port is free
+// before it is opened again; the caller holds the lock.
+func (s *SyslogServer) closeLocked() {
+	if s.cancel != nil {
+		s.cancel() // also closes the open TCP connections
+		s.cancel = nil
+	}
+	if s.pc != nil {
+		s.pc.Close()
+		s.pc = nil
+	}
+	if s.ln != nil {
+		s.ln.Close()
+		s.ln = nil
+	}
+}
+
+// apply opens the listeners required by the configuration.
+func (s *SyslogServer) apply() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.closeLocked()
+	s.errUDP, s.errTCP = "", ""
+	if !s.cfg.Enabled || s.parent.Err() != nil {
+		log.Printf("syslog: reception disabled")
+		return
+	}
+	ctx, cancel := context.WithCancel(s.parent)
+	s.cancel = cancel
+	var open []string
+	if s.cfg.UDP {
+		pc, err := net.ListenPacket("udp", s.addr)
+		if err != nil {
+			s.errUDP = err.Error()
+			log.Printf("syslog udp: %v", err)
+		} else {
+			if uc, ok := pc.(*net.UDPConn); ok {
+				_ = uc.SetReadBuffer(8 << 20)
+			}
+			s.pc = pc
+			go serveUDP(ctx, pc, s.sink)
+			open = append(open, "udp")
+		}
+	}
+	if s.cfg.TCP {
+		ln, err := net.Listen("tcp", s.addr)
+		if err != nil {
+			s.errTCP = err.Error()
+			log.Printf("syslog tcp: %v", err)
+		} else {
+			s.ln = ln
+			go serveTCP(ctx, ln, s.sink)
+			open = append(open, "tcp")
+		}
+	}
+	log.Printf("syslog: listening on %s %v", s.addr, open)
+}
+
+// Configure saves and applies a new configuration.
+func (s *SyslogServer) Configure(cfg syslogConfig) error {
+	s.mu.Lock()
+	s.cfg = cfg
+	s.mu.Unlock()
+	s.apply()
+	return writeJSONFile(s.cfgPath, cfg)
+}
+
+// Status is the state shown in Settings > Sources.
+func (s *SyslogServer) Status() map[string]any {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return map[string]any{
+		"enabled":      s.cfg.Enabled,
+		"udp":          s.cfg.UDP,
+		"tcp":          s.cfg.TCP,
+		"udpListening": s.pc != nil,
+		"tcpListening": s.ln != nil,
+		"udpError":     s.errUDP,
+		"tcpError":     s.errTCP,
+		"port":         s.publicPort,
+		"internalAddr": s.addr,
+	}
+}