all: Refactor cmd/syncthing creating lib/syncthing (ref #4085) (#5805)

* add skeleton for lib/syncthing

* copy syncthingMain to lib/syncthing (verbatim)

* Remove code to deduplicate copies of syncthingMain

* fix simple build errors

* move stuff from main to syncthing with minimal mod

* merge runtime options

* actually use syncthing.App

* pass io.writer to lib/syncthing for auditing

* get rid of env stuff in lib/syncthing

* add .Error() and comments

* review: Remove fs interactions from lib

* and go 1.13 happened

* utility functions

lib/syncthing/auditservice.go

@@ -0,0 +1,69 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package syncthing
+
+import (
+	"encoding/json"
+	"io"
+
+	"github.com/syncthing/syncthing/lib/events"
+)
+
+// The auditService subscribes to events and writes these in JSON format, one
+// event per line, to the specified writer.
+type auditService struct {
+	w       io.Writer     // audit destination
+	stop    chan struct{} // signals time to stop
+	started chan struct{} // signals startup complete
+	stopped chan struct{} // signals stop complete
+}
+
+func newAuditService(w io.Writer) *auditService {
+	return &auditService{
+		w:       w,
+		stop:    make(chan struct{}),
+		started: make(chan struct{}),
+		stopped: make(chan struct{}),
+	}
+}
+
+// Serve runs the audit service.
+func (s *auditService) Serve() {
+	defer close(s.stopped)
+	sub := events.Default.Subscribe(events.AllEvents)
+	defer events.Default.Unsubscribe(sub)
+	enc := json.NewEncoder(s.w)
+
+	// We're ready to start processing events.
+	close(s.started)
+
+	for {
+		select {
+		case ev := <-sub.C():
+			enc.Encode(ev)
+		case <-s.stop:
+			return
+		}
+	}
+}
+
+// Stop stops the audit service.
+func (s *auditService) Stop() {
+	close(s.stop)
+}
+
+// WaitForStart returns once the audit service is ready to receive events, or
+// immediately if it's already running.
+func (s *auditService) WaitForStart() {
+	<-s.started
+}
+
+// WaitForStop returns once the audit service has stopped.
+// (Needed by the tests.)
+func (s *auditService) WaitForStop() {
+	<-s.stopped
+}