lib/config, cmd/syncthing: Handle committing configuration better (fixes #3077)

This slightly changes the interface used for committing configuration
changes. The two parts are now:

 - VerifyConfiguration, which runs synchronously and locked, and can
   abort the config change. These callbacks shouldn't *do* anything
   apart from looking at the config changes and saying yes or no. No
   change from previously.

 - CommitConfiguration, which runs asynchronously (one goroutine per
   call) *after* replacing the config and releasing any locks. Returning
   false from these methods sets the "requires restart" flag, which now
   lives in the config.Wrapper.

This should be deadlock free as the CommitConfiguration calls can take
as long as they like and can wait for locks to be released when they
need to tweak things. I think this should be safe compared to before as
the CommitConfiguration calls were always made from a random background
goroutine (typically one from the HTTP server), so it was always
concurrent with everything else anyway.

Hence the CommitResponse type is gone, instead you get an error back on
verification failure only, and need to explicitly check
w.RequiresRestart() afterwards if you care.

As an added bonus this fixes a bug where we would reset the "requires
restart" indicator if a config that did not require restart was saved,
even if we already were in the requires-restart state.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3386

lib/config/wrapper.go

@@ -8,6 +8,7 @@ package config
 
 import (
 	"os"
+	"sync/atomic"
 
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/osutil"
@@ -41,11 +42,6 @@ type Committer interface {
 	String() string
 }
 
-type CommitResponse struct {
-	ValidationError error
-	RequiresRestart bool
-}
-
 // A wrapper around a Configuration that manages loads, saves and published
 // notifications of changes to registered Handlers
 
@@ -58,6 +54,8 @@ type Wrapper struct {
 	replaces  chan Configuration
 	subs      []Committer
 	mut       sync.Mutex
+
+	requiresRestart uint32 // an atomic bool
 }
 
 // Wrap wraps an existing Configuration structure and ties it to a file on
@@ -128,32 +126,21 @@ func (w *Wrapper) Raw() Configuration {
 }
 
 // Replace swaps the current configuration object for the given one.
-func (w *Wrapper) Replace(cfg Configuration) CommitResponse {
+func (w *Wrapper) Replace(cfg Configuration) error {
 	w.mut.Lock()
 	defer w.mut.Unlock()
+
 	return w.replaceLocked(cfg)
 }
 
-func (w *Wrapper) replaceLocked(to Configuration) CommitResponse {
+func (w *Wrapper) replaceLocked(to Configuration) error {
 	from := w.cfg
 
 	for _, sub := range w.subs {
 		l.Debugln(sub, "verifying configuration")
 		if err := sub.VerifyConfiguration(from, to); err != nil {
 			l.Debugln(sub, "rejected config:", err)
-			return CommitResponse{
-				ValidationError: err,
-			}
-		}
-	}
-
-	allOk := true
-	for _, sub := range w.subs {
-		l.Debugln(sub, "committing configuration")
-		ok := sub.CommitConfiguration(from, to)
-		if !ok {
-			l.Debugln(sub, "requires restart")
-			allOk = false
+			return err
 		}
 	}
 
@@ -161,8 +148,22 @@ func (w *Wrapper) replaceLocked(to Configuration) CommitResponse {
 	w.deviceMap = nil
 	w.folderMap = nil
 
-	return CommitResponse{
-		RequiresRestart: !allOk,
+	w.notifyListeners(from, to)
+
+	return nil
+}
+
+func (w *Wrapper) notifyListeners(from, to Configuration) {
+	for _, sub := range w.subs {
+		go w.notifyListener(sub, from, to)
+	}
+}
+
+func (w *Wrapper) notifyListener(sub Committer, from, to Configuration) {
+	l.Debugln(sub, "committing configuration")
+	if !sub.CommitConfiguration(from, to) {
+		l.Debugln(sub, "requires restart")
+		w.setRequiresRestart()
 	}
 }
 
@@ -182,7 +183,7 @@ func (w *Wrapper) Devices() map[protocol.DeviceID]DeviceConfiguration {
 
 // SetDevice adds a new device to the configuration, or overwrites an existing
 // device with the same ID.
-func (w *Wrapper) SetDevice(dev DeviceConfiguration) CommitResponse {
+func (w *Wrapper) SetDevice(dev DeviceConfiguration) error {
 	w.mut.Lock()
 	defer w.mut.Unlock()
 
@@ -218,7 +219,7 @@ func (w *Wrapper) Folders() map[string]FolderConfiguration {
 
 // SetFolder adds a new folder to the configuration, or overwrites an existing
 // folder with the same ID.
-func (w *Wrapper) SetFolder(fld FolderConfiguration) CommitResponse {
+func (w *Wrapper) SetFolder(fld FolderConfiguration) error {
 	w.mut.Lock()
 	defer w.mut.Unlock()
 
@@ -246,7 +247,7 @@ func (w *Wrapper) Options() OptionsConfiguration {
 }
 
 // SetOptions replaces the current options configuration object.
-func (w *Wrapper) SetOptions(opts OptionsConfiguration) CommitResponse {
+func (w *Wrapper) SetOptions(opts OptionsConfiguration) error {
 	w.mut.Lock()
 	defer w.mut.Unlock()
 	newCfg := w.cfg.Copy()
@@ -262,7 +263,7 @@ func (w *Wrapper) GUI() GUIConfiguration {
 }
 
 // SetGUI replaces the current GUI configuration object.
-func (w *Wrapper) SetGUI(gui GUIConfiguration) CommitResponse {
+func (w *Wrapper) SetGUI(gui GUIConfiguration) error {
 	w.mut.Lock()
 	defer w.mut.Unlock()
 	newCfg := w.cfg.Copy()
@@ -332,3 +333,11 @@ func (w *Wrapper) ListenAddresses() []string {
 	}
 	return util.UniqueStrings(addresses)
 }
+
+func (w *Wrapper) RequiresRestart() bool {
+	return atomic.LoadUint32(&w.requiresRestart) != 0
+}
+
+func (w *Wrapper) setRequiresRestart() {
+	atomic.StoreUint32(&w.requiresRestart, 1)
+}