lib/config: Decouple VerifyConfiguration from Committer (#7939)

... and remove 8/10 implementations, which were no-ops. This saves code
and time copying configurations.

lib/config/wrapper.go

@@ -31,14 +31,14 @@ const (
 
 var errTooManyModifications = errors.New("too many concurrent config modifications")
 
-// The Committer interface is implemented by objects that need to know about
-// or have a say in configuration changes.
+// The Committer and Verifier interfaces are implemented by objects
+// that need to know about or have a say in configuration changes.
 //
 // When the configuration is about to be changed, VerifyConfiguration() is
-// called for each subscribing object, with the old and new configuration. A
-// nil error is returned if the new configuration is acceptable (i.e. does not
-// contain any errors that would prevent it from being a valid config).
-// Otherwise an error describing the problem is returned.
+// called for each subscribing object that implements it, with copies of the
+// old and new configuration. A nil error is returned if the new configuration
+// is acceptable (i.e., does not contain any errors that would prevent it from
+// being a valid config). Otherwise an error describing the problem is returned.
 //
 // If any subscriber returns an error from VerifyConfiguration(), the
 // configuration change is not committed and an error is returned to whoever
@@ -55,11 +55,16 @@ var errTooManyModifications = errors.New("too many concurrent config modificatio
 // configuration (e.g. calling Wrapper.SetFolder), that are also acquired in any
 // methods of the Committer interface.
 type Committer interface {
-	VerifyConfiguration(from, to Configuration) error
 	CommitConfiguration(from, to Configuration) (handled bool)
 	String() string
 }
 
+// A Verifier can determine if a new configuration is acceptable.
+// See the description for Committer, above.
+type Verifier interface {
+	VerifyConfiguration(from, to Configuration) error
+}
+
 // Waiter allows to wait for the given config operation to complete.
 type Waiter interface {
 	Wait()
@@ -300,6 +305,10 @@ func (w *wrapper) replaceLocked(to Configuration) (Waiter, error) {
 	}
 
 	for _, sub := range w.subs {
+		sub, ok := sub.(Verifier)
+		if !ok {
+			continue
+		}
 		l.Debugln(sub, "verifying configuration")
 		if err := sub.VerifyConfiguration(from.Copy(), to.Copy()); err != nil {
 			l.Debugln(sub, "rejected config:", err)