Make configsource Watchable an optional interface

config/experimental/configsource/component.go

@@ -31,12 +31,6 @@ var ErrSessionClosed = errors.New("parent session was closed")
 // specific error must use errors.Is.
 var ErrValueUpdated = errors.New("configuration must retrieve the updated value")
 
-// ErrWatcherNotSupported is returned by WatchForUpdate functions when the configuration
-// source can't watch for updates on the value.
-// This error can be wrapped with additional information. Callers trying to identify this
-// specific error must use errors.Is.
-var ErrWatcherNotSupported = errors.New("value watcher is not supported")
-
 // ConfigSource is the interface to be implemented by objects used by the collector
 // to retrieve external configuration information.
 type ConfigSource interface {
@@ -84,14 +78,13 @@ type Session interface {
 type Retrieved interface {
  // Value is the retrieved data that will be injected on the configuration.
  Value() interface{}
+}
 
+// Watchable is an optional interface that Retrieved can implement if the given source
+// supports monitoring for updates.
+type Watchable interface {
  // WatchForUpdate is used to monitor for updates on the retrieved value.
- //
- // If a watcher is not supported by the configuration store in general or for the specific
- // retrieved value the WatchForUpdate must immediately return ErrWatcherNotSupported or
- // an error wrapping it.
- //
- // When watching is supported WatchForUpdate must not return until one of the following happens:
+ // It must not return until one of the following happens:
  //
  // 1. An update is detected for the monitored value. In this case the function should
  // return ErrValueUpdated or an error wrapping it.

config/internal/configsource/manager.go

@@ -161,7 +161,7 @@ type Manager struct {
  // into the configuration.
  sessions map[string]configsource.Session
  // watchers keeps track of all WatchForUpdate functions for retrieved values.
- watchers []func() error
+ watchers []configsource.Watchable
  // watchersWG is used to ensure that Close waits for all WatchForUpdate calls
  // to complete.
  watchersWG sync.WaitGroup
@@ -220,18 +220,13 @@ func (m *Manager) WatchForUpdate() error {
  doneCh := make(chan struct{})
  defer close(doneCh)
 
- for _, watcher := range m.watchers {
+ for i := range m.watchers {
+ watcher := m.watchers[i]
  m.watchersWG.Add(1)
- watcherFn := watcher
  go func() {
  defer m.watchersWG.Done()
-
- err := watcherFn()
+ err := watcher.WatchForUpdate()
  switch {
- case errors.Is(err, configsource.ErrWatcherNotSupported):
- // The watcher for the retrieved value is not supported, nothing to
- // do, just exit from the goroutine.
- return
  case errors.Is(err, configsource.ErrSessionClosed):
  // The Session from which this watcher was retrieved is being closed.
  // There is no error to report, just exit from the goroutine.
@@ -356,7 +351,9 @@ func (m *Manager) expandConfigSource(ctx context.Context, cfgSrc configsource.Co
  return nil, fmt.Errorf("config source %q failed to retrieve value: %w", cfgSrcName, err)
  }
 
- m.watchers = append(m.watchers, retrieved.WatchForUpdate)
+ if watcher, okWatcher := retrieved.(configsource.Watchable); okWatcher {
+ m.watchers = append(m.watchers, watcher)
+ }
 
  return retrieved.Value(), nil
 }

config/internal/configsource/manager_test.go

@@ -602,17 +602,17 @@ func (t *testConfigSource) Retrieve(ctx context.Context, selector string, params
  return nil, fmt.Errorf("no value for selector %q", selector)
  }
 
- watchForUpdateFn := func() error {
- return configsource.ErrWatcherNotSupported
- }
-
  if entry.WatchForUpdateFn != nil {
- watchForUpdateFn = entry.WatchForUpdateFn
+ return &watchableRetrieved{
+ retrieved: retrieved{
+ value: entry.Value,
+ },
+ watchForUpdateFn: entry.WatchForUpdateFn,
+ }, nil
  }
 
  return &retrieved{
- value: entry.Value,
- watchForUpdateFn: watchForUpdateFn,
+ value: entry.Value,
  }, nil
 }
 
@@ -625,8 +625,7 @@ func (t *testConfigSource) Close(context.Context) error {
 }
 
 type retrieved struct {
- value interface{}
- watchForUpdateFn func() error
+ value interface{}
 }
 
 var _ configsource.Retrieved = (*retrieved)(nil)
@@ -635,6 +634,11 @@ func (r *retrieved) Value() interface{} {
  return r.value
 }
 
-func (r *retrieved) WatchForUpdate() error {
+type watchableRetrieved struct {
+ retrieved
+ watchForUpdateFn func() error
+}
+
+func (r *watchableRetrieved) WatchForUpdate() error {
  return r.watchForUpdateFn()
 }