Promtail: show a clearer reason in "disable watchConfig" log message …

…when server is disabled (#11420)

**What this PR does / why we need it**:
This PR improves the clarity of log message when the Promtail server is
disabled (if `server.disable` is true in config file, or flag
`-server.disable` is used).

Previously, the reason "promtailServer cast fail" in log message (as
below) was ambiguous, leading to confusion about potential error in
config file.
```
level=warn ts=2023-12-09T15:40:24.064376445Z caller=promtail.go:260 msg="disable watchConfig" reason="promtailServer cast fail"
```

The updated message is more understandable, indicating directly that the
Promtail server is disabled.
```
level=warn ts=2023-12-09T15:50:58.723168763Z caller=promtail.go:260 msg="disable watchConfig" reason="Promtail server is disabled"
```

**Which issue(s) this PR fixes**:
N/A.

**Special notes for your reviewer**:
Should we turn down the log level to INFO?

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [x] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](d10549e)
- [ ] If the change is deprecating or removing a configuration option,
update the `deprecated-config.yaml` and `deleted-config.yaml` files
respectively in the `tools/deprecated-config-checker` directory.
[Example
PR](0d4416a)

CHANGELOG.md

@@ -77,6 +77,7 @@
 
 * [10677](https://github.com/grafana/loki/pull/10677) **chaudum** Remove deprecated `stream_lag_labels` setting from both the `options` and `client` configuration sections.
 * [10689](https://github.com/grafana/loki/pull/10689) **dylanguedes**: Ingester: Make jitter to be 20% of flush check period instead of 1%.
+* [11420](https://github.com/grafana/loki/pull/11420) **zry98**: Show a clearer reason in "disable watchConfig" log message when server is disabled.
 
 ##### Fixes
 

clients/pkg/promtail/promtail.go

@@ -255,25 +255,29 @@ func (p *Promtail) watchConfig() {
 		level.Warn(p.logger).Log("msg", "disable watchConfig", "reason", "Promtail newConfig func is Empty")
 		return
 	}
-	promtailServer, ok := p.server.(*server.PromtailServer)
-	if !ok {
-		level.Warn(p.logger).Log("msg", "disable watchConfig", "reason", "promtailServer cast fail")
+	switch srv := p.server.(type) {
+	case *server.NoopServer:
+		level.Warn(p.logger).Log("msg", "disable watchConfig", "reason", "Promtail server is disabled")
 		return
-	}
-	level.Warn(p.logger).Log("msg", "enable watchConfig")
-	hup := make(chan os.Signal, 1)
-	signal.Notify(hup, syscall.SIGHUP)
-	for {
-		select {
-		case <-hup:
-			_ = p.reload()
-		case rc := <-promtailServer.Reload():
-			if err := p.reload(); err != nil {
-				rc <- err
-			} else {
-				rc <- nil
+	case *server.PromtailServer:
+		level.Warn(p.logger).Log("msg", "enable watchConfig")
+		hup := make(chan os.Signal, 1)
+		signal.Notify(hup, syscall.SIGHUP)
+		for {
+			select {
+			case <-hup:
+				_ = p.reload()
+			case rc := <-srv.Reload():
+				if err := p.reload(); err != nil {
+					rc <- err
+				} else {
+					rc <- nil
+				}
 			}
 		}
+	default:
+		level.Warn(p.logger).Log("msg", "disable watchConfig", "reason", "Unknown Promtail server type")
+		return
 	}
 }
 

clients/pkg/promtail/server/server.go

@@ -321,25 +321,25 @@ func computeExternalURL(u string, port int) (*url.URL, error) {
 	return eu, nil
 }
 
-type noopServer struct {
+type NoopServer struct {
 	log  log.Logger
 	sigs chan os.Signal
 }
 
-func newNoopServer(log log.Logger) *noopServer {
-	return &noopServer{
+func newNoopServer(log log.Logger) *NoopServer {
+	return &NoopServer{
 		log:  log,
 		sigs: make(chan os.Signal, 1),
 	}
 }
 
-func (s *noopServer) Run() error {
+func (s *NoopServer) Run() error {
 	signal.Notify(s.sigs, syscall.SIGINT, syscall.SIGTERM)
 	sig := <-s.sigs
 	level.Info(s.log).Log("msg", "received shutdown signal", "sig", sig)
 	return nil
 }
 
-func (s *noopServer) Shutdown() {
+func (s *NoopServer) Shutdown() {
 	s.sigs <- syscall.SIGTERM
 }