Add cert watcher for target allocator TLS config

Author: musa-asad

receiver/prometheusreceiver/targetallocator/manager.go

@@ -15,6 +15,7 @@ import (
 	"sort"
 	"time"
 
+	"github.com/fsnotify/fsnotify"
 	commonconfig "github.com/prometheus/common/config"
 	"github.com/prometheus/common/model"
 	promconfig "github.com/prometheus/prometheus/config"
@@ -36,6 +37,7 @@ type Manager struct {
 	scrapeManager          *scrape.Manager
 	discoveryManager       *discovery.Manager
 	enableNativeHistograms bool
+	watcher 			   *fsnotify.Watcher
 }
 
 func NewManager(set receiver.Settings, cfg *Config, promCfg *promconfig.Config, enableNativeHistograms bool) *Manager {
@@ -90,11 +92,70 @@ func (m *Manager) Start(ctx context.Context, host component.Host, sm *scrape.Man
 			}
 		}
 	}()
+
+	certFile := m.cfg.TLSSetting.CertFile
+	keyFile := m.cfg.TLSSetting.KeyFile
+	caFile := m.cfg.TLSSetting.CAFile
+
+	if (certFile != "") || (keyFile != "") || (caFile != "") {
+		watcher, werr := fsnotify.NewWatcher()
+		if werr != nil {
+			m.settings.Logger.Warn("Failed to create fsnotify watcher", zap.Error(werr))
+		} else {
+			m.watcher = watcher
+			go m.watchTLSFiles(watcher, &savedHash, httpClient)
+
+			if certFile != "" {
+				_ = watcher.Add(certFile)
+			}
+			if keyFile != "" {
+				_ = watcher.Add(keyFile)
+			}
+			if caFile != "" {
+				_ = watcher.Add(caFile)
+			}
+		}
+	}
+
 	return nil
 }
 
+func (m *Manager) watchTLSFiles(watcher *fsnotify.Watcher, savedHash *uint64, httpClient *http.Client) {
+	for {
+		select {
+		case event, ok := <-watcher.Events:
+			if !ok {
+				return
+			}
+			if event.Op&(fsnotify.Write|fsnotify.Create|fsnotify.Remove) != 0 {
+				m.settings.Logger.Info("TLS file changed; re-syncing config", zap.String("file", event.Name), zap.String("op", event.Op.String()))
+				if event.Op&fsnotify.Remove != 0 {
+					time.Sleep(200 * time.Millisecond)
+					_ = watcher.Add(event.Name)
+				}
+				hash, newErr := m.sync(*savedHash, httpClient)
+				if newErr != nil {
+					m.settings.Logger.Error(newErr.Error())
+					continue
+				}
+				*savedHash = hash
+			}
+		case werr, ok := <-watcher.Errors:
+			if !ok {
+				return
+			}
+			m.settings.Logger.Warn("fsnotify error", zap.Error(werr))
+		case <-m.shutdown:
+			return
+		}
+	}
+}
+
 func (m *Manager) Shutdown() {
 	close(m.shutdown)
+	if m.watcher != nil {
+		_ = m.watcher.Close()
+	}
 }
 
 // sync request jobs from targetAllocator and update underlying receiver, if the response does not match the provided compareHash.