Narrow down Mutex locked scope

While working on #112, I experienced some delay when replaying the whole
Icinga 2 Objects API state after an Event Stream connection loss. After
taking some pprof snapshots, some big unlocks occurred.

Thus, I minimized the scopes or code areas being mutex protected.
Especially in the GetCurrent function creates a long Lock even over SQL
queries.

However, in most experimental sessions the locks were insignificant,
while the SQL internals required huge time slots.

internal/incident/incidents.go

@@ -17,7 +17,7 @@ import (
 
 var (
 	currentIncidents   = make(map[*object.Object]*Incident)
-	currentIncidentsMu sync.Mutex
+	currentIncidentsMu sync.RWMutex
 )
 
 // LoadOpenIncidents loads all active (not yet closed) incidents from the database and restores all their states.
@@ -59,11 +59,11 @@ func GetCurrent(
 	ctx context.Context, db *icingadb.DB, obj *object.Object, logger *logging.Logger, runtimeConfig *config.RuntimeConfig,
 	create bool,
 ) (*Incident, bool, error) {
-	currentIncidentsMu.Lock()
-	defer currentIncidentsMu.Unlock()
-
 	created := false
+
+	currentIncidentsMu.RLock()
 	currentIncident := currentIncidents[obj]
+	currentIncidentsMu.RUnlock()
 
 	if currentIncident == nil {
 		ir := &IncidentRow{}
@@ -94,7 +94,9 @@ func GetCurrent(
 		}
 
 		if currentIncident != nil {
+			currentIncidentsMu.Lock()
 			currentIncidents[obj] = currentIncident
+			currentIncidentsMu.Unlock()
 		}
 	}
 
@@ -114,17 +116,16 @@ func RemoveCurrent(obj *object.Object) {
 	currentIncidentsMu.Lock()
 	defer currentIncidentsMu.Unlock()
 
-	currentIncident := currentIncidents[obj]
-
-	if currentIncident != nil {
-		delete(currentIncidents, obj)
-	}
+	delete(currentIncidents, obj)
 }
 
 // GetCurrentIncidents returns a map of all incidents for debugging purposes.
+//
+// NOTE: This map contains the same pointers as stored in currentIncidents, but are no longer protected with its mutex.
+// For accessing each Incident, use their internal sync.Mutex functions.
 func GetCurrentIncidents() map[int64]*Incident {
-	currentIncidentsMu.Lock()
-	defer currentIncidentsMu.Unlock()
+	currentIncidentsMu.RLock()
+	defer currentIncidentsMu.RUnlock()
 
 	m := make(map[int64]*Incident)
 	for _, incident := range currentIncidents {

internal/object/object.go

@@ -53,14 +53,13 @@ func FromEvent(ctx context.Context, db *icingadb.DB, ev *event.Event) (*Object,
 	id := ID(ev.SourceId, ev.Tags)
 
 	cacheMu.Lock()
-	defer cacheMu.Unlock()
-
 	object, ok := cache[id.String()]
 	if !ok {
 		object = NewObject(db, ev)
 		object.ID = id
 		cache[id.String()] = object
 	}
+	cacheMu.Unlock()
 
 	tx, err := db.BeginTxx(ctx, nil)
 	if err != nil {
@@ -104,10 +103,12 @@ func FromEvent(ctx context.Context, db *icingadb.DB, ev *event.Event) (*Object,
 		return nil, fmt.Errorf("can't commit object database transaction: %w", err)
 	}
 
+	cacheMu.Lock()
 	object.ExtraTags = ev.ExtraTags
 	object.Tags = ev.Tags
 	object.Name = ev.Name
 	object.URL = ev.URL
+	cacheMu.Unlock()
 
 	return object, nil
 }