fix(discovery): prevent ResumeScanning crash when message arrives in Idle state

When the Discovery actor was rate-limited, it sent delayed ResumeScanning
messages. However, immediate ResumeScanning messages from discoverChildren
could finish all work before the delayed message arrived, causing a state
transition to Idle. The delayed message then arrived with no handler,
crashing the actor.

Fix by tracking pending delayed messages and preventing discoverChildren
from sending immediate messages when one is pending. This keeps queued
work in queuedListOperations, preventing premature Idle transitions.

Also fixes summary map not being reset between discovery runs, which caused
stale counts to accumulate across runs.

Author: JeroenSoeters

internal/metastructure/discovery/discovery.go

@@ -104,6 +104,9 @@ type DiscoveryData struct {
 	// will not start new list operations, but will allow outstanding operations
 	// to complete. Uses reference counting to support multiple concurrent pausers.
 	pauseCount int
+	// hasPendingResumeScan tracks whether a delayed ResumeScanning message is in flight.
+	// This prevents sending redundant delayed messages when one is already pending.
+	hasPendingResumeScan bool
 }
 
 func (d *DiscoveryData) SetTargets(targets []*pkgmodel.Target) {
@@ -238,6 +241,7 @@ func resumeDiscovery(state gen.Atom, data DiscoveryData, message messages.Resume
 func discover(state gen.Atom, data DiscoveryData, message Discover, proc gen.Process) (gen.Atom, DiscoveryData, []statemachine.Action, error) {
 	data.isScheduledDiscovery = !message.Once
 	data.timeStarted = time.Now()
+	data.summary = make(map[string]int)
 
 	if state == StateDiscovering {
 		proc.Log().Debug("Discovery already running, consider configuring a longer interval")
@@ -310,6 +314,9 @@ func discover(state gen.Atom, data DiscoveryData, message Discover, proc gen.Pro
 }
 
 func resumeScanning(state gen.Atom, data DiscoveryData, message ResumeScanning, proc gen.Process) (gen.Atom, DiscoveryData, []statemachine.Action, error) {
+	// The delayed message has arrived - clear the pending flag
+	data.hasPendingResumeScan = false
+
 	// Don't start new list operations if Discovery is paused
 	if data.pauseCount > 0 {
 		proc.Log().Debug("Discovery is paused, skipping resumeScanning", "pauseCount", data.pauseCount)
@@ -344,11 +351,13 @@ func resumeScanning(state gen.Atom, data DiscoveryData, message ResumeScanning,
 	}
 	for namespace, done := range finished {
 		if !done {
+			// Send a delayed message to continue processing
 			_, err := proc.SendAfter(proc.PID(), ResumeScanning{}, 1*time.Second)
 			if err != nil {
 				proc.Log().Error("Discovery failed to send ResumeScanning message: %v", err)
 				return state, data, nil, gen.TerminateReasonPanic
 			}
+			data.hasPendingResumeScan = true
 			return state, data, nil, nil
 		}
 		delete(data.queuedListOperations, namespace)
@@ -604,10 +613,14 @@ func discoverChildren(op ListOperation, data DiscoveryData, proc gen.Process) er
 				ParentKSUID:  parent.Ksuid,
 				ListParams:   util.MapToString(listParams),
 			})
-			err = proc.Send(proc.PID(), ResumeScanning{})
-			if err != nil {
-				proc.Log().Error("Failed to send ResumeScanning", "error", err)
-				return fmt.Errorf("failed to send ResumeScanning: %w", err)
+			// Only send immediate ResumeScanning if no delayed message is pending.
+			// If a delayed message is pending, it will process the queued work when it arrives.
+			if !data.hasPendingResumeScan {
+				err = proc.Send(proc.PID(), ResumeScanning{})
+				if err != nil {
+					proc.Log().Error("Failed to send ResumeScanning", "error", err)
+					return fmt.Errorf("failed to send ResumeScanning: %w", err)
+				}
 			}
 		}
 	}