fix(mirror): eliminate race conditions by using fresh factory inside task closures

Affected endpoints: apiMirrorsDrop, apiMirrorsUpdate.

Both endpoints shared the same architectural flaw as the previously fixed
publish, repos, and snapshot endpoints: operations were performed outside
the task lock, with stale DB state used inside the lock.

Issues Fixed:

1. apiMirrorsDrop - Collections created before task lock
   Problem: mirrorCollection and snapshotCollection created before task lock.
   Snapshot dependency check done with stale factory. Concurrent drops both
   load pre-task state, both see same snapshot dependencies. If snapshots
   created after pre-task check, can delete mirror used by snapshots.

   Fix: Create fresh taskCollectionFactory inside task, fresh load of mirror
   after lock acquired, fresh snapshot check with current factory, drop using
   fresh collections.

2. apiMirrorsUpdate - Mirror loaded before task lock
   Problem: remote loaded outside task, rename duplicate check with stale
   factory. Concurrent updates both load pre-task state, long-running update
   uses stale mirror reference. TOCTOU race: rename check passes, another
   creates mirror with same name, update saves with stale data.

   Fix: Create fresh taskCollectionFactory inside task, fresh load of mirror
   after lock acquired, pre-task rename validation, fresh rename check inside
   lock, use fresh mirror and collections for all operations.

Root cause analysis:

The fundamental issue is the split between pre-task work and task-protected
work. Collections and objects were being loaded before lock acquisition, then
stale copies used inside the lock.

Correct pattern (from fixed publish.go, repos.go, and snapshot.go):

1. HTTP Handler (before task lock):
   - Shallow load for 404 check only
   - Extract resource keys
   - Submit task with resources

2. Task Closure (after lock acquired):
   - Create fresh collectionFactory
   - Fresh load of all objects
   - LoadComplete on fresh copies
   - All mutations on fresh state
   - All checks atomic inside lock
   - Save using fresh collections

This ensures:
- Concurrent operations are serialized by task queue
- No stale DB state used for mutations
- No lost updates from concurrent modifications
- No TOCTOU races on duplicate checks
- No loss of mirrors used by snapshots
- No stale data in long-running updates

api/mirror.go

@@ -216,9 +216,9 @@ func apiMirrorsDrop(c *gin.Context) {
 	name := c.Params.ByName("name")
 	force := c.Request.URL.Query().Get("force") == "1"
 
+	// Phase 1: Pre-task validation (shallow load for 404 check only)
 	collectionFactory := context.NewCollectionFactory()
 	mirrorCollection := collectionFactory.RemoteRepoCollection()
-	snapshotCollection := collectionFactory.SnapshotCollection()
 
 	repo, err := mirrorCollection.ByName(name)
 	if err != nil {
@@ -228,21 +228,34 @@ func apiMirrorsDrop(c *gin.Context) {
 
 	resources := []string{string(repo.Key())}
 	taskName := fmt.Sprintf("Delete mirror %s", name)
+
 	maybeRunTaskInBackground(c, taskName, resources, func(_ aptly.Progress, _ *task.Detail) (*task.ProcessReturnValue, error) {
-		err := repo.CheckLock()
+		// Phase 2: Inside task lock - create fresh collections
+		taskCollectionFactory := context.NewCollectionFactory()
+		taskMirrorCollection := taskCollectionFactory.RemoteRepoCollection()
+		taskSnapshotCollection := taskCollectionFactory.SnapshotCollection()
+
+		// Fresh load after lock acquired
+		repo, err := taskMirrorCollection.ByName(name)
+		if err != nil {
+			return &task.ProcessReturnValue{Code: http.StatusInternalServerError, Value: nil}, fmt.Errorf("unable to drop: %v", err)
+		}
+
+		err = repo.CheckLock()
 		if err != nil {
 			return &task.ProcessReturnValue{Code: http.StatusInternalServerError, Value: nil}, fmt.Errorf("unable to drop: %v", err)
 		}
 
 		if !force {
-			snapshots := snapshotCollection.ByRemoteRepoSource(repo)
+			// Fresh checks with current collections
+			snapshots := taskSnapshotCollection.ByRemoteRepoSource(repo)
 
 			if len(snapshots) > 0 {
 				return &task.ProcessReturnValue{Code: http.StatusForbidden, Value: nil}, fmt.Errorf("won't delete mirror with snapshots, use 'force=1' to override")
 			}
 		}
 
-		err = mirrorCollection.Drop(repo)
+		err = taskMirrorCollection.Drop(repo)
 		if err != nil {
 			return &task.ProcessReturnValue{Code: http.StatusInternalServerError, Value: nil}, fmt.Errorf("unable to drop: %v", err)
 		}
@@ -550,6 +563,7 @@ func apiMirrorsUpdate(c *gin.Context) {
 		return
 	}
 
+	// Pre-task validation of new name if provided
 	if b.Name != remote.Name {
 		_, err = collection.ByName(b.Name)
 		if err == nil {
@@ -566,9 +580,26 @@ func apiMirrorsUpdate(c *gin.Context) {
 
 	resources := []string{string(remote.Key())}
 	maybeRunTaskInBackground(c, "Update mirror "+b.Name, resources, func(out aptly.Progress, detail *task.Detail) (*task.ProcessReturnValue, error) {
+		// Phase 2: Inside task lock - create fresh factory
+		taskCollectionFactory := context.NewCollectionFactory()
+		taskCollection := taskCollectionFactory.RemoteRepoCollection()
+
+		// Fresh load after lock acquired
+		remote, err := taskCollection.ByName(c.Params.ByName("name"))
+		if err != nil {
+			return &task.ProcessReturnValue{Code: http.StatusInternalServerError, Value: nil}, fmt.Errorf("unable to update: %s", err)
+		}
+
+		// Fresh rename check inside lock (if renaming)
+		if b.Name != remote.Name {
+			_, err := taskCollection.ByName(b.Name)
+			if err == nil {
+				return &task.ProcessReturnValue{Code: http.StatusConflict, Value: nil}, fmt.Errorf("unable to rename: mirror %s already exists", b.Name)
+			}
+		}
 
 		downloader := context.NewDownloader(out)
-		err := remote.Fetch(downloader, verifier, b.IgnoreSignatures)
+		err = remote.Fetch(downloader, verifier, b.IgnoreSignatures)
 		if err != nil {
 			return &task.ProcessReturnValue{Code: http.StatusInternalServerError, Value: nil}, fmt.Errorf("unable to update: %s", err)
 		}
@@ -780,8 +811,8 @@ func apiMirrorsUpdate(c *gin.Context) {
 		}
 
 		log.Info().Msgf("%s: Finalizing download...", b.Name)
-		_ = remote.FinalizeDownload(collectionFactory, out)
-		err = collectionFactory.RemoteRepoCollection().Update(remote)
+		_ = remote.FinalizeDownload(taskCollectionFactory, out)
+		err = taskCollection.Update(remote)
 		if err != nil {
 			return &task.ProcessReturnValue{Code: http.StatusInternalServerError, Value: nil}, fmt.Errorf("unable to update: %s", err)
 		}