fix(scanner): prevent race condition when scanning deleted folders

When the watcher detects changes in a folder that gets deleted before
the scanner runs (due to the 10-second delay), the scanner was
prematurely removing these folders from the tracking map, preventing
them from being marked as missing.

The issue occurred because `newFolderEntry` was calling `popLastUpdate`
before verifying the folder actually exists on the filesystem.

Changes:
- Move fs.Stat check before newFolderEntry creation in loadDir to
  ensure deleted folders remain in lastUpdates for finalize() to handle
- Add early existence check in walkDirTree to skip non-existent target
  folders with a warning log
- Add unit test verifying non-existent folders aren't removed from
  lastUpdates prematurely
- Add integration test for deleted folder scenario with ScanFolders

Fixes the issue where deleting entire folders (e.g., /music/AC_DC)
wouldn't mark tracks as missing when using selective folder scanning.

scanner/scanner_test.go

@@ -478,6 +478,56 @@ var _ = Describe("Scanner", Ordered, func() {
 			Expect(mf.Missing).To(BeFalse())
 		})
 
+		It("marks tracks as missing when scanning a deleted folder with ScanFolders", func() {
+			By("Adding a third track to Revolver to have more test data")
+			fsys.Add("The Beatles/Revolver/03 - I'm Only Sleeping.mp3", revolver(track(3, "I'm Only Sleeping")))
+			Expect(runScanner(ctx, false)).To(Succeed())
+
+			By("Verifying initial state has 5 tracks")
+			Expect(ds.MediaFile(ctx).CountAll(model.QueryOptions{
+				Filters: squirrel.Eq{"missing": false},
+			})).To(Equal(int64(5)))
+
+			By("Removing the entire Revolver folder from filesystem")
+			fsys.Remove("The Beatles/Revolver/01 - Taxman.mp3")
+			fsys.Remove("The Beatles/Revolver/02 - Eleanor Rigby.mp3")
+			fsys.Remove("The Beatles/Revolver/03 - I'm Only Sleeping.mp3")
+
+			By("Scanning the parent folder (simulating watcher behavior)")
+			targets := []model.ScanTarget{
+				{LibraryID: lib.ID, FolderPath: "The Beatles"},
+			}
+			_, err := s.ScanFolders(ctx, false, targets)
+			Expect(err).To(Succeed())
+
+			By("Checking all Revolver tracks are marked as missing")
+			mf, err := findByPath("The Beatles/Revolver/01 - Taxman.mp3")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(mf.Missing).To(BeTrue())
+
+			mf, err = findByPath("The Beatles/Revolver/02 - Eleanor Rigby.mp3")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(mf.Missing).To(BeTrue())
+
+			mf, err = findByPath("The Beatles/Revolver/03 - I'm Only Sleeping.mp3")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(mf.Missing).To(BeTrue())
+
+			By("Checking the Help! tracks are not affected")
+			mf, err = findByPath("The Beatles/Help!/01 - Help!.mp3")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(mf.Missing).To(BeFalse())
+
+			mf, err = findByPath("The Beatles/Help!/02 - The Night Before.mp3")
+			Expect(err).ToNot(HaveOccurred())
+			Expect(mf.Missing).To(BeFalse())
+
+			By("Verifying only 2 non-missing tracks remain (Help! tracks)")
+			Expect(ds.MediaFile(ctx).CountAll(model.QueryOptions{
+				Filters: squirrel.Eq{"missing": false},
+			})).To(Equal(int64(2)))
+		})
+
 		It("does not override artist fields when importing an undertagged file", func() {
 			By("Making sure artist in the DB contains MBID and sort name")
 			aa, err := ds.Artist(ctx).GetAll(model.QueryOptions{

scanner/walk_dir_tree.go

@@ -32,9 +32,18 @@ func walkDirTree(ctx context.Context, job *scanJob, targetFolders ...string) (<-
 				return
 			}
 
+			// Check if target folder exists before walking it
+			// If it doesn't exist (e.g., deleted between watcher detection and scan execution),
+			// skip it so it remains in job.lastUpdates and gets handled in following steps
+			_, err := fs.Stat(job.fs, folderPath)
+			if err != nil {
+				log.Warn(ctx, "Scanner: Target folder does not exist.", "path", folderPath, err)
+				continue
+			}
+
 			// Create checker and push patterns from root to this folder
 			checker := newIgnoreChecker(job.fs)
-			err := checker.PushAllParents(ctx, folderPath)
+			err = checker.PushAllParents(ctx, folderPath)
 			if err != nil {
 				log.Error(ctx, "Scanner: Error pushing ignore patterns for target folder", "path", folderPath, err)
 				continue
@@ -82,13 +91,16 @@ func walkFolder(ctx context.Context, job *scanJob, currentFolder string, checker
 }
 
 func loadDir(ctx context.Context, job *scanJob, dirPath string, checker *IgnoreChecker) (folder *folderEntry, children []string, err error) {
-	folder = newFolderEntry(job, dirPath)
-
+	// Check if directory exists before creating the folder entry
+	// This is important to avoid removing the folder from lastUpdates if it doesn't exist
 	dirInfo, err := fs.Stat(job.fs, dirPath)
 	if err != nil {
 		log.Warn(ctx, "Scanner: Error stating dir", "path", dirPath, err)
 		return nil, nil, err
 	}
+
+	// Now that we know the folder exists, create the entry (which removes it from lastUpdates)
+	folder = newFolderEntry(job, dirPath)
 	folder.modTime = dirInfo.ModTime()
 
 	dir, err := job.fs.Open(dirPath)

scanner/walk_dir_tree_test.go

@@ -182,6 +182,38 @@ var _ = Describe("walk_dir_tree", func() {
 				Expect(folders).ToNot(HaveKey("Artist/Album2"))
 				Expect(folders).ToNot(HaveKey("Artist/Album2/Sub"))
 			})
+
+			It("should skip non-existent target folders and preserve them in lastUpdates", func() {
+				// Setup job with lastUpdates for both existing and non-existing folders
+				job.lastUpdates = map[string]model.FolderUpdateInfo{
+					model.FolderID(job.lib, "Artist/Album1"):             {},
+					model.FolderID(job.lib, "NonExistent/DeletedFolder"): {},
+					model.FolderID(job.lib, "OtherArtist/Album3"):        {},
+				}
+
+				// Try to scan existing folder and non-existing folder
+				results, err := walkDirTree(ctx, job, "Artist/Album1", "NonExistent/DeletedFolder")
+				Expect(err).ToNot(HaveOccurred())
+
+				// Collect results
+				folders := map[string]struct{}{}
+				for folder := range results {
+					folders[folder.path] = struct{}{}
+				}
+
+				// Should only include the existing folder
+				Expect(folders).To(HaveKey("Artist/Album1"))
+				Expect(folders).ToNot(HaveKey("NonExistent/DeletedFolder"))
+
+				// The non-existent folder should still be in lastUpdates (not removed by popLastUpdate)
+				Expect(job.lastUpdates).To(HaveKey(model.FolderID(job.lib, "NonExistent/DeletedFolder")))
+
+				// The existing folder should have been removed from lastUpdates
+				Expect(job.lastUpdates).ToNot(HaveKey(model.FolderID(job.lib, "Artist/Album1")))
+
+				// Folders not in targets should remain in lastUpdates
+				Expect(job.lastUpdates).To(HaveKey(model.FolderID(job.lib, "OtherArtist/Album3")))
+			})
 		})
 	})