fix(scanner): add nil guards to cursor wrapping (#5139)

* fix(persistence): add nil guards to cursor wrapping in folder and mediafile repos

Prevent SIGSEGV panic when queryWithStableResults yields a zero-value
struct on the rows.Err() path (e.g., "database is locked" during
concurrent scanning). Extract cursor wrapping into wrapFolderCursor and
wrapMediaFileCursor with nil checks matching the existing pattern in
album_repository.go.

Fixes #5138

* fix(persistence): wrap original cursor error in nil guard messages

Use %w to preserve the underlying error (e.g., "database is locked")
so callers can use errors.Is/As for root cause analysis. Tests now
verify the original error is accessible via errors.Is.

* fix(persistence): add nil guards and error wrapping in album, folder, and mediafile cursor functions

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

persistence/album_repository.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"iter"
 	"maps"
 	"slices"
 	"strings"
@@ -302,17 +303,21 @@ func (r *albumRepository) GetTouchedAlbums(libID int) (model.AlbumCursor, error)
 	if err != nil {
 		return nil, err
 	}
+	return wrapAlbumCursor(cursor), nil
+}
+
+func wrapAlbumCursor(cursor iter.Seq2[dbAlbum, error]) model.AlbumCursor {
 	return func(yield func(model.Album, error) bool) {
 		for a, err := range cursor {
 			if a.Album == nil {
-				yield(model.Album{}, fmt.Errorf("unexpected nil album: %v", a))
+				yield(model.Album{}, fmt.Errorf("unexpected nil album (%v): %w", a, err))
 				return
 			}
 			if !yield(*a.Album, err) || err != nil {
 				return
 			}
 		}
-	}, nil
+	}
 }
 
 // RefreshPlayCounts updates the play count and last play date annotations for all albums, based

persistence/album_repository_test.go

@@ -1,6 +1,7 @@
 package persistence
 
 import (
+	"errors"
 	"fmt"
 	"time"
 
@@ -743,6 +744,46 @@ var _ = Describe("AlbumRepository", func() {
 			_, _ = albumRepo.executeSQL(squirrel.Delete("album").Where(squirrel.Eq{"id": album.ID}))
 		})
 	})
+
+	Describe("wrapAlbumCursor", func() {
+		It("does not panic when the cursor yields a dbAlbum with nil Album", func() {
+			// Simulate what queryWithStableResults does on the rows.Err() path:
+			// it yields a zero-value dbAlbum (where Album is nil) with an error.
+			dbErr := fmt.Errorf("database is locked")
+			cursor := func(yield func(dbAlbum, error) bool) {
+				var empty dbAlbum // Album pointer is nil
+				yield(empty, dbErr)
+			}
+
+			// wrapAlbumCursor should handle the nil Album without panicking
+			wrappedCursor := wrapAlbumCursor(cursor)
+			var gotErr error
+			Expect(func() {
+				for _, err := range wrappedCursor {
+					gotErr = err
+				}
+			}).ToNot(Panic())
+			Expect(gotErr).To(HaveOccurred())
+			Expect(gotErr.Error()).To(ContainSubstring("unexpected nil album"))
+			Expect(errors.Is(gotErr, dbErr)).To(BeTrue(), "should wrap the original cursor error")
+		})
+
+		It("yields albums from a valid cursor", func() {
+			album := &model.Album{ID: "a1", Name: "Test"}
+			cursor := func(yield func(dbAlbum, error) bool) {
+				yield(dbAlbum{Album: album}, nil)
+			}
+
+			wrappedCursor := wrapAlbumCursor(cursor)
+			var albums []model.Album
+			for a, err := range wrappedCursor {
+				Expect(err).ToNot(HaveOccurred())
+				albums = append(albums, a)
+			}
+			Expect(albums).To(HaveLen(1))
+			Expect(albums[0].ID).To(Equal("a1"))
+		})
+	})
 })
 
 func _p(id, name string, sortName ...string) model.Participant {

persistence/folder_repository.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"iter"
 	"maps"
 	"os"
 	"path/filepath"
@@ -218,13 +219,21 @@ func (r folderRepository) GetTouchedWithPlaylists() (model.FolderCursor, error)
 	if err != nil {
 		return nil, err
 	}
+	return wrapFolderCursor(cursor), nil
+}
+
+func wrapFolderCursor(cursor iter.Seq2[dbFolder, error]) model.FolderCursor {
 	return func(yield func(model.Folder, error) bool) {
 		for f, err := range cursor {
+			if f.Folder == nil {
+				yield(model.Folder{}, fmt.Errorf("unexpected nil folder (%v): %w", f, err))
+				return
+			}
 			if !yield(*f.Folder, err) || err != nil {
 				return
 			}
 		}
-	}, nil
+	}
 }
 
 func (r folderRepository) purgeEmpty(libraryIDs ...int) error {

persistence/folder_repository_test.go

@@ -2,6 +2,7 @@ package persistence
 
 import (
 	"context"
+	"errors"
 	"fmt"
 
 	"github.com/navidrome/navidrome/log"
@@ -210,4 +211,44 @@ var _ = Describe("FolderRepository", func() {
 			})
 		})
 	})
+
+	Describe("wrapFolderCursor", func() {
+		It("does not panic when the cursor yields a dbFolder with nil Folder", func() {
+			// Simulate what queryWithStableResults does on the rows.Err() path:
+			// it yields a zero-value dbFolder (where Folder is nil) with an error.
+			dbErr := fmt.Errorf("database is locked")
+			cursor := func(yield func(dbFolder, error) bool) {
+				var empty dbFolder // Folder pointer is nil
+				yield(empty, dbErr)
+			}
+
+			// wrapFolderCursor should handle the nil Folder without panicking
+			wrappedCursor := wrapFolderCursor(cursor)
+			var gotErr error
+			Expect(func() {
+				for _, err := range wrappedCursor {
+					gotErr = err
+				}
+			}).ToNot(Panic())
+			Expect(gotErr).To(HaveOccurred())
+			Expect(gotErr.Error()).To(ContainSubstring("unexpected nil folder"))
+			Expect(errors.Is(gotErr, dbErr)).To(BeTrue(), "should wrap the original cursor error")
+		})
+
+		It("yields folders from a valid cursor", func() {
+			folder := &model.Folder{ID: "f1", Name: "Test"}
+			cursor := func(yield func(dbFolder, error) bool) {
+				yield(dbFolder{Folder: folder}, nil)
+			}
+
+			wrappedCursor := wrapFolderCursor(cursor)
+			var folders []model.Folder
+			for f, err := range wrappedCursor {
+				Expect(err).ToNot(HaveOccurred())
+				folders = append(folders, f)
+			}
+			Expect(folders).To(HaveLen(1))
+			Expect(folders[0].ID).To(Equal("f1"))
+		})
+	})
 })

persistence/mediafile_repository.go

@@ -3,6 +3,7 @@ package persistence
 import (
 	"context"
 	"fmt"
+	"iter"
 	"slices"
 	"strconv"
 	"strings"
@@ -231,17 +232,7 @@ func (r *mediaFileRepository) GetCursor(options ...model.QueryOptions) (model.Me
 	if err != nil {
 		return nil, err
 	}
-	return func(yield func(model.MediaFile, error) bool) {
-		for m, err := range cursor {
-			if m.MediaFile == nil {
-				yield(model.MediaFile{}, fmt.Errorf("unexpected nil mediafile: %v", m))
-				return
-			}
-			if !yield(*m.MediaFile, err) || err != nil {
-				return
-			}
-		}
-	}, nil
+	return wrapMediaFileCursor(cursor), nil
 }
 
 // FindByPaths finds media files by their paths.
@@ -371,13 +362,21 @@ func (r *mediaFileRepository) GetMissingAndMatching(libId int) (model.MediaFileC
 	if err != nil {
 		return nil, err
 	}
+	return wrapMediaFileCursor(cursor), nil
+}
+
+func wrapMediaFileCursor(cursor iter.Seq2[dbMediaFile, error]) model.MediaFileCursor {
 	return func(yield func(model.MediaFile, error) bool) {
 		for m, err := range cursor {
+			if m.MediaFile == nil {
+				yield(model.MediaFile{}, fmt.Errorf("unexpected nil mediafile (%v): %w", m, err))
+				return
+			}
 			if !yield(*m.MediaFile, err) || err != nil {
 				return
 			}
 		}
-	}, nil
+	}
 }
 
 // FindRecentFilesByMBZTrackID finds recently added files by MusicBrainz Track ID in other libraries

persistence/mediafile_repository_test.go

@@ -2,6 +2,8 @@ package persistence
 
 import (
 	"context"
+	"errors"
+	"fmt"
 	"time"
 
 	"github.com/Masterminds/squirrel"
@@ -711,4 +713,44 @@ var _ = Describe("MediaRepository", func() {
 			Expect(results).To(BeEmpty())
 		})
 	})
+
+	Describe("wrapMediaFileCursor", func() {
+		It("does not panic when the cursor yields a dbMediaFile with nil MediaFile", func() {
+			// Simulate what queryWithStableResults does on the rows.Err() path:
+			// it yields a zero-value dbMediaFile (where MediaFile is nil) with an error.
+			dbErr := fmt.Errorf("database is locked")
+			cursor := func(yield func(dbMediaFile, error) bool) {
+				var empty dbMediaFile // MediaFile pointer is nil
+				yield(empty, dbErr)
+			}
+
+			// wrapMediaFileCursor should handle the nil MediaFile without panicking
+			wrappedCursor := wrapMediaFileCursor(cursor)
+			var gotErr error
+			Expect(func() {
+				for _, err := range wrappedCursor {
+					gotErr = err
+				}
+			}).ToNot(Panic())
+			Expect(gotErr).To(HaveOccurred())
+			Expect(gotErr.Error()).To(ContainSubstring("unexpected nil mediafile"))
+			Expect(errors.Is(gotErr, dbErr)).To(BeTrue(), "should wrap the original cursor error")
+		})
+
+		It("yields mediafiles from a valid cursor", func() {
+			mf := &model.MediaFile{ID: "mf1", Title: "Test"}
+			cursor := func(yield func(dbMediaFile, error) bool) {
+				yield(dbMediaFile{MediaFile: mf}, nil)
+			}
+
+			wrappedCursor := wrapMediaFileCursor(cursor)
+			var mediafiles []model.MediaFile
+			for m, err := range wrappedCursor {
+				Expect(err).ToNot(HaveOccurred())
+				mediafiles = append(mediafiles, m)
+			}
+			Expect(mediafiles).To(HaveLen(1))
+			Expect(mediafiles[0].ID).To(Equal("mf1"))
+		})
+	})
 })