feat: the library's folders are set in Admin › Library, and changing a folder moves its tracks with it, keeping their plays, likes and playlists (#6129)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/scanner.go

@@ -12,6 +12,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/dhowden/tag"
@@ -87,7 +88,11 @@ type Stats struct {
 type Scanner struct {
 	pool   *pgxpool.Pool
 	logger *slog.Logger
-	paths  []string
+	// paths are the library's folders, changed by SetRoots (#6129). Read them
+	// through Roots.
+	paths        []string
+	rootsMu      sync.RWMutex
+	rootsChanged chan struct{}
 	// fingerprint derives a file's acoustic identity (M400). A field so an
 	// integration test can substitute a deterministic one: CI has no real audio
 	// to fingerprint, and what the test pins is WHEN the scan fingerprints, not
@@ -99,7 +104,8 @@ type Scanner struct {
 }
 
 func New(pool *pgxpool.Pool, logger *slog.Logger, paths []string, settings *FingerprintSettingsService) *Scanner {
-	return &Scanner{pool: pool, logger: logger, paths: paths, fingerprint: computeFingerprint, settings: settings}
+	return &Scanner{pool: pool, logger: logger, paths: paths, rootsChanged: make(chan struct{}, 1),
+		fingerprint: computeFingerprint, settings: settings}
 }
 
 // Scan walks every configured root and upserts any audio file whose mtime is
@@ -206,7 +212,7 @@ func (s *Scanner) enumerate(
 ) ([]string, int) {
 	paths := make([]string, 0, 8192)
 	errs := 0
-	for _, root := range s.paths {
+	for _, root := range s.Roots() {
 		// WalkDir's own error return is folded into the per-entry handler below,
 		// so a bad root is counted rather than aborting the whole scan — one
 		// unreadable root shouldn't discard the others' results.
@@ -691,4 +697,11 @@ func IsAudioFile(path string) bool {
 }
 
 // Roots are the folders the scanner reads: the library.
-func (s *Scanner) Roots() []string { return append([]string(nil), s.paths...) }
+func (s *Scanner) Roots() []string {
+	if s == nil {
+		return nil
+	}
+	s.rootsMu.RLock()
+	defer s.rootsMu.RUnlock()
+	return append([]string(nil), s.paths...)
+}