refactor(scanner): consolidate split-exception wiring and drop hot-path lock

ArtistSplitExceptionsRx is called per tag mapping per scanned file across
concurrent goroutines; replace the mutex+joined-key cache with an atomic
pointer compared via slices.Equal. Route all participant call sites through
WithParticipantExceptions and a shared splitParticipantValues helper.

model/metadata/map_participants.go

@@ -94,15 +94,11 @@ func (md Metadata) processPerformers(participants model.Participants, rolesMbzId
 		roleIdx[role] = 0
 	}
 
-	conf := model.TagRolesConf()
-	conf.ExceptionsRx = model.ArtistSplitExceptionsRx()
+	conf := model.TagRolesConf().WithParticipantExceptions(model.TagPerformer)
 	titleCaser := cases.Title(language.Und)
 	for _, performer := range md.Pairs(model.TagPerformer) {
 		subRole := titleCaser.String(performer.Key())
-		names := []string{performer.Value()}
-		if len(conf.Split) > 0 {
-			names = filterDuplicatedOrEmptyValues(conf.SplitTagValue(names))
-		}
+		names := splitParticipantValues(conf, []string{performer.Value()})
 		for _, name := range names {
 			artist := model.Artist{
 				ID:              md.artistID(name),
@@ -177,6 +173,16 @@ func (md Metadata) buildArtists(names, sorts, mbids []string) []model.Artist {
 	return artists
 }
 
+// splitParticipantValues splits values by the conf separators, dropping
+// duplicated or empty entries. Values are returned unchanged when the conf
+// has no separators.
+func splitParticipantValues(conf model.TagConf, values []string) []string {
+	if len(conf.Split) == 0 {
+		return values
+	}
+	return filterDuplicatedOrEmptyValues(conf.SplitTagValue(values))
+}
+
 // getRoleValues returns the values of a role tag, splitting them if necessary
 func (md Metadata) getRoleValues(role model.TagName) []string {
 	values := md.Strings(role)
@@ -187,12 +193,8 @@ func (md Metadata) getRoleValues(role model.TagName) []string {
 	if conf.Split == nil {
 		conf = model.TagRolesConf()
 	}
-	if len(conf.Split) > 0 {
-		conf.ExceptionsRx = model.ArtistSplitExceptionsRx()
-		values = conf.SplitTagValue(values)
-		return filterDuplicatedOrEmptyValues(values)
-	}
-	return values
+	conf = conf.WithParticipantExceptions(role)
+	return splitParticipantValues(conf, values)
 }
 
 // getArtistValues returns the values of a single or multi artist tag, splitting them if necessary
@@ -209,12 +211,8 @@ func (md Metadata) getArtistValues(single, multi model.TagName) []string {
 	if conf.Split == nil {
 		conf = model.TagArtistsConf()
 	}
-	if len(conf.Split) > 0 {
-		conf.ExceptionsRx = model.ArtistSplitExceptionsRx()
-		vSingle = conf.SplitTagValue(vSingle)
-		return filterDuplicatedOrEmptyValues(vSingle)
-	}
-	return vSingle
+	conf = conf.WithParticipantExceptions(single)
+	return splitParticipantValues(conf, vSingle)
 }
 
 func (md Metadata) mapDisplayName(singularTagName, pluralTagName model.TagName) string {

model/tag_mappings.go

@@ -7,6 +7,7 @@ import (
 	"slices"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"unicode"
 	"unicode/utf8"
 
@@ -129,25 +130,25 @@ func compileExceptionsRegex(exceptions []string) *regexp.Regexp {
 	return rx
 }
 
-var artistSplitExceptions struct {
-	sync.Mutex
-	key string
-	rx  *regexp.Regexp
+type artistSplitExceptionsCache struct {
+	names []string
+	rx    *regexp.Regexp
 }
 
+var artistSplitExceptions atomic.Pointer[artistSplitExceptionsCache]
+
 // ArtistSplitExceptionsRx returns the regex for Scanner.ArtistSplitExceptions,
 // or nil if none are configured. Compiled lazily (config hooks only run once
 // per process, before tests can override the option) and cached until the
-// configured list changes.
+// configured list changes. Lock-free on the cache-hit path, as this is called
+// per tag mapping per scanned file, across concurrent scanner goroutines.
 func ArtistSplitExceptionsRx() *regexp.Regexp {
-	c := &artistSplitExceptions
-	c.Lock()
-	defer c.Unlock()
-	key := strings.Join(conf.Server.Scanner.ArtistSplitExceptions, "\x00")
-	if c.key != key {
-		c.key = key
-		c.rx = compileExceptionsRegex(conf.Server.Scanner.ArtistSplitExceptions)
+	names := conf.Server.Scanner.ArtistSplitExceptions
+	if c := artistSplitExceptions.Load(); c != nil && slices.Equal(c.names, names) {
+		return c.rx
 	}
+	c := &artistSplitExceptionsCache{names: slices.Clone(names), rx: compileExceptionsRegex(names)}
+	artistSplitExceptions.Store(c)
 	return c.rx
 }