index: Index MusicBrainz album IDs from music files.

Index "MusicBrainz Album ID" ID3v2 frames as
"musicbrainzalbumid" media tags to facilitate downloading
cover art from coverartarchive.org.

Change-Id: Ie81017dd6f76ec355ee0d1daedfb7180cb70ad59

pkg/index/indextest/tests.go

@@ -406,6 +406,7 @@ func Index(t *testing.T, initIdx func() *index.Index) {
 		{"artist", "Test Artist"},
 		{"album", "Test Album"},
 		{"genre", "(20)Alternative"},
+		{"musicbrainzalbumid", "00000000-0000-0000-0000-000000000000"},
 		{"year", "1992"},
 		{"track", "1"},
 		{"disc", "2"},

pkg/index/receive.go

@@ -567,15 +567,16 @@ func indexMusic(r types.SizeReaderAt, wholeRef blob.Ref, mm *mutationMap) {
 	// Note: if you add to this map, please update
 	// pkg/search/query.go's MediaTagConstraint Tag docs.
 	tags := map[string]string{
-		"title":      tag.Title(),
-		"artist":     tag.Artist(),
-		"album":      tag.Album(),
-		"genre":      tag.Genre(),
-		"year":       yearStr,
-		"track":      trackStr,
-		"disc":       discStr,
-		"mediaref":   mediaRef.String(),
-		"durationms": durationStr,
+		"title":              tag.Title(),
+		"artist":             tag.Artist(),
+		"album":              tag.Album(),
+		"genre":              tag.Genre(),
+		"musicbrainzalbumid": tag.CustomFrames()["MusicBrainz Album Id"],
+		"year":               yearStr,
+		"track":              trackStr,
+		"disc":               discStr,
+		"mediaref":           mediaRef.String(),
+		"durationms":         durationStr,
 	}
 
 	for tag, value := range tags {

pkg/search/query.go

@@ -333,7 +333,7 @@ type FileConstraint struct {
 
 type MediaTagConstraint struct {
 	// Tag is the tag to match.
-	// For ID3, this includes: title, artist, album, genre, year, track, disc, mediaref, durationms.
+	// For ID3, this includes: title, artist, album, genre, musicbrainzalbumid, year, track, disc, mediaref, durationms.
 	Tag string `json:"tag"`
 
 	String *StringConstraint `json:"string,omitempty"`