fix(scanner): restore basic tag extraction fallback mechanism for improved metadata parsing (#4401)

* feat: add basic tag extraction fallback mechanism

Added basic tag extraction from TagLib's generic Tag interface as a fallback
when PropertyMap doesn't contain standard metadata fields. This ensures that
essential tags like title, artist, album, comment, genre, year, and track
are always available even when they're not present in format-specific
property maps.

Changes include:
- Extract basic tags (__title, __artist, etc.) in C++ wrapper
- Add parseBasicTag function to process basic tags in Go extractor
- Refactor parseProp function to be reusable across property parsing
- Ensure basic tags are preferred over PropertyMap when available

* feat(taglib): update tag parsing to use double underscores for properties

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

---------

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

adapters/taglib/taglib.go

@@ -43,23 +43,21 @@ func (e extractor) extractMetadata(filePath string) (*metadata.Info, error) {
 
 	// Parse audio properties
 	ap := metadata.AudioProperties{}
-	if length, ok := tags["_lengthinmilliseconds"]; ok && len(length) > 0 {
-		millis, _ := strconv.Atoi(length[0])
-		if millis > 0 {
-			ap.Duration = (time.Millisecond * time.Duration(millis)).Round(time.Millisecond * 10)
-		}
-		delete(tags, "_lengthinmilliseconds")
-	}
-	parseProp := func(prop string, target *int) {
-		if value, ok := tags[prop]; ok && len(value) > 0 {
-			*target, _ = strconv.Atoi(value[0])
-			delete(tags, prop)
-		}
-	}
-	parseProp("_bitrate", &ap.BitRate)
-	parseProp("_channels", &ap.Channels)
-	parseProp("_samplerate", &ap.SampleRate)
-	parseProp("_bitspersample", &ap.BitDepth)
+	ap.BitRate = parseProp(tags, "__bitrate")
+	ap.Channels = parseProp(tags, "__channels")
+	ap.SampleRate = parseProp(tags, "__samplerate")
+	ap.BitDepth = parseProp(tags, "__bitspersample")
+	length := parseProp(tags, "__lengthinmilliseconds")
+	ap.Duration = (time.Millisecond * time.Duration(length)).Round(time.Millisecond * 10)
+
+	// Extract basic tags
+	parseBasicTag(tags, "__title", "title")
+	parseBasicTag(tags, "__artist", "artist")
+	parseBasicTag(tags, "__album", "album")
+	parseBasicTag(tags, "__comment", "comment")
+	parseBasicTag(tags, "__genre", "genre")
+	parseBasicTag(tags, "__year", "year")
+	parseBasicTag(tags, "__track", "tracknumber")
 
 	// Parse track/disc totals
 	parseTuple := func(prop string) {
@@ -107,6 +105,31 @@ var tiplMapping = map[string]string{
 	"DJ-mix":   "djmixer",
 }
 
+// parseProp parses a property from the tags map and sets it to the target integer.
+// It also deletes the property from the tags map after parsing.
+func parseProp(tags map[string][]string, prop string) int {
+	if value, ok := tags[prop]; ok && len(value) > 0 {
+		v, _ := strconv.Atoi(value[0])
+		delete(tags, prop)
+		return v
+	}
+	return 0
+}
+
+// parseBasicTag checks if a basic tag (like __title, __artist, etc.) exists in the tags map.
+// If it does, it moves the value to a more appropriate tag name (like title, artist, etc.),
+// and deletes the basic tag from the map. If the target tag already exists, it ignores the basic tag.
+func parseBasicTag(tags map[string][]string, basicName string, tagName string) {
+	basicValue := tags[basicName]
+	if len(basicValue) == 0 {
+		return
+	}
+	delete(tags, basicName)
+	if len(tags[tagName]) == 0 {
+		tags[tagName] = basicValue
+	}
+}
+
 // parseTIPL parses the ID3v2.4 TIPL frame string, which is received from TagLib in the format:
 //
 //	"arranger Andrew Powell engineer Chris Blair engineer Pat Stapley producer Eric Woolfson".

adapters/taglib/taglib_wrapper.cpp

@@ -45,31 +45,63 @@ int taglib_read(const FILENAME_CHAR_T *filename, unsigned long id) {
 
   // Add audio properties to the tags
   const TagLib::AudioProperties *props(f.audioProperties());
-  goPutInt(id, (char *)"_lengthinmilliseconds", props->lengthInMilliseconds());
-  goPutInt(id, (char *)"_bitrate", props->bitrate());
-  goPutInt(id, (char *)"_channels", props->channels());
-  goPutInt(id, (char *)"_samplerate", props->sampleRate());
+  goPutInt(id, (char *)"__lengthinmilliseconds", props->lengthInMilliseconds());
+  goPutInt(id, (char *)"__bitrate", props->bitrate());
+  goPutInt(id, (char *)"__channels", props->channels());
+  goPutInt(id, (char *)"__samplerate", props->sampleRate());
 
+  // Extract bits per sample for supported formats
+  int bitsPerSample = 0;
   if (const auto* apeProperties{ dynamic_cast<const TagLib::APE::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  apeProperties->bitsPerSample());
-  if (const auto* asfProperties{ dynamic_cast<const TagLib::ASF::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  asfProperties->bitsPerSample());
+      bitsPerSample = apeProperties->bitsPerSample();
+  else if (const auto* asfProperties{ dynamic_cast<const TagLib::ASF::Properties*>(props) })
+      bitsPerSample = asfProperties->bitsPerSample();
   else if (const auto* flacProperties{ dynamic_cast<const TagLib::FLAC::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  flacProperties->bitsPerSample());
+      bitsPerSample = flacProperties->bitsPerSample();
   else if (const auto* mp4Properties{ dynamic_cast<const TagLib::MP4::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  mp4Properties->bitsPerSample());
+      bitsPerSample = mp4Properties->bitsPerSample();
   else if (const auto* wavePackProperties{ dynamic_cast<const TagLib::WavPack::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  wavePackProperties->bitsPerSample());
+      bitsPerSample = wavePackProperties->bitsPerSample();
   else if (const auto* aiffProperties{ dynamic_cast<const TagLib::RIFF::AIFF::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  aiffProperties->bitsPerSample());
+      bitsPerSample = aiffProperties->bitsPerSample();
   else if (const auto* wavProperties{ dynamic_cast<const TagLib::RIFF::WAV::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  wavProperties->bitsPerSample());
+      bitsPerSample = wavProperties->bitsPerSample();
   else if (const auto* dsfProperties{ dynamic_cast<const TagLib::DSF::Properties*>(props) })
-      goPutInt(id, (char *)"_bitspersample",  dsfProperties->bitsPerSample());
+      bitsPerSample = dsfProperties->bitsPerSample();
+  
+  if (bitsPerSample > 0) {
+      goPutInt(id, (char *)"__bitspersample", bitsPerSample);
+  }
 
   // Send all properties to the Go map
   TagLib::PropertyMap tags = f.file()->properties();
 
+  // Make sure at least the basic properties are extracted
+  TagLib::Tag *basic = f.file()->tag();
+  if (!basic->isEmpty()) {
+    if (!basic->title().isEmpty()) {
+      tags.insert("__title", basic->title());
+    }
+    if (!basic->artist().isEmpty()) {
+      tags.insert("__artist", basic->artist());
+    }
+    if (!basic->album().isEmpty()) {
+      tags.insert("__album", basic->album());
+    }
+    if (!basic->comment().isEmpty()) {
+      tags.insert("__comment", basic->comment());
+    }
+    if (!basic->genre().isEmpty()) {
+      tags.insert("__genre", basic->genre());
+    }
+    if (basic->year() > 0) {
+      tags.insert("__year", TagLib::String::number(basic->year()));
+    }
+    if (basic->track() > 0) {
+      tags.insert("__track", TagLib::String::number(basic->track()));
+    }
+  }
+
   TagLib::ID3v2::Tag *id3Tags = NULL;
 
   // Get some extended/non-standard ID3-only tags (ex: iTunes extended frames)