tag_ape: return false if no usable tag was found

Ignore APE tags that have no usable tags, and use the ID3 tag instead.
This is useful when the APE tag only contains replay gain, and the
real tags are stored as ID3.  This implements feature request Mantis
#0003521.

NEWS

@@ -3,6 +3,7 @@ ver 0.17.1 (2012/??/??)
   - require appropriate permissions for searchadd{,pl}
 * tags:
   - aiff: support the AIFC format
+  - ape: check for ID3 if no usable APE tag was found
 * playlist:
   - cue: support file types "MP3", "AIFF"
 * output:

src/tag_ape.c

@@ -40,43 +40,55 @@ tag_ape_name_parse(const char *name)
 	return type;
 }
 
-static void
+/**
+ * @return true if the item was recognized
+ */
+static bool
 tag_ape_import_item(unsigned long flags,
 		    const char *key, const char *value, size_t value_length,
 		    const struct tag_handler *handler, void *handler_ctx)
 {
 	/* we only care about utf-8 text tags */
 	if ((flags & (0x3 << 1)) != 0)
-		return;
+		return false;
 
 	tag_handler_invoke_pair(handler, handler_ctx, key, value);
 
 	enum tag_type type = tag_ape_name_parse(key);
 	if (type == TAG_NUM_OF_ITEM_TYPES)
-		return;
+		return false;
 
+	bool recognized = false;
 	const char *end = value + value_length;
 	while (true) {
 		/* multiple values are separated by null bytes */
 		const char *n = memchr(value, 0, end - value);
 		if (n != NULL) {
-			if (n > value)
+			if (n > value) {
 				tag_handler_invoke_tag(handler, handler_ctx,
 						       type, value);
+				recognized = true;
+			}
+
 			value = n + 1;
 		} else {
 			char *p = g_strndup(value, end - value);
 			tag_handler_invoke_tag(handler, handler_ctx,
 					       type, p);
 			g_free(p);
+			recognized = true;
 			break;
 		}
 	}
+
+	return recognized;
 }
 
 struct tag_ape_ctx {
 	const struct tag_handler *handler;
 	void *handler_ctx;
+
+	bool recognized;
 };
 
 static bool
@@ -85,8 +97,8 @@ tag_ape_callback(unsigned long flags, const char *key,
 {
 	struct tag_ape_ctx *ctx = _ctx;
 
-	tag_ape_import_item(flags, key, value, value_length,
-			    ctx->handler, ctx->handler_ctx);
+	ctx->recognized |= tag_ape_import_item(flags, key, value, value_length,
+					       ctx->handler, ctx->handler_ctx);
 	return true;
 }
 
@@ -97,7 +109,9 @@ tag_ape_scan2(const char *path_fs,
 	struct tag_ape_ctx ctx = {
 		.handler = handler,
 		.handler_ctx = handler_ctx,
+		.recognized = false,
 	};
 
-	return tag_ape_scan(path_fs, tag_ape_callback, &ctx);
+	return tag_ape_scan(path_fs, tag_ape_callback, &ctx) &&
+		ctx.recognized;
 }