Add a prolog and epilog for safe reading of the metadata bundle

metadata_hub.c

@@ -130,6 +130,18 @@ void metadata_hub_modify_epilog(int modified) {
   }
 }
 
+void metadata_hub_read_prolog(void) {
+  // always run this before reading an entry or a sequence of entries in the metadata_hub
+  // debug(1, "locking metadata hub for reading");
+  pthread_rwlock_rdlock(&metadata_hub_re_lock);
+}
+
+void metadata_hub_read_epilog(void) {
+  // always run this after reading an entry or a sequence of entries in the metadata_hub
+  // debug(1, "unlocking metadata hub for reading");
+  pthread_rwlock_unlock(&metadata_hub_re_lock);
+}
+
 char *metadata_write_image_file(const char *buf, int len) {
 
   // warning -- this removes all files from the directory apart from this one, if it exists

metadata_hub.h

@@ -87,7 +87,7 @@ typedef struct metadata_bundle {
 
   int speaker_volume; // this is the actual speaker volume, allowing for the main volume and the
                       // speaker volume control
-  int previous_speaker_volume; // this is needed to prevent a loop
+  // int previous_speaker_volume; // this is needed to prevent a loop
 
   metadata_watcher watchers[number_of_watchers]; // functions to call if the metadata is changed.
   void *watchers_data[number_of_watchers];       // their individual data
@@ -107,3 +107,7 @@ void metadata_hub_release_track_artwork(void);
 // afterwards
 void metadata_hub_modify_prolog(void);
 void metadata_hub_modify_epilog(int modified); // set to true if modifications occured, 0 otherwise
+
+// these are for safe reading
+void metadata_hub_read_prolog(void);
+void metadata_hub_read_epilog(void);