medialib: acquire priority access for Control

Contrary to Get and List requests, which acquire the priority access
since 7ef4e1ef, not all Control requests
may acquire the priority access safely currently.

Indeed, some of them indirectly acquire the internal medialibrary mutex
(for example, MediaLibrary::reload() calls startDiscoverer()), which may
be locked by a non-priority caller.

Acquire priority access for Control requests which do not suffer from
this problem.

modules/misc/medialibrary/medialibrary.cpp

@@ -506,6 +506,19 @@ bool MediaLibrary::Start()
 
 int MediaLibrary::Control( int query, va_list args )
 {
+    /*
+     * Contrary to Get and List requests, not all Control requests may acquire
+     * the priority access safely (without deadlocks) currently: some of them
+     * indirectly acquire the internal medialibrary mutex (for example,
+     * MediaLibrary::reload() calls startDiscoverer()), which may be locked by
+     * a non-priority caller.
+     *
+     * Therefore, priority access is only acquired for Control requests which
+     * do not suffer for this problem.
+     *
+     * See <https://code.videolan.org/videolan/vlc/-/merge_requests/223>
+     */
+
     switch ( query )
     {
         case VLC_ML_ADD_FOLDER:
@@ -553,6 +566,8 @@ int MediaLibrary::Control( int query, va_list args )
         }
         case VLC_ML_IS_INDEXED:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto mrl = va_arg( args, const char* );
             auto res = va_arg( args, bool* );
             *res = m_ml->isIndexed( mrl );
@@ -578,6 +593,8 @@ int MediaLibrary::Control( int query, va_list args )
             break;
         case VLC_ML_NEW_EXTERNAL_MEDIA:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto mrl = va_arg( args, const char* );
             auto media = m_ml->addExternalMedia( mrl, -1 );
             if ( media == nullptr )
@@ -587,6 +604,8 @@ int MediaLibrary::Control( int query, va_list args )
         }
         case VLC_ML_NEW_STREAM:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto mrl = va_arg( args, const char* );
             auto media = m_ml->addStream( mrl );
             if ( media == nullptr )
@@ -596,6 +615,8 @@ int MediaLibrary::Control( int query, va_list args )
         }
         case VLC_ML_MEDIA_GENERATE_THUMBNAIL:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto mediaId = va_arg( args, int64_t );
             auto sizeType = va_arg( args, int );
             auto width = va_arg( args, uint32_t );
@@ -618,9 +639,15 @@ int MediaLibrary::Control( int query, va_list args )
         case VLC_ML_MEDIA_REMOVE_BOOKMARK:
         case VLC_ML_MEDIA_REMOVE_ALL_BOOKMARKS:
         case VLC_ML_MEDIA_UPDATE_BOOKMARK:
+        {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             return controlMedia( query, args );
+        }
         case VLC_ML_PLAYLIST_CREATE:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto name = va_arg( args, const char * );
             auto playlist = m_ml->createPlaylist( name );
             if ( playlist == nullptr )
@@ -631,12 +658,16 @@ int MediaLibrary::Control( int query, va_list args )
         }
         case VLC_ML_PLAYLIST_DELETE:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             if ( m_ml->deletePlaylist( va_arg( args, int64_t ) ) == false )
                 return VLC_EGENERIC;
             return VLC_SUCCESS;
         }
         case VLC_ML_PLAYLIST_APPEND:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto playlist = m_ml->playlist( va_arg( args, int64_t ) );
             if ( playlist == nullptr )
                 return VLC_EGENERIC;
@@ -646,6 +677,8 @@ int MediaLibrary::Control( int query, va_list args )
         }
         case VLC_ML_PLAYLIST_INSERT:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto playlist = m_ml->playlist( va_arg( args, int64_t ) );
             if ( playlist == nullptr )
                 return VLC_EGENERIC;
@@ -657,6 +690,8 @@ int MediaLibrary::Control( int query, va_list args )
         }
         case VLC_ML_PLAYLIST_MOVE:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto playlist = m_ml->playlist( va_arg( args, int64_t ) );
             if ( playlist == nullptr )
                 return VLC_EGENERIC;
@@ -668,6 +703,8 @@ int MediaLibrary::Control( int query, va_list args )
         }
         case VLC_ML_PLAYLIST_REMOVE:
         {
+            auto priorityAccess = m_ml->acquirePriorityAccess();
+
             auto playlist = m_ml->playlist( va_arg( args, int64_t ) );
             if ( playlist == nullptr )
                 return VLC_EGENERIC;