player: deal with some corner cases with playlist navigation

The purpose is making accessing the current playlist entry saner when
commands are executed during initialization, termination, or after
playlist navigation commands.

For example, the "playlist_remove current" command will invalidate
playlist->current - but some things still access the playlist entry even
on uninit. Until now, checking stop_play implicitly took care of it, so
it worked, but it was still messy.

Introduce the mpctx->playing field, which points to the current playlist
entry, even if the entry was removed and/or the playlist's current entry
was moved (e.g. due to playlist navigation).

common/playlist.c

@@ -106,10 +106,18 @@ static void playlist_unlink(struct playlist *pl, struct playlist_entry *entry)
     entry->pl = NULL;
 }
 
+void playlist_entry_unref(struct playlist_entry *e)
+{
+    e->reserved--;
+    if (e->reserved < 0)
+        talloc_free(e);
+}
+
 void playlist_remove(struct playlist *pl, struct playlist_entry *entry)
 {
     playlist_unlink(pl, entry);
-    talloc_free(entry);
+    entry->removed = true;
+    playlist_entry_unref(entry);
 }
 
 void playlist_clear(struct playlist *pl)

common/playlist.h

@@ -41,6 +41,11 @@ struct playlist_entry {
     bool playback_short : 1;
     // Set to true if not at least 1 frame (audio or video) could be played.
     bool init_failed : 1;
+    // Entry was removed with playlist_remove (etc.), but not deallocated.
+    bool removed : 1;
+    // Additional refcount. Normally (reserved==0), the entry is owned by the
+    // playlist, and this can be used to keep the entry alive.
+    int reserved;
     // Used to reject loading of unsafe entries from external playlists.
     // Can have any of the following bit flags set:
     //  STREAM_SAFE_ONLY: only allow streams marked with is_safe
@@ -89,4 +94,6 @@ struct playlist_entry *playlist_entry_from_index(struct playlist *pl, int index)
 struct mpv_global;
 struct playlist *playlist_parse_file(const char *file, struct mpv_global *global);
 
+void playlist_entry_unref(struct playlist_entry *e);
+
 #endif

player/core.h

@@ -187,7 +187,8 @@ typedef struct MPContext {
     struct osd_progbar_state osd_progbar;
 
     struct playlist *playlist;
-    char *filename; // currently playing file
+    struct playlist_entry *playing; // currently playing file
+    char *filename; // always the same as playing->filename (or NULL)
     struct mp_resolve_result *resolve_result;
     enum stop_play_reason stop_play;
     unsigned int initialized_flags;  // which subsystems have been initialized

player/loadfile.c

@@ -1024,11 +1024,12 @@ static void play_current_file(struct MPContext *mpctx)
 
     reset_playback_state(mpctx);
 
-    if (mpctx->playlist->current)
-        mpctx->filename = mpctx->playlist->current->filename;
-
-    if (!mpctx->filename)
+    mpctx->playing = mpctx->playlist->current;
+    if (!mpctx->playing || !mpctx->playing->filename)
         goto terminate_playback;
+    mpctx->playing->reserved += 1;
+
+    mpctx->filename = mpctx->playing->filename;
 
     mpctx->add_osd_seek_info &= OSD_SEEK_INFO_EDITION;
 
@@ -1050,8 +1051,8 @@ static void play_current_file(struct MPContext *mpctx)
     if (opts->position_resume)
         mp_load_playback_resume(mpctx, mpctx->filename);
 
-    load_per_file_options(mpctx->mconfig, mpctx->playlist->current->params,
-                          mpctx->playlist->current->num_params);
+    load_per_file_options(mpctx->mconfig, mpctx->playing->params,
+                          mpctx->playing->num_params);
 
 #if HAVE_LIBASS
     if (opts->ass_style_override)
@@ -1083,7 +1084,7 @@ static void play_current_file(struct MPContext *mpctx)
     }
     int stream_flags = STREAM_READ;
     if (!opts->load_unsafe_playlists)
-        stream_flags |= mpctx->playlist->current->stream_flags;
+        stream_flags |= mpctx->playing->stream_flags;
     mpctx->stream = stream_create(stream_filename, stream_flags, mpctx->global);
     if (!mpctx->stream) { // error...
         mp_process_input(mpctx);
@@ -1321,18 +1322,14 @@ terminate_playback:
     if (mpctx->stop_play != PT_RESTART)
         m_config_restore_backups(mpctx->mconfig);
 
-    mpctx->filename = NULL;
     mpctx->resolve_result = NULL;
-    talloc_free(tmp);
 
-    // Played/paused for longer than 3 seconds -> ok
-    bool playback_short = mpctx->stop_play == AT_END_OF_FILE &&
-                (playback_start < 0 || mp_time_sec() - playback_start < 3.0);
-    bool init_failed = mpctx->stop_play == AT_END_OF_FILE &&
-                (mpctx->shown_aframes == 0 && mpctx->shown_vframes == 0);
-    if (mpctx->playlist->current && !mpctx->playlist->current_was_replaced) {
-        mpctx->playlist->current->playback_short = playback_short;
-        mpctx->playlist->current->init_failed = init_failed;
+    if (mpctx->playing && mpctx->stop_play == AT_END_OF_FILE) {
+        // Played/paused for longer than 3 seconds -> ok
+        mpctx->playing->playback_short =
+            playback_start < 0 || mp_time_sec() - playback_start < 3.0;
+        mpctx->playing->init_failed =
+            mpctx->shown_aframes == 0 && mpctx->shown_vframes == 0;
     }
 
     mp_notify(mpctx, MPV_EVENT_TRACKS_CHANGED, NULL);
@@ -1348,6 +1345,13 @@ terminate_playback:
     default:                end_event.reason = -1; break;
     };
     mp_notify(mpctx, MPV_EVENT_END_FILE, &end_event);
+
+    if (mpctx->playing)
+        playlist_entry_unref(mpctx->playing);
+    mpctx->playing = NULL;
+    mpctx->filename = NULL;
+
+    talloc_free(tmp);
 }
 
 // Determine the next file to play. Note that if this function returns non-NULL,