add album_save_post_processors to plug-in API.

I wanted to extend the M3U playlist plugin to automatically generate an .m3u8
when Albums get saved.

To be able to do so, I needed to add album_save_post_processors so that I could
get access to an album on save.

File save is done in a thread, so I needed to implement a way to wait for those
threads to finish by using the file_save_post_processors hook.

picard/album.py

@@ -753,3 +753,20 @@ def register_album_post_removal_processor(function, priority=PluginPriority.NORM
 
 def run_album_post_removal_processors(album_object):
     _album_post_removal_processors.run(album_object)
+
+_album_post_save_processors = PluginFunctions(label='album_post_save_processors')
+
+
+def register_album_post_save_processor(function, priority=PluginPriority.NORMAL):
+    """Registers an album-removed processor.
+    Args:
+        function: function to call after album save, it will be passed the album object
+        priority: optional, PluginPriority.NORMAL by default
+    Returns:
+        None
+    """
+    _album_post_save_processors.register(function.__module__, function, priority)
+
+
+def run_album_post_save_processors(album_object):
+    _album_post_save_processors.run(album_object)

picard/tagger.py

@@ -76,6 +76,7 @@
     Album,
     NatAlbum,
     run_album_post_removal_processors,
+    run_album_post_save_processors,
 )
 from picard.browser.browser import BrowserIntegration
 from picard.browser.filelookup import FileLookup
@@ -98,7 +99,7 @@
 )
 from picard.dataobj import DataObject
 from picard.disc import Disc
-from picard.file import File
+from picard.file import File, register_file_post_save_processor
 from picard.formats import open_ as open_file
 from picard.i18n import setup_gettext
 from picard.pluginmanager import PluginManager
@@ -294,6 +295,10 @@ def __init__(self, picard_args, unparsed_args, localedir, autoupdate):
         if self.autoupdate_enabled:
             self.updatecheckmanager = UpdateCheckManager(parent=self.window)
 
+        # track all file save completions for album save
+        self._album_saves = []
+        register_file_post_save_processor(self._file_post_save)
+
     def register_cleanup(self, func):
         self.exit_cleanup.append(func)
 
@@ -658,9 +663,14 @@ def get_files_from_objects(self, objects, save=False):
 
     def save(self, objects):
         """Save the specified objects."""
-        files = self.get_files_from_objects(objects, save=True)
-        for file in files:
-            file.save()
+        for o in objects:
+            log.debug('save object %r', o)
+            files = self.get_files_from_objects([o, ], save=True)
+            for file in files:
+                file.save()
+            if isinstance(o, Album):
+                # track the album and its files pending save
+                self._album_saves.append((o, files))
 
     def load_album(self, album_id, discid=None):
         album_id = self.mbid_redirects.get(album_id, album_id)
@@ -773,6 +783,17 @@ def remove(self, objects):
         if files:
             self.remove_files(files)
 
+    # run album_save hooks when the last file is saved
+    def _file_post_save(self, file):
+        for album, files in self._album_saves:
+            if file in files:
+                files.remove(file)
+            if not files:
+                log.debug("Album %r saved, running post_save_processors", album)
+                run_album_post_save_processors(album)
+                self._album_saves.remove((album, files))
+                return
+
     def _lookup_disc(self, disc, result=None, error=None):
         self.restore_cursor()
         if error is not None: