[meld] filediff, matchers.helpers: Improve matcher process clean up (#565)

[Kai Willadsen <kaiw src gnome org>]

commit 8ed5befe2a7c9d5bbacb109112b3c01978c54005
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sat Mar 20 09:37:50 2021 +1000

    filediff, matchers.helpers: Improve matcher process clean up (#565)
    
    We try to handle shut down and clean up of our multiprocessing matcher
    process when we remove FileDiff tabs, but this wasn't working properly.
    We know, for a start, that we have some garbage collection issues around
    FileDiff in general.
    
    Since __del__ is not guaranteed to get called (and in fact was not
    getting called at all for this specific case in some minimal testing),
    this commit moves the logic to a specific stop() call, which is invoked
    in our FileDiff close handling.
    
    This commit also cleans up our handling slightly to avoid spurious
    errors when we close a file comparison tab before the matcher process
    has been started (since it starts on idle).

 meld/filediff.py         | 10 +++++-----
 meld/matchers/helpers.py | 11 +++++++----
 2 files changed, 12 insertions(+), 9 deletions(-)
---
diff --git a/meld/filediff.py b/meld/filediff.py
index e40f1293..d06c538f 100644
--- a/meld/filediff.py
+++ b/meld/filediff.py
@@ -1246,12 +1246,12 @@ class FileDiff(Gtk.VBox, MeldDoc):
             for buf in self.textbuffer:
                 buf.data.disconnect_monitor()
 
-            # TODO: This should not be necessary; remove if and when we
-            # figure out what's keeping MeldDocs alive for too long.
             try:
-                del self._cached_match
-            except AttributeError:
-                pass
+                self._cached_match.stop()
+            except Exception:
+                # Ignore any cross-process exceptions that happen when
+                # shutting down our matcher process.
+                log.exception('Failed to shut down matcher process')
             # TODO: Base the return code on something meaningful for VC tools
             self.close_signal.emit(0)
         return response
diff --git a/meld/matchers/helpers.py b/meld/matchers/helpers.py
index 3a3d1372..7e0f7e88 100644
--- a/meld/matchers/helpers.py
+++ b/meld/matchers/helpers.py
@@ -68,11 +68,14 @@ class CachedSequenceMatcher:
         self.queued_matches = {}
         GLib.idle_add(self.thread.start)
 
-    def __del__(self):
+    def stop(self) -> None:
         self.tasks.put((MatcherWorker.END_TASK, ('', '')))
-        self.thread.join(self.TASK_GRACE_PERIOD)
-        if self.thread.exitcode is None:
-            self.thread.terminate()
+        if self.thread.is_alive():
+            self.thread.join(self.TASK_GRACE_PERIOD)
+            if self.thread.exitcode is None:
+                self.thread.terminate()
+        self.cache = {}
+        self.queued_matches = {}
 
     def match(self, text1, textn, cb):
         texts = (text1, textn)