Properly wait for multiprocesses to exit

* Fix for #693

core/pe/matchblock.py

@@ -9,7 +9,6 @@
 import logging
 import multiprocessing
 from itertools import combinations
-from time import sleep
 
 from hscommon.util import extract, iterconsume
 from hscommon.trans import tr
@@ -255,9 +254,7 @@ def getmatches(pictures, cache_path, threshold, match_scaled=False, j=job.nulljo
             ref.dimensions  # pre-read dimensions for display in results
             other.dimensions
             result.append(get_match(ref, other, percentage))
-    # HACK this is a workaround for when the progress bar gets stuck sometimes,
-    # as we enter a deadlock somewhere and the sub-processes become zombies.
-    sleep(1)
+    pool.join()
     return result