Fix memory leak related to MerginProject object (fixes #78)

It turns out that we are creating MerginProject objects way too much,
and the cleanup of it may not be handled properly because of a cyclic
reference between MerginProject and its internal GeoDiff context object.

It is going to be both safer and faster to create MerginProject for
a particular working directory once and then keep it cached.

Fun fact: the leak is not showing up in "top" commend, only in
docker's stats about memory...

Author: wonder-sk

dbsync.py

@@ -173,9 +173,28 @@ def _print_mergin_changes(diff_dict):
         print("  removed: " + item['path'])
 
 
+# Dictionary used by _get_mergin_project() function below.
+# key = path to a local dir with Mergin project, value = cached MerginProject object
+cached_mergin_project_objects = {}
+
+
+def _get_mergin_project(work_path):
+    """
+    Returns a cached MerginProject object or creates one if it does not exist yet.
+    This is to avoid creating many of these objects (e.g. every pull/push) because it does
+    initialization of geodiff as well, so things should be 1. a bit faster, and 2. safer.
+    (Safer because we are having a cycle of refs between GeoDiff and MerginProject objects
+    related to logging - and untangling those would need some extra calls when we are done
+    with MerginProject. But since we use the object all the time, it's better to cache it anyway.)
+    """
+    if work_path not in cached_mergin_project_objects:
+        cached_mergin_project_objects[work_path] = MerginProject(work_path)
+    return cached_mergin_project_objects[work_path]
+
+
 def _get_project_version(work_path):
     """ Returns the current version of the project """
-    mp = MerginProject(work_path)
+    mp = _get_mergin_project(work_path)
     return mp.metadata["version"]
 
 
@@ -234,7 +253,7 @@ def pull(conn_cfg, mc):
     _check_has_working_dir(work_dir)
     _check_has_sync_file(gpkg_full_path)
 
-    mp = MerginProject(work_dir)
+    mp = _get_mergin_project(work_dir)
     mp.set_tables_to_skip(ignored_tables)
     if mp.geodiff is None:
         raise DbSyncError("Mergin Maps client installation problem: geodiff not available")
@@ -322,7 +341,7 @@ def status(conn_cfg, mc):
     _check_has_sync_file(gpkg_full_path)
 
     # get basic information
-    mp = MerginProject(work_dir)
+    mp = _get_mergin_project(work_dir)
     mp.set_tables_to_skip(ignored_tables)
     if mp.geodiff is None:
         raise DbSyncError("Mergin Maps client installation problem: geodiff not available")
@@ -394,7 +413,7 @@ def push(conn_cfg, mc):
     _check_has_working_dir(work_dir)
     _check_has_sync_file(gpkg_full_path)
 
-    mp = MerginProject(work_dir)
+    mp = _get_mergin_project(work_dir)
     mp.set_tables_to_skip(ignored_tables)
     if mp.geodiff is None:
         raise DbSyncError("Mergin Maps client installation problem: geodiff not available")