Try to fix sync issue in python client (#119)

Overwriting geopackages is dangerous and should be avoided.
If a geopackage is open in some other client (and there are wal/shm files next to it),
it may loose data or even completely break the database file.

Author: wonder-sk

mergin/client.py

@@ -271,7 +271,6 @@ def get_pull_changes(self, server_files):
         if not self.geodiff:
             return changes
 
-        size_limit = int(os.environ.get('DIFFS_LIMIT_SIZE', 1024 * 1024)) # with smaller values than limit download full file instead of diffs
         not_updated = []
         for file in changes['updated']:
             # for small geodiff files it does not make sense to download diff and then apply it (slow)
@@ -294,8 +293,7 @@ def get_pull_changes(self, server_files):
                     break  # we found force update in history, does not make sense to download diffs
 
             if is_updated:
-                if diffs and file['size'] > size_limit and diffs_size < file['size']/2:
-                    file['diffs'] = diffs
+                file['diffs'] = diffs
             else:
                 not_updated.append(file)
 
@@ -455,9 +453,19 @@ def apply_pull_changes(self, changes, temp_dir):
                             if os.path.exists(f'{dest}-shm'):
                                 os.remove(f'{dest}-shm')
                     else:
-                        # just use server version of file to update both project file and its basefile
-                        shutil.copy(src, dest)
-                        shutil.copy(src, basefile)
+                        # The local file is not modified -> no rebase needed.
+                        # We just apply the diff between our copy and server to both the local copy and its basefile
+                        try:
+                            server_diff = self.fpath(f'{path}-server_diff', temp_dir)  # diff between server file and local basefile
+                            self.geodiff.create_changeset(basefile, src, server_diff)
+                            self.geodiff.apply_changeset(dest, server_diff)
+                            self.geodiff.apply_changeset(basefile, server_diff)
+                        except (pygeodiff.GeoDiffLibError, pygeodiff.GeoDiffLibConflictError):
+                            # something bad happened and we have failed to patch our local files - this should not happen if there
+                            # wasn't a schema change or something similar that geodiff can't handle.
+                            # FIXME: this is a last resort and may corrupt data! (we should warn user)
+                            shutil.copy(src, dest)
+                            shutil.copy(src, basefile)
                 else:
                     # backup if needed
                     if path in modified and item['checksum'] != local_files_map[path]['checksum']: