Add contrib script to resolve conflicts

Asks the user when two iCalender objects conflict during a sync,
which one to take.

contrib/conflict_resolution/resolve_interactively.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python3
+"""Ask user to resolve a vdirsyncer sync conflict interactively.
+
+Needs a way to ask the user.
+The use of https://apps.kde.org/kdialog/ for GNU/Linix is hardcoded.
+
+Depends on python>3.5 and KDialog.
+
+Usage:
+  Ensure the file executable and use it in the vdirsyncer.conf file, e.g.
+
+  conflict_resolution = ["command", "/home/bern/vdirsyncer/resolve_interactively.py"]
+
+This file is Free Software under the following license:
+SPDX-License-Identifier: Apache-2.0
+SPDX-FileCopyrightText: 2021 Intevation GmbH <https://intevation.de>
+Author: <bernhard.reiter@intevation.de>
+"""
+from pathlib import Path
+import re
+import subprocess
+import sys
+
+KDIALOG = "/usr/bin/kdialog"
+
+SUMMARY_PATTERN = re.compile("^(SUMMARY:.*)$", re.MULTILINE)
+
+def get_summary(icalendar_text:str):
+    """Get the first SUMMARY: line from an iCalendar text.
+
+    Do not care about the line being continued.
+    """
+    match = re.search(SUMMARY_PATTERN, icalendar_text)
+    return match[1]
+
+
+def main(ical1_filename, ical2_filename):
+    ical1 = ical1_filename.read_text()
+    ical2 = ical2_filename.read_text()
+
+    additional_args = ["--yes-label", "take first"] # return code == 0
+    additional_args += ["--no-label", "take second"] # return code == 1
+    additional_args += ["--cancel-label", "do not resolve"] # return code == 2
+
+    r = subprocess.run(args = [
+        KDIALOG,
+        "--warningyesnocancel",
+        "There was a sync conflict, do you prefer the first entry: \n" +
+        get_summary(ical1) + "...\n(full contents: " + str(ical1_filename) +
+        " )\n\nor the second entry: \n" +
+        get_summary(ical2) + "...\n(full contents: " + str(ical2_filename) + 
+        " )?"
+    ] + additional_args)
+
+    if r.returncode == 2:
+        # cancel was pressed
+        return # shall lead to items not changed, because not copied
+
+    if r.returncode == 0:
+        # we want to take the first item, so overwrite the second
+        ical2_filename.write_text(ical1)
+    else: # r.returncode == 1, we want the second item, so overwrite the first
+        ical1_filename.write_text(ical2)
+
+if len(sys.argv) != 3:
+    sys.stdout.write(__doc__)
+else:
+    main(Path(sys.argv[1]), Path(sys.argv[2]))