tito: Update changelog automatically on release

Fixes #983

.tito/custom/abrt/__init__.py

@@ -1,13 +1,43 @@
-from fileinput import FileInput
 import os
 import re
 import shlex
+from datetime import date
+from fileinput import FileInput
 
 from tito.common import info_out, run_command
 from tito.tagger import VersionTagger
 
 
 class Tagger(VersionTagger):
+    CHANGELOG_FILE = 'CHANGELOG.md'
+
+    def _update_changelog(self, new_version: str):
+        """
+        Update changelog with the new version. This entails renaming headings
+        in the Markdown file and updating links to compare the corresponding
+        commits on GitHub.
+        """
+        with FileInput(self.CHANGELOG_FILE, inplace=True) as changelog:
+            for line in changelog:
+                if line.startswith('## [Unreleased]'):
+                    # Add a heading for the release right below "Unreleased",
+                    # inheriting its contents. This means that changes that were
+                    # unreleased until now have become released in this new version.
+                    release_date = date.today().strftime('%Y-%m-%d')
+                    line += f'\n## [{new_version}] - {release_date}\n'
+                elif line.startswith('[Unreleased]:'):
+                    # Update link to comparison of changes on GitHub.
+                    match = re.search(r'(https://.+/compare/)(.+)\.\.\.HEAD', line)
+                    assert match is not None
+                    url_prefix = match[1]
+                    old_version = match[2]
+                    line = (f'[Unreleased]: {url_prefix}{new_version}...HEAD\n'
+                            f'[{new_version}]: {url_prefix}{new_version}...{old_version}\n')
+
+                print(line, end='')
+
+        run_command(f'git add -- {self.CHANGELOG_FILE}')
+
     def _bump_version(self, release=False, zstream=False):
         version = super()._bump_version().split('-', maxsplit=1)[0]
         pattern = re.compile(r'(?<=^m4_define\(\[faf_version\], \[)'
@@ -27,6 +57,7 @@ def _tag_release(self):
         version = self._bump_version()
 
         self._check_tag_does_not_exist(version)
+        self._update_changelog(version)
         self._clear_package_metadata()
 
         metadata_file = os.path.join(self.rel_eng_dir, 'packages', self.project_name)