Honor GIT_AUTHOR_DATE from env

When invoked from a git hook we can use the environment to detect
the date for a commit instead of just using the current system date.

This makes handling more robust when amending a commit

.pylintrc

@@ -294,7 +294,7 @@ indent-string='    '
 max-line-length=120
 
 # Maximum number of lines in a module.
-max-module-lines=1000
+max-module-lines=2000
 
 # Allow the body of a class to be on the same line as the declaration if body
 # contains single statement.

Changelog.md

@@ -10,6 +10,7 @@ _WIP_
 * Make detection of comment style based on suffix case insensitive
 * Fix handling of files with unicode BOM
 * Allow selection of latest year vs range of years
+* Fix handling of author years when amending a commit
 
 v2.6.2
 ------

license_tools/__init__.py

@@ -62,6 +62,46 @@ def current_year():
             DateUtils._current_year = int(year)
         return DateUtils._current_year
 
+    @staticmethod
+    def parse_git_date(git_date: str) -> datetime.datetime:
+        """
+        Returns a datetime parsed from a git date string
+        as documented at https://git-scm.com/docs/git-commit/2.24.0#_date_formats
+        """
+        # Git internal format
+        try:
+            if git_date.startswith('@'):
+                unix_timestamp, tz_offset = git_date[1:].split(' ', maxsplit=1)
+                unix_timestamp = int(unix_timestamp)
+                timezone = datetime.timezone(datetime.datetime.strptime(tz_offset, '%z').utcoffset())
+                return datetime.datetime.fromtimestamp(unix_timestamp, tz=timezone)
+        except:  # pylint: disable=bare-except
+            pass
+
+        # RFC 2822
+        try:
+            from email.utils import parsedate_to_datetime  # pylint: disable=import-outside-toplevel
+            author_date = parsedate_to_datetime(git_date)
+            if author_date:
+                return author_date
+        except:  # pylint: disable=bare-except
+            pass
+
+        # ISO 8601
+        try:
+            return datetime.datetime.fromisoformat(git_date)
+        except:  # pylint: disable=bare-except
+            pass
+
+        # plain formats
+        for format in ['%Y.%m.%d', '%m/%d/%Y', '%d.%m.%Y']:
+            try:
+                return datetime.datetime.strptime(git_date, format)
+            except:  # pylint: disable=bare-except
+                pass
+
+        raise RuntimeError(f"Not a supported git date format: {git_date}")
+
 
 class Style(enum.Enum):
     """Enumerates the different known comment styles"""
@@ -258,7 +298,11 @@ def __init__(self, name: str, year_from: int = None,
         self.git_repo = git_repo
         self.name_from_git = git_repo is not None
         if year_to is None:
-            year_to = DateUtils.current_year()
+            git_author_date = os.environ.get('GIT_AUTHOR_DATE', None)
+            if self.name_from_git and git_author_date:
+                year_to = DateUtils.parse_git_date(git_author_date).year
+            else:
+                year_to = DateUtils.current_year()
         self.year_to = year_to
         if year_from:
             self.year_from = year_from

test.py

@@ -18,6 +18,7 @@
 # limitations under the License.
 
 from asyncio import subprocess
+import datetime
 import functools
 import os
 import pathlib
@@ -95,6 +96,23 @@ def test_recursive_includes(self):
                 raise
 
 
+class TestDateUtils(unittest.TestCase):
+
+    def test_git_date_format(self):
+        EXPRESSIONS = [
+            ('Thu, 07 Apr 2005 22:13:13 +0200', 2005, 4, 7),  # RFC 2822
+            ('2005-04-07T22:13:13', 2005, 4, 7),  # ISO 8601
+            ('1998.02.01', 1998, 2, 1),
+            ('01/02/1987', 1987, 1, 2),
+            ('@1453791344 +0100', 2016, 1, 26),  # git internal format
+        ]
+        for expression, year, month, day in EXPRESSIONS:
+            date = license_tools.DateUtils.parse_git_date(expression)
+            self.assertEqual(date.year, year)
+            self.assertEqual(date.month, month)
+            self.assertEqual(date.day, day)
+
+
 def parser_test(file: pathlib.Path):
     def file_wrapper(func):
         @functools.wraps(func)