glamr: add backup_users management command

This can be used as a cron job on the production deployment to extract
backups of the user data.

mibios/glamr/management/commands/backup_users.py

@@ -0,0 +1,63 @@
+from datetime import datetime
+from pathlib import Path
+import shutil
+import tempfile
+
+from django.core.management import call_command
+from django.core.management.base import BaseCommand, CommandError
+
+
+class Command(BaseCommand):
+    help = 'Save backup of user data if data has changed since earlier backup.'
+
+    BACKUP_BASE = 'user_backup_'
+    BACKUP_SUFFIX = '.json'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--outdir',
+            default=None,
+            help='Directory to which backups get written.'
+        )
+
+    def handle(self, *args, outdir=None, **options):
+        if outdir is None:
+            outdir = Path()
+        else:
+            outdir = Path(outdir)
+
+        if not outdir.is_dir():
+            raise CommandError(f'no such directory: {outdir}')
+
+        # get most recent backup
+        old = None
+        for i in outdir.glob(f'{self.BACKUP_BASE}*{self.BACKUP_SUFFIX}'):
+            if old is None or old.name < i.name:
+                old = i
+
+        today = datetime.now().date()  # prints as YYYY-MM-DD
+        fname = self.BACKUP_BASE + str(today) + self.BACKUP_SUFFIX
+        with tempfile.TemporaryDirectory() as tmpd:
+            new = Path(tmpd) / fname
+            call_command(
+                'dumpdata',
+                'auth.User',
+                format='json',
+                indent=4,
+                database='default',
+                natural_foreign=True,
+                natural_primary=True,
+                output=new,
+            )
+
+            if old and old.read_text() == new.read_text():
+                self.stdout.write(f'No changes, {old} is already up-to-date\n')
+                return
+
+            dst = outdir / new.name
+            shutil.copyfile(new, dst)
+            self.stdout.write(f'Backup written to {dst}\n')
+            if old and old.name == new.name:
+                # multiple backups for same day
+                self.stdout.write('[WARNING] File with same name existed and '
+                                  'got overwritten!')