[WIP] Fix Modoboa crashes when Postfix maps become modified

[ntninja]

Description of the issue/feature this PR addresses:

Modoboa's Postfix map file generator will generate a broken modoboa-postfix-maps.chk file if it fails to validate the existing map files due to user changes. On next start (ie: the second start after a user map file change!) modoboa will then crash inexplicably.

Explanation of each commit:

1. Don't create invalid map files when detecting unexpected changes in a map file
2. Don't crash on startup when encountering invalid lines in the checksum file
3. Don't crash on startup when missing checksum file entries for only some (but not all) files

As can be seen from the commit dates these commits are rather old – it's been a while since I encountered this issue last.

Current behavior before PR:

modoboa    | Traceback (most recent call last):
modoboa    |   File "manage.py", line 22, in <module>
modoboa    |     execute_from_command_line(sys.argv)
modoboa    |   File "/usr/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
modoboa    |     utility.execute()
modoboa    |   File "/usr/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
modoboa    |     self.fetch_command(subcommand).run_from_argv(self.argv)
modoboa    |   File "/usr/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
modoboa    |     self.execute(*args, **cmd_options)
modoboa    |   File "/usr/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
modoboa    |     output = self.handle(*args, **options)
modoboa    |   File "/usr/lib/python3.6/site-packages/modoboa/core/management/commands/generate_postfix_maps.py", line 158, in handle
modoboa    |     self.__load_checksums(destdir)
modoboa    |   File "/usr/lib/python3.6/site-packages/modoboa/core/management/commands/generate_postfix_maps.py", line 53, in __load_checksums
modoboa    |     fname, dbtype, checksum = line.split(":")
modoboa    | ValueError: too many values to unpack (expected 3)

Desired behavior after PR is merged:

Modoboa should only log message: Cannot upgrade '…' map because it has been modified.

[ntninja]

Example of a generated broken checksum file:

sql-domains.cf:postgres:1a4b72269319d42e03660993c634dfc6
sql-domain-aliases.cf:postgres:3eba920e79502e0c6dfb16d07ad5005f
sql-aliases.cf:postgres:12c47c08e2f828ab1937962b5244b097
sql-maintain.cf:postgres:c799b6e34e9c23173e96544966ba6543
sql-sender-login-map.cf:postgres:{'dbtype': 'postgres', 'checksum': '91716f8df65c736cbde6abec6f7c8c2d'}
sql-transport.cf:postgres:2a6d892cebccb696d8afa5cf0d12fcd1
sql-relaydomains.cf:postgres:73639173f9966e099c012ec5a060d3c6
sql-spliteddomains-transport.cf:postgres:2bf57b102e7509835771ecc1fd0e8227
sql-relay-recipient-verification.cf:postgres:14289023719219c5d6d31e8294b2a505

[ntninja]

Hmm… the contents of mapobject.filename appears to be inconsistent: Sometimes it's a string, sometimes it's a dict…

[tonioo]

Are you sure about this line?

[tonioo]

Hmm… the contents of mapobject.filename appears to be inconsistent: Sometimes it's a string, sometimes it's a dict…

Do you have an example?