add group to make 3dt read journal[DO NOT MERGE]

3dt needs to read the journal. In order to do that it needs to be in
systemd-journal group. The issue is that coreos uses several backends for
users and groups. One is a regular /etc/passwd, /etc/groups and the other is
located under /usr/share/baselayout. If we try to append dcos_3dt user with
flag -G this will not work. (Tested on CoreOS beta (1068.3.0))
This seems to be a long running issue with CoreOS
coreos/bugs#312
However if we try to change a user's primary group with -g flag all works
as expected.

This is not meant to be merged, used for testing at this point.

packages/3dt/buildinfo.json

@@ -8,5 +8,6 @@
     "ref_origin": "master"
   },
   "username": "dcos_3dt",
+  "group": "systemd-journal",
   "state_directory": true
 }

pkgpanda/__init__.py

@@ -172,6 +172,10 @@ def state_directory(self):
     def username(self):
         return self.__pkginfo.get('username', None)
 
+    @property
+    def group(self):
+        return self.__pkginfo.get('group', None)
+
     def __repr__(self):
         return str(self.__id)
 
@@ -442,7 +446,14 @@ def validate_username(username):
         if not re.match(username_regex, username):
             raise ValidationError("Username must begin with `dcos_` and only have a-z and underscore after that")
 
-    def add_user(self, username):
+    @staticmethod
+    def validate_group(group):
+        try:
+            check_output(['getent', 'group', group])
+        except CalledProcessError as ex:
+            raise ValidationError("Group {} does not exist on the system".format(group))
+
+    def add_user(self, username, group):
         UserManagement.validate_username(username)
 
         if not self._manage_users:
@@ -463,14 +474,23 @@ def add_user(self, username):
                                   "automatic user addition is disabled".format(username))
 
         # Add the user:
+        add_user_cmd = [
+            'useradd',
+            '--system',
+            '--home-dir', '/opt/mesosphere',
+            '--shell', '/sbin/nologin',
+            '-c', 'DCOS System User',
+        ]
+
+        if group is not None:
+            add_user_cmd += [
+                '-g', group
+            ]
+
+        add_user_cmd += [username]
+
         try:
-            check_output([
-                'useradd',
-                '--system',
-                '--home-dir', '/opt/mesosphere',
-                '--shell', '/sbin/nologin',
-                '-c', 'DCOS System User',
-                username])
+            check_output(add_user_cmd)
             self._users.add(username)
         except CalledProcessError as ex:
             raise ValidationError("User {} doesn't exist and couldn't be created because of: {}"
@@ -681,7 +701,7 @@ def symlink_all(src, dest):
             # to something incompatible. We survive the first upgrade because everything goes from
             # root to specific users, and root can access all user files.
             if package.username is not None:
-                sysusers.add_user(package.username)
+                sysusers.add_user(package.username, package.group)
 
             # Ensure the state directory in `/var/lib/dcos` exists
             # TODO(cmaloney): On upgrade take a snapshot?

pkgpanda/build/__init__.py

@@ -846,6 +846,18 @@ def cache_abs(filename):
             raise BuildError("username in buildinfo.json didn't meet the validation rules. {}".format(ex))
         pkginfo['username'] = username
 
+    group = None
+    if builder.has('group'):
+        group = builder.take('group')
+        if not isinstance(group, str):
+            raise BuildError("group in buildinfo.json must be either not set (use default group for this user)"
+                             ", or group must be a string")
+        try:
+            pkgpanda.UserManagement.validate_group(group)
+        except ValidationError as ex:
+            raise BuildError("groupd in buildinfo.json didn't meet the validation rules. {}".format(ex))
+        pkginfo['group'] = group
+
     # Packages need directories inside the fake install root (otherwise docker
     # will try making the directories on a readonly filesystem), so build the
     # install root now, and make the package directories in it as we go.