stages: add org.osbuild.ostree.passwd

This stage takes /usr/lib/passwd and /usr/etc/passwd from an OSTree
checkout, merges them into one file, and store it as /etc/passwd in the
buildroot.

It does the same for /etc/group.

The reason for doing this is that there is an issue with unstable UIDs
and GIDs when creating OSTree commits from scratch. When there is a
package that creates a system user or a system group, it can change the
UID and GID of users and groups that are created later.

This is not a problem in traditional deployments because already created
users and groups never change their UIDs and GIDs, but with OSTree we
recreate the files from scratch and then replace the previous one so it
can actually change.

By copying the files to the build root before doing any other
operations, we can make sure that the UIDs and GIDs of already existing
users and groups won't change.

Co-author: Christian Kellner <christian@kellner.me>

osbuild/util/ostree.py

@@ -139,3 +139,50 @@ def deployment_path(root: PathLike, osname: str, ref: str, serial: int):
     sysroot = f"{stateroot}/deploy/{commit}.{serial}"
 
     return sysroot
+
+
+class PasswdLike:
+    """Representation of a file with structure like /etc/passwd
+
+    If each line in a file contains a key-value pair separated by the
+    first colon on the line, it can be considered "passwd"-like. This
+    class can parse the the list, manipulate it, and export it to file
+    again.
+    """
+    def __init__(self):
+        """Initialize an empty PasswdLike object"""
+        self.db = dict()
+
+    @classmethod
+    def from_file(cls, path: PathLike, allow_missing_file: bool=False):
+        """Initialize a PasswdLike object from an existing file"""
+        ret = cls()
+        if allow_missing_file:
+            if not os.path.isfile(path):
+                return ret
+
+        with open(path, "r") as p:
+            ret.db = cls._passwd_lines_to_dict(p.readlines())
+        return ret
+
+    def merge_with_file(self, path: PathLike, allow_missing_file: bool=False):
+        """Extend the database with entries from another file"""
+        if allow_missing_file:
+            if not os.path.isfile(path):
+                return
+
+        with open(path, "r") as p:
+            additional_passwd_dict = self._passwd_lines_to_dict(p.readlines())
+            for name, passwd_line in additional_passwd_dict.items():
+                if name not in self.db:
+                    self.db[name] = passwd_line
+
+    def dump_to_file(self, path: PathLike):
+        """Write the current database to a file"""
+        with open(path, "w") as p:
+            p.writelines(list(self.db.values()))
+
+    @staticmethod
+    def _passwd_lines_to_dict(lines):
+        """Take a list of passwd lines and produce a "name": "line" dictionary"""
+        return {line.split(':')[0]: line for line in lines}