Copy host's users into buildroot on-demand

Fixes: rpm-software-management#1091

mock/docs/site-defaults.cfg

@@ -603,3 +603,11 @@
 # because by design Mock doesn't have to fully isolate, we disable seccomp for
 # those containerization tools by default.
 #config_opts["seccomp"] = False
+
+# List of usernames (strings) that will be pre-created in buildroot.  The UID
+# and GID in-chroot is going to be the same as on-host.  This option is for
+# example useful for the 'pesign' use-cases that both (a) bind-mount
+# pesign-owned socket-files into the chroot and (b) install the
+# 'BuildRequires: pesign' package which would overwrite the ownership of the
+# socket file.  See https://github.com/rpm-software-management/mock/issues/1091
+#config_opts["copy_host_users"] = []

mock/py/mockbuild/buildroot.py

@@ -476,6 +476,11 @@ def _make_users(self):
         )
         self._enable_chrootuser_account()
 
+        # Other users
+        for user in self.config.get("copy_host_users", []):
+            self.shadow_utils.copy_from_host(user)
+
+
     @traceLog()
     def _enable_chrootuser_account(self):
         passwd = self.make_chroot_path('/etc/passwd')

mock/py/mockbuild/config.py

@@ -333,6 +333,8 @@ def setup_default_config_opts():
 
     config_opts["seccomp"] = False
 
+    config_opts["copy_host_users"] = []
+
     # mapping from target_arch (or forcearch) to arch in /usr/bin/qemu-*-static
     config_opts["qemu_user_static_mapping"] = {
         'aarch64': 'aarch64',

mock/py/mockbuild/shadow_utils.py

@@ -59,3 +59,32 @@ def create_user(self, username, uid=None, gid=None, home=None):
         if home is not None:
             command += ["-d", str(home)]
         self._execute_command(command)
+
+    def copy_from_host(self, username):
+        """
+        Copy user (with uid/gid/group_name) from Host into the self.root.
+        """
+        try:
+            info = pwd.getpwnam(username)
+            uid = info.pw_uid
+            gid = info.pw_gid
+        except KeyError as err:
+            raise RuntimeError(
+                f"Can not find the requested user {username} "
+                "on host") from err
+
+        try:
+            group_name = grp.getgrgid(gid).gr_name
+        except KeyError as err:
+            raise RuntimeError(
+                f"Can not find the requested GID {gid} "
+                "on host") from err
+
+        self.delete_user(username, can_fail=True)
+        # This might fail because the group doesn't exist in the chroot (OK to
+        # ignore), or because there still are other users in the group (a
+        # serious error case, but OK to ignore because the subsequent
+        # 'crate_group' attempt will fail anyway).
+        self.delete_group(group_name, can_fail=True)
+        self.create_group(group_name, gid=gid)
+        self.create_user(group_name, uid=uid, gid=gid)