Add support for --transient

Adds support for the --transient option on all transactions. Passing
--transient on a bootc system will call `bootc usr-overlay` to create a
transient writeable /usr and continue the transaction.

Specifying --transient on a non-bootc system will throw an error; we
don't want to mislead users to thinking this feature works on non-bootc
systems.

If --transient is not specified and the bootc system is in a locked
state, the operation will be aborted and a message will be printed
suggesting to try again with --transient.

dnf/cli/cli.py

@@ -205,28 +205,50 @@ class BaseCli(dnf.Base):
             else:
                 self.output.reportDownloadSize(install_pkgs, install_only)
 
+        bootc_unlock_requested = False
+
         if trans or self._moduleContainer.isChanged() or \
                 (self._history and (self._history.group or self._history.env)):
             # confirm with user
             if self.conf.downloadonly:
                 logger.info(_("{prog} will only download packages for the transaction.").format(
                     prog=dnf.util.MAIN_PROG_UPPER))
+
             elif 'test' in self.conf.tsflags:
                 logger.info(_("{prog} will only download packages, install gpg keys, and check the "
                               "transaction.").format(prog=dnf.util.MAIN_PROG_UPPER))
-            if dnf.util._is_bootc_host() and \
-                    os.path.realpath(self.conf.installroot) == "/" and \
-                    not self.conf.downloadonly:
-                _bootc_host_msg = _("""
-*** Error: system is configured to be read-only; for more
-*** information run `bootc --help`.
-""")
-                logger.info(_bootc_host_msg)
-                raise CliError(_("Operation aborted."))
+
+            is_bootc_transaction = dnf.util._is_bootc_host() and \
+                os.path.realpath(self.conf.installroot) == "/" and \
+                not self.conf.downloadonly
+
+            # Handle bootc transactions. `--transient` must be specified if
+            # /usr is not already writeable.
+            if is_bootc_transaction:
+                if self.conf.persistence == "persist":
+                    logger.info(_("Persistent transactions aren't supported on bootc systems."))
+                    raise CliError(_("Operation aborted."))
+                assert self.conf.persistence in ("auto", "transient")
+                if not dnf.util._is_bootc_unlocked():
+                    if self.conf.persistence == "auto":
+                        logger.info(_("This bootc system is configured to be read-only. Pass --transient to "
+                                      "perform this and subsequent transactions in a transient overlay which "
+                                      "will reset when the system reboots."))
+                        raise CliError(_("Operation aborted."))
+                    assert self.conf.persistence == "transient"
+                    logger.info(_("A transient overlay will be created on /usr that will be discarded on reboot. "
+                                  "Keep in mind that changes to /etc and /var will still persist, and packages "
+                                  "commonly modify these directories."))
+                    bootc_unlock_requested = True
+            elif self.conf.persistence == "transient":
+                raise CliError(_("Transient transactions are only supported on bootc systems."))
 
             if self._promptWanted():
                 if self.conf.assumeno or not self.output.userconfirm():
                     raise CliError(_("Operation aborted."))
+
+            if bootc_unlock_requested:
+                dnf.util._bootc_unlock()
         else:
             logger.info(_('Nothing to do.'))
             return

dnf/cli/option_parser.py

@@ -317,6 +317,9 @@ class OptionParser(argparse.ArgumentParser):
         general_grp.add_argument("--downloadonly", dest="downloadonly",
                                  action="store_true", default=False,
                                  help=_("only download packages"))
+        general_grp.add_argument("--transient", dest="persistence",
+                                 action="store_const", const="transient", default=None,
+                                 help=_("Use a transient overlay which will reset on reboot"))
         general_grp.add_argument("--comment", dest="comment", default=None,
                                  help=_("add a comment to transaction"))
         # Updateinfo options...

dnf/conf/config.py

@@ -343,7 +343,7 @@ class MainConf(BaseConfig):
                        'best', 'assumeyes', 'assumeno', 'clean_requirements_on_remove', 'gpgcheck',
                        'showdupesfromrepos', 'plugins', 'ip_resolve',
                        'rpmverbosity', 'disable_excludes', 'color',
-                       'downloadonly', 'exclude', 'excludepkgs', 'skip_broken',
+                       'downloadonly', 'persistence', 'exclude', 'excludepkgs', 'skip_broken',
                        'tsflags', 'arch', 'basearch', 'ignorearch', 'cacheonly', 'comment']
 
         for name in config_args:

dnf/util.py

@@ -38,6 +38,7 @@ import logging
 import os
 import pwd
 import shutil
+import subprocess
 import sys
 import tempfile
 import time
@@ -642,15 +643,32 @@ def _is_file_pattern_present(specs):
 
 
 def _is_bootc_host():
-    """Returns true is the system is managed as an immutable container,
-       false otherwise.  If msg is True, a warning message is displayed
-       for the user.
-    """
-    ostree_booted = '/run/ostree-booted'
-    usr = '/usr/'
-    # Check if usr is writtable and we are in a running ostree system.
-    # We want this code to return true only when the system is in locked state. If someone ran
-    # bootc overlay or ostree admin unlock we would want normal DNF path to be ran as it will be
-    # temporary changes (until reboot).
-    return os.path.isfile(ostree_booted) and not os.access(usr, os.W_OK)
+    """Returns true is the system is managed as an immutable container, false
+    otherwise."""
+    ostree_booted = "/run/ostree-booted"
+    return os.path.isfile(ostree_booted)
 
+
+def _is_bootc_unlocked():
+    """Check whether /usr is writeable, e.g. if we are in a normal mutable
+    system or if we are in a bootc after `bootc usr-overlay` or `ostree admin
+    unlock` was run."""
+    usr = "/usr"
+    return os.access(usr, os.W_OK)
+
+
+def _bootc_unlock():
+    """Set up a writeable overlay on bootc systems."""
+
+    if _is_bootc_unlocked():
+        return
+
+    unlock_command = ["bootc", "usr-overlay"]
+
+    try:
+        completed_process = subprocess.run(unlock_command, text=True)
+        completed_process.check_returncode()
+    except FileNotFoundError:
+        raise dnf.exceptions.Error(_("bootc command not found. Is this a bootc system?"))
+    except subprocess.CalledProcessError:
+        raise dnf.exceptions.Error(_("Failed to unlock system: %s", completed_process.stderr))