bootc: Add support for --transient

[evan-goode]

This is just a proof-of-concept. I am not sure whether we want to implement the bootc support this way or keep the original plan of keeping things contained to a DNF plugin. But this approach has the big advantage that we get all DNF operations (builddep, install, remove, upgrade, everything) working in "transient mode" for free without reimplementing any DNF commands within the bootc plugin.

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.

[pep8speaks]

Hello @evan-goode! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2024-11-12 19:59:46 UTC

[evan-goode]

Requires rpm-software-management/libdnf#1678.

[dcantrell]

Since we discussed transient mode is kind of the only workable option, does it need to be exposed as a CLI option? If we're trying to lay down groundwork for future dnf5 handling, that's fine too.

I do like modifying the main command this way, even though it bumps part out in anothter package. I had originally suggested a hybrid approach like that, but I think everyone was more comfortable with the plugin approach.

I would also walk this patch set and pull out common strings and replace them with macros or variable names. I do this with the thinking that down the road we will need additional or different name support for things like /usr or /bootc. Put all those strings in one place and macro name them then get rid of the multiple string duplications.

[evan-goode]

I changed the libdnf option to persistence with the values auto, transient, and persist, with auto being the default. We could possibly rename this to bootc_behavior or something.

Currently, persistence=persist throws an error on bootc systems (locked or unlocked) and persistence=transient throws an error on non-bootc systems. persistence=auto will throw an error on locked bootc systems prompting the user to specify --transient, and proceed as usual on unlocked bootc systems.

Thinking it over some more, I think we should make the user explicitly pass --transient, or set persistence=transient in dnf.conf. Some UX requirements/nice-to-haves are:

• It should be very clear to bootc users that changes are not going to persist across reboots like they would on regular mutable systems.
• It should be clear to non-bootc users that --transient doesn't work on their system (i.e. error message printed when they specify --transient).
• If the usr overlay already exists, DNF should proceed without complaint, even if the user doesn't explicitly specify --transient.
• Users should be able to make transient mode the default in dnf.conf, e.g. by setting persistence=transient.
• In the future, if/when we add a --persist flag to DNF (4 or 5), the UX should not have to change much.

I moved a couple strings to variables, although for these cases, if we were to change the strings, we'd probably want to change the variable names also.