Dear Norman,
Maybe it is beneficial to break down the problem even further. In fact, depot archive types do not arbitrary depend on one another. Specifically, binary archives cannot depend on each other. Also raw archives have no dependencies. Src archives can only depend on api archives but not on other src archives. Also api archives cannot have dependencies. For this current discussion, I'd leave out src and api archives anyway.
The only case where a dependency tree of multiple levels is formed are pkg archives depending on other pkg archives. With this observation, I would only look at pkg archives at first. Scanning the depot for the list of pkg archives should be quick enough. For each pkg, I would ask: "should this pkg be removed?". The answer is given by the <config>. To implement this step, there is no need to build an internal data structure.
Then, after having removed pkg archives, I'd read the content of all remaining 'archives' files present in the depot, putting each line into a dictionary (removing duplicates that way). Now we know all archives that are still required.
Sorry, I was not very clear. I agree, at first we only traverse archives of type PKG to collect 'archives' dependency files.
With this list (dictionary) gathered, we can again go through the depot. For each bin or raw archive, we'd look whether it is featured in our list or not. If not, we can remove the sub directory. For each pkg archive, we look if it is either featured in our list or if it is tagged as manually installed by the user. If neither is the case, we can remove it as well, and remember that we should do another iteration of garbage collection (now with the pkg removed, further removals may become possible).
There is no need to create a complete implementation of a Graph data structure. As you describe with the Dictionary, I have something similar in mind to collect archives dependencies. I have named the top-level class that holds the Dictionary "graph". I should not if this is confusing.
The Dictionary would be used to associate an archive path with a list of PKG archive types it is referenced in. Thus, archives with no references after PKG deletion are identified, and archives referenced by a deleted PKG but still referenced by any other PKG(s) can be kept.
But what if a pkg was manually installed by the user (lets say "blue_backdrop") and also happens to be a dependency of another dependent pkg (like "blue_backdrop_with_logo") installed separately?
In this case, I would expect to keep the "blue_backdrop" when uninstalling only the dependent pkg "blue_backdrop_with_logo". If the "blue_backdrop" had been installed as a mere side effect of installing "blue_backdrop_with_logo", I would expect to have it automatically removed along with "blue_backdrop_with_logo".
To take this decision, I think we have to preserve the information of how each pkg entered the depot. Hence, my suggestion to explicitly mark the pkg archives that entered the depot by user intent.
You are correct. I missed that. Thank you for explaining in details!
I have a first implementation on the 'depot_remove' [1] branch. It can be improved or changed. Please note that this is a partial implementation. There are some TODOs comments. I also commented on it as much as possible for clarity.
[1] https://github.com/a-dmg/genode/tree/depot_remove
Points that remain to be addressed:
- Identify BIN archives, and provide 'arch' attribute to the configuration for this purpose.
- Make the PKG deletion in place to remove PKG references in the Dictionary.
- Collect orphan archive reference by no PKG. Make that last step optional? As it requires traversing the depot for any other archive types. I am questioning myself if this is necessary.
- The configuration does not implement all config's nodes as discussed, only '<remove />' for instance.
You might be interested in the 'depot.h' file. I would suggest reading it from bottom to top. You can use the 'depot_remove' runscript, which has debug logs describing what's happening.
Let me know what you think about it. If you want it simplified, and if you have further suggestions?
I hope this is digestible enough for a pleasant review. Thank you very much for your time.
Cheers,
Alice