Stabilize automatic garbage collection.

[ehuss]

This proposes to stabilize automatic garbage collection of Cargo's global cache data in the cargo home directory.

What is being stabilized?

This PR stabilizes automatic garbage collection, which is triggered at most once per day by default. This automatic gc will delete old, unused files in cargo's home directory.

It will delete files that need to be downloaded from the network after 3 months, and files that can be generated without network access after 1 month. These thresholds are intended to balance the intent of reducing cargo's disk usage versus deleting too often forcing cargo to do extra work when files are missing.

Tracking of the last-use data is stored in a sqlite database in the cargo home directory. Cargo updates timestamps in that database whenever it accesses a file in the cache. This part is already stabilized.

This PR also stabilizes the gc.auto.frequency configuration option. The primary use case for when a user may want to set that is to set it to "never" to disable gc should the need arise to avoid it.

When gc is initiated, and there are files to delete, there will be a progress bar while it is deleting them. The progress bar will disappear when it finishes. If the user runs with -v verbose option, then cargo will also display which files it deletes.

If there is an error while cleaning, cargo will only display a warning, and otherwise continue.

What is not being stabilized?

The manual garbage collection option (via cargo clean gc) is not proposed to be stabilized at this time. That still needs some design work. This is tracked in #13060.

Additionally, there are several low-level config options currently implemented which define the thresholds for when it will delete files. I think these options are probably too low-level and specific. This is tracked in #13061.

Garbage collection of build artifacts is not yet implemented, and tracked in #13136.

Background

This feature is tracked in #12633 and was implemented in a variety of PRs, primarily #12634.

The tests for this feature are located in https://github.com/rust-lang/cargo/blob/master/tests/testsuite/global_cache_tracker.rs.

Cargo started tracking the last-use data on stable via #13492 in 1.78 which was released 2024-05-02. This PR is proposing to stabilize automatic deletion in 1.82 which will be released in 2024-10-17.

Risks

Users who frequently use versions of Rust older than 1.78 will not have the last-use data tracking updated. If they infrequently use 1.78 or newer, and use the same cache files, then the last-use tracking will only be updated by the newer versions. If that time frame is more than 1 month (or 3 months for downloaded data), then cargo will delete files that the older versions are still using. This means the next time they run the older version, it will have to re-download or re-extract the files.

The effects of deleting cache data in environments where cargo's cache is modified by external tools is not fully known. For example, CI caching systems may save and restore cargo's cache. Similarly, things like Docker images that try to save the cache in a layer, or mount the cache in a read-only filesystem may have undesirable interactions.

The once-a-day performance hit might be noticeable to some people. I've been using this for several months, and almost never notice it. However, slower systems, or situations where there is a lot of data to delete might take a while (on the order of seconds hopefully).

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[ehuss]

@rfcbot fcp merge

[rfcbot]

Team member @ehuss has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Eh2406
• @Muscraft
• @arlosi
• @ehuss
• @epage
• @joshtriplett
• @weihanglo

Concerns:

• field-name (Stabilize automatic garbage collection. #14287 (comment))

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[epage]

@rfcbot concern field-name

Something that came up on the tracking issue is whether people will get confused with the gc name (#12633 (comment)). I suspect that won't be a problem but I want to make sure we acknowledge it and agree it isn't a problem first.

I am also somewhat concerned about how to organize and name the config for if/when we get to GC within target directories. I wonder if this would affect the top-level table name (e.g. calling it global-cache). In other ways, only stabilizing auto gives us a lot of leeway in what it does.

(sorry, thought of this after the conversation about stabilizing this)

[cessen]

Something that came up on the tracking issue is whether people will get confused with the gc name

I agree that this isn't likely to be an issue in practice.

However, I also feel like we might as well be more accurate and call it something like "cache cleaning", which is what it actually is. As I mentioned in the other thread, garbage collection is usually related to some concept of reachability and resultant high confidence that something is no longer used. This doesn't reflect how this feature actually works, or ever reasonably could work.

And unless I'm missing something here (which is always possible), I don't think there's any cost to simply naming this feature more accurately, aside from taking a bit of time to decide on that name.

[elenakrittik]

Sorry if this is the wrong place to ask, but will there be a way to disable this behaviour?

[Skgland]

Sorry if this is the wrong place to ask, but will there be a way to disable this behaviour?

I think the PR description already answers this:

This PR also stabilizes the gc.auto.frequency configuration option. The primary use case for when a user may want to set that is to set it to "never" to disable gc should the need arise to avoid it.

[ehuss]

Sure, would be happy to rename it. I opened #14292 to have that suggestion, please express your thoughts over there.

[weihanglo]

“months” is an approximate number.

cargo/src/cargo/core/gc.rs

Lines 373 to 381 in ea14e86

	let factor = match right {
	"second" | "seconds" => 1,
	"minute" | "minutes" => 60,
	"hour" | "hours" => 60 * 60,
	"day" | "days" => 24 * 60 * 60,
	"week" | "weeks" => 7 * 24 * 60 * 60,
	"month" | "months" => 2_629_746, // average is 30.436875 days
	_ => return None,
	};

I can foresee someone will interpret months as monthly and think it will be clean on the same day when set, while it is not true especially in February. I don't think this is a thing we can't change after stabilization. Just calling it out if someone disagrees.

[teohhanhui]

Why not just remove months? It'd be counter-intuitive to anyone trying to use it... The user can already specify something like 180 days for ~6 months, no?

[epage]

For myself, being able to say "6 months" is much easier than calculating out the number of days and reading the number of days.

[teohhanhui]

@epage But would anyone expect it to be 182.62125 days? Principle of least surprise...

[epage]

There is an inherent approximation when giving a unit. The larger the unit, the larger the approximation. If you say "6 months", you shouldn't care whether thats 180, 186, 182, or 182.62125.

btw laughing emoji's in a technical discussion like this come across as rude.

[teohhanhui]

btw laughing emoji's in a technical discussion like this come across as rude.

Sigh... Here we go again. Intent does not carry across text (or emojis), so please don't jump to conclusions like that.

I was not even disagreeing with what you said. Just pointing out that it'd be surpirising for the user, as the OP of this thread has already pointed out (a different surprising aspect).