Add a 'dart pub cache path' command

[gspencergoog]

Description

This adds a path subcommand to the dart pub cache command which simply prints the effective pub cache path.

This is useful for determining the cache directory in effect when building CI rules.

For instance a Github action could now be created that looks like this:

      - name: Get pub cache path
        id: pub-cache
        run: echo "dir=$(dart pub cache path)" >> $GITHUB_OUTPUT
      - name: Cache Pub dependencies
        uses: actions/cache@v4
        with:
          path: ${{ steps.pub-cache.outputs.dir }}
          key: ${{ runner.os }}-pub-${{ hashFiles('**/pubspec.lock') }}
          restore-keys: ${{ runner.os }}-pub-

Otherwise, you have to know what the cache directory resolution scheme is on each platform, and the PUB_CACHE env var, and the path is platform specific.

It's also useful if an AI agent wants to know where the cache is.

Tests

• Added a test.

[natebosch]

I agree that this is worth landing. I'll let the pub team review.

[gspencergoog]

@jonasfj Did you have bandwidth to review this?

[natebosch]

Oops, looks like Jonas is out, I should have checked when assigning originally.

[sigurdm]

We include the cache path in .dart_tool/package_config.json . I think it is better to read it from there.

It is under the "pubCache" key.

[gspencergoog]

The reason I wanted to add this command is that figuring out the value requires complicated and obscure knowledge. For instance, even though you've mentioned that it's stored in a file, I still don't know if setting PUB_CACHE overrides that. And what do I do if the file hasn't been created yet? Also, having it in a file means I need to have a JSON parser and parse the key out of the file. This way, I get the value with one simple documented command.

With this documented command, it's easy to get a canonical value, that I know is the one being used by Dart, that is using the same logic as Dart does, so if it ever changes, I don't need to do anything.

[sigurdm]

In the CI case I think it is better to set the cache to a fixed path by hard coding PUB_CACHE.

[gspencergoog]

In the CI case I think it is better to set the cache to a fixed path by hard coding PUB_CACHE.

If I set the PUB_CACHE to something when the CI docker image doesn't, or to something different, then I'm going to have to download more each time I update the cached directory. Sure, I can probably predict that the docker image already has PUB_CACHE set and use that, or that it's in the HOME dir, but that's just extra logic I need to write each time.

Also, CI isn't the only use case. Any time I have a script that wants to know where the cache is, I need to replicate the "is PUB_CACHE set? No, is it in package_config.json? Is it in HOME?" logic.

And the AI use case is also made simpler by having a single command to execute.