How to extend `Package` in v2?

[Frederick888]

I'd like to always use the latest Git version of java-test and java-debug-adapter. So when I was on Mason v1, I

1. Cloned those two Git repos
2. Added a custom user.mason-repository in my config
3. Wrote a LocalPackage that extended Package
   a. LocalPackage:get_installed_version returned the SHA of Git HEAD
   b. LocalPackage:check_new_version fetched Git remote and returned the SHA of Git origin/HEAD
4. Put the two packages in lua/user/mason-registry
   a. They used LocalPackage.new()
   b. Their install ran git reset --hard origin/HEAD, built the package, and finally symlinked the files

Now I'm working on v2 migration in lazyvim15 branch, and I'm getting the error below:

.lua:28: attempt to index field 'instances' (a nil value)
stack traceback:
        .../nvim/lazy/mason.nvim/lua/mason-registry/sources/lua.lua:28: in function 'get_package'
        ...l/share/nvim/lazy/mason.nvim/lua/mason-registry/init.lua:26: in function <...l/share/nvim/lazy/mason.nvim/lua/mason-registry/init.lua:24>
        vim/shared.lua: in function 'get_all_packages'
        ...cal/share/nvim/lazy/mason.nvim/lua/mason/ui/instance.lua:727: in main chunk
        [C]: in function 'require'
        .../.local/share/nvim/lazy/mason.nvim/lua/mason/ui/init.lua:9: in function 'open'
        ...cal/share/nvim/lazy/mason.nvim/lua/mason/api/command.lua:5: in function <...cal/share/nvim/lazy/mason.nvim/lua/mason/api/command.lua:4>
        [C]: in function 'cmd'
        .../share/nvim/lazy/lazy.nvim/lua/lazy/core/handler/cmd.lua:48: in function <.../share/nvim/lazy/lazy.nvim/lua/lazy/core/handler/cmd.lua:16>
        [C]: in function 'nvim_exec2'
        vim/_editor.lua: in function 'action'
        ...cal/share/nvim/lazy/snacks.nvim/lua/snacks/dashboard.lua:693: in function <...cal/share/nvim/lazy/snacks.nvim/lua/snacks/dashboard.lua:692>

I'm mainly stuck on how I can pass the registry itself to the new Package:new(spec, reg) at the moment. Hopefully that's my last problem. Or maybe the whole thing is an overkill and there's a much simpler way to do Git + build + symlink?

I'd appreciate a little guidance. Thank you!

---

Update: Also tried getting rid of Package:new() let Mason hydrate the packages itself https://git.tsundere.moe/Frederick888/frederick-settings/-/commit/9b14146d0dbf6e87c60186295e8c03e0508d8d6b

local LocalPackage = require('user.mason-registry.local-package')

local pkg = setmetatable({
  name = 'java-test',
  description = [[
    The Test Runner for Java works with java-debug-adapter to provide the following features:
    - Run/Debug test cases
    - Customize test configurations
    - View test report
    - View tests in Test Explorer

    Enables support for the following test frameworks:

    - JUnit 4 (v4.8.0+)
    - JUnit 5 (v5.1.0+)
    - TestNG (v6.8.0+)
  ]],
  homepage = 'https://github.com/microsoft/vscode-java-test',
  licenses = { 'MIT' },
  languages = { 'Java' },
  categories = { 'DAP', 'User' },
  source = {
    id = 'pkg:mason/vscode-java-test',
    ---@param ctx InstallContext
    install = function(ctx)
      local package_spec
      for _, s in ipairs(ctx.package.registry:get_all_package_specs()) do
        if s.name == ctx.package.name then
          package_spec = s
          break
        end
      end
      local requested_version = ctx.requested_version or 'origin/HEAD'
      local source_path = package_spec:get_source_path()
      ctx.spawn.git({ '-C', source_path, 'reset', '--hard', requested_version })
      ctx.spawn.npm({ '-C', source_path, 'install' })
      ctx.spawn.npm({ '-C', source_path, 'run', 'build-plugin' })
      require('user.mason-registry.symlink-installer')('%.jar$', 'extension/server')(ctx)
    end,
  },
}, { __index = LocalPackage })

function pkg:get_source_path()
  return vim.fs.normalize('~/Programming/JavaScript/vscode-java-test')
end

function pkg:get_artefact_path()
  return vim.fs.normalize('~/Programming/JavaScript/vscode-java-test/server')
end

return pkg

Installation worked. However since all my LocalPackage methods, including get_latest_version() and get_installed_version(), are now only in the package specs rather than packages themselves, Mason cannot show / detect their versions correctly as it just uses the default ones from AbstractPackage.