[WIP] parallelising ghcide tests

[komikat]

Using getCurrentDirectory and setCurrentDirectory makes it difficult to run tests in parallel since one thread modifying the global CWD variable would affect all other test threads. The goal of this PR is to deliberate on ways to avoid using the aforementioned functions (by making paths absolute) and have type guarantees for these absolute paths.

Note [Root Directory] from Development.IDE.Core.Shake

We keep track of the root directory explicitly, which is the directory of the project root.
We might be setting it via these options with decreasing priority:

1. from LSP workspace root, resRootPath in LanguageContextEnv.
2. command line (--cwd)
3. default to the current directory.

Using getCurrentDirectory makes it more difficult to run the tests, as we spawn one thread of HLS per test case.
If we modify the global Variable CWD, via setCurrentDirectory, all other test threads are suddenly affected,
forcing us to run all integration tests sequentially.

Also, there might be a race condition if we depend on the current directory, as some plugin might change it.
e.g. stylish's loadConfig. #4234

But according to https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_workspaceFolders
The root dir is deprecated, that means we should cleanup dependency on the project root(Or $CWD) thing gradually,
so multi-workspaces can actually be supported when we use absolute path everywhere(might also need some high level design).
That might not be possible unless we have everything adapted to it, like 'hlint' and 'evaluation of template haskell'.
But we should still be working towards the goal.

We can drop it in the future once:

1. We can get rid all the usages of root directory in the codebase.
2. LSP version we support actually removes the root directory from the protocol.

Thank you @fendor for mentioning haskell/cabal#9718 which seems to be doing something similar albeit for completely different reasons.

• Consider using a typed path library #4277
  ref Path newtype declaration #4372
• remove references to setCurrentDirectory where possible
  • hls-stylish-plugin
• investigate using lspEnv's resRootPath as the source of truth for the root directory.
  Details

  `doInitialize` uses the `defaultRoot` parameter, which is passed down from `setupLSP` (`argsProjectRoot`) in `Arguments{..}` in `defaultArguments` which seems to be taken from `getCurrentDirectory`.

[komikat]

Tracking setCurrentDirectory usage

• hls-test-utils/src/Test/
  runSessionWithTestConfig uses keepCurrentDirectory if testShiftRoot