`mode :: Mode (CmdArgs Cmd) ` memoizes async exceptions, breaking HLS Shake restarts
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Description
The top-level mode :: Mode (CmdArgs Cmd) in HLint is initialized using unsafePerformIO. If an asynchronous exception occurs during this initialization, the exception is captured and memoized in the CAF. As a result, every subsequent access to mode rethrows the same async exception.
In practice, this causes HLS to retain the cancel exception from an old Shake session and propagate it into the new session, immediately killing it.
See https://github.com/haskell/haskell-language-server/issues/4718#issue-3370604152
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Research direction
Start at the top-level mode :: Mode (CmdArgs Cmd) initialization in HLint and trace its unsafePerformIO usage. Use the linked HLS issue and the described old/new Shake-session scenario as reproduction context. Done means an asynchronous cancellation from an old session is not retained by later accesses to mode.
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Assessment
- Tech stack
- haskell
- Domain
- tooling
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100