Deadlock in registerNewDebugSession threads

[alt-romes]

Dear @dmjio,

Me and @mpickering have been implementing a debugger for GHC and we started using the dap library for the DAP part of the debugger.
However, we ran into a nasty deadlock while doing something seemingly standard in the debugger threads that are launched by registerNewDebugSession.

We had something that looked like

  (readDebuggerOutput, writeDebuggerOutput) <- liftIO P.createPipe
  registerNewDebugSession (maybe "debug-session" T.pack __sessionId) DAS{..}
    [ debuggerThread ... writeDebuggerOutput
    , outputEventsThread readDebuggerOutput
    ]

And this resulted in a deadlock within dap outlined as:

1. Initialisation starts, starts writing to a pipe
2. Worker thread tries to output things from the pipe, but can't because that requires taking a lock on the MVar
3. Step 1 blocks because the pipe gets full
4. Step 2 blocks, since it is waiting for step 1 to finish

[alt-romes]

I meant to open this issue in the dap library, sorry.