Fix some JET errors around matching methods for kill(...) and process_exited(...) (#172)

* Fix some JET errors around matching methods for `kill(...)` and `process_exited(...)`

```
│┌ manage(manager::Distributed.LocalManager, id::Int64, config::Distributed.WorkerConfig, op::Symbol) @ Distributed /workpath/Distributed.jl/src/managers.jl:529
││ no matching method found `kill(::Nothing, ::Int64)` (1/2 union split): Distributed.kill((config::Distributed.WorkerConfig).process::Union{Nothing, Base.Process}, 2)
│└────────────────────
```

```
│┌ (::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig})() @ Distributed /workpath/Distributed.jl/src/managers.jl:757
│ no matching method found `kill(::Nothing, ::Int64)` (1/2 union split): Distributed.kill((getfield(#self#::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig}, :config)::Distributed.WorkerConfig).process::Union{Nothing, Base.Process}, (profile_sig::Tuple{String, Int64})[2]::Int64)
└────────────────────
```

```
┌ (::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig})() @ Distributed /workpath/Distributed.jl/src/managers.jl:761
│ no matching method found `kill(::Nothing, ::Int64)` (1/2 union split): Distributed.kill((getfield(#self#::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig}, :config)::Distributed.WorkerConfig).process::Union{Nothing, Base.Process}, (Distributed.Base).SIGQUIT::Int64)
└────────────────────
```

```
┌ (::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig})() @ Distributed /workpath/Distributed.jl/src/managers.jl:766
│ no matching method found `kill(::Nothing, ::Int64)` (1/2 union split): Distributed.kill((getfield(#self#::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig}, :config)::Distributed.WorkerConfig).process::Union{Nothing, Base.Process}, (Distributed.Base).SIGKILL::Int64)
└────────────────────
```

```
┌ (::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig})() @ Distributed /workpath/Distributed.jl/src/managers.jl:752
│ no matching method found `process_exited(::Nothing)` (1/2 union split): Distributed.process_exited((getfield(#self#::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig}, :config)::Distributed.WorkerConfig).process::Union{Nothing, Base.Process})
└────────────────────
```

```
┌ (::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig})() @ Distributed /workpath/Distributed.jl/src/managers.jl:764
│ no matching method found `process_exited(::Nothing)` (1/2 union split): Distributed.process_exited((getfield(#self#::Distributed.var"#200#201"{Int64, Int64, Int64, Int64, Distributed.WorkerConfig}, :config)::Distributed.WorkerConfig).process::Union{Nothing, Base.Process})
└────────────────────
```

(cherry picked from commit e19a4bfe30fd580cc8e8baa5033e2d7a2c1e1dc9)

* Apply suggestions from code review

Co-authored-by: James Wrigley <JamesWrigley@users.noreply.github.com>

* Define `process = config.process::Process` in one place, and then re-use the `process` variable when needed

---------

Co-authored-by: James Wrigley <JamesWrigley@users.noreply.github.com>
(cherry picked from commit 231da28ef52953b68280a295419b990cb5c7e3f7)

Author: DilumAluthge

src/managers.jl

@@ -553,7 +553,7 @@ end
 
 function manage(manager::LocalManager, id::Integer, config::WorkerConfig, op::Symbol)
     if op === :interrupt
-        kill(config.process, 2)
+        kill(config.process::Process, 2)
     end
 end
 
@@ -776,21 +776,22 @@ function kill(manager::LocalManager, pid::Int, config::WorkerConfig; profile_wai
         sleep(exit_timeout)
 
         # Check to see if our child exited, and if not, send an actual kill signal
-        if !process_exited(config.process)
+        process = config.process::Process
+        if !process_exited(process)
             @warn "Failed to gracefully kill worker $(pid)"
             profile_sig = Sys.iswindows() ? nothing : Sys.isbsd() ? ("SIGINFO", 29) : ("SIGUSR1" , 10)
             if profile_sig !== nothing
                 @warn("Sending profile $(profile_sig[1]) to worker $(pid)")
-                kill(config.process, profile_sig[2])
+                kill(process, profile_sig[2])
                 sleep(profile_wait)
             end
             @warn("Sending SIGQUIT to worker $(pid)")
-            kill(config.process, Base.SIGQUIT)
+            kill(process, Base.SIGQUIT)
 
             sleep(term_timeout)
-            if !process_exited(config.process)
+            if !process_exited(process)
                 @warn("Worker $(pid) ignored SIGQUIT, sending SIGKILL")
-                kill(config.process, Base.SIGKILL)
+                kill(process, Base.SIGKILL)
             end
         end
     end