manually closing window when engine is shown `show_msession()`

[musm]

ref https://github.com/JuliaInterop/MATLAB.jl/pull/79

The only problematic issue here is if the user closes the matlab engine window manually (i.e. clicking the X button) instead of using the hide_msession() function. Because then the pointer to the engine is still attached to a session that does not exist. I'm not sure what is the best way to prevent this from occurring or on how to test if the pointer is still attached to a live engine window; the reason I bring this issue up is that it's possible to segfault julia if you close the window instead of hide_msession().

[ChrisRackauckas]

This can only be a problem when using this interactively, and so I don't think it's too much of an issue. It's something to mention in the README, i.e. "don't do it!", but it shouldn't harm any scripts.

[musm]

That is a very good point. Yeah it's only possible to do this if used interactively. Still would be nice if there was a way to NULL the ptr or something after the user closes the engine, but it isn't at all obvious how this would be done.

Perhaps @yuyichao has some thoughts.

[yuyichao]

IIUC these problems are usually solved by keeping a reference count so that the pointer does not become invalid, or having a callback before the pointer becomes invalid. I'm not sure which/any one of these are application given the matlab api.

Segfault under certain user action should be fixed if possible and should be clearly documented if not.

[musm]

Reproducible with

using MATLAB
julia> show_msession()  # close window manually

julia> restart_default_msession()
ERROR: MATLAB.MEngineError("failed to close a MATLAB engine session (err = 1)")
 in close(::MATLAB.MSession) at C:\Users\Mus\.julia\v0.5\MATLAB\src\engine.jl:57
 in restart_default_msession(::Int64) at C:\Users\Mus\.julia\v0.5\MATLAB\src\engine.jl:69
 in restart_default_msession() at C:\Users\Mus\.julia\v0.5\MATLAB\src\engine.jl:67

julia> restart_default_msession()
Please submit a bug report with steps to reproduce this fault, and any error messages that follow (in their entirety). Thanks.
Exception: EXCEPTION_ACCESS_VIOLATION at 0xe2b208d -- engEvalString at C:\Program Files\MATLAB\R2016b\bin\win64\libeng.DLL (unknown line)
while loading no file, in expression starting on line 0
engEvalString at C:\Program Files\MATLAB\R2016b\bin\win64\libeng.DLL (unknown line)
engClose at C:\Program Files\MATLAB\R2016b\bin\win64\libeng.DLL (unknown line)
close at C:\Users\Mus\.julia\v0.5\MATLAB\src\engine.jl:56
unknown function (ip: 00000000152942F6)
jl_call_method_internal at /home/Administrator/buildbot/slave/package_win6_2-x64/build/src/home/Administrator/buildbot/slave/package_win6_2-x64/build/src\julia_internal.h:189 [inlined]
jl_apply_generic at /home/Administrator/buildbot/slave/package_win6_2-x64/build/

[musm]

having a callback before the pointer becomes invalid. I'm not sure which/any one of these are application given the matlab api.

How would this look like? Ideally I would need to register a callback when the user closes the window.

In this case, I don't think reference counting would work.