disable VisualStudioInstanceProvider on mono

[corngood]

Since #998 I've been getting:

System.PlatformNotSupportedException: Operation is not supported on this platform.
  at OmniSharp.MSBuild.Discovery.Interop.GetSetupConfiguration () [0x0000a] in <fc86425b83fa4820a94c3447774fdb30>:0
  at OmniSharp.MSBuild.Discovery.Providers.VisualStudioInstanceProvider.GetInstances () [0x0000e] in <fc86425b83fa4820a94c3447774fdb30>:0
  at OmniSharp.MSBuild.Discovery.MSBuildLocator.GetInstances () [0x0001e] in <fc86425b83fa4820a94c3447774fdb30>:0
  at OmniSharp.CompositionHostBuilder.Build () [0x00053] in <fc86425b83fa4820a94c3447774fdb30>:0
  at OmniSharp.Stdio.Host..ctor (System.IO.TextReader input, OmniSharp.Stdio.Services.ISharedTextWriter writer, OmniSharp.IOmniSharpEnvironment environment, Microsoft.Extensions.Configuration.IConfiguration configuration, System.IServiceProvider serviceProvider, OmniSharp.CompositionHostBuilder compositionHostBuilder, Microsoft.Extensions.Logging.ILoggerFactory loggerFactory, System.Threading.CancellationTokenSource cancellationTokenSource) [0x00052] in <6ba7b54f3edf42509e7ecb6c40595f1f>:0
  at OmniSharp.Stdio.Program+<>c__DisplayClass0_0.<Main>b__1 () [0x00087] in <6ba7b54f3edf42509e7ecb6c40595f1f>:0
  at OmniSharp.CommandLineApplication+<>c__DisplayClass11_0.<OnExecute>b__0 () [0x0000b] in <fc86425b83fa4820a94c3447774fdb30>:0
  at Microsoft.Extensions.CommandLineUtils.CommandLineApplication.Execute (System.String[] args) [0x0035b] in <e56ebbc3ed87488b8e26736bbadaa5d3>:0
  at OmniSharp.CommandLineApplication.Execute (System.Collections.Generic.IEnumerable`1[T] args) [0x00042] in <fc86425b83fa4820a94c3447774fdb30>:0
  at OmniSharp.Stdio.Program+<>c__DisplayClass0_1.<Main>b__0 () [0x00028] in <6ba7b54f3edf42509e7ecb6c40595f1f>:0
  at OmniSharp.HostHelpers.Start (System.Func`1[TResult] action) [0x0001c] in <fc86425b83fa4820a94c3447774fdb30>:0

I've worked around this by disabling the VS instance provider on mono, but I'm still not exactly sure why it's failing. I've tried x64 and x86 mono, and according to http://www.mono-project.com/docs/advanced/com-interop/, COM interop is supposed to work.

[corngood]

Actually it looks like it isn't implemented in mono:

https://github.com/mono/mono/blob/813c1308fe53d16b419d22777f4d1124f06e7c9e/mcs/class/corlib/System.Runtime.InteropServices/Marshal.cs#L725

[DustinCampbell]

Are you running OmniSharp on Mono on Windows?

(and if so, why?)

[corngood]

Yeah, unfortunately. There's a bug somewhere that causes net46 processes to hang when started as a child process from cygwin emacs (nothing ever shows up on stdout). This doesn't happen with earlier runtimes (e.g. 3.5) or mono.

[DustinCampbell]

Got it. That kind of stinks. It'd be great if we could track down that bug. Do you know if it happens with the Windows Linux Subsystem?

[DustinCampbell]

Could you remove the first condition?

if (PlatformHelper.IsMono)
{
    return NoInstances;
}

[corngood]

Sure, but I figured that was also there for non-windows non-mono platforms e.g. netcore?

[DustinCampbell]

OmniSharp does not run on .NET Core. It requires Mono or Desktop CLR. So, that change should be fine.

[corngood]

Cool. Done.

[corngood]

I don't think it's possible to fork/exec a net46 process from WSL. I'm sure running emacs on WSL w/ mono omnisharp would be fine, but I prefer the GUI/filesystem integration in cygwin.

I do plan on investigating the bug, but I have a feeling it's something to do with file handle/tty stuff in .net. I'll at least try to make a simple repro.