Sunday, 15 November 2020

MapGuide dev diary: The beginnings of clearing the final hurdle

I've stated many times in this long and arduous MapGuide Open Source 4.0 development cycle that the final hurdle that must be cleared before 4.0 could ever be considered final is when we can finally generate language bindings for .net, Java and PHP with a vanilla and un-modified version of SWIG.

The reasons for needing to do this were already explained in my previous introductory post to these new bindings, but to re-iterate the cliff notes version:

  • We need to support and bundle PHP 7. This is non-negotiable. The current bundled PHP 5.6 is too old and long past EOL and it is a bad look to have to bundle this version of PHP for a production MapGuide deployment/installation.
  • The latest release of SWIG can generate bindings for PHP 7
  • The cross-platform .net core has grown in leaps and bounds over traditional windows-only .net Framework in terms of adoption. The just released .net 5.0 is a sign that the current windows-only .net Framework is dead/legacy and the future of .net is a cross-platform one.
  • As a result, if we're going to be supporting .net in MapGuide, we should be generating a .net binding that can work in both Windows and Linux.
  • And if we need to do that, we might as well do it with the latest release of SWIG
  • And if 2/3 languages require vanilla SWIG, we might as well go for the trifecta and generate our Java binding with it as well!

As this final hurdle involves many steps, I figure this journey is worth documenting with its own mini dev diary series.

So what has changed since the initial announcement of these experimental bindings?

Firstly, I have decided to bring the current binding work into the official MapGuide source in a new vanilla_swig sandbox branch. All development work will continue in this branch. The previous GitHub repo housing this work will no longer be maintained and I will eventually archive/delete this repo. Going from Git back to SVN might sound like a downgrade (technically yes), but my developer "inner loop" has sped up a lot by having everything in the same repo and not having to coordinate files/changes across 2 different repos in 2 different locations. Maybe one day we'll permanently migrate the MapGuide source on GitHub, but today is not that day.

Secondly, before I tackle the PHP 7 support, I wanted to see whether the .net/Java bindings were still functional and what other final improvements we can make before all attention is fully diverted to the PHP 7 binding.

For Java, after some minor fix ups, the binding and its test suite were still A-OK. So onto the .net binding.

When I introduced these new experimental bindings, the .net one was back to a single monolithic assembly (MapGuideDotNetApi). I wasn't fully comfortable with the monolithic assembly as it would hamper any attempts to write code that could work in both MapGuide and mg-desktop. The mg-desktop .net support was hanging off of the currently Foundation/Geometry/PlatformBase split assemblies. Having our new .net binding go back to a monolithic assembly would hamper our ability to write such shared code, so if it was possible we should try to replicate the Foundation/Geometry/PlatformBase/MapGuideCommon/Web split layout in the new .net binding.

Using the current .net binding as a point of reference, splitting the monolithic MapGuideDotNetApi assembly back to the 5 constituent parts was a relatively simple affair. Thanks to the dramatically simplified csproj format we now have 5 hand-crafted C# projects targeting netstandard2.0 that reference each other and that SWIG dumps all its generated C# source into (without having to add each .cs file into the project itself) for easy compilation that automatically publishes out to respective nuget packages like so.

And because our 5 projects reference each other, those dependencies are also expressed in the nuget packages themselves. That is to say, if you install the MapGuideCommon package, it will automatically install the PlatformBase, Geometry and Foundation packages as well as they were defined as project dependencies of the MapGuideCommon C# project file.

And the final cherry on top? These nuget packages are still self-contained and bundle the native dlls that the .net binding is wrapping. The current nuget packages are already self-contained, but they are only consumable in legacy .net Framework, are windows-only and require kludgy powershell hacks to make sure all the native dlls are copied out to the project's output directory. Our new nuget packages take advantage of the fact that native libraries are now first class citizens in the .net core packaging world.

By adding such dlls to the runtimes/win-x64/native folder of a C# project, they will automatically be bundled into any nuget package created and the .net core project system knows to automatically copy these dlls out to the right location where the .net assembly can P/invoke them. 

Now for a multi-platform .net binding to work, we have to get SWIG to generate the same C++ glue code, but this time to be compiled on Linux but with the same library names so our SWIG-generated C# code will correctly P/Invoke into the respective Windows .dll or Linux .so, and pack those compiled .so files into the runtimes/linux-x64/native folder of our 5 C# projects.for automatic bundling into our nuget package.

How we are able to do this will be the topic of a future post once I've figured it all out.

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