Godot's code base is split into independent modules that can be enabled and disabled at will. However many games require custom native code and thus need to define their own modules. The simplest way to do this (which, I'm told, game developers quite often do) is to fork the upstream repo and put your your code in it. This works and is a good solution for one-off projects that write all extra code by themselves. This approach also has its downsides. The two major ones are that updating to a newer version of the engine can turn into a rebasing hell and that it is difficult to combine multiple third party modules.
Ideally what you'd want to do is to take upstream Godot and then take a third party module for, say, physics and a second module written by completely different people that does sound effects processing, combine all three and have things just work. Typically those modules are developed in their own repositories. Thus we'd end up with the following kind of a dependency graph.
This would indicate a circular dependency between the two repositories: in order to build the external module you need to depend on the Godot repo and in order to build the Godot you need to depend on the external repository. This is bad. If there's one thing you don't want in your source code it is circular dependencies.
Solving it using Meson's primitives
If you look at the picture in more detail you can tell that there is no circular dependencies between individual targets. Thus to solve the problem you need some way to tell the external dependency how to get the core libraries and headers it needs and conversely a way for the main build to extract from the external project what modules it has built and where they are. As Meson subprojects are built in isolation one can't just blindly poke the innards of other projects as one can do if everything is in a single megaproject.
The way Godot's current build is set up is that first it defines the core libraries, then all the modules and finally things on top of that (like the editor and main executable). We need to extend this so that external modules are set up at the same time as internal modules and then joined into one. Thus the final link won't even be able to tell the difference between external and internal modules.
First we need to set up the dependency info for the core libraries, which is done like this:
First we set up a dependency object that encapsulates everything needed to build extension modules and then specify that whenever a dependency called godotcore is looked up, Meson will return the newly defined object. This even works inside subprojects that are otherwise isolated from the master project.
Assuming we have a list of external module subprojects available, we can go through them one by one and build them.
The first line runs the subproject, the latter two are ignored for now, we'll come back to them. The subproject's meson.build file starts by getting the dependency info.
godotcore_dep = dependency('godotcore')
Then it does whatever it needs to build the extension module. Finally it defines the information that the main Godot application needs to use the module:
In this case I have converted Godot's internal tga module to build as an external module hence the name. This concludes the subproject and execution resumes in the master project and the two remaining lines grab the module build information and name and append them to the list of modules to use.
This is basically it. There are obviously more details needed like integrating with Godot's documentation system for modules but the basic principle for those is the same. With this approach the integration of multiple external modules is simple: you need to place them in the main project's subprojects directory and add their names to the list of external module subprojects. All of this can be done with no code changes to the main Godot repo so updating it to the newest upstream version is just a matter of doing a git pull.