Introduce system mods to mod loading

[sciwhiz12]

This PR implements and closes #8049 by introducing the concept of system mods to mod loading.

System mods are mods which are required to exist in the environment during mod loading. These may be specially provided mods (for example, the minecraft mod), or mods which are vital to the framework which FML is connected to (for example, Minecraft Forge and the forge mod). If any of these configured mod IDs cannot be found during mod loading, an exception is immediately thrown to halt the game.

These system mods are used as the only existing mods in the mod list if mod sorting or dependency verification fails. This allows later steps in the which use resources from these mod files to work correctly (up to when the error screen is shown and the game exits).

As an example: in #7993, because the forge mod was missing from this list, the forge universal JAR was not included when the Access Transformers subsystem was searching for AT configuration files to load. This caused a JVM error to be thrown when the first field (that was assumed to be transformed by AT at that point) tried to be accessed by Forge code.

Implementation notes

The set of mod IDs of system mods are passed in as FML service provider arguments: the coreGameMods option (when referenced in the command line: --fml.systemMods), which takes in comma-separated mod IDs for those core game mods.

These are then passed down through ModDiscoverer into ModSorter, which does the finding of the core game mods and adding them to the systemMods field (previously named forgeAndMC). The FML-only project is configured with only the minecraft mod as the core game mod, while the Forge project is configured with both minecraft and forge as core game mods.

The set of mod IDs of system mods are configured from the value of the FML-System-Mods manifest entry from first ModFile whose locator is MinecraftLocator.

[cpw]

I see.. good idea..

[cpw]

Not quite sure about passing it all the way in from launch, but otherwise OK.

[sciwhiz12]

I was/am unsure if there is a better way to provide the set of core game mod IDs to the mod validator from FMLoader.
I thought about a file resource containing the mod IDs per line, but that would mean somehow configuring the path to the JAR so the resource can be loaded, which would possibly be a bit of a mess.
If anyone has a better idea, I'd like to hear it so we can see if it can be implemented cleanly. Otherwise, this PR is as ready as it'll ever be (pending reviews from Triage/Core for any possible defects).

[cpw]

Manifest? We could add a manifest entry to the forge jar listing the "core mods"?

[cpw]

Also, lets call them something other than "core mods". We have core mods already, and this would just confuse matters. These should be call "system mods" because they're part of ensuring basic system function.

[sciwhiz12]

I agree with the rename to "system mods", for less confusion with "coremods".

For the manifest idea, I've found a prospective solution which involves iterating over the list of ModFiles in ModSorter#buildUniqueList, filtering out those which do not use MinecraftLocator, and taking the first FML-System-Mods manifest value from these mod files manifests.

However, this does have one big gotcha: the minecraft mod is now hardcoded as an always-existing system mod, because there is no clean way for me to introduce a manifest for the FMLOnly jar in the development environment. We could potentially change that by making the minecraft mod be added as the only existing system mod if there are no system mods configured via the manifest, but I leave that decision up to you.

[cpw]

Is having minecraft hardcoded as a systemmod a bad thing? Given it's a requirement for basically everything else, it seems like a very small loss of generality... Is FML-System-Mod going to act as a flag? Do we expect many other mods to need this? It requires some very special setup to be able to play in this space, so exposing it generally will come at a very high potential for fuck ups..

[sciwhiz12]

On further thought, minecraft being a hardcoded system mod is not really bad, since it is always present as added by MinecraftLocator, but I did want to note it's hardcodedness at least.

And FML-System-Mods is not a flag, but rather a comma-separated list of mod IDs which are designated as system mods. Given that the manifest value is (a) only taken from mod files whose locator is MinecraftLocator, which excludes any mod provided either via the regular locators or a custom locator, and (b) limited to the first found manifest entry from those mod files, it is highly unlikely if not impossible for mod developers to actually affect FML by inserting this manifest entry in their own JARs.

[cpw]

OK, so your idea is to use the MinecraftLocator to locate the "tag". That seems reasonable, agree.

[cpw]

Okay that looks much better