What's left to make this work

Start with CacheHorizonPlugin.java lines 131-135 and 139-203, then investigate the Gradle task-execution APIs referenced by the copied implementation. Determine whether a task can be disabled during execution and how to implement isCachedOrUpToDate(Task task); done means both questions have validated answers and the plugin behavior works.

There are two open questions:

1. Does gradle let you disable a task during task execution? That's our whole premise, so hopefully the answer to this is yes!

https://github.com/diffplug/cache-horizon/blob/13424bc79e322285c8f8916700109874e24edaf2/src/main/java/com/diffplug/cachehorizon/CacheHorizonPlugin.java#L131-L135

2. How can we implement boolean isCachedOrUpToDate(Task task)?

This is how far we got, by copying out the relevant-looking parts of Gradle's task execution. It definitely doesn't work as written, but it points towards the internal APIs that Gradle uses for this.

https://github.com/diffplug/cache-horizon/blob/13424bc79e322285c8f8916700109874e24edaf2/src/main/java/com/diffplug/cachehorizon/CacheHorizonPlugin.java#L139-L203

That's as far as we got, and as far as we are likely to get for the foreseeable future. We're happy to merge PRs and add committers here if you don't want to deal with publishing and maintaining, also we're also happy to shut this project down and link to yours if you find a way forward and want to maintain it.

Our intent is to use this issue as the "forum" for solving the two questions above, or for linking out to other solutions that frame the problem differently.