Fix cacheKeyForTree & OneShot incompatibility #1100
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The fix in #1064 turns out to be insufficient. The problem is that `cacheKeyForTree` can provide a cached tree at any point in the broccoli graph. We can't tell just by looking at the top level node for an addon whether it contains, deep down inside it, some trees that have already been run by other builders via OneShot. I don't like `cacheKeyForTree`. It's trying to solve a real problem, but at the wrong layer. Instead of sharing work between many addon instances, it should have been preventing there from *being* many addon instances in the first place. I'm moving Embroider to a solution more like that. We already have the `reduceInstances` hook, which is a more blunt and powerful way to through away whole Addon instances. We can give it a default implementation that will allow two addons with *entirely* identical cacheKeyForTree to deduplicate at the *instance* level, not the tree level.
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The fix in #1064 turns out to be insufficient.
The problem is that
cacheKeyForTreecan provide a cached tree at any point in the broccoli graph. We can't tell just by looking at the top level node for an addon whether it contains, deep down inside it, some trees that have already been run by other builders via OneShot.I don't like
cacheKeyForTree. It's trying to solve a real problem, but at the wrong layer. Instead of sharing work between many addon instances, it should have been preventing there from being many addon instances in the first place.This PR moves Embroider to a solution more like that. We already have the
reduceInstanceshook. Here I give it a default implementation that will allow two addon instances with identicalcacheKeyForTreefor all trees to deduplicate at the instance level, not the tree level.It's also important to point out that there are up to three different kinds of deduplication going on in a classic build via
cacheKeyForTree:Our
reduceInstanceshook intentionally only covers the first case.For the second case (when you want to deduplicate different copies of a package) you can do that in your package manager instead via yarn resolutions or NPM overrides.
For the third case (when you want to deduplicate between entirely unrelated packages)... no. Just don't. That is the worst kind of hidden global monkey patching.