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OrderedSetMultimap. This maintains insertion order while eliminating duplicates.
Certain rules, in particular, otherwise break this invariant:
https://github.com/bazelbuild/bazel/blo[]e3e28274cca5b87f48abe33884edb84016dd3/src/main/java/com/google/devtools/build/lib/skyframe/ConfiguredTargetFunction.java#L403
There's no reason (to my knowledge) to need multiple instances of the same <Attribute, Dependency> pair.
More context from Google code review:
(Michael Staib):
> There are many things which pass around a dependentNodeMap or help construct one or modify one. We want an interface which has the right guarantees.
> ListMultimap is not the right interface because it has no guarantee of unique elements, which we want - we don't want the problem that this CL ran into, and there's no reason (that we know of, to be confirmed) that anyone would want multiple identical Dependencies.
> SetMultimap is not the right interface because it has no guarantee of deterministic iteration order or efficient iteration, which we want - dependency order sometimes matters (e.g., Java classpath or C++ link order).
> We agreed that the best way to get what we want is to define our own interface with its own simultaneous uniqueness and iterability guarantees. Unspoken in the discussion was why we wouldn't just use LinkedHashMultimap as the thing we pass around. IMO the reason for that is that we don't care that it be a LinkedHashMultimap specifically; if tomorrow Guava comes out with a faster cooler map that has deterministic and efficient iteration and guarantees element uniqueness, we want it.
> In this case we're going to make the "interface" be a (final?) class: OrderedSetMultimap, an extension of ForwardingSetMultimap which delegates to LinkedHashMultimap, an implementation which does support both of those guarantees.
> I had mentioned in the conversation that none of the Multimap implementations make guarantees about key iteration order, but this is not true - LinkedHashMultimap preserves key insertion order. We should perhaps declare this as part of the OrderedSetMultimap contract as well.
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Googler
Greg Estren