Change UnboundedSourceAsSdfWrapperFn to share the cache across instances.

[scwhittle]

Add a utility class to enable sharing across all deserialized instances of a DoFn and use it in UnboundedSourceAsSdfWrapperFn to cache Readers across dofn instances.

fixes #32968

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[scwhittle]

R: @kennknowles

I don't really like the utility class name, let me know if you have some better idea. It seems like this could be a common use within DoFns. For example, @sjvanrossum made me think of this concern in his work on Kafka source in #32986

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[scwhittle]

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[scwhittle]

@sjvanrossum I tried to abstract your caching in the KafkaSDF here since there would be similar concerns about a static cache for this dofn.

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[sjvanrossum]

The time-based eviction policy seems to be aligned with the reader cache in the Dataflow runner, but the size-based eviction policy is not present in the Dataflow runner. I'm thinking this would negatively impact Read instances with > 100 splits.

[sjvanrossum]

Aside from that, the manual cache management implemented in the restriction tracker looks like it could be replaced by applying #expireAfterAccess(long, TimeUnit) and #build(CacheLoader), but that's a separate issue.

[sjvanrossum]

@sjvanrossum I tried to abstract your caching in the KafkaSDF here since there would be similar concerns about a static cache for this dofn.

That looks good to me, thanks for distilling it into a utility class! As to naming, this is essentially a Supplier with additional guarantees about the return value of #get().

Drawing inspiration from Guava's Suppliers #memoize(Supplier), #memoizeWithExpiration(Supplier, long, TimeUnit) and #ofInstance​(T) backed by MemoizingSupplier, ExpiringMemoizingSupplier and SupplierOfInstance respectively, I think SerializableSupplierMemoizingPerInstantiation conveys the intended application of this class.