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As explained in SERVER-46373 currently $lookup is not allowed to target a sharded collection in a multi document transaction due to 2 known reasons:
- That a shard targeting itself will deadlock trying to re-acquire the same session
- That if a different shard needs to be targeted, the shard does not know how to correctly start the multi-doc txn on that other shard.
After SERVER-81190 unsharded collection will be tracked and $lookup as a consequence won't be allowed on a unsharded collection as well (within a transaction)
The goal of this ticket is to allow $lookup on an unsplittable collection if that collection is still on the primary. This is just a temporary fix before SERVER-46373 to enable SERVER-81190 to be merged and start tracking collections.
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