The config.cache.chunks.* collections are created as part of a shard refreshing the metadata (routing table) for a sharded collection. A complete copy of the config.cache.chunks.* collections exist on each shard (they are so-called reference tables). It is therefore always valid for a shard to read from a config.cache.chunks.* namespace locally. (The existing $lookup behavior is to do the merging on the primary shard for the database.)

The from parameter to the $lookup stage should be made to support the following syntax:

{from: {db: <string>, coll: <string>}}

It must be an error to use this new syntax with any namespace other than config.cache.chunks.*

Additionally, the $lookup stage must not attempt to route the request for config.cache.chunks.* to any other shard, even when the internalQueryAllowShardedLookup server parameter is enabled. Consider generalizing the check for the local database in the attachCursorToPipeline() function to be NamespaceString::isNamespaceAlwaysUnsharded().