mongos doesn't raise validation errors but returns empty cursors when the database doesn't exist

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    • Type: Bug
    • Resolution: Fixed
    • Priority: Major - P3
    • 7.2.0-rc0
    • Affects Version/s: None
    • Component/s: None
    • None
    • Query Execution
    • Minor Change
    • ALL
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      mongos returns an empty cursor with this aggregation when the db does not exist.

      db.aggregate([{$match: {size: "medium"}}, {$documents: [{_id: 1, size: "medium"}, {_id: 2}]}]); 

      This command should return an error since $documents can only be used in the first stage in a pipeline.

      Additionally, mongos will return an empty cursor with this aggregation. This aggregation should also raise an error since $unionWith must be used with a collection.

      db.aggregate([
  {
     $unionWith: { 
         pipeline: [{ $documents: {$map: {input: {$range: [0, 5]}, in : {x: "$$this"}}}}] 
     }       
   }, 
   { $group: {_id: "$x", x: {$first: "$x"}}},
   { $project: {_id: 0}},    
 ])    
 .toArray();  

      Discovered in SERVER-63811.  The empty cursor is being returned by this logic:

          if (executionNsRoutingInfoStatus.isOK()) {
        cm = std::move(executionNsRoutingInfoStatus.getValue());
    } else if (!(hasChangeStream &&
                 executionNsRoutingInfoStatus == ErrorCodes::NamespaceNotFound)) {
        appendEmptyResultSetWithStatus(
            opCtx, namespaces.requestedNss, executionNsRoutingInfoStatus.getStatus(), result);
        return Status::OK();
    }

      However validation checks should have occurred before this logic and returned an error.

            Assignee:
            Colin Stolley
            Reporter:
            Gil Alon
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              Created:
              Updated:
              Resolved: