For either copy operation the concurrent queries check could help, however, issuing a copy query to the server has a different flow than regular queries.
For CopyOut it goes:

COPY TO STDOUT query sent
Server responds with CopyOutResponse docs
The server then sends zero or more CopyData messages (docs) where each message represents a row as a single collection of bytes
Once all the rows have been sent, the server finalizes the copy with a CopyDone message docs
With all rows of the target table copied to the client, the server send the customary CommandComplete followed by the ReadyForQuery messages.

So for CopyOut, if we want to push each row to the query result, we can hook into the existing query processing with special cases for the CopyOut command. My only concern here would be holding a large amount of data in memory since the CopyOut command is usually used for bulk data processing. An iterator might help if the client is able to process incoming rows fairly quickly and limit the memory consumption for bulk operations, but again, that would pose it's own issues.

For CopyIn:

COPY FROM STDIN query sent
Server response with CopyInResponse docs
This now lets the user send zero or more rows to the server as CopyData messages (see docs above)
Once the user is done pushing data to the server, the user must send a CopyDone mesage (see docs above)
The server then responds with the customary CommandComplete followed by the ReadyForQuery messages.

So for CopyIn, since the client is in control of completing the copy operation, the client might accidentally send a regular query message while a copy in is active which (to my understanding) will disrupt the message flow and lead to a connection state I cannot account for.

All of this is to say that the Copy command presents some challenges to how the event handling works with postgres connections. What I could do is set the query promise to a non empty value for the duration of the copy operation to avoid querying the server while in copy mode. This also might be made a little easier if I was to create dedicated copyIn/copyOut methods on the PostgreSQLConnection class since currently, the copy messages are treated as raw queries.

Uh oh!

Postgresql COPY command #419

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