Re: Adding Support for Copy callback functionality on COPY TO api

Hi Bilva,

Thank you for registering this patch!

I had a few suggestions:

1. Please run pg_indent[1] on your code. Make sure you add
copy_data_destination_cb to src/tools/pgindent/typedefs.list. Please
run pg_indent on only the files you changed (it will take files as
command line args)

2. For features such as this, it is often helpful to find a use case
within backend/utility/extension code that demonstrate thes callback and
to include the code to exercise it with the patch. Refer how
copy_read_data() is used as copy_data_source_cb, to copy the data from
the query results from the WAL receiver (Refer: copy_table()). Finding
a similar use case in the source tree will make a stronger case
for this patch.

3. Wouldn't we want to return the number of bytes written from
copy_data_destination_cb? (Similar to copy_data_source_cb) We should
also think about how to represent failure. Looking at CopySendEndOfRow(),
we should error out like we do for the other copy_dests after checking the
return value for the callback invocation.

4.
> bool pipe = (cstate->filename == NULL) && (cstate->data_destination_cb == NULL);

I think a similar change should also be applied to BeginCopyFrom() and
CopyFrom(). Or even better, such code could be refactored to have a
separate destination type COPY_PIPE. This of course, will be a separate
patch. I think the above line is okay for this patch.

Regards,
Soumyadeep