Logical Replication - detail message with names of missing columns

Hi,

I observed that, in logical replication when a subscriber is missing
some columns, it currently emits an error message that says "some"
columns are missing(see logicalrep_rel_open()), but it doesn't specify
what the missing column names are. The comment there also says that
finding the missing column names is a todo item(/* TODO, detail
message with names of missing columns */).

I propose a patch to find the missing columns on the subscriber
relation using the publisher relation columns and show them in the
error message which can make the error more informative to the user.

Here's a snapshot how the error looks with the patch:
2020-09-04 01:00:36.721 PDT [843128] ERROR: logical replication
target relation "public.test_1" is missing "b1, d1" replicated columns
2020-09-04 01:00:46.784 PDT [843132] ERROR: logical replication
target relation "public.test_1" is missing "b1" replicated columns
2020-09-06 21:24:53.645 PDT [902945] ERROR: logical replication
target relation "public.test_1" is missing "a1, c1, d1, b1" replicated
columns

Thoughts?

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com