I am using Debezium Engine 2.6.1 Final, and Debezium Postgres Connector 2.6.1 Final, and I have been facing the same issue.

While trying to perform the ad-hoc blocking snapshot, using file signal channel, for tables that have only snapshot events and there are no changes in the database after the replication slot was created, the ad-hoc would not work.

In the database, I have 3 tables: products, items, and test, however in the table.include.list I have included only 2 of them as follows:

props.setProperty("table.include.list", "public.products,public.items");{}

Started the engine and all the snapshot events were retrieved successfully. (So far, no changes in the database).

I sent a signal through the signal file, to perform an ad-hoc for the products table, but nothing happened.

 (Signal file:_ {{}"id"{}:{}"unique-signal-id-0-file"{},{}"type"{}:{}"execute-snapshot"{},{}"data"{}:{{}"type"{}:{}"BLOCKING"{},{}"data-collections"{}:[{_}{_}"public.products"{_}{_}]}}{}){_}

Then, I added 1 row in the “test” table (which is not in the table.include.list) and the blocking ad-hoc snapshot of the products table that I required earlier started right after this change. 

From the log, it looks like the streaming won’t be paused unless there is a new LSN: 

Then, right after that, I performed another ad-hoc, for the “items” table, and it was performed successfully. (Processing signal line: {"id":"unique-signal-id-2","type":"execute-snapshot","data":{"type":"BLOCKING","data-collections":["public.items"]}})

As it has already been mentioned, the ad-hoc would not be performed if there is no change in the transaction log.

For your convenience, I have attached the TRACE log file as well as Thread dumbs.

Could you please take a look at this issue?

Thank you!