Hi,
While working on an output plugin that uses streaming protocol, I hit an assertion failure. Further investigations revealed a possible bug in core Postgres. This must be new to PG14 since streaming support is new to this release. I extended the test_decoding regression test to demonstrate the failure. PFA
```
2021-05-25 11:32:19.493 IST client backend[68321] pg_regress/stream STATEMENT: SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '
1', 'stream-changes', '1');
TRAP: FailedAssertion("txn->size == 0", File: "reorderbuffer.c", Line: 3476, PID: 68321)
```
From my preliminary analysis, it looks like we fail to adjust the memory accounting after streaming toasted tuples. More concretely, after `ReorderBufferProcessPartialChange()` processes the in-progress transaction, `ReorderBufferTruncateTXN()` truncates the accumulated changed in the transaction, but fails to adjust the buffer size for toast chunks. Maybe we are missing a call to `ReorderBufferToastReset()` somewhere?
From what I see, the assertion only triggers when data is inserted via COPY (multi-insert).
Let me know if anything else is needed to reproduce this.
Thanks,