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## Problem In the test for https://github.com/neondatabase/neon/pull/6776, a test cases uses tiny layer sizes and tiny stripe sizes. This hits a scenario where a shard's checkpoint interval spans a region where none of the content in the WAL is ingested by this shard. Since there is no layer to flush, we do not advance disk_consistent_lsn, and this causes the test to fail while waiting for LSN to advance. ## Summary of changes - Pass an LSN through `layer_flush_start_tx`. This is the LSN to which we have frozen at the time we ask the flush to flush layers frozen up to this point. - In the layer flush task, if the layers we flush do not reach `frozen_to_lsn`, then advance disk_consistent_lsn up to this point. - In `maybe_freeze_ephemeral_layer`, handle the case where last_record_lsn has advanced without writing a layer file: this ensures that disk_consistent_lsn and remote_consistent_lsn advance anyway. The net effect is that the disk_consistent_lsn is allowed to advance past regions in the WAL where a shard ingests no data, and that we uphold our guarantee that remote_consistent_lsn always eventually reaches the tip of the WAL. The case of no layer at all is hard to test at present due to >0 shards being polluted with SLRU writes, but I have tested it locally with a branch that disables SLRU writes on shards >0. We can tighten up the testing on this in future as/when we refine shard filtering (currently shards >0 need the SLRU because they use it to figure out cutoff in GC using timestamp-to-lsn).