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## Problem For any given tenant shard, pageservers receive all of the tenant's WAL from the safekeeper. This soft-blocks us from using larger shard counts due to bandwidth concerns and CPU overhead of filtering out the records. ## Summary of changes This PR lifts the decoding and interpretation of WAL from the pageserver into the safekeeper. A customised PG replication protocol is used where instead of sending raw WAL, the safekeeper sends filtered, interpreted records. The receiver drives the protocol selection, so, on the pageserver side, usage of the new protocol is gated by a new pageserver config: `wal_receiver_protocol`. More granularly the changes are: 1. Optionally inject the protocol and shard identity into the arguments used for starting replication 2. On the safekeeper side, implement a new wal sending primitive which decodes and interprets records before sending them over 3. On the pageserver side, implement the ingestion of this new replication message type. It's very similar to what we already have for raw wal (minus decoding and interpreting). ## Notes * This PR currently uses my [branch of rust-postgres](https://github.com/neondatabase/rust-postgres/tree/vlad/interpreted-wal-record-replication-support) which includes the deserialization logic for the new replication message type. PR for that is open [here](https://github.com/neondatabase/rust-postgres/pull/32). * This PR contains changes for both pageservers and safekeepers. It's safe to merge because the new protocol is disabled by default on the pageserver side. We can gradually start enabling it in subsequent releases. * CI tests are running on https://github.com/neondatabase/neon/pull/9747 ## Links Related: https://github.com/neondatabase/neon/issues/9336 Epic: https://github.com/neondatabase/neon/issues/9329