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a2a44ea213
Instead of thinking in terms of 'latest' and 'lsn' of the request, each request has two LSNs: the request LSN and 'not_modified_since' LSN. The request is nominally made at the request LSN, that determines what page version we want to see. But as a hint, we also include 'not_modified_since'. It tells the pageserver that the page has not been modified since that LSN, which allows the pageserver to skip waiting for newer WAL to arrive, and could allow more optimizations in the future. Refactor the internal functions to calculate the request LSN to calculate both LSNs. Sending two LSNs to the pageserver requires using the new protocol version 2. The previous commit added the server support for it, but we still default to the old protocol for compatibility with old pageservers. The 'neon.protocol_version' GUC can be used to use the new protocol. The new protocol addresses one cause of issue #6211, although you can still get the same error if you have a standby that is lagging behind so that the page version it needs is genuinely GC'd away.