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## Problem Historically, the pageserver used an "uninit mark" file on disk for two purposes: - Track which timeline dirs are incomplete for handling on restart - Avoid trying to create the same timeline twice at the same time. The original purpose of handling restarts is now defunct, as we use remote storage as the source of truth and clean up any trash timeline dirs on startup. Using the file to mutually exclude creation operations is error prone compared with just doing it in memory, and the existing checks happened some way into the creation operation, and could expose errors as 500s (anyhow::Errors) rather than something clean. ## Summary of changes - Creations are now mutually excluded in memory (using `Tenant::timelines_creating`), rather than relying on a file on disk for coordination. - Acquiring unique access to the timeline ID now happens earlier in the request. - Creating the same timeline which already exists is now a 201: this simplifies retry handling for clients. - 409 is still returned if a timeline with the same ID is still being created: if this happens it is probably because the client timed out an earlier request and has retried. - Colliding timeline creation requests should no longer return 500 errors This paves the way to entirely removing uninit markers in a subsequent change. --------- Co-authored-by: Joonas Koivunen <joonas@neon.tech>