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RequestContext is used to track each "operation" or "task" in a way that's not tied to tokio tasks. It provides support for fine-grained cancellation of individual operations, or all tasks working on an active tenant or timeline. Most async functions now take a RequestContext argument. RequestContexts form a hierarchy, so that you have a top-level context e.g. for a TCP listener task, a child context for each task handling a connection, and perhaps a grandchild context for each individual client request. In addition to the hierarchy, each RequestContext can be associated with a Tenant or Timeline object. This is used to prevent a Tenant or Timeline from being deleted or detached while there are still tasks accessing it. This fixes a long-standing race conditions between GC/compaction and deletion (see issues #2914 and compiler in any way, but the functions like `get_active_timeline` make it easy to do the right thing. This replaces most of the machinery in `task_mgr.rs`. We don't track running tasks as such anymore, only RequestContexts. In practice, every task holds onto a RequestContext. In addition to supporting cancellation, the RequestContext specifies the desired behavior if a remote layer is needed for the operation. This replaces the `with_ondemand_download_sync` and `no_ondemand_download` macros. The on-demand download now happens deep in the call stack, in get_reconstruct_data(), and the caller is no longer involved in the download, except by passing a RequestContext that specifies whether to do on-demand download or not. The PageReconstructResult type is gone but the PageReconstructError::NeedsDownload variant remains. It's now returned if the context specified "don't do on-demand download", and a layer is missing. TODO: - Enforce better that you hold a RequestContext associated with a Tenant or Timeline. - All the fields in RequestContext are currently 'pub', but things will break if you modify the tenant/timeline fields directly. Make that more safe. - When you create a subcontext, should it inherit the Tenant / Timeline of its parent? - Can the walreceiver::TaskHandle stuff be replaced with this? - Extract smaller patches: - What else could we extract?