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f4f300732a
This is preliminary work for/from #4220 (async `Layer::get_value_reconstruct_data`). The motivation is to avoid locking `Tenant::timelines` in places that can't be `async`, because in #4333 we want to convert Tenant::timelines from `std::sync::Mutex` to `tokio::sync::Mutex`. But, the changes here are useful in general because they clean up & document tenant state transitions. That also paves the way for #4350, which is an alternative to #4333 that refactors the pageserver code so that we can keep the `Tenant::timelines` mutex sync. This patch consists of the following core insights and changes: * spawn_load and spawn_attach own the tenant state until they're done * once load()/attach() calls are done ... * if they failed, transition them to Broken directly (we know that there's no background activity because we didn't call activate yet) * if they succeed, call activate. We can make it infallible. How? Later. * set_broken() and set_stopping() are changed to wait for spawn_load() / spawn_attach() to finish. * This sounds scary because it might hinder detach or shutdown, but actually, concurrent attach+detach, or attach+shutdown, or load+shutdown, or attach+shutdown were just racy before this PR. So, with this change, they're not anymore. In the future, we can add a `CancellationToken` stored in Tenant to cancel `load` and `attach` faster, i.e., make `spawn_load` / `spawn_attach` transition them to Broken state sooner. See the doc comments on TenantState for the state transitions that are now possible. It might seem scary, but actually, this patch reduces the possible state transitions. We introduce a new state `TenantState::Activating` to avoid grabbing the `Tenant::timelines` lock inside the `send_modify` closure. These were the humble beginnings of this PR (see Motivation section), and I think it's still the right thing to have this `Activating` state, even if we decide against async `Tenant::timelines` mutex. The reason is that `send_modify` locks internally, and by moving locking of Tenant::timelines out of the closure, the internal locking of `send_modify` becomes a leaf of the lock graph, and so, we eliminate deadlock risk. Co-authored-by: Joonas Koivunen <joonas@neon.tech>