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In safekeepers we have several background tasks. Previously `WAL backup` task was spawned by another task called `wal_backup_launcher`. That task received notifications via `wal_backup_launcher_rx` and decided to spawn or kill existing backup task associated with the timeline. This was inconvenient because each code segment that touched shared state was responsible for pushing notification into `wal_backup_launcher_tx` channel. This was error prone because it's easy to miss and could lead to deadlock in some cases, if notification pushing was done in the wrong order. We also had a similar issue with `is_active` timeline flag. That flag was calculated based on the state and code modifying the state had to call function to update the flag. We had a few bugs related to that, when we forgot to update `is_active` flag in some places where it could change. To fix these issues, this PR adds a new `timeline_manager` background task associated with each timeline. This task is responsible for managing all background tasks, including `is_active` flag which is used for pushing broker messages. It is subscribed for updates in timeline state in a loop and decides to spawn/kill background tasks when needed. There is a new structure called `TimelinesSet`. It stores a set of `Arc<Timeline>` and allows to copy the set to iterate without holding the mutex. This is what replaced `is_active` flag for the broker. Now broker push task holds a reference to the `TimelinesSet` with active timelines and use it instead of iterating over all timelines and filtering by `is_active` flag. Also added some metrics for manager iterations and active backup tasks. Ideally manager should be doing not too many iterations and we should not have a lot of backup tasks spawned at the same time. Fixes #7751 --------- Co-authored-by: Arseny Sher <sher-ars@yandex.ru>