Introduce virtual tasks - task manager tasks which cover
cluster-wide operations.
Virtual tasks aren't kept in memory, instead their statuses
are retrieved from associated service when user requests
them with task manager API. From API users' perspective,
virtual tasks behave similarly to regular tasks, but they can
be queried from any node in a cluster.
Virtual tasks cannot have a parent task. They can have
children on each node in a cluster, but do not keep references
to them. So, if a direct child of a virtual task is unregistered
from task manager, it will no longer be shown in parent's
children vector.
virtual_task class corresponds to all virtual tasks in one
group. If users want to list all tasks in a module, a virtual_task
returns all recent supported operations; if they request virtual
task's status - info about the one specified operation is
presented. Time to live, number of tracked operations etc.
depend on the implementation of individual virtual_task.
All virtual_tasks are kept only on shard 0.
Refs: https://github.com/scylladb/scylladb/issues/15852
New feature, no backport needed.
Closesscylladb/scylladb#16374
* github.com:scylladb/scylladb:
docs: describe virtual tasks
db: node_ops: filter topology request entries
test: add a topology suite for testing tasks
node_ops: service: create streaming tasks
node_ops: register node_ops_virtual_task in task manager
service: node_ops: keep node ops module in storage service
node_ops: implement node_ops_virtual_task methods
db: service: modify methods to get topology_requests data
db: service: add request type column to topology_requests
node_ops: add task manager module and node_ops_virtual_task
tasks: api: add virtual task support to get_task_status_recursively
tasks: api: add virtual task support
tasks: api: add virtual tasks support to get_tasks
tasks: add task_handler to hide task and virtual_task differences from user
tasks: modify invoke_on_task
tasks: implement task_manager::virtual_task::impl::get_children
tasks: keep virtual tasks in task manager
tasks: introduce task_manager::virtual_task
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