In these changes, we pair draining the view builder with its start.
To better understand what was done and why, let's first look at the
situation before this commit and the context of it:
(a) The following things happened in order:
1. The view builder would be constructed.
2. Right after that, a deferred lambda would be created to stop the
view builder during shutdown.
3. group0_service would be started.
4. A deferred lambda stopping group0_service would be created right
after that.
5. The view builder would be started.
(b) Because the view builder depends on group0_client, it couldn't be
started before starting group0_service. On the other hand, other
services depend on the view builder, e.g. the stream manager. That
makes changing the order of initialization a difficult problem,
so we want to avoid doing that unless we're sure it's the right
choice.
(c) Since the view builder uses group0_client, there was a possibility
of running into a segmentation fault issue in the following
scenario:
1. A call to `view_builder::mark_view_build_success()` is issued.
2. We stop group0_service.
3. `view_builder::mark_view_build_success()` calls
`announce_with_raft()`, which leads to a use-after-free because
group0_service has already been destroyed.
This very scenario took place in scylladb/scylladb#20772.
Initially, we decided to solve the issue by initializing
group0_service a bit earlier (scylladb/scylladb@7bad8378c7).
Unfortunately, it led to other issues described in scylladb/scylladb#21534.
We reverted that change in the previous commit. These changes are the
second attempt to the problem where we want to solve it in a safer manner.
The solution we came up with is to pair the start of the view builder
with a deferred lambda that deinitializes it by calling
`view_builder::drain()`. No other component of the system should be
able to use the view builder anymore, so it's safe to do that.
Furthermore, that pairing makes the analysis of
initialization/deinitialization order much easier. We also solve the
aformentioned use-after-free issue because the view builder itself
will no longer attempt to use group0_client.
Note that we still pair a deferred lambda calling `view_builder::stop()`
with the construction of the view builder; that function will also call
`view_builder::drain()`. Another notable thing is `view_builder::drain()`
may be called earlier by `storage_service::do_drain()`. In other words,
these changes cover the situation when Scylla runs into a problem when
starting up.
Fixesscylladb/scylladb#20772
(cherry picked from commit 06ce976370)
1200d3b735