view: Plug view update generator to database

The database is low-level service and currently view update generator
implicitly depend on it via storage proxy. However, database does need
to push view updates with the help of mutate_MV helper, thus adding the
dependency loop.

This patch exploits the fact that view updates start being pushed late
enough, by that time all other service, including proxy and view update
generator, seem to be up and running. This allows a "weak dependency"
from database to view update generator, like there's one from database
to system keyspace already.

So in this patch the v.u.g. puts the shared-from-this pointer onto the
database at the time it starts. On stop it removes this pointer after
database is drained and (hopefully) all view updates are pushed.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

db/view/view_update_generator.cc

@@ -97,6 +97,7 @@ view_update_generator::view_update_generator(replica::database& db, sharded<serv
     setup_metrics();
     discover_staging_sstables();
     (void)_proxy;
+    _db.plug_view_update_generator(*this);
 }
 
 view_update_generator::~view_update_generator() {}
@@ -206,6 +207,7 @@ future<> view_update_generator::start() {
 }
 
 future<> view_update_generator::stop() {
+    _db.unplug_view_update_generator();
     _as.request_abort();
     _pending_sstables.signal();
     return std::move(_started).then([this] {

db/view/view_update_generator.hh

@@ -28,7 +28,7 @@ class storage_proxy;
 
 namespace db::view {
 
-class view_update_generator {
+class view_update_generator : public async_sharded_service<view_update_generator> {
 public:
     static constexpr size_t registration_queue_size = 5;