querier_cache: implement stop

Close the _closing_gate to wait on background
close of dropped queries, and close all remaining queriers.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

database.cc

@@ -2016,7 +2016,9 @@ database::stop() {
                 _read_concurrency_sem.stop(),
                 _streaming_concurrency_sem.stop(),
                 _compaction_concurrency_sem.stop(),
-                _system_read_concurrency_sem.stop()).discard_result();
+                _system_read_concurrency_sem.stop()).discard_result().finally([this] {
+            return _querier_cache.stop();
+        });
     });
 }
 

querier.cc

@@ -348,7 +348,8 @@ std::optional<Querier> querier_cache::lookup_querier(
     ++stats.drops;
 
     // Close and drop the querier in the background.
-    // TODO: wait on _closing_gate in querier_cache::close()
+    // It is safe to do so, since _closing_gate is closed and
+    // waited on in querier_cache::stop()
     (void)with_gate(_closing_gate, [this, q = std::move(q)] () mutable {
         return q.close().finally([q = std::move(q)] {});
     });
@@ -437,6 +438,18 @@ future<> querier_cache::evict_all_for_table(const utils::UUID& schema_id) noexce
     co_return;
 }
 
+future<> querier_cache::stop() noexcept {
+    co_await _closing_gate.close();
+
+    for (auto* ip : {&_data_querier_index, &_mutation_querier_index, &_shard_mutation_querier_index}) {
+        auto& idx = *ip;
+        for (auto it = idx.begin(); it != idx.end(); it = idx.erase(it)) {
+            co_await it->second->close();
+            --_stats.population;
+        }
+    }
+}
+
 querier_cache_context::querier_cache_context(querier_cache& cache, utils::UUID key, query::is_first_page is_first_page)
     : _cache(&cache)
     , _key(key)

querier.hh

@@ -450,6 +450,11 @@ public:
     /// Should be used when dropping a table.
     future<> evict_all_for_table(const utils::UUID& schema_id) noexcept;
 
+    /// Close all queriers and wait on background work.
+    ///
+    /// Should be used before destroying the querier_cache.
+    future<> stop() noexcept;
+
     const stats& get_stats() const {
         return _stats;
     }

test/boost/querier_cache_test.cc

@@ -177,6 +177,7 @@ public:
     }
 
     ~test_querier_cache() {
+        _cache.stop().get();
         _sem.stop().get();
     }