Historical queries cache size soft limit

.daily_canary

@@ -3,4 +3,4 @@
  (  V  ) /  .  \  |    +---=---'
 /--x-m- /--n-n---xXx--/--yY------>>>----<<<>>]]{{}}---||-/\---..
 2024__
-!..!!
+!..!

doc/build_apps/api.rst

@@ -125,7 +125,7 @@ Historical Queries
 
 .. doxygenclass:: ccf::historical::AbstractStateCache
    :project: CCF
-   :members: set_default_expiry_duration, get_state_at, get_store_at, get_store_range, drop_cached_states
+   :members: set_default_expiry_duration, set_soft_cache_limit, get_state_at, get_store_at, get_store_range, drop_cached_states
 
 .. doxygenstruct:: ccf::historical::State
    :project: CCF

include/ccf/historical_queries_interface.h

@@ -49,6 +49,8 @@ namespace ccf::historical
 
   using ExpiryDuration = std::chrono::seconds;
 
+  using CacheSize = size_t;
+
   /** Stores the progress of historical query requests.
    *
    * A request will generally need to be made multiple times (with the same
@@ -79,6 +81,13 @@ namespace ccf::historical
     virtual void set_default_expiry_duration(
       ExpiryDuration seconds_until_expiry) = 0;
 
+    /** Set the cache limit (in bytes) to evict least recently used requests
+     * from the cache after its size grows beyond this limit. The limit is not
+     * strict. It is estimated based on serialized states' sizes approximation
+     * and is checked once per tick, and so it can overflow for a short time.
+     */
+    virtual void set_soft_cache_limit(CacheSize cache_limit) = 0;
+
     /** EXPERIMENTAL: Set the tracking of deletes on missing keys for historical
      * queries.
      *

samples/apps/logging/logging.cpp

@@ -460,6 +460,12 @@ namespace loggingapp
         PUBLIC_RECORDS, context, 10000, 20);
       context.get_indexing_strategies().install_strategy(index_per_public_key);
 
+      // According to manual obvervation it's enough to start evicting old
+      // requests on historical perf test, but not too small to get stuck
+      // because of a single request being larget than the cache.
+      constexpr size_t cache_limit = 1024 * 1024 * 10; // MB
+      context.get_historical_state().set_soft_cache_limit(cache_limit);
+
       const ccf::AuthnPolicies auth_policies = {
         ccf::jwt_auth_policy,
         ccf::user_cert_auth_policy,