filter state: add hashable string factory

docs/root/configuration/advanced/well_known_filter_state.rst

@@ -136,6 +136,11 @@ configuration with a :ref:`factory lookup key
   This creates a filter state entry named ``my.custom.key`` containing the string ``my-value``.
   The value can be accessed in access logs using ``%FILTER_STATE(my.custom.key)%``.
 
+``envoy.hashable_string``
+  Same as ``envoy.string`` but supports connection pool hashing when :ref:`shared with the upstream
+  <arch_overview_advanced_filter_state_sharing>`. Please use with care as it can lead to significant
+  increase in the number of upstream connections when used with HTTP upstreams.
+
 ``envoy.network.ip``
   A factory to create IP addresses from ``IPv4`` and ``IPv6`` address strings.
 

docs/root/configuration/security/secret.rst

@@ -410,7 +410,7 @@ the downstream filter chain, e.g. using the following filter configuration:
     "@type": type.googleapis.com/envoy.extensions.filters.network.set_filter_state.v3.Config
     on_new_connection:
     - object_key: envoy.tls.certificate_mappers.on_demand_secret
-      factory_key: envoy.string
+      factory_key: envoy.hashable_string
       format_string:
         text_format_source:
           inline_string: my_secret_name

source/extensions/filters/common/set_filter_state/BUILD

@@ -13,6 +13,7 @@ envoy_cc_library(
     srcs = ["filter_config.cc"],
     hdrs = ["filter_config.h"],
     deps = [
+        "//envoy/common:hashable_interface",
         "//envoy/formatter:substitution_formatter_interface",
         "//envoy/registry",
         "//envoy/stream_info:filter_state_interface",

source/extensions/filters/common/set_filter_state/filter_config.cc

@@ -1,5 +1,6 @@
 #include "source/extensions/filters/common/set_filter_state/filter_config.h"
 
+#include "envoy/common/hashable.h"
 #include "envoy/registry/registry.h"
 
 #include "source/common/formatter/substitution_format_string.h"
@@ -22,6 +23,25 @@ class GenericStringObjectFactory : public StreamInfo::FilterState::ObjectFactory
 
 REGISTER_FACTORY(GenericStringObjectFactory, StreamInfo::FilterState::ObjectFactory);
 
+class HashableString : public Router::StringAccessorImpl, public Hashable {
+public:
+  HashableString(absl::string_view value) : StringAccessorImpl(value) {}
+
+  // Hashable
+  absl::optional<uint64_t> hash() const override { return HashUtil::xxHash64(asString()); }
+};
+
+class GenericHashableStringObjectFactory : public StreamInfo::FilterState::ObjectFactory {
+public:
+  std::string name() const override { return "envoy.hashable_string"; }
+  std::unique_ptr<StreamInfo::FilterState::Object>
+  createFromBytes(absl::string_view data) const override {
+    return std::make_unique<HashableString>(data);
+  }
+};
+
+REGISTER_FACTORY(GenericHashableStringObjectFactory, StreamInfo::FilterState::ObjectFactory);
+
 std::vector<Value>
 Config::parse(const Protobuf::RepeatedPtrField<FilterStateValueProto>& proto_values,
               Server::Configuration::GenericFactoryContext& context) const {

test/extensions/filters/common/set_filter_state/BUILD

@@ -13,6 +13,7 @@ envoy_cc_test(
     srcs = ["filter_config_test.cc"],
     rbe_pool = "6gig",
     deps = [
+        "//envoy/common:hashable_interface",
         "//source/common/formatter:formatter_extension_lib",
         "//source/common/router:string_accessor_lib",
         "//source/extensions/filters/common/set_filter_state:filter_config_lib",

test/extensions/filters/common/set_filter_state/filter_config_test.cc

@@ -1,3 +1,5 @@
+#include "envoy/common/hashable.h"
+
 #include "source/common/router/string_accessor_impl.h"
 #include "source/extensions/filters/common/set_filter_state/filter_config.h"
 #include "source/server/generic_factory_context.h"
@@ -93,6 +95,19 @@ TEST_F(ConfigTest, SetValueWithFactory) {
   EXPECT_EQ(0, info_.filterState()->objectsSharedWithUpstreamConnection()->size());
 }
 
+TEST_F(ConfigTest, SetHashableValueWithFactory) {
+  initialize({R"YAML(
+    object_key: my_key
+    factory_key: envoy.hashable_string
+    format_string:
+      text_format_source:
+        inline_string: "XXX"
+  )YAML"});
+  update();
+  const auto* foo = info_.filterState()->getDataReadOnly<Hashable>("my_key");
+  ASSERT_NE(nullptr, foo);
+}
+
 TEST_F(ConfigTest, SetValueConnection) {
   initialize({R"YAML(
     object_key: foo