matcher: add support for new any matcher mechanism

docs/root/version_history/current.rst

@@ -33,6 +33,7 @@ Removed Config or Runtime
 New Features
 ------------
 * http3: downstream HTTP/3 support is now GA! Upstream HTTP/3 also GA for specific deployments. See :ref:`here <arch_overview_http3>` for details.
+* matching: the matching API can now express a match tree that will always match by omitting a matcher at the top level.
 
 
 Deprecated

source/common/matcher/matcher.h

@@ -61,6 +61,22 @@ static inline MaybeMatchResult evaluateMatch(MatchTree<DataType>& match_tree,
 
 template <class DataType> using FieldMatcherFactoryCb = std::function<FieldMatcherPtr<DataType>()>;
 
+/**
+ * A matcher that will always resolve to associated on_no_match. This is used when
+ * the matcher is configured without a matcher, allowing for a tree that always resolves
+ * to a specific OnMatch.
+ */
+template <class DataType> class AnyMatcher : public MatchTree<DataType> {
+public:
+  explicit AnyMatcher(absl::optional<OnMatch<DataType>> on_no_match)
+      : on_no_match_(std::move(on_no_match)) {}
+
+  typename MatchTree<DataType>::MatchResult match(const DataType&) override {
+    return {MatchState::MatchComplete, on_no_match_};
+  }
+  const absl::optional<OnMatch<DataType>> on_no_match_;
+};
+
 /**
  * Recursively constructs a MatchTree from a protobuf configuration.
  * @param DataType the type used as a source for DataInputs
@@ -83,8 +99,7 @@ class MatchTreeFactory : public OnMatchFactory<DataType> {
     case MatcherType::kMatcherList:
       return createListMatcher(config);
     case MatcherType::MATCHER_TYPE_NOT_SET:
-      IS_ENVOY_BUG("match fail");
-      return nullptr;
+      return createAnyMatcher(config);
     }
     return nullptr;
   }
@@ -100,6 +115,15 @@ class MatchTreeFactory : public OnMatchFactory<DataType> {
   }
 
 private:
+  template <class MatcherType>
+  MatchTreeFactoryCb<DataType> createAnyMatcher(const MatcherType& config) {
+    auto on_no_match = createOnMatch(config.on_no_match());
+
+    return [on_no_match]() {
+      return std::make_unique<AnyMatcher<DataType>>(
+          on_no_match ? absl::make_optional((*on_no_match)()) : absl::nullopt);
+    };
+  }
   template <class MatcherType>
   MatchTreeFactoryCb<DataType> createListMatcher(const MatcherType& config) {
     std::vector<std::pair<FieldMatcherFactoryCb<DataType>, OnMatchFactoryCb<DataType>>>

test/common/matcher/matcher_test.cc

@@ -17,6 +17,7 @@
 #include "test/test_common/utility.h"
 
 #include "gtest/gtest.h"
+#include "xds/type/matcher/v3/matcher.pb.validate.h"
 
 namespace Envoy {
 namespace Matcher {
@@ -83,6 +84,29 @@ TEST_F(MatcherTest, TestMatcher) {
   EXPECT_NE(result.on_match_->action_cb_, nullptr);
 }
 
+TEST_F(MatcherTest, TestAnyMatcher) {
+  const std::string yaml = R"EOF(
+on_no_match:
+  action:
+    name: test_action
+    typed_config:
+      "@type": type.googleapis.com/google.protobuf.StringValue
+      value: match!!
+  )EOF";
+
+  xds::type::matcher::v3::Matcher matcher;
+  MessageUtil::loadFromYaml(yaml, matcher, ProtobufMessage::getStrictValidationVisitor());
+
+  TestUtility::validate(matcher);
+
+  auto match_tree = factory_.create(matcher);
+
+  const auto result = match_tree()->match(TestData());
+  EXPECT_EQ(result.match_state_, MatchState::MatchComplete);
+  EXPECT_TRUE(result.on_match_.has_value());
+  EXPECT_NE(result.on_match_->action_cb_, nullptr);
+}
+
 TEST_F(MatcherTest, CustomGenericInput) {
   const std::string yaml = R"EOF(
 matcher_list: