upstream: fix subset_lb crash when host configured with no metadata

source/common/upstream/subset_lb.cc

@@ -128,19 +128,21 @@ void SubsetLoadBalancer::rebuildSingle() {
   for (const auto& host_set : original_priority_set_.hostSetsPerPriority()) {
     for (const auto& host : host_set->hosts()) {
       MetadataConstSharedPtr metadata = host->metadata();
-      const auto& filter_metadata = metadata->filter_metadata();
-      auto filter_it = filter_metadata.find(Config::MetadataFilters::get().ENVOY_LB);
-      if (filter_it != filter_metadata.end()) {
-        const auto& fields = filter_it->second.fields();
-        auto fields_it = fields.find(single_key_);
-        if (fields_it != fields.end()) {
-          auto [iterator, did_insert] =
-              single_host_per_subset_map_.try_emplace(fields_it->second, host);
-          UNREFERENCED_PARAMETER(iterator);
-          if (!did_insert) {
-            // Two hosts with the same metadata value were found. Ignore all but one of them, and
-            // set a metric for how many times this happened.
-            collision_count++;
+      if (metadata != nullptr) {
+        const auto& filter_metadata = metadata->filter_metadata();
+        auto filter_it = filter_metadata.find(Config::MetadataFilters::get().ENVOY_LB);
+        if (filter_it != filter_metadata.end()) {
+          const auto& fields = filter_it->second.fields();
+          auto fields_it = fields.find(single_key_);
+          if (fields_it != fields.end()) {
+            auto [iterator, did_insert] =
+                single_host_per_subset_map_.try_emplace(fields_it->second, host);
+            UNREFERENCED_PARAMETER(iterator);
+            if (!did_insert) {
+              // Two hosts with the same metadata value were found. Ignore all but one of them, and
+              // set a metric for how many times this happened.
+              collision_count++;
+            }
           }
         }
       }

test/integration/http_subset_lb_integration_test.cc

@@ -216,4 +216,26 @@ TEST_P(HttpSubsetLbIntegrationTest, SubsetLoadBalancer) {
   runTest(type_b_request_headers_, "b");
 }
 
+// Tests subset-compatible load balancer policy with empty metadata
+TEST_P(HttpSubsetLbIntegrationTest, SubsetLoadBalancer_null_metadata) {
+  config_helper_.addConfigModifier([&](envoy::config::bootstrap::v3::Bootstrap& bootstrap) {
+    auto* static_resources = bootstrap.mutable_static_resources();
+    auto* cluster = static_resources->mutable_clusters(0);
+
+    // Set single_host_per_subset to be true
+    auto* subset_selector = cluster->mutable_lb_subset_config()->mutable_subset_selectors(0);
+    subset_selector->set_single_host_per_subset(true);
+
+    // Clear the metadata for each host
+    auto* load_assignment = cluster->mutable_load_assignment();
+    auto* endpoints = load_assignment->mutable_endpoints(0);
+    for (uint32_t i = 0; i < num_hosts_; i++) {
+      auto* lb_endpoint = endpoints->mutable_lb_endpoints(i);
+      lb_endpoint->clear_metadata();
+    }
+  });
+
+  initialize();
+}
+
 } // namespace Envoy