Merge pull request #8263 from markdroth/grpclb_resolver_semantic_fix

Centralized logic for choosing the right LB policy.

1 files changed, 28 insertions, 4 deletions

diff --git a/src/core/ext/client_config/client_channel.c b/src/core/ext/client_config/client_channel.c
index 3b4747bebc..f3befe9b47 100644
--- a/src/core/ext/client_config/client_channel.c
+++ b/src/core/ext/client_config/client_channel.c
@@ -185,10 +185,34 @@ static void on_resolver_result_changed(grpc_exec_ctx *exec_ctx, void *arg,
     lb_policy_args.additional_args =
         grpc_resolver_result_get_lb_policy_args(chand->resolver_result);
     lb_policy_args.client_channel_factory = chand->client_channel_factory;
-    lb_policy = grpc_lb_policy_create(
-        exec_ctx,
-        grpc_resolver_result_get_lb_policy_name(chand->resolver_result),
-        &lb_policy_args);
+
+    // Special case: If all of the addresses are balancer addresses,
+    // assume that we should use the grpclb policy, regardless of what the
+    // resolver actually specified.
+    const char* lb_policy_name =
+        grpc_resolver_result_get_lb_policy_name(chand->resolver_result);
+    bool found_backend_address = false;
+    for (size_t i = 0; i < lb_policy_args.addresses->num_addresses; ++i) {
+      if (!lb_policy_args.addresses->addresses[i].is_balancer) {
+        found_backend_address = true;
+        break;
+      }
+    }
+    if (!found_backend_address) {
+      if (lb_policy_name != NULL && strcmp(lb_policy_name, "grpclb") != 0) {
+        gpr_log(GPR_INFO,
+                "resolver requested LB policy %s but provided only balancer "
+                "addresses, no backend addresses -- forcing use of grpclb LB "
+                "policy", (lb_policy_name == NULL ? "(none)" : lb_policy_name));
+      }
+      lb_policy_name = "grpclb";
+    }
+    // Use pick_first if nothing was specified and we didn't select grpclb
+    // above.
+    if (lb_policy_name == NULL) lb_policy_name = "pick_first";
+
+    lb_policy =
+        grpc_lb_policy_create(exec_ctx, lb_policy_name, &lb_policy_args);
     if (lb_policy != NULL) {
       GRPC_LB_POLICY_REF(lb_policy, "config_change");
       GRPC_ERROR_UNREF(state_error);