Refine model used to enter transient failure for pick_first

src/core/ext/lb_policy/pick_first/pick_first.c

@@ -315,11 +315,14 @@ static void pf_connectivity_changed(grpc_exec_ctx *exec_ctx, void *arg,
             &p->checking_connectivity, &p->connectivity_changed);
         break;
       case GRPC_CHANNEL_TRANSIENT_FAILURE:
+        p->checking_subchannel =
+            (p->checking_subchannel + 1) % p->num_subchannels;
+        if (p->checking_subchannel == 0) {
+          /* only trigger transient failure when we've tried all alternatives */
           grpc_connectivity_state_set(exec_ctx, &p->state_tracker,
                                       GRPC_CHANNEL_TRANSIENT_FAILURE,
                                       "connecting_transient_failure");
-        p->checking_subchannel =
-            (p->checking_subchannel + 1) % p->num_subchannels;
+        }
         p->checking_connectivity = grpc_subchannel_check_connectivity(
             p->subchannels[p->checking_subchannel]);
         if (p->checking_connectivity == GRPC_CHANNEL_TRANSIENT_FAILURE) {