grpclb: Add support for balancer telling client to enter fallback mode.

src/core/ext/filters/client_channel/lb_policy/grpclb/grpclb.cc

@@ -1157,6 +1157,25 @@ void GrpcLb::BalancerCallState::OnBalancerMessageReceivedLocked(
         }
         break;
       }
+      case response.FALLBACK: {
+        if (!grpclb_policy->fallback_mode_) {
+          gpr_log(GPR_INFO,
+                  "[grpclb %p] Entering fallback mode as requested by balancer",
+                  grpclb_policy);
+          if (grpclb_policy->fallback_at_startup_checks_pending_) {
+            grpclb_policy->fallback_at_startup_checks_pending_ = false;
+            grpc_timer_cancel(&grpclb_policy->lb_fallback_timer_);
+            grpclb_policy->CancelBalancerChannelConnectivityWatchLocked();
+          }
+          grpclb_policy->fallback_mode_ = true;
+          grpclb_policy->CreateOrUpdateChildPolicyLocked();
+          // Reset serverlist, so that if the balancer exits fallback
+          // mode by sending the same serverlist we were previously
+          // using, we don't incorrectly ignore it as a duplicate.
+          grpclb_policy->serverlist_.reset();
+        }
+        break;
+      }
     }
   }
   grpc_slice_unref_internal(response_slice);

src/core/ext/filters/client_channel/lb_policy/grpclb/load_balancer_api.cc

@@ -181,6 +181,12 @@ bool GrpcLbResponseParse(const grpc_slice& encoded_grpc_grpclb_response,
     }
     return true;
   }
+  // Handle fallback.
+  if (grpc_lb_v1_LoadBalanceResponse_has_fallback_response(response)) {
+    result->type = result->FALLBACK;
+    return true;
+  }
+  // Unknown response type.
   return false;
 }
 

src/core/ext/filters/client_channel/lb_policy/grpclb/load_balancer_api.h

@@ -48,7 +48,7 @@ struct GrpcLbServer {
 };
 
 struct GrpcLbResponse {
-  enum { INITIAL, SERVERLIST } type;
+  enum { INITIAL, SERVERLIST, FALLBACK } type;
   grpc_millis client_stats_report_interval = 0;
   std::vector<GrpcLbServer> serverlist;
 };

src/core/ext/upb-generated/src/proto/grpc/lb/v1/load_balancer.upb.c

@@ -71,20 +71,22 @@ const upb_msglayout grpc_lb_v1_ClientStats_msginit = {
   UPB_SIZE(40, 48), 6, false,
 };
 
-static const upb_msglayout *const grpc_lb_v1_LoadBalanceResponse_submsgs[2] = {
+static const upb_msglayout *const grpc_lb_v1_LoadBalanceResponse_submsgs[3] = {
+  &grpc_lb_v1_FallbackResponse_msginit,
   &grpc_lb_v1_InitialLoadBalanceResponse_msginit,
   &grpc_lb_v1_ServerList_msginit,
 };
 
-static const upb_msglayout_field grpc_lb_v1_LoadBalanceResponse__fields[2] = {
-  {1, UPB_SIZE(0, 0), UPB_SIZE(-5, -9), 0, 11, 1},
-  {2, UPB_SIZE(0, 0), UPB_SIZE(-5, -9), 1, 11, 1},
+static const upb_msglayout_field grpc_lb_v1_LoadBalanceResponse__fields[3] = {
+  {1, UPB_SIZE(0, 0), UPB_SIZE(-5, -9), 1, 11, 1},
+  {2, UPB_SIZE(0, 0), UPB_SIZE(-5, -9), 2, 11, 1},
+  {3, UPB_SIZE(0, 0), UPB_SIZE(-5, -9), 0, 11, 1},
 };
 
 const upb_msglayout grpc_lb_v1_LoadBalanceResponse_msginit = {
   &grpc_lb_v1_LoadBalanceResponse_submsgs[0],
   &grpc_lb_v1_LoadBalanceResponse__fields[0],
-  UPB_SIZE(8, 16), 2, false,
+  UPB_SIZE(8, 16), 3, false,
 };
 
 static const upb_msglayout *const grpc_lb_v1_InitialLoadBalanceResponse_submsgs[1] = {
@@ -129,5 +131,11 @@ const upb_msglayout grpc_lb_v1_Server_msginit = {
   UPB_SIZE(24, 48), 4, false,
 };
 
+const upb_msglayout grpc_lb_v1_FallbackResponse_msginit = {
+  NULL,
+  NULL,
+  UPB_SIZE(0, 0), 0, false,
+};
+
 #include "upb/port_undef.inc"
 

src/core/ext/upb-generated/src/proto/grpc/lb/v1/load_balancer.upb.h

@@ -28,6 +28,7 @@ struct grpc_lb_v1_LoadBalanceResponse;
 struct grpc_lb_v1_InitialLoadBalanceResponse;
 struct grpc_lb_v1_ServerList;
 struct grpc_lb_v1_Server;
+struct grpc_lb_v1_FallbackResponse;
 typedef struct grpc_lb_v1_LoadBalanceRequest grpc_lb_v1_LoadBalanceRequest;
 typedef struct grpc_lb_v1_InitialLoadBalanceRequest grpc_lb_v1_InitialLoadBalanceRequest;
 typedef struct grpc_lb_v1_ClientStatsPerToken grpc_lb_v1_ClientStatsPerToken;
@@ -36,6 +37,7 @@ typedef struct grpc_lb_v1_LoadBalanceResponse grpc_lb_v1_LoadBalanceResponse;
 typedef struct grpc_lb_v1_InitialLoadBalanceResponse grpc_lb_v1_InitialLoadBalanceResponse;
 typedef struct grpc_lb_v1_ServerList grpc_lb_v1_ServerList;
 typedef struct grpc_lb_v1_Server grpc_lb_v1_Server;
+typedef struct grpc_lb_v1_FallbackResponse grpc_lb_v1_FallbackResponse;
 extern const upb_msglayout grpc_lb_v1_LoadBalanceRequest_msginit;
 extern const upb_msglayout grpc_lb_v1_InitialLoadBalanceRequest_msginit;
 extern const upb_msglayout grpc_lb_v1_ClientStatsPerToken_msginit;
@@ -44,6 +46,7 @@ extern const upb_msglayout grpc_lb_v1_LoadBalanceResponse_msginit;
 extern const upb_msglayout grpc_lb_v1_InitialLoadBalanceResponse_msginit;
 extern const upb_msglayout grpc_lb_v1_ServerList_msginit;
 extern const upb_msglayout grpc_lb_v1_Server_msginit;
+extern const upb_msglayout grpc_lb_v1_FallbackResponse_msginit;
 struct google_protobuf_Duration;
 struct google_protobuf_Timestamp;
 extern const upb_msglayout google_protobuf_Duration_msginit;
@@ -221,6 +224,7 @@ UPB_INLINE char *grpc_lb_v1_LoadBalanceResponse_serialize(const grpc_lb_v1_LoadB
 typedef enum {
   grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_initial_response = 1,
   grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_server_list = 2,
+  grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_fallback_response = 3,
   grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_NOT_SET = 0
 } grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_oneofcases;
 UPB_INLINE grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_oneofcases grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_case(const grpc_lb_v1_LoadBalanceResponse* msg) { return (grpc_lb_v1_LoadBalanceResponse_load_balance_response_type_oneofcases)UPB_FIELD_AT(msg, int32_t, UPB_SIZE(4, 8)); }
@@ -229,6 +233,8 @@ UPB_INLINE bool grpc_lb_v1_LoadBalanceResponse_has_initial_response(const grpc_l
 UPB_INLINE const grpc_lb_v1_InitialLoadBalanceResponse* grpc_lb_v1_LoadBalanceResponse_initial_response(const grpc_lb_v1_LoadBalanceResponse *msg) { return UPB_READ_ONEOF(msg, const grpc_lb_v1_InitialLoadBalanceResponse*, UPB_SIZE(0, 0), UPB_SIZE(4, 8), 1, NULL); }
 UPB_INLINE bool grpc_lb_v1_LoadBalanceResponse_has_server_list(const grpc_lb_v1_LoadBalanceResponse *msg) { return _upb_has_oneof_field(msg, UPB_SIZE(4, 8), 2); }
 UPB_INLINE const grpc_lb_v1_ServerList* grpc_lb_v1_LoadBalanceResponse_server_list(const grpc_lb_v1_LoadBalanceResponse *msg) { return UPB_READ_ONEOF(msg, const grpc_lb_v1_ServerList*, UPB_SIZE(0, 0), UPB_SIZE(4, 8), 2, NULL); }
+UPB_INLINE bool grpc_lb_v1_LoadBalanceResponse_has_fallback_response(const grpc_lb_v1_LoadBalanceResponse *msg) { return _upb_has_oneof_field(msg, UPB_SIZE(4, 8), 3); }
+UPB_INLINE const grpc_lb_v1_FallbackResponse* grpc_lb_v1_LoadBalanceResponse_fallback_response(const grpc_lb_v1_LoadBalanceResponse *msg) { return UPB_READ_ONEOF(msg, const grpc_lb_v1_FallbackResponse*, UPB_SIZE(0, 0), UPB_SIZE(4, 8), 3, NULL); }
 
 UPB_INLINE void grpc_lb_v1_LoadBalanceResponse_set_initial_response(grpc_lb_v1_LoadBalanceResponse *msg, grpc_lb_v1_InitialLoadBalanceResponse* value) {
   UPB_WRITE_ONEOF(msg, grpc_lb_v1_InitialLoadBalanceResponse*, UPB_SIZE(0, 0), value, UPB_SIZE(4, 8), 1);
@@ -254,6 +260,18 @@ UPB_INLINE struct grpc_lb_v1_ServerList* grpc_lb_v1_LoadBalanceResponse_mutable_
   }
   return sub;
 }
+UPB_INLINE void grpc_lb_v1_LoadBalanceResponse_set_fallback_response(grpc_lb_v1_LoadBalanceResponse *msg, grpc_lb_v1_FallbackResponse* value) {
+  UPB_WRITE_ONEOF(msg, grpc_lb_v1_FallbackResponse*, UPB_SIZE(0, 0), value, UPB_SIZE(4, 8), 3);
+}
+UPB_INLINE struct grpc_lb_v1_FallbackResponse* grpc_lb_v1_LoadBalanceResponse_mutable_fallback_response(grpc_lb_v1_LoadBalanceResponse *msg, upb_arena *arena) {
+  struct grpc_lb_v1_FallbackResponse* sub = (struct grpc_lb_v1_FallbackResponse*)grpc_lb_v1_LoadBalanceResponse_fallback_response(msg);
+  if (sub == NULL) {
+    sub = (struct grpc_lb_v1_FallbackResponse*)upb_msg_new(&grpc_lb_v1_FallbackResponse_msginit, arena);
+    if (!sub) return NULL;
+    grpc_lb_v1_LoadBalanceResponse_set_fallback_response(msg, sub);
+  }
+  return sub;
+}
 
 /* grpc.lb.v1.InitialLoadBalanceResponse */
 
@@ -350,6 +368,22 @@ UPB_INLINE void grpc_lb_v1_Server_set_drop(grpc_lb_v1_Server *msg, bool value) {
   UPB_FIELD_AT(msg, bool, UPB_SIZE(4, 4)) = value;
 }
 
+/* grpc.lb.v1.FallbackResponse */
+
+UPB_INLINE grpc_lb_v1_FallbackResponse *grpc_lb_v1_FallbackResponse_new(upb_arena *arena) {
+  return (grpc_lb_v1_FallbackResponse *)upb_msg_new(&grpc_lb_v1_FallbackResponse_msginit, arena);
+}
+UPB_INLINE grpc_lb_v1_FallbackResponse *grpc_lb_v1_FallbackResponse_parse(const char *buf, size_t size,
+                        upb_arena *arena) {
+  grpc_lb_v1_FallbackResponse *ret = grpc_lb_v1_FallbackResponse_new(arena);
+  return (ret && upb_decode(buf, size, ret, &grpc_lb_v1_FallbackResponse_msginit, arena)) ? ret : NULL;
+}
+UPB_INLINE char *grpc_lb_v1_FallbackResponse_serialize(const grpc_lb_v1_FallbackResponse *msg, upb_arena *arena, size_t *len) {
+  return upb_encode(msg, &grpc_lb_v1_FallbackResponse_msginit, arena, len);
+}
+
+
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif

src/proto/grpc/lb/v1/load_balancer.proto

@@ -94,6 +94,11 @@ message LoadBalanceResponse {
     // Contains the list of servers selected by the load balancer. The client
     // should send requests to these servers in the specified order.
     ServerList server_list = 2;
+
+    // If this field is set, then the client should eagerly enter fallback
+    // mode (even if there are existing, healthy connections to backends).
+    // See go/grpclb-explicit-fallback for more details.
+    FallbackResponse fallback_response = 3;
   }
 }
 
@@ -148,3 +153,5 @@ message Server {
 
   reserved 5;
 }
+
+message FallbackResponse {}

test/cpp/end2end/grpclb_end2end_test.cc

@@ -1371,7 +1371,7 @@ TEST_F(SingleBalancerTest, FallbackEarlyWhenBalancerChannelFails) {
 TEST_F(SingleBalancerTest, FallbackEarlyWhenBalancerCallFails) {
   const int kFallbackTimeoutMs = 10000 * grpc_test_slowdown_factor();
   ResetStub(kFallbackTimeoutMs);
-  // Return an unreachable balancer and one fallback backend.
+  // Return one balancer and one fallback backend.
   std::vector<AddressData> addresses;
   addresses.emplace_back(AddressData{balancers_[0]->port_, true, ""});
   addresses.emplace_back(AddressData{backends_[0]->port_, false, ""});
@@ -1384,6 +1384,47 @@ TEST_F(SingleBalancerTest, FallbackEarlyWhenBalancerCallFails) {
                  /* wait_for_ready */ false);
 }
 
+TEST_F(SingleBalancerTest, FallbackControlledByBalancer_BeforeFirstServerlist) {
+  const int kFallbackTimeoutMs = 10000 * grpc_test_slowdown_factor();
+  ResetStub(kFallbackTimeoutMs);
+  // Return one balancer and one fallback backend.
+  std::vector<AddressData> addresses;
+  addresses.emplace_back(AddressData{balancers_[0]->port_, true, ""});
+  addresses.emplace_back(AddressData{backends_[0]->port_, false, ""});
+  SetNextResolution(addresses);
+  // Balancer explicitly tells client to fallback.
+  LoadBalanceResponse resp;
+  resp.mutable_fallback_response();
+  ScheduleResponseForBalancer(0, resp, 0);
+  // Send RPC with deadline less than the fallback timeout and make sure it
+  // succeeds.
+  CheckRpcSendOk(/* times */ 1, /* timeout_ms */ 1000,
+                 /* wait_for_ready */ false);
+}
+
+TEST_F(SingleBalancerTest, FallbackControlledByBalancer_AfterFirstServerlist) {
+  // Return one balancer and one fallback backend (backend 0).
+  std::vector<AddressData> addresses;
+  addresses.emplace_back(AddressData{balancers_[0]->port_, true, ""});
+  addresses.emplace_back(AddressData{backends_[0]->port_, false, ""});
+  SetNextResolution(addresses);
+  // Balancer initially sends serverlist, then tells client to fall back,
+  // then sends the serverlist again.
+  // The serverlist points to backend 1.
+  LoadBalanceResponse serverlist_resp =
+      BalancerServiceImpl::BuildResponseForBackends({backends_[1]->port_}, {});
+  LoadBalanceResponse fallback_resp;
+  fallback_resp.mutable_fallback_response();
+  ScheduleResponseForBalancer(0, serverlist_resp, 0);
+  ScheduleResponseForBalancer(0, fallback_resp, 100);
+  ScheduleResponseForBalancer(0, serverlist_resp, 100);
+  // Requests initially go to backend 1, then go to backend 0 in
+  // fallback mode, then go back to backend 1 when we exit fallback.
+  WaitForBackend(1);
+  WaitForBackend(0);
+  WaitForBackend(1);
+}
+
 TEST_F(SingleBalancerTest, BackendsRestart) {
   SetNextResolutionAllBalancers();
   const size_t kNumRpcsPerAddress = 100;