client channel: refactor call objects in preparation for promise conversion (#32223)

This is a prerequisite for converting the client_channel filter to
promises. This refactors two objects:
- `ClientChannel::CallData`, which is primarily responsible for applying
the service config to the call
- `ClientChannel::LoadBalancedCall`, which is responsible for doing the
LB pick for the call attempt

Each of those classes has been split into two pieces:
- a base class with the functionality to be shared between the legacy
filter stack implementation and the new promise-based implementation
- a subclass providing the legacy filter stack implementation

A subsequent PR will add another subclass that provides the
promise-based implementation.

BUILD

@@ -2785,6 +2785,7 @@ grpc_cc_library(
     ],
     external_deps = [
         "absl/base:core_headers",
+        "absl/cleanup",
         "absl/container:flat_hash_set",
         "absl/container:inlined_vector",
         "absl/status",

src/core/ext/filters/client_channel/client_channel.h

@@ -69,7 +69,6 @@
 #include "src/core/lib/service_config/service_config.h"
 #include "src/core/lib/service_config/service_config_call_data.h"
 #include "src/core/lib/service_config/service_config_parser.h"
-#include "src/core/lib/slice/slice.h"
 #include "src/core/lib/slice/slice_buffer.h"
 #include "src/core/lib/surface/channel.h"
 #include "src/core/lib/transport/connectivity_state.h"
@@ -110,6 +109,7 @@ class ClientChannel {
   static const grpc_channel_filter kFilterVtable;
 
   class LoadBalancedCall;
+  class FilterBasedLoadBalancedCall;
 
   // Flag that this object gets stored in channel args as a raw pointer.
   struct RawPointerChannelArgTag {};
@@ -163,7 +163,7 @@ class ClientChannel {
   void RemoveConnectivityWatcher(
       AsyncConnectivityStateWatcherInterface* watcher);
 
-  OrphanablePtr<LoadBalancedCall> CreateLoadBalancedCall(
+  OrphanablePtr<FilterBasedLoadBalancedCall> CreateLoadBalancedCall(
       const grpc_call_element_args& args, grpc_polling_entity* pollent,
       grpc_closure* on_call_destruction_complete,
       ConfigSelector::CallDispatchController* call_dispatch_controller,
@@ -177,6 +177,7 @@ class ClientChannel {
 
  private:
   class CallData;
+  class FilterBasedCallData;
   class ResolverResultHandler;
   class SubchannelWrapper;
   class ClientChannelControlHelper;
@@ -297,7 +298,7 @@ class ClientChannel {
   //
   mutable Mutex resolution_mu_;
   // List of calls queued waiting for resolver result.
-  absl::flat_hash_set<grpc_call_element*> resolver_queued_calls_
+  absl::flat_hash_set<CallData*> resolver_queued_calls_
       ABSL_GUARDED_BY(resolution_mu_);
   // Data from service config.
   absl::Status resolver_transient_failure_error_
@@ -371,11 +372,12 @@ class ClientChannel {
 class ClientChannel::LoadBalancedCall
     : public InternallyRefCounted<LoadBalancedCall, kUnrefCallDtor> {
  public:
+  // TODO(roth): Make this private.
   class LbCallState : public LbCallStateInternal {
    public:
     explicit LbCallState(LoadBalancedCall* lb_call) : lb_call_(lb_call) {}
 
-    void* Alloc(size_t size) override { return lb_call_->arena_->Alloc(size); }
+    void* Alloc(size_t size) override { return lb_call_->arena()->Alloc(size); }
 
     // Internal API to allow first-party LB policies to access per-call
     // attributes set by the ConfigSelector.
@@ -385,37 +387,132 @@ class ClientChannel::LoadBalancedCall
     LoadBalancedCall* lb_call_;
   };
 
+  LoadBalancedCall(
+      ClientChannel* chand, grpc_call_context_element* call_context,
+      ConfigSelector::CallDispatchController* call_dispatch_controller,
+      bool is_transparent_retry);
+  ~LoadBalancedCall() override;
+
+  void Orphan() override;
+
+  // Called by channel when removing a call from the list of queued calls.
+  void RemoveCallFromLbQueuedCallsLocked()
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::lb_mu_);
+
+  // Called by the channel for each queued call when a new picker
+  // becomes available.
+  virtual void RetryPickLocked()
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::lb_mu_) = 0;
+
+ protected:
+  ClientChannel* chand() const { return chand_; }
+  ConfigSelector::CallDispatchController* call_dispatch_controller() const {
+    return call_dispatch_controller_;
+  }
+  CallTracer::CallAttemptTracer* call_attempt_tracer() const {
+    return call_attempt_tracer_;
+  }
+  gpr_cycle_counter lb_call_start_time() const { return lb_call_start_time_; }
+  ConnectedSubchannel* connected_subchannel() const {
+    return connected_subchannel_.get();
+  }
+  LoadBalancingPolicy::SubchannelCallTrackerInterface*
+  lb_subchannel_call_tracker() const {
+    return lb_subchannel_call_tracker_.get();
+  }
+
+  // Attempts an LB pick.  The following outcomes are possible:
+  // - No pick result is available yet.  The call will be queued and
+  //   nullopt will be returned.  The channel will later call
+  //   RetryPickLocked() when a new picker is available and the pick
+  //   should be retried.
+  // - The pick failed.  If the call is not wait_for_ready, a non-OK
+  //   status will be returned.  (If the call *is* wait_for_ready,
+  //   it will be queued instead.)
+  // - The pick completed successfully.  A connected subchannel is
+  //   stored and an OK status will be returned.
+  absl::optional<absl::Status> PickSubchannel(bool was_queued);
+
+  void RecordCallCompletion(absl::Status status,
+                            grpc_metadata_batch* recv_trailing_metadata,
+                            grpc_transport_stream_stats* transport_stream_stats,
+                            absl::string_view peer_address);
+
+ private:
+  class Metadata;
+  class BackendMetricAccessor;
+
+  virtual Arena* arena() const = 0;
+  virtual grpc_call_context_element* call_context() const = 0;
+  virtual grpc_polling_entity* pollent() const = 0;
+  virtual grpc_metadata_batch* send_initial_metadata() const = 0;
+
+  // Helper function for performing an LB pick with a specified picker.
+  // Returns true if the pick is complete.
+  bool PickSubchannelImpl(LoadBalancingPolicy::SubchannelPicker* picker,
+                          grpc_error_handle* error);
+  // Adds the call to the channel's list of queued picks if not already present.
+  void AddCallToLbQueuedCallsLocked()
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::lb_mu_);
+
+  // Called when adding the call to the LB queue.
+  virtual void OnAddToQueueLocked()
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::lb_mu_) {}
+
+  ClientChannel* chand_;
+
+  ConfigSelector::CallDispatchController* call_dispatch_controller_;
+  CallTracer::CallAttemptTracer* call_attempt_tracer_;
+
+  gpr_cycle_counter lb_call_start_time_ = gpr_get_cycle_counter();
+
+  RefCountedPtr<ConnectedSubchannel> connected_subchannel_;
+  const BackendMetricData* backend_metric_data_ = nullptr;
+  std::unique_ptr<LoadBalancingPolicy::SubchannelCallTrackerInterface>
+      lb_subchannel_call_tracker_;
+};
+
+class ClientChannel::FilterBasedLoadBalancedCall
+    : public ClientChannel::LoadBalancedCall {
+ public:
   // If on_call_destruction_complete is non-null, then it will be
   // invoked once the LoadBalancedCall is completely destroyed.
   // If it is null, then the caller is responsible for checking whether
   // the LB call has a subchannel call and ensuring that the
   // on_call_destruction_complete closure passed down from the surface
   // is not invoked until after the subchannel call stack is destroyed.
-  LoadBalancedCall(
+  FilterBasedLoadBalancedCall(
       ClientChannel* chand, const grpc_call_element_args& args,
       grpc_polling_entity* pollent, grpc_closure* on_call_destruction_complete,
       ConfigSelector::CallDispatchController* call_dispatch_controller,
       bool is_transparent_retry);
-  ~LoadBalancedCall() override;
+  ~FilterBasedLoadBalancedCall() override;
 
   void Orphan() override;
 
   void StartTransportStreamOpBatch(grpc_transport_stream_op_batch* batch);
 
-  void PickSubchannel(bool was_queued);
-
-  // Called by channel when removing a call from the list of queued calls.
-  void RemoveCallFromLbQueuedCallsLocked()
-      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::lb_mu_);
-
   RefCountedPtr<SubchannelCall> subchannel_call() const {
     return subchannel_call_;
   }
 
  private:
   class LbQueuedCallCanceller;
-  class Metadata;
+
-  class BackendMetricAccessor;
+  // Work-around for Windows compilers that don't allow nested classes
+  // to access protected members of the enclosing class's parent class.
+  using LoadBalancedCall::call_dispatch_controller;
+  using LoadBalancedCall::chand;
+
+  Arena* arena() const override { return arena_; }
+  grpc_call_context_element* call_context() const override {
+    return call_context_;
+  }
+  grpc_polling_entity* pollent() const override { return pollent_; }
+  grpc_metadata_batch* send_initial_metadata() const override {
+    return pending_batches_[0]
+        ->payload->send_initial_metadata.send_initial_metadata;
+  }
 
   // Returns the index into pending_batches_ to be used for batch.
   static size_t GetBatchIndex(grpc_transport_stream_op_batch* batch);
@@ -451,36 +548,28 @@ class ClientChannel::LoadBalancedCall
   static void RecvMessageReady(void* arg, grpc_error_handle error);
   static void RecvTrailingMetadataReady(void* arg, grpc_error_handle error);
 
-  void RecordCallCompletion(absl::Status status);
+  // Called to perform a pick, both when the call is initially started
+  // and when it is queued and the channel gets a new picker.
+  void TryPick(bool was_queued);
 
-  void CreateSubchannelCall();
+  void OnAddToQueueLocked() override
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::lb_mu_);
 
-  // Helper function for performing an LB pick with a specified picker.
+  void RetryPickLocked() override
-  // Returns true if the pick is complete.
-  bool PickSubchannelImpl(LoadBalancingPolicy::SubchannelPicker* picker,
-                          grpc_error_handle* error);
-  // Adds the call to the channel's list of queued picks if not already present.
-  void AddCallToLbQueuedCallsLocked()
       ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::lb_mu_);
 
-  ClientChannel* chand_;
+  void CreateSubchannelCall();
 
   // TODO(roth): Instead of duplicating these fields in every filter
   // that uses any one of them, we should store them in the call
   // context.  This will save per-call memory overhead.
-  Slice path_;  // Request path.
   Timestamp deadline_;
   Arena* arena_;
+  grpc_call_context_element* call_context_;
   grpc_call_stack* owning_call_;
   CallCombiner* call_combiner_;
-  grpc_call_context_element* call_context_;
   grpc_polling_entity* pollent_;
   grpc_closure* on_call_destruction_complete_;
-  ConfigSelector::CallDispatchController* call_dispatch_controller_;
-
-  CallTracer::CallAttemptTracer* call_attempt_tracer_;
-
-  gpr_cycle_counter lb_call_start_time_ = gpr_get_cycle_counter();
 
   // Set when we get a cancel_stream op.
   grpc_error_handle cancel_error_;
@@ -492,11 +581,6 @@ class ClientChannel::LoadBalancedCall
   LbQueuedCallCanceller* lb_call_canceller_
       ABSL_GUARDED_BY(&ClientChannel::lb_mu_) = nullptr;
 
-  RefCountedPtr<ConnectedSubchannel> connected_subchannel_;
-  const BackendMetricData* backend_metric_data_ = nullptr;
-  std::unique_ptr<LoadBalancingPolicy::SubchannelCallTrackerInterface>
-      lb_subchannel_call_tracker_;
-
   RefCountedPtr<SubchannelCall> subchannel_call_;
 
   // For intercepting send_initial_metadata on_complete.
@@ -531,6 +615,7 @@ class ClientChannel::LoadBalancedCall
 // A sub-class of ServiceConfigCallData used to access the
 // CallDispatchController.  Allocated on the arena, stored in the call
 // context, and destroyed when the call is destroyed.
+// TODO(roth): Combine this with lb_call_state_internal.h.
 class ClientChannelServiceConfigCallData : public ServiceConfigCallData {
  public:
   ClientChannelServiceConfigCallData(

src/core/ext/filters/client_channel/config_selector.h

@@ -28,7 +28,6 @@
 #include "absl/strings/string_view.h"
 
 #include <grpc/grpc.h>
-#include <grpc/slice.h>
 #include <grpc/support/log.h>
 
 #include "src/core/lib/channel/channel_fwd.h"
@@ -39,6 +38,7 @@
 #include "src/core/lib/service_config/service_config.h"
 #include "src/core/lib/service_config/service_config_call_data.h"
 #include "src/core/lib/service_config/service_config_parser.h"
+#include "src/core/lib/slice/slice.h"
 #include "src/core/lib/transport/metadata_batch.h"
 
 // Channel arg key for ConfigSelector.
@@ -65,7 +65,6 @@ class ConfigSelector : public RefCounted<ConfigSelector> {
   };
 
   struct GetCallConfigArgs {
-    grpc_slice* path;
     grpc_metadata_batch* initial_metadata;
     Arena* arena;
   };
@@ -132,8 +131,10 @@ class DefaultConfigSelector : public ConfigSelector {
 
   absl::StatusOr<CallConfig> GetCallConfig(GetCallConfigArgs args) override {
     CallConfig call_config;
+    Slice* path = args.initial_metadata->get_pointer(HttpPathMetadata());
+    GPR_ASSERT(path != nullptr);
     call_config.method_configs =
-        service_config_->GetMethodParsedConfigVector(*args.path);
+        service_config_->GetMethodParsedConfigVector(path->c_slice());
     call_config.service_config = service_config_;
     return call_config;
   }

src/core/ext/filters/client_channel/resolver/xds/xds_resolver.cc

@@ -47,6 +47,7 @@
 #include <grpc/grpc.h>
 
 #include "src/core/lib/gprpp/unique_type_name.h"
+#include "src/core/lib/slice/slice.h"
 
 #define XXH_INLINE_ALL
 #include "xxhash.h"
@@ -84,7 +85,6 @@
 #include "src/core/lib/resource_quota/arena.h"
 #include "src/core/lib/service_config/service_config.h"
 #include "src/core/lib/service_config/service_config_impl.h"
-#include "src/core/lib/slice/slice_internal.h"
 #include "src/core/lib/transport/metadata_batch.h"
 #include "src/core/lib/uri/uri_parser.h"
 
@@ -658,8 +658,10 @@ absl::optional<uint64_t> HeaderHashHelper(
 
 absl::StatusOr<ConfigSelector::CallConfig>
 XdsResolver::XdsConfigSelector::GetCallConfig(GetCallConfigArgs args) {
+  Slice* path = args.initial_metadata->get_pointer(HttpPathMetadata());
+  GPR_ASSERT(path != nullptr);
   auto route_index = XdsRouting::GetRouteForRequest(
-      RouteListIterator(&route_table_), StringViewFromSlice(*args.path),
+      RouteListIterator(&route_table_), path->as_string_view(),
       args.initial_metadata);
   if (!route_index.has_value()) {
     return absl::UnavailableError(

src/core/ext/filters/client_channel/retry_filter.cc

@@ -448,7 +448,7 @@ class RetryFilter::CallData {
 
     CallData* calld_;
     AttemptDispatchController attempt_dispatch_controller_;
-    OrphanablePtr<ClientChannel::LoadBalancedCall> lb_call_;
+    OrphanablePtr<ClientChannel::FilterBasedLoadBalancedCall> lb_call_;
     bool lb_call_committed_ = false;
 
     grpc_timer per_attempt_recv_timer_;
@@ -555,7 +555,8 @@ class RetryFilter::CallData {
   void AddClosureToStartTransparentRetry(CallCombinerClosureList* closures);
   static void StartTransparentRetry(void* arg, grpc_error_handle error);
 
-  OrphanablePtr<ClientChannel::LoadBalancedCall> CreateLoadBalancedCall(
+  OrphanablePtr<ClientChannel::FilterBasedLoadBalancedCall>
+  CreateLoadBalancedCall(
       ConfigSelector::CallDispatchController* call_dispatch_controller,
       bool is_transparent_retry);
 
@@ -586,7 +587,7 @@ class RetryFilter::CallData {
   // LB call used when we've committed to a call attempt and the retry
   // state for that attempt is no longer needed.  This provides a fast
   // path for long-running streaming calls that minimizes overhead.
-  OrphanablePtr<ClientChannel::LoadBalancedCall> committed_call_;
+  OrphanablePtr<ClientChannel::FilterBasedLoadBalancedCall> committed_call_;
 
   // When are are not yet fully committed to a particular call (i.e.,
   // either we might still retry or we have committed to the call but
@@ -881,7 +882,7 @@ namespace {
 void StartBatchInCallCombiner(void* arg, grpc_error_handle /*ignored*/) {
   grpc_transport_stream_op_batch* batch =
       static_cast<grpc_transport_stream_op_batch*>(arg);
-  auto* lb_call = static_cast<ClientChannel::LoadBalancedCall*>(
+  auto* lb_call = static_cast<ClientChannel::FilterBasedLoadBalancedCall*>(
       batch->handler_private.extra_arg);
   // Note: This will release the call combiner.
   lb_call->StartTransportStreamOpBatch(batch);
@@ -2301,7 +2302,7 @@ void RetryFilter::CallData::StartTransportStreamOpBatch(
   call_attempt_->StartRetriableBatches();
 }
 
-OrphanablePtr<ClientChannel::LoadBalancedCall>
+OrphanablePtr<ClientChannel::FilterBasedLoadBalancedCall>
 RetryFilter::CallData::CreateLoadBalancedCall(
     ConfigSelector::CallDispatchController* call_dispatch_controller,
     bool is_transparent_retry) {