[transport] Move call spine related types into their own file (#35689)

Part of a continued hygiene effort for transport.h (pure code movement, no refactoring at this time) Closes #35689 COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35689 from ctiller:channel-ez 3366310cc3 PiperOrigin-RevId: 602416330
2024-01-29 09:47:00 -08:00 · 2024-01-29 09:47:00 -08:00 · fb14685fd6
parent 5b444972f3
commit fb14685fd6
20 changed files with 579 additions and 473 deletions
--- a/2
+++ b/2
@ -1533,6 +1533,7 @@ grpc_cc_library(
        "//src/core:bitset",
        "//src/core:call_filters",
        "//src/core:call_final_info",
+        "//src/core:call_spine",
        "//src/core:cancel_callback",
        "//src/core:channel_args",
        "//src/core:channel_args_endpoint_config",
@ -1584,7 +1585,6 @@ grpc_cc_library(
        "//src/core:pollset_set",
        "//src/core:posix_event_engine_base_hdrs",
        "//src/core:posix_event_engine_endpoint",
-        "//src/core:prioritized_race",
        "//src/core:promise_status",
        "//src/core:race",
        "//src/core:random_early_detection",
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@ -2528,6 +2528,7 @@ add_library(grpc
  src/core/lib/transport/bdp_estimator.cc
  src/core/lib/transport/call_filters.cc
  src/core/lib/transport/call_final_info.cc
+  src/core/lib/transport/call_spine.cc
  src/core/lib/transport/connectivity_state.cc
  src/core/lib/transport/error_utils.cc
  src/core/lib/transport/handshaker.cc
@ -3243,6 +3244,7 @@ add_library(grpc_unsecure
  src/core/lib/transport/bdp_estimator.cc
  src/core/lib/transport/call_filters.cc
  src/core/lib/transport/call_final_info.cc
+  src/core/lib/transport/call_spine.cc
  src/core/lib/transport/connectivity_state.cc
  src/core/lib/transport/error_utils.cc
  src/core/lib/transport/handshaker.cc
@ -5330,6 +5332,7 @@ add_library(grpc_authorization_provider
  src/core/lib/transport/batch_builder.cc
  src/core/lib/transport/call_filters.cc
  src/core/lib/transport/call_final_info.cc
+  src/core/lib/transport/call_spine.cc
  src/core/lib/transport/connectivity_state.cc
  src/core/lib/transport/error_utils.cc
  src/core/lib/transport/handshaker.cc
--- a/2
+++ b/2
@ -1710,6 +1710,7 @@ LIBGRPC_SRC = \
    src/core/lib/transport/bdp_estimator.cc \
    src/core/lib/transport/call_filters.cc \
    src/core/lib/transport/call_final_info.cc \
+    src/core/lib/transport/call_spine.cc \
    src/core/lib/transport/connectivity_state.cc \
    src/core/lib/transport/error_utils.cc \
    src/core/lib/transport/handshaker.cc \
@ -2259,6 +2260,7 @@ LIBGRPC_UNSECURE_SRC = \
    src/core/lib/transport/bdp_estimator.cc \
    src/core/lib/transport/call_filters.cc \
    src/core/lib/transport/call_final_info.cc \
+    src/core/lib/transport/call_spine.cc \
    src/core/lib/transport/connectivity_state.cc \
    src/core/lib/transport/error_utils.cc \
    src/core/lib/transport/handshaker.cc \
--- a/Package.swift
+++ b/Package.swift
@ -1903,6 +1903,8 @@ let package = Package(
        "src/core/lib/transport/call_filters.h",
        "src/core/lib/transport/call_final_info.cc",
        "src/core/lib/transport/call_final_info.h",
+        "src/core/lib/transport/call_spine.cc",
+        "src/core/lib/transport/call_spine.h",
        "src/core/lib/transport/connectivity_state.cc",
        "src/core/lib/transport/connectivity_state.h",
        "src/core/lib/transport/custom_metadata.h",
--- a/build_autogenerated.yaml
+++ b/build_autogenerated.yaml
@ -1179,6 +1179,7 @@ libs:
  - src/core/lib/transport/bdp_estimator.h
  - src/core/lib/transport/call_filters.h
  - src/core/lib/transport/call_final_info.h
+  - src/core/lib/transport/call_spine.h
  - src/core/lib/transport/connectivity_state.h
  - src/core/lib/transport/custom_metadata.h
  - src/core/lib/transport/error_utils.h
@ -1982,6 +1983,7 @@ libs:
  - src/core/lib/transport/bdp_estimator.cc
  - src/core/lib/transport/call_filters.cc
  - src/core/lib/transport/call_final_info.cc
+  - src/core/lib/transport/call_spine.cc
  - src/core/lib/transport/connectivity_state.cc
  - src/core/lib/transport/error_utils.cc
  - src/core/lib/transport/handshaker.cc
@ -2628,6 +2630,7 @@ libs:
  - src/core/lib/transport/bdp_estimator.h
  - src/core/lib/transport/call_filters.h
  - src/core/lib/transport/call_final_info.h
+  - src/core/lib/transport/call_spine.h
  - src/core/lib/transport/connectivity_state.h
  - src/core/lib/transport/custom_metadata.h
  - src/core/lib/transport/error_utils.h
@ -3047,6 +3050,7 @@ libs:
  - src/core/lib/transport/bdp_estimator.cc
  - src/core/lib/transport/call_filters.cc
  - src/core/lib/transport/call_final_info.cc
+  - src/core/lib/transport/call_spine.cc
  - src/core/lib/transport/connectivity_state.cc
  - src/core/lib/transport/error_utils.cc
  - src/core/lib/transport/handshaker.cc
@ -4669,6 +4673,7 @@ libs:
  - src/core/lib/transport/batch_builder.h
  - src/core/lib/transport/call_filters.h
  - src/core/lib/transport/call_final_info.h
+  - src/core/lib/transport/call_spine.h
  - src/core/lib/transport/connectivity_state.h
  - src/core/lib/transport/custom_metadata.h
  - src/core/lib/transport/error_utils.h
@ -4965,6 +4970,7 @@ libs:
  - src/core/lib/transport/batch_builder.cc
  - src/core/lib/transport/call_filters.cc
  - src/core/lib/transport/call_final_info.cc
+  - src/core/lib/transport/call_spine.cc
  - src/core/lib/transport/connectivity_state.cc
  - src/core/lib/transport/error_utils.cc
  - src/core/lib/transport/handshaker.cc
--- a/config.m4
+++ b/config.m4
@ -838,6 +838,7 @@ if test "$PHP_GRPC" != "no"; then
    src/core/lib/transport/bdp_estimator.cc \
    src/core/lib/transport/call_filters.cc \
    src/core/lib/transport/call_final_info.cc \
+    src/core/lib/transport/call_spine.cc \
    src/core/lib/transport/connectivity_state.cc \
    src/core/lib/transport/error_utils.cc \
    src/core/lib/transport/handshaker.cc \
--- a/config.w32
+++ b/config.w32
@ -803,6 +803,7 @@ if (PHP_GRPC != "no") {
    "src\\core\\lib\\transport\\bdp_estimator.cc " +
    "src\\core\\lib\\transport\\call_filters.cc " +
    "src\\core\\lib\\transport\\call_final_info.cc " +
+    "src\\core\\lib\\transport\\call_spine.cc " +
    "src\\core\\lib\\transport\\connectivity_state.cc " +
    "src\\core\\lib\\transport\\error_utils.cc " +
    "src\\core\\lib\\transport\\handshaker.cc " +
--- a/gRPC-C++.podspec
+++ b/gRPC-C++.podspec
@ -1283,6 +1283,7 @@ Pod::Spec.new do |s|
                      'src/core/lib/transport/bdp_estimator.h',
                      'src/core/lib/transport/call_filters.h',
                      'src/core/lib/transport/call_final_info.h',
+                      'src/core/lib/transport/call_spine.h',
                      'src/core/lib/transport/connectivity_state.h',
                      'src/core/lib/transport/custom_metadata.h',
                      'src/core/lib/transport/error_utils.h',
@ -2537,6 +2538,7 @@ Pod::Spec.new do |s|
                              'src/core/lib/transport/bdp_estimator.h',
                              'src/core/lib/transport/call_filters.h',
                              'src/core/lib/transport/call_final_info.h',
+                              'src/core/lib/transport/call_spine.h',
                              'src/core/lib/transport/connectivity_state.h',
                              'src/core/lib/transport/custom_metadata.h',
                              'src/core/lib/transport/error_utils.h',
--- a/gRPC-Core.podspec
+++ b/gRPC-Core.podspec
@ -2012,6 +2012,8 @@ Pod::Spec.new do |s|
                      'src/core/lib/transport/call_filters.h',
                      'src/core/lib/transport/call_final_info.cc',
                      'src/core/lib/transport/call_final_info.h',
+                      'src/core/lib/transport/call_spine.cc',
+                      'src/core/lib/transport/call_spine.h',
                      'src/core/lib/transport/connectivity_state.cc',
                      'src/core/lib/transport/connectivity_state.h',
                      'src/core/lib/transport/custom_metadata.h',
@ -3314,6 +3316,7 @@ Pod::Spec.new do |s|
                              'src/core/lib/transport/bdp_estimator.h',
                              'src/core/lib/transport/call_filters.h',
                              'src/core/lib/transport/call_final_info.h',
+                              'src/core/lib/transport/call_spine.h',
                              'src/core/lib/transport/connectivity_state.h',
                              'src/core/lib/transport/custom_metadata.h',
                              'src/core/lib/transport/error_utils.h',
--- a/grpc.gemspec
+++ b/grpc.gemspec
@ -1905,6 +1905,8 @@ Gem::Specification.new do |s|
  s.files += %w( src/core/lib/transport/call_filters.h )
  s.files += %w( src/core/lib/transport/call_final_info.cc )
  s.files += %w( src/core/lib/transport/call_final_info.h )
+  s.files += %w( src/core/lib/transport/call_spine.cc )
+  s.files += %w( src/core/lib/transport/call_spine.h )
  s.files += %w( src/core/lib/transport/connectivity_state.cc )
  s.files += %w( src/core/lib/transport/connectivity_state.h )
  s.files += %w( src/core/lib/transport/custom_metadata.h )
--- a/grpc.gyp
+++ b/grpc.gyp
@ -1024,6 +1024,7 @@
        'src/core/lib/transport/bdp_estimator.cc',
        'src/core/lib/transport/call_filters.cc',
        'src/core/lib/transport/call_final_info.cc',
+        'src/core/lib/transport/call_spine.cc',
        'src/core/lib/transport/connectivity_state.cc',
        'src/core/lib/transport/error_utils.cc',
        'src/core/lib/transport/handshaker.cc',
@ -1513,6 +1514,7 @@
        'src/core/lib/transport/bdp_estimator.cc',
        'src/core/lib/transport/call_filters.cc',
        'src/core/lib/transport/call_final_info.cc',
+        'src/core/lib/transport/call_spine.cc',
        'src/core/lib/transport/connectivity_state.cc',
        'src/core/lib/transport/error_utils.cc',
        'src/core/lib/transport/handshaker.cc',
@ -2282,6 +2284,7 @@
        'src/core/lib/transport/batch_builder.cc',
        'src/core/lib/transport/call_filters.cc',
        'src/core/lib/transport/call_final_info.cc',
+        'src/core/lib/transport/call_spine.cc',
        'src/core/lib/transport/connectivity_state.cc',
        'src/core/lib/transport/error_utils.cc',
        'src/core/lib/transport/handshaker.cc',
--- a/package.xml
+++ b/package.xml
@ -1887,6 +1887,8 @@
    <file baseinstalldir="/" name="src/core/lib/transport/call_filters.h" role="src" />
    <file baseinstalldir="/" name="src/core/lib/transport/call_final_info.cc" role="src" />
    <file baseinstalldir="/" name="src/core/lib/transport/call_final_info.h" role="src" />
+    <file baseinstalldir="/" name="src/core/lib/transport/call_spine.cc" role="src" />
+    <file baseinstalldir="/" name="src/core/lib/transport/call_spine.h" role="src" />
    <file baseinstalldir="/" name="src/core/lib/transport/connectivity_state.cc" role="src" />
    <file baseinstalldir="/" name="src/core/lib/transport/connectivity_state.h" role="src" />
    <file baseinstalldir="/" name="src/core/lib/transport/custom_metadata.h" role="src" />
--- a/src/core/BUILD
+++ b/src/core/BUILD
@ -6788,6 +6788,30 @@ grpc_cc_library(
    ],
 )

+grpc_cc_library(
+    name = "call_spine",
+    srcs = [
+        "lib/transport/call_spine.cc",
+    ],
+    hdrs = [
+        "lib/transport/call_spine.h",
+    ],
+    deps = [
+        "1999",
+        "for_each",
+        "if",
+        "latch",
+        "message",
+        "metadata",
+        "pipe",
+        "prioritized_race",
+        "promise_status",
+        "status_flag",
+        "try_seq",
+        "//:gpr",
+    ],
+)
+
 grpc_cc_library(
    name = "metadata_batch",
    srcs = [
--- a/src/core/lib/transport/call_spine.cc
+++ b/src/core/lib/transport/call_spine.cc
@ -0,0 +1,103 @@
+// Copyright 2024 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <grpc/support/port_platform.h>
+
+#include "src/core/lib/transport/call_spine.h"
+
+namespace grpc_core {
+
+void ForwardCall(CallHandler call_handler, CallInitiator call_initiator,
+                 ClientMetadataHandle client_initial_metadata) {
+  // Send initial metadata.
+  call_initiator.SpawnGuarded(
+      "send_initial_metadata",
+      [client_initial_metadata = std::move(client_initial_metadata),
+       call_initiator]() mutable {
+        return call_initiator.PushClientInitialMetadata(
+            std::move(client_initial_metadata));
+      });
+  // Read messages from handler into initiator.
+  call_handler.SpawnGuarded("read_messages", [call_handler,
+                                              call_initiator]() mutable {
+    return Seq(ForEach(OutgoingMessages(call_handler),
+                       [call_initiator](MessageHandle msg) mutable {
+                         // Need to spawn a job into the initiator's activity to
+                         // push the message in.
+                         return call_initiator.SpawnWaitable(
+                             "send_message",
+                             [msg = std::move(msg), call_initiator]() mutable {
+                               return call_initiator.CancelIfFails(
+                                   call_initiator.PushMessage(std::move(msg)));
+                             });
+                       }),
+               [call_initiator](StatusFlag result) mutable {
+                 call_initiator.SpawnInfallible(
+                     "finish-downstream", [call_initiator, result]() mutable {
+                       if (result.ok()) {
+                         call_initiator.FinishSends();
+                       } else {
+                         call_initiator.Cancel();
+                       }
+                       return Empty{};
+                     });
+                 return result;
+               });
+  });
+  call_initiator.SpawnInfallible("read_the_things", [call_initiator,
+                                                     call_handler]() mutable {
+    return Seq(
+        call_initiator.CancelIfFails(TrySeq(
+            call_initiator.PullServerInitialMetadata(),
+            [call_handler,
+             call_initiator](absl::optional<ServerMetadataHandle> md) mutable {
+              const bool has_md = md.has_value();
+              call_handler.SpawnGuarded(
+                  "recv_initial_metadata",
+                  [md = std::move(md), call_handler]() mutable {
+                    return call_handler.PushServerInitialMetadata(
+                        std::move(md));
+                  });
+              return If(
+                  has_md,
+                  ForEach(OutgoingMessages(call_initiator),
+                          [call_handler](MessageHandle msg) mutable {
+                            return call_handler.SpawnWaitable(
+                                "recv_message",
+                                [msg = std::move(msg), call_handler]() mutable {
+                                  return call_handler.CancelIfFails(
+                                      call_handler.PushMessage(std::move(msg)));
+                                });
+                          }),
+                  []() -> StatusFlag { return Success{}; });
+            })),
+        call_initiator.PullServerTrailingMetadata(),
+        [call_handler](ServerMetadataHandle md) mutable {
+          call_handler.SpawnGuarded(
+              "recv_trailing_metadata",
+              [md = std::move(md), call_handler]() mutable {
+                return call_handler.PushServerTrailingMetadata(std::move(md));
+              });
+          return Empty{};
+        });
+  });
+}
+
+CallInitiatorAndHandler MakeCall(
+    grpc_event_engine::experimental::EventEngine* event_engine, Arena* arena) {
+  auto spine = CallSpine::Create(event_engine, arena);
+  return {CallInitiator(spine), CallHandler(spine)};
+}
+
+}  // namespace grpc_core
--- a/src/core/lib/transport/call_spine.h
+++ b/src/core/lib/transport/call_spine.h
@ -0,0 +1,418 @@
+// Copyright 2024 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GRPC_SRC_CORE_LIB_TRANSPORT_CALL_SPINE_H
+#define GRPC_SRC_CORE_LIB_TRANSPORT_CALL_SPINE_H
+
+#include <grpc/support/port_platform.h>
+
+#include <grpc/support/log.h>
+
+#include "src/core/lib/promise/detail/status.h"
+#include "src/core/lib/promise/for_each.h"
+#include "src/core/lib/promise/if.h"
+#include "src/core/lib/promise/latch.h"
+#include "src/core/lib/promise/party.h"
+#include "src/core/lib/promise/pipe.h"
+#include "src/core/lib/promise/prioritized_race.h"
+#include "src/core/lib/promise/status_flag.h"
+#include "src/core/lib/promise/try_seq.h"
+#include "src/core/lib/transport/message.h"
+#include "src/core/lib/transport/metadata.h"
+
+namespace grpc_core {
+
+// The common middle part of a call - a reference is held by each of
+// CallInitiator and CallHandler - which provide interfaces that are appropriate
+// for each side of a call.
+// The spine will ultimately host the pipes, filters, and context for one part
+// of a call: ie top-half client channel, sub channel call, server call.
+// TODO(ctiller): eventually drop this when we don't need to reference into
+// legacy promise calls anymore
+class CallSpineInterface {
+ public:
+  virtual ~CallSpineInterface() = default;
+  virtual Pipe<ClientMetadataHandle>& client_initial_metadata() = 0;
+  virtual Pipe<ServerMetadataHandle>& server_initial_metadata() = 0;
+  virtual Pipe<MessageHandle>& client_to_server_messages() = 0;
+  virtual Pipe<MessageHandle>& server_to_client_messages() = 0;
+  virtual Pipe<ServerMetadataHandle>& server_trailing_metadata() = 0;
+  virtual Latch<ServerMetadataHandle>& cancel_latch() = 0;
+  // Add a callback to be called when server trailing metadata is received.
+  void OnDone(absl::AnyInvocable<void()> fn) {
+    if (on_done_ == nullptr) {
+      on_done_ = std::move(fn);
+      return;
+    }
+    on_done_ = [first = std::move(fn), next = std::move(on_done_)]() mutable {
+      first();
+      next();
+    };
+  }
+  void CallOnDone() {
+    if (on_done_ != nullptr) std::exchange(on_done_, nullptr)();
+  }
+  virtual Party& party() = 0;
+  virtual void IncrementRefCount() = 0;
+  virtual void Unref() = 0;
+
+  // Cancel the call with the given metadata.
+  // Regarding the `MUST_USE_RESULT absl::nullopt_t`:
+  // Most cancellation calls right now happen in pipe interceptors;
+  // there `nullopt` indicates terminate processing of this pipe and close with
+  // error.
+  // It's convenient then to have the Cancel operation (setting the latch to
+  // terminate the call) be the last thing that occurs in a pipe interceptor,
+  // and this construction supports that (and has helped the author not write
+  // some bugs).
+  GRPC_MUST_USE_RESULT absl::nullopt_t Cancel(ServerMetadataHandle metadata) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
+    auto& c = cancel_latch();
+    if (c.is_set()) return absl::nullopt;
+    c.Set(std::move(metadata));
+    CallOnDone();
+    client_initial_metadata().sender.CloseWithError();
+    server_initial_metadata().sender.CloseWithError();
+    client_to_server_messages().sender.CloseWithError();
+    server_to_client_messages().sender.CloseWithError();
+    return absl::nullopt;
+  }
+
+  auto WaitForCancel() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
+    return cancel_latch().Wait();
+  }
+
+  // Wrap a promise so that if it returns failure it automatically cancels
+  // the rest of the call.
+  // The resulting (returned) promise will resolve to Empty.
+  template <typename Promise>
+  auto CancelIfFails(Promise promise) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
+    using P = promise_detail::PromiseLike<Promise>;
+    using ResultType = typename P::Result;
+    return Map(std::move(promise), [this](ResultType r) {
+      if (!IsStatusOk(r)) {
+        std::ignore = Cancel(StatusCast<ServerMetadataHandle>(r));
+      }
+      return r;
+    });
+  }
+
+  // Spawn a promise that returns Empty{} and save some boilerplate handling
+  // that detail.
+  template <typename PromiseFactory>
+  void SpawnInfallible(absl::string_view name, PromiseFactory promise_factory) {
+    party().Spawn(name, std::move(promise_factory), [](Empty) {});
+  }
+
+  // Spawn a promise that returns some status-like type; if the status
+  // represents failure automatically cancel the rest of the call.
+  template <typename PromiseFactory>
+  void SpawnGuarded(absl::string_view name, PromiseFactory promise_factory) {
+    using FactoryType =
+        promise_detail::OncePromiseFactory<void, PromiseFactory>;
+    using PromiseType = typename FactoryType::Promise;
+    using ResultType = typename PromiseType::Result;
+    static_assert(
+        std::is_same<bool,
+                     decltype(IsStatusOk(std::declval<ResultType>()))>::value,
+        "SpawnGuarded promise must return a status-like object");
+    party().Spawn(name, std::move(promise_factory), [this](ResultType r) {
+      if (!IsStatusOk(r)) {
+        if (grpc_trace_promise_primitives.enabled()) {
+          gpr_log(GPR_DEBUG, "SpawnGuarded sees failure: %s",
+                  r.ToString().c_str());
+        }
+        std::ignore = Cancel(StatusCast<ServerMetadataHandle>(std::move(r)));
+      }
+    });
+  }
+
+ private:
+  absl::AnyInvocable<void()> on_done_{nullptr};
+};
+
+class CallSpine final : public CallSpineInterface, public Party {
+ public:
+  static RefCountedPtr<CallSpine> Create(
+      grpc_event_engine::experimental::EventEngine* event_engine,
+      Arena* arena) {
+    return RefCountedPtr<CallSpine>(arena->New<CallSpine>(event_engine, arena));
+  }
+
+  Pipe<ClientMetadataHandle>& client_initial_metadata() override {
+    return client_initial_metadata_;
+  }
+  Pipe<ServerMetadataHandle>& server_initial_metadata() override {
+    return server_initial_metadata_;
+  }
+  Pipe<MessageHandle>& client_to_server_messages() override {
+    return client_to_server_messages_;
+  }
+  Pipe<MessageHandle>& server_to_client_messages() override {
+    return server_to_client_messages_;
+  }
+  Pipe<ServerMetadataHandle>& server_trailing_metadata() override {
+    return server_trailing_metadata_;
+  }
+  Latch<ServerMetadataHandle>& cancel_latch() override { return cancel_latch_; }
+  Party& party() override { return *this; }
+  void IncrementRefCount() override { Party::IncrementRefCount(); }
+  void Unref() override { Party::Unref(); }
+
+ private:
+  friend class Arena;
+  CallSpine(grpc_event_engine::experimental::EventEngine* event_engine,
+            Arena* arena)
+      : Party(arena, 1), event_engine_(event_engine) {}
+
+  class ScopedContext : public ScopedActivity,
+                        public promise_detail::Context<Arena> {
+   public:
+    explicit ScopedContext(CallSpine* spine)
+        : ScopedActivity(&spine->party()), Context<Arena>(spine->arena()) {}
+  };
+
+  bool RunParty() override {
+    ScopedContext context(this);
+    return Party::RunParty();
+  }
+
+  void PartyOver() override {
+    Arena* a = arena();
+    {
+      ScopedContext context(this);
+      CancelRemainingParticipants();
+      a->DestroyManagedNewObjects();
+    }
+    this->~CallSpine();
+    a->Destroy();
+  }
+
+  grpc_event_engine::experimental::EventEngine* event_engine() const override {
+    return event_engine_;
+  }
+
+  // Initial metadata from client to server
+  Pipe<ClientMetadataHandle> client_initial_metadata_{arena()};
+  // Initial metadata from server to client
+  Pipe<ServerMetadataHandle> server_initial_metadata_{arena()};
+  // Messages travelling from the application to the transport.
+  Pipe<MessageHandle> client_to_server_messages_{arena()};
+  // Messages travelling from the transport to the application.
+  Pipe<MessageHandle> server_to_client_messages_{arena()};
+  // Trailing metadata from server to client
+  Pipe<ServerMetadataHandle> server_trailing_metadata_{arena()};
+  // Latch that can be set to terminate the call
+  Latch<ServerMetadataHandle> cancel_latch_;
+  // Event engine associated with this call
+  grpc_event_engine::experimental::EventEngine* const event_engine_;
+};
+
+class CallInitiator {
+ public:
+  explicit CallInitiator(RefCountedPtr<CallSpineInterface> spine)
+      : spine_(std::move(spine)) {}
+
+  auto PushClientInitialMetadata(ClientMetadataHandle md) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return Map(spine_->client_initial_metadata().sender.Push(std::move(md)),
+               [](bool ok) { return StatusFlag(ok); });
+  }
+
+  auto PullServerInitialMetadata() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return Map(spine_->server_initial_metadata().receiver.Next(),
+               [](NextResult<ServerMetadataHandle> md)
+                   -> ValueOrFailure<absl::optional<ServerMetadataHandle>> {
+                 if (!md.has_value()) {
+                   if (md.cancelled()) return Failure{};
+                   return absl::optional<ServerMetadataHandle>();
+                 }
+                 return absl::optional<ServerMetadataHandle>(std::move(*md));
+               });
+  }
+
+  auto PullServerTrailingMetadata() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return PrioritizedRace(
+        Map(spine_->server_trailing_metadata().receiver.Next(),
+            [spine = spine_](
+                NextResult<ServerMetadataHandle> md) -> ServerMetadataHandle {
+              GPR_ASSERT(md.has_value());
+              return std::move(*md);
+            }),
+        spine_->WaitForCancel());
+  }
+
+  auto PullMessage() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return spine_->server_to_client_messages().receiver.Next();
+  }
+
+  auto PushMessage(MessageHandle message) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return Map(
+        spine_->client_to_server_messages().sender.Push(std::move(message)),
+        [](bool r) { return StatusFlag(r); });
+  }
+
+  void FinishSends() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    spine_->client_to_server_messages().sender.Close();
+  }
+
+  template <typename Promise>
+  auto CancelIfFails(Promise promise) {
+    return spine_->CancelIfFails(std::move(promise));
+  }
+
+  void Cancel() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    std::ignore =
+        spine_->Cancel(ServerMetadataFromStatus(absl::CancelledError()));
+  }
+
+  template <typename PromiseFactory>
+  void SpawnGuarded(absl::string_view name, PromiseFactory promise_factory) {
+    spine_->SpawnGuarded(name, std::move(promise_factory));
+  }
+
+  template <typename PromiseFactory>
+  void SpawnInfallible(absl::string_view name, PromiseFactory promise_factory) {
+    spine_->SpawnInfallible(name, std::move(promise_factory));
+  }
+
+  template <typename PromiseFactory>
+  auto SpawnWaitable(absl::string_view name, PromiseFactory promise_factory) {
+    return spine_->party().SpawnWaitable(name, std::move(promise_factory));
+  }
+
+  Arena* arena() { return spine_->party().arena(); }
+
+ private:
+  RefCountedPtr<CallSpineInterface> spine_;
+};
+
+class CallHandler {
+ public:
+  explicit CallHandler(RefCountedPtr<CallSpineInterface> spine)
+      : spine_(std::move(spine)) {}
+
+  auto PullClientInitialMetadata() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return Map(spine_->client_initial_metadata().receiver.Next(),
+               [](NextResult<ClientMetadataHandle> md)
+                   -> ValueOrFailure<ClientMetadataHandle> {
+                 if (!md.has_value()) return Failure{};
+                 return std::move(*md);
+               });
+  }
+
+  auto PushServerInitialMetadata(absl::optional<ServerMetadataHandle> md) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return If(
+        md.has_value(),
+        [&md, this]() {
+          return Map(
+              spine_->server_initial_metadata().sender.Push(std::move(*md)),
+              [](bool ok) { return StatusFlag(ok); });
+        },
+        [this]() {
+          spine_->server_initial_metadata().sender.Close();
+          return []() -> StatusFlag { return Success{}; };
+        });
+  }
+
+  auto PushServerTrailingMetadata(ServerMetadataHandle md) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    spine_->server_initial_metadata().sender.Close();
+    spine_->server_to_client_messages().sender.Close();
+    spine_->client_to_server_messages().receiver.CloseWithError();
+    spine_->CallOnDone();
+    return Map(spine_->server_trailing_metadata().sender.Push(std::move(md)),
+               [](bool ok) { return StatusFlag(ok); });
+  }
+
+  auto PullMessage() {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return spine_->client_to_server_messages().receiver.Next();
+  }
+
+  auto PushMessage(MessageHandle message) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    return Map(
+        spine_->server_to_client_messages().sender.Push(std::move(message)),
+        [](bool ok) { return StatusFlag(ok); });
+  }
+
+  void Cancel(ServerMetadataHandle status) {
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
+    std::ignore = spine_->Cancel(std::move(status));
+  }
+
+  void OnDone(absl::AnyInvocable<void()> fn) { spine_->OnDone(std::move(fn)); }
+
+  template <typename Promise>
+  auto CancelIfFails(Promise promise) {
+    return spine_->CancelIfFails(std::move(promise));
+  }
+
+  template <typename PromiseFactory>
+  void SpawnGuarded(absl::string_view name, PromiseFactory promise_factory) {
+    spine_->SpawnGuarded(name, std::move(promise_factory));
+  }
+
+  template <typename PromiseFactory>
+  void SpawnInfallible(absl::string_view name, PromiseFactory promise_factory) {
+    spine_->SpawnInfallible(name, std::move(promise_factory));
+  }
+
+  template <typename PromiseFactory>
+  auto SpawnWaitable(absl::string_view name, PromiseFactory promise_factory) {
+    return spine_->party().SpawnWaitable(name, std::move(promise_factory));
+  }
+
+  Arena* arena() { return spine_->party().arena(); }
+
+ private:
+  RefCountedPtr<CallSpineInterface> spine_;
+};
+
+struct CallInitiatorAndHandler {
+  CallInitiator initiator;
+  CallHandler handler;
+};
+
+CallInitiatorAndHandler MakeCall(
+    grpc_event_engine::experimental::EventEngine* event_engine, Arena* arena);
+
+template <typename CallHalf>
+auto OutgoingMessages(CallHalf h) {
+  struct Wrapper {
+    CallHalf h;
+    auto Next() { return h.PullMessage(); }
+  };
+  return Wrapper{std::move(h)};
+}
+
+// Forward a call from `call_handler` to `call_initiator` (with initial metadata
+// `client_initial_metadata`)
+void ForwardCall(CallHandler call_handler, CallInitiator call_initiator,
+                 ClientMetadataHandle client_initial_metadata);
+
+}  // namespace grpc_core
+
+#endif  // GRPC_SRC_CORE_LIB_TRANSPORT_CALL_SPINE_H
--- a/src/core/lib/transport/transport.cc
+++ b/src/core/lib/transport/transport.cc
@ -215,89 +215,3 @@ grpc_transport_stream_op_batch* grpc_make_transport_stream_op(
  op->op.on_complete = &op->outer_on_complete;
  return &op->op;
 }
-
-namespace grpc_core {
-
-void ForwardCall(CallHandler call_handler, CallInitiator call_initiator,
-                 ClientMetadataHandle client_initial_metadata) {
-  // Send initial metadata.
-  call_initiator.SpawnGuarded(
-      "send_initial_metadata",
-      [client_initial_metadata = std::move(client_initial_metadata),
-       call_initiator]() mutable {
-        return call_initiator.PushClientInitialMetadata(
-            std::move(client_initial_metadata));
-      });
-  // Read messages from handler into initiator.
-  call_handler.SpawnGuarded("read_messages", [call_handler,
-                                              call_initiator]() mutable {
-    return Seq(ForEach(OutgoingMessages(call_handler),
-                       [call_initiator](MessageHandle msg) mutable {
-                         // Need to spawn a job into the initiator's activity to
-                         // push the message in.
-                         return call_initiator.SpawnWaitable(
-                             "send_message",
-                             [msg = std::move(msg), call_initiator]() mutable {
-                               return call_initiator.CancelIfFails(
-                                   call_initiator.PushMessage(std::move(msg)));
-                             });
-                       }),
-               [call_initiator](StatusFlag result) mutable {
-                 call_initiator.SpawnInfallible(
-                     "finish-downstream", [call_initiator, result]() mutable {
-                       if (result.ok()) {
-                         call_initiator.FinishSends();
-                       } else {
-                         call_initiator.Cancel();
-                       }
-                       return Empty{};
-                     });
-                 return result;
-               });
-  });
-  call_initiator.SpawnInfallible("read_the_things", [call_initiator,
-                                                     call_handler]() mutable {
-    return Seq(
-        call_initiator.CancelIfFails(TrySeq(
-            call_initiator.PullServerInitialMetadata(),
-            [call_handler,
-             call_initiator](absl::optional<ServerMetadataHandle> md) mutable {
-              const bool has_md = md.has_value();
-              call_handler.SpawnGuarded(
-                  "recv_initial_metadata",
-                  [md = std::move(md), call_handler]() mutable {
-                    return call_handler.PushServerInitialMetadata(
-                        std::move(md));
-                  });
-              return If(
-                  has_md,
-                  ForEach(OutgoingMessages(call_initiator),
-                          [call_handler](MessageHandle msg) mutable {
-                            return call_handler.SpawnWaitable(
-                                "recv_message",
-                                [msg = std::move(msg), call_handler]() mutable {
-                                  return call_handler.CancelIfFails(
-                                      call_handler.PushMessage(std::move(msg)));
-                                });
-                          }),
-                  []() -> StatusFlag { return Success{}; });
-            })),
-        call_initiator.PullServerTrailingMetadata(),
-        [call_handler](ServerMetadataHandle md) mutable {
-          call_handler.SpawnGuarded(
-              "recv_trailing_metadata",
-              [md = std::move(md), call_handler]() mutable {
-                return call_handler.PushServerTrailingMetadata(std::move(md));
-              });
-          return Empty{};
-        });
-  });
-}
-
-CallInitiatorAndHandler MakeCall(
-    grpc_event_engine::experimental::EventEngine* event_engine, Arena* arena) {
-  auto spine = CallSpine::Create(event_engine, arena);
-  return {CallInitiator(spine), CallHandler(spine)};
-}
-
-}  // namespace grpc_core
--- a/src/core/lib/transport/transport.h
+++ b/src/core/lib/transport/transport.h
@ -42,7 +42,6 @@

 #include "src/core/lib/channel/context.h"
 #include "src/core/lib/debug/trace.h"
-#include "src/core/lib/gprpp/debug_location.h"
 #include "src/core/lib/gprpp/orphanable.h"
 #include "src/core/lib/gprpp/ref_counted.h"
 #include "src/core/lib/iomgr/call_combiner.h"
@ -53,16 +52,12 @@
 #include "src/core/lib/iomgr/polling_entity.h"
 #include "src/core/lib/promise/arena_promise.h"
 #include "src/core/lib/promise/context.h"
-#include "src/core/lib/promise/detail/status.h"
-#include "src/core/lib/promise/if.h"
 #include "src/core/lib/promise/latch.h"
-#include "src/core/lib/promise/party.h"
 #include "src/core/lib/promise/pipe.h"
-#include "src/core/lib/promise/prioritized_race.h"
-#include "src/core/lib/promise/status_flag.h"
 #include "src/core/lib/resource_quota/arena.h"
 #include "src/core/lib/slice/slice_buffer.h"
 #include "src/core/lib/transport/call_final_info.h"
+#include "src/core/lib/transport/call_spine.h"
 #include "src/core/lib/transport/connectivity_state.h"
 #include "src/core/lib/transport/message.h"
 #include "src/core/lib/transport/metadata.h"
@ -160,386 +155,6 @@ struct CallArgs {
 using NextPromiseFactory =
    std::function<ArenaPromise<ServerMetadataHandle>(CallArgs)>;

-// The common middle part of a call - a reference is held by each of
-// CallInitiator and CallHandler - which provide interfaces that are appropriate
-// for each side of a call.
-// The spine will ultimately host the pipes, filters, and context for one part
-// of a call: ie top-half client channel, sub channel call, server call.
-// TODO(ctiller): eventually drop this when we don't need to reference into
-// legacy promise calls anymore
-class CallSpineInterface {
- public:
-  virtual ~CallSpineInterface() = default;
-  virtual Pipe<ClientMetadataHandle>& client_initial_metadata() = 0;
-  virtual Pipe<ServerMetadataHandle>& server_initial_metadata() = 0;
-  virtual Pipe<MessageHandle>& client_to_server_messages() = 0;
-  virtual Pipe<MessageHandle>& server_to_client_messages() = 0;
-  virtual Pipe<ServerMetadataHandle>& server_trailing_metadata() = 0;
-  virtual Latch<ServerMetadataHandle>& cancel_latch() = 0;
-  // Add a callback to be called when server trailing metadata is received.
-  void OnDone(absl::AnyInvocable<void()> fn) {
-    if (on_done_ == nullptr) {
-      on_done_ = std::move(fn);
-      return;
-    }
-    on_done_ = [first = std::move(fn), next = std::move(on_done_)]() mutable {
-      first();
-      next();
-    };
-  }
-  void CallOnDone() {
-    if (on_done_ != nullptr) std::exchange(on_done_, nullptr)();
-  }
-  virtual Party& party() = 0;
-  virtual void IncrementRefCount() = 0;
-  virtual void Unref() = 0;
-
-  // Cancel the call with the given metadata.
-  // Regarding the `MUST_USE_RESULT absl::nullopt_t`:
-  // Most cancellation calls right now happen in pipe interceptors;
-  // there `nullopt` indicates terminate processing of this pipe and close with
-  // error.
-  // It's convenient then to have the Cancel operation (setting the latch to
-  // terminate the call) be the last thing that occurs in a pipe interceptor,
-  // and this construction supports that (and has helped the author not write
-  // some bugs).
-  GRPC_MUST_USE_RESULT absl::nullopt_t Cancel(ServerMetadataHandle metadata) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
-    auto& c = cancel_latch();
-    if (c.is_set()) return absl::nullopt;
-    c.Set(std::move(metadata));
-    CallOnDone();
-    client_initial_metadata().sender.CloseWithError();
-    server_initial_metadata().sender.CloseWithError();
-    client_to_server_messages().sender.CloseWithError();
-    server_to_client_messages().sender.CloseWithError();
-    return absl::nullopt;
-  }
-
-  auto WaitForCancel() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
-    return cancel_latch().Wait();
-  }
-
-  // Wrap a promise so that if it returns failure it automatically cancels
-  // the rest of the call.
-  // The resulting (returned) promise will resolve to Empty.
-  template <typename Promise>
-  auto CancelIfFails(Promise promise) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
-    using P = promise_detail::PromiseLike<Promise>;
-    using ResultType = typename P::Result;
-    return Map(std::move(promise), [this](ResultType r) {
-      if (!IsStatusOk(r)) {
-        std::ignore = Cancel(StatusCast<ServerMetadataHandle>(r));
-      }
-      return r;
-    });
-  }
-
-  // Spawn a promise that returns Empty{} and save some boilerplate handling
-  // that detail.
-  template <typename PromiseFactory>
-  void SpawnInfallible(absl::string_view name, PromiseFactory promise_factory) {
-    party().Spawn(name, std::move(promise_factory), [](Empty) {});
-  }
-
-  // Spawn a promise that returns some status-like type; if the status
-  // represents failure automatically cancel the rest of the call.
-  template <typename PromiseFactory>
-  void SpawnGuarded(absl::string_view name, PromiseFactory promise_factory) {
-    using FactoryType =
-        promise_detail::OncePromiseFactory<void, PromiseFactory>;
-    using PromiseType = typename FactoryType::Promise;
-    using ResultType = typename PromiseType::Result;
-    static_assert(
-        std::is_same<bool,
-                     decltype(IsStatusOk(std::declval<ResultType>()))>::value,
-        "SpawnGuarded promise must return a status-like object");
-    party().Spawn(name, std::move(promise_factory), [this](ResultType r) {
-      if (!IsStatusOk(r)) {
-        if (grpc_trace_promise_primitives.enabled()) {
-          gpr_log(GPR_DEBUG, "SpawnGuarded sees failure: %s",
-                  r.ToString().c_str());
-        }
-        std::ignore = Cancel(StatusCast<ServerMetadataHandle>(std::move(r)));
-      }
-    });
-  }
-
- private:
-  absl::AnyInvocable<void()> on_done_{nullptr};
-};
-
-class CallSpine final : public CallSpineInterface, public Party {
- public:
-  static RefCountedPtr<CallSpine> Create(
-      grpc_event_engine::experimental::EventEngine* event_engine,
-      Arena* arena) {
-    return RefCountedPtr<CallSpine>(arena->New<CallSpine>(event_engine, arena));
-  }
-
-  Pipe<ClientMetadataHandle>& client_initial_metadata() override {
-    return client_initial_metadata_;
-  }
-  Pipe<ServerMetadataHandle>& server_initial_metadata() override {
-    return server_initial_metadata_;
-  }
-  Pipe<MessageHandle>& client_to_server_messages() override {
-    return client_to_server_messages_;
-  }
-  Pipe<MessageHandle>& server_to_client_messages() override {
-    return server_to_client_messages_;
-  }
-  Pipe<ServerMetadataHandle>& server_trailing_metadata() override {
-    return server_trailing_metadata_;
-  }
-  Latch<ServerMetadataHandle>& cancel_latch() override { return cancel_latch_; }
-  Party& party() override { return *this; }
-  void IncrementRefCount() override { Party::IncrementRefCount(); }
-  void Unref() override { Party::Unref(); }
-
- private:
-  friend class Arena;
-  CallSpine(grpc_event_engine::experimental::EventEngine* event_engine,
-            Arena* arena)
-      : Party(arena, 1), event_engine_(event_engine) {}
-
-  class ScopedContext : public ScopedActivity,
-                        public promise_detail::Context<Arena> {
-   public:
-    explicit ScopedContext(CallSpine* spine)
-        : ScopedActivity(&spine->party()), Context<Arena>(spine->arena()) {}
-  };
-
-  bool RunParty() override {
-    ScopedContext context(this);
-    return Party::RunParty();
-  }
-
-  void PartyOver() override {
-    Arena* a = arena();
-    {
-      ScopedContext context(this);
-      CancelRemainingParticipants();
-      a->DestroyManagedNewObjects();
-    }
-    this->~CallSpine();
-    a->Destroy();
-  }
-
-  grpc_event_engine::experimental::EventEngine* event_engine() const override {
-    return event_engine_;
-  }
-
-  // Initial metadata from client to server
-  Pipe<ClientMetadataHandle> client_initial_metadata_{arena()};
-  // Initial metadata from server to client
-  Pipe<ServerMetadataHandle> server_initial_metadata_{arena()};
-  // Messages travelling from the application to the transport.
-  Pipe<MessageHandle> client_to_server_messages_{arena()};
-  // Messages travelling from the transport to the application.
-  Pipe<MessageHandle> server_to_client_messages_{arena()};
-  // Trailing metadata from server to client
-  Pipe<ServerMetadataHandle> server_trailing_metadata_{arena()};
-  // Latch that can be set to terminate the call
-  Latch<ServerMetadataHandle> cancel_latch_;
-  // Event engine associated with this call
-  grpc_event_engine::experimental::EventEngine* const event_engine_;
-};
-
-class CallInitiator {
- public:
-  explicit CallInitiator(RefCountedPtr<CallSpineInterface> spine)
-      : spine_(std::move(spine)) {}
-
-  auto PushClientInitialMetadata(ClientMetadataHandle md) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return Map(spine_->client_initial_metadata().sender.Push(std::move(md)),
-               [](bool ok) { return StatusFlag(ok); });
-  }
-
-  auto PullServerInitialMetadata() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return Map(spine_->server_initial_metadata().receiver.Next(),
-               [](NextResult<ServerMetadataHandle> md)
-                   -> ValueOrFailure<absl::optional<ServerMetadataHandle>> {
-                 if (!md.has_value()) {
-                   if (md.cancelled()) return Failure{};
-                   return absl::optional<ServerMetadataHandle>();
-                 }
-                 return absl::optional<ServerMetadataHandle>(std::move(*md));
-               });
-  }
-
-  auto PullServerTrailingMetadata() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return PrioritizedRace(
-        Map(spine_->server_trailing_metadata().receiver.Next(),
-            [spine = spine_](
-                NextResult<ServerMetadataHandle> md) -> ServerMetadataHandle {
-              GPR_ASSERT(md.has_value());
-              return std::move(*md);
-            }),
-        spine_->WaitForCancel());
-  }
-
-  auto PullMessage() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return spine_->server_to_client_messages().receiver.Next();
-  }
-
-  auto PushMessage(MessageHandle message) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return Map(
-        spine_->client_to_server_messages().sender.Push(std::move(message)),
-        [](bool r) { return StatusFlag(r); });
-  }
-
-  void FinishSends() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    spine_->client_to_server_messages().sender.Close();
-  }
-
-  template <typename Promise>
-  auto CancelIfFails(Promise promise) {
-    return spine_->CancelIfFails(std::move(promise));
-  }
-
-  void Cancel() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    std::ignore =
-        spine_->Cancel(ServerMetadataFromStatus(absl::CancelledError()));
-  }
-
-  template <typename PromiseFactory>
-  void SpawnGuarded(absl::string_view name, PromiseFactory promise_factory) {
-    spine_->SpawnGuarded(name, std::move(promise_factory));
-  }
-
-  template <typename PromiseFactory>
-  void SpawnInfallible(absl::string_view name, PromiseFactory promise_factory) {
-    spine_->SpawnInfallible(name, std::move(promise_factory));
-  }
-
-  template <typename PromiseFactory>
-  auto SpawnWaitable(absl::string_view name, PromiseFactory promise_factory) {
-    return spine_->party().SpawnWaitable(name, std::move(promise_factory));
-  }
-
-  Arena* arena() { return spine_->party().arena(); }
-
- private:
-  RefCountedPtr<CallSpineInterface> spine_;
-};
-
-class CallHandler {
- public:
-  explicit CallHandler(RefCountedPtr<CallSpineInterface> spine)
-      : spine_(std::move(spine)) {}
-
-  auto PullClientInitialMetadata() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return Map(spine_->client_initial_metadata().receiver.Next(),
-               [](NextResult<ClientMetadataHandle> md)
-                   -> ValueOrFailure<ClientMetadataHandle> {
-                 if (!md.has_value()) return Failure{};
-                 return std::move(*md);
-               });
-  }
-
-  auto PushServerInitialMetadata(absl::optional<ServerMetadataHandle> md) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return If(
-        md.has_value(),
-        [&md, this]() {
-          return Map(
-              spine_->server_initial_metadata().sender.Push(std::move(*md)),
-              [](bool ok) { return StatusFlag(ok); });
-        },
-        [this]() {
-          spine_->server_initial_metadata().sender.Close();
-          return []() -> StatusFlag { return Success{}; };
-        });
-  }
-
-  auto PushServerTrailingMetadata(ServerMetadataHandle md) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    spine_->server_initial_metadata().sender.Close();
-    spine_->server_to_client_messages().sender.Close();
-    spine_->client_to_server_messages().receiver.CloseWithError();
-    spine_->CallOnDone();
-    return Map(spine_->server_trailing_metadata().sender.Push(std::move(md)),
-               [](bool ok) { return StatusFlag(ok); });
-  }
-
-  auto PullMessage() {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return spine_->client_to_server_messages().receiver.Next();
-  }
-
-  auto PushMessage(MessageHandle message) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    return Map(
-        spine_->server_to_client_messages().sender.Push(std::move(message)),
-        [](bool ok) { return StatusFlag(ok); });
-  }
-
-  void Cancel(ServerMetadataHandle status) {
-    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
-    std::ignore = spine_->Cancel(std::move(status));
-  }
-
-  void OnDone(absl::AnyInvocable<void()> fn) { spine_->OnDone(std::move(fn)); }
-
-  template <typename Promise>
-  auto CancelIfFails(Promise promise) {
-    return spine_->CancelIfFails(std::move(promise));
-  }
-
-  template <typename PromiseFactory>
-  void SpawnGuarded(absl::string_view name, PromiseFactory promise_factory) {
-    spine_->SpawnGuarded(name, std::move(promise_factory));
-  }
-
-  template <typename PromiseFactory>
-  void SpawnInfallible(absl::string_view name, PromiseFactory promise_factory) {
-    spine_->SpawnInfallible(name, std::move(promise_factory));
-  }
-
-  template <typename PromiseFactory>
-  auto SpawnWaitable(absl::string_view name, PromiseFactory promise_factory) {
-    return spine_->party().SpawnWaitable(name, std::move(promise_factory));
-  }
-
-  Arena* arena() { return spine_->party().arena(); }
-
- private:
-  RefCountedPtr<CallSpineInterface> spine_;
-};
-
-struct CallInitiatorAndHandler {
-  CallInitiator initiator;
-  CallHandler handler;
-};
-
-CallInitiatorAndHandler MakeCall(
-    grpc_event_engine::experimental::EventEngine* event_engine, Arena* arena);
-
-template <typename CallHalf>
-auto OutgoingMessages(CallHalf h) {
-  struct Wrapper {
-    CallHalf h;
-    auto Next() { return h.PullMessage(); }
-  };
-  return Wrapper{std::move(h)};
-}
-
-// Forward a call from `call_handler` to `call_initiator` (with initial metadata
-// `client_initial_metadata`)
-void ForwardCall(CallHandler call_handler, CallInitiator call_initiator,
-                 ClientMetadataHandle client_initial_metadata);
-
 }  // namespace grpc_core

 // forward declarations
--- a/src/python/grpcio/grpc_core_dependencies.py
+++ b/src/python/grpcio/grpc_core_dependencies.py
@ -812,6 +812,7 @@ CORE_SOURCE_FILES = [
    'src/core/lib/transport/bdp_estimator.cc',
    'src/core/lib/transport/call_filters.cc',
    'src/core/lib/transport/call_final_info.cc',
+    'src/core/lib/transport/call_spine.cc',
    'src/core/lib/transport/connectivity_state.cc',
    'src/core/lib/transport/error_utils.cc',
    'src/core/lib/transport/handshaker.cc',
--- a/tools/doxygen/Doxyfile.c++.internal
+++ b/tools/doxygen/Doxyfile.c++.internal
@ -2904,6 +2904,8 @@ src/core/lib/transport/call_filters.cc \
 src/core/lib/transport/call_filters.h \
 src/core/lib/transport/call_final_info.cc \
 src/core/lib/transport/call_final_info.h \
+src/core/lib/transport/call_spine.cc \
+src/core/lib/transport/call_spine.h \
 src/core/lib/transport/connectivity_state.cc \
 src/core/lib/transport/connectivity_state.h \
 src/core/lib/transport/custom_metadata.h \
--- a/tools/doxygen/Doxyfile.core.internal
+++ b/tools/doxygen/Doxyfile.core.internal
@ -2685,6 +2685,8 @@ src/core/lib/transport/call_filters.cc \
 src/core/lib/transport/call_filters.h \
 src/core/lib/transport/call_final_info.cc \
 src/core/lib/transport/call_final_info.h \
+src/core/lib/transport/call_spine.cc \
+src/core/lib/transport/call_spine.h \
 src/core/lib/transport/connectivity_state.cc \
 src/core/lib/transport/connectivity_state.h \
 src/core/lib/transport/custom_metadata.h \