//
//
// Copyright 2015 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_TRANSPORT_H
#define GRPC_SRC_CORE_LIB_TRANSPORT_TRANSPORT_H

#include <grpc/impl/connectivity_state.h>
#include <grpc/slice.h>
#include <grpc/status.h>
#include <grpc/support/port_platform.h>
#include <grpc/support/time.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>

#include <functional>
#include <optional>
#include <string>
#include <utility>

#include "src/core/call/call_destination.h"
#include "src/core/call/call_spine.h"
#include "src/core/call/message.h"
#include "src/core/call/metadata.h"
#include "src/core/call/metadata_batch.h"
#include "src/core/channelz/channelz.h"
#include "src/core/ext/transport/chttp2/transport/http2_status.h"
#include "src/core/lib/debug/trace.h"
#include "src/core/lib/experiments/experiments.h"
#include "src/core/lib/iomgr/call_combiner.h"
#include "src/core/lib/iomgr/closure.h"
#include "src/core/lib/iomgr/endpoint.h"
#include "src/core/lib/iomgr/error.h"
#include "src/core/lib/iomgr/iomgr_fwd.h"
#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/if.h"
#include "src/core/lib/promise/latch.h"
#include "src/core/lib/promise/pipe.h"
#include "src/core/lib/promise/promise.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/connectivity_state.h"
#include "src/core/lib/transport/transport_fwd.h"
#include "src/core/util/orphanable.h"
#include "src/core/util/ref_counted.h"
#include "absl/functional/any_invocable.h"
#include "absl/log/log.h"
#include "absl/status/status.h"
#include "absl/strings/string_view.h"

// Minimum and maximum protocol accepted versions.
#define GRPC_PROTOCOL_VERSION_MAX_MAJOR 2
#define GRPC_PROTOCOL_VERSION_MAX_MINOR 1
#define GRPC_PROTOCOL_VERSION_MIN_MAJOR 2
#define GRPC_PROTOCOL_VERSION_MIN_MINOR 1

#define GRPC_ARG_TRANSPORT "grpc.internal.transport"

/** A comma separated list of supported transport protocols. If non-empty,
 allows the client and server to attempt to negotiate transport protocols.
 NOTE: This is an experimental feature. It is not fully implemented and is not
 currently functional.
 TODO(gtcooke94) - update with specific details when implementing. */
#define GRPC_ARG_TRANSPORT_PROTOCOLS "grpc.internal.transport_protocols"

namespace grpc_core {

// Move only type that tracks call startup.
// Allows observation of when client_initial_metadata has been processed by the
// end of the local call stack.
// Interested observers can call Wait() to obtain a promise that will resolve
// when all local client_initial_metadata processing has completed.
// The result of this token is either true on successful completion, or false
// if the metadata was not sent.
// To set a successful completion, call Complete(true). For failure, call
// Complete(false).
// If Complete is not called, the destructor of a still held token will complete
// with failure.
// Transports should hold this token until client_initial_metadata has passed
// any flow control (eg MAX_CONCURRENT_STREAMS for http2).
class ClientInitialMetadataOutstandingToken {
 public:
  static ClientInitialMetadataOutstandingToken Empty() {
    return ClientInitialMetadataOutstandingToken();
  }
  static ClientInitialMetadataOutstandingToken New(
      Arena* arena = GetContext<Arena>()) {
    ClientInitialMetadataOutstandingToken token;
    token.latch_ = arena->New<Latch<bool>>();
    return token;
  }

  ClientInitialMetadataOutstandingToken(
      const ClientInitialMetadataOutstandingToken&) = delete;
  ClientInitialMetadataOutstandingToken& operator=(
      const ClientInitialMetadataOutstandingToken&) = delete;
  ClientInitialMetadataOutstandingToken(
      ClientInitialMetadataOutstandingToken&& other) noexcept
      : latch_(std::exchange(other.latch_, nullptr)) {}
  ClientInitialMetadataOutstandingToken& operator=(
      ClientInitialMetadataOutstandingToken&& other) noexcept {
    if (IsMetadataOutstandingTokenRefactorEnabled()) {
      MaybeSet(false);
    }
    latch_ = std::exchange(other.latch_, nullptr);
    return *this;
  }
  ~ClientInitialMetadataOutstandingToken() {
    if (IsMetadataOutstandingTokenRefactorEnabled()) {
      MaybeSet(false);
    } else {
      if (latch_ != nullptr) latch_->Set(false);
    }
  }
  void Complete(bool success) {
    if (IsMetadataOutstandingTokenRefactorEnabled()) {
      MaybeSet(success);
    } else {
      if (latch_ != nullptr) std::exchange(latch_, nullptr)->Set(success);
    }
  }

  // Returns a promise that will resolve when this object (or its moved-from
  // ancestor) is dropped. If the token is Empty(), the promise resolves to
  // false immediately (when the refactor experiment is enabled).
  auto Wait() {
    return If(
        latch_ != nullptr, [latch = latch_]() { return latch->Wait(); },
        []() { return Immediate(false); });
  }

 private:
  ClientInitialMetadataOutstandingToken() = default;

  void MaybeSet(bool status) {
    if (latch_ != nullptr && !latch_->is_set()) {
      latch_->Set(status);
    }
  }

  Latch<bool>* latch_ = nullptr;
};

using ClientInitialMetadataOutstandingTokenWaitType =
    decltype(std::declval<ClientInitialMetadataOutstandingToken>().Wait());

struct CallArgs {
  // Initial metadata from the client to the server.
  // During promise setup this can be manipulated by filters (and then
  // passed on to the next filter).
  ClientMetadataHandle client_initial_metadata;
  // Token indicating that client_initial_metadata is still being processed.
  // This should be moved around and only destroyed when the transport is
  // satisfied that the metadata has passed any flow control measures it has.
  ClientInitialMetadataOutstandingToken client_initial_metadata_outstanding;
  // Latch that will ultimately contain the polling entity for the call.
  // TODO(ctiller): remove once event engine lands
  Latch<grpc_polling_entity>* polling_entity;
  // Initial metadata from the server to the client.
  // Set once when it's available.
  // During promise setup filters can substitute their own latch for this
  // and consequently intercept the sent value and mutate/observe it.
  PipeSender<ServerMetadataHandle>* server_initial_metadata;
  // Messages travelling from the application to the transport.
  PipeReceiver<MessageHandle>* client_to_server_messages;
  // Messages travelling from the transport to the application.
  PipeSender<MessageHandle>* server_to_client_messages;
};

using NextPromiseFactory =
    std::function<ArenaPromise<ServerMetadataHandle>(CallArgs)>;

}  // namespace grpc_core

// forward declarations

// grpc_stream doesn't actually exist. It's used as a typesafe
// opaque pointer for whatever data the transport wants to track
// for a stream.
typedef struct grpc_stream grpc_stream;

typedef struct grpc_stream_refcount {
  grpc_core::RefCount refs;
  grpc_closure destroy;
#ifndef NDEBUG
  const char* object_type;
#endif
} grpc_stream_refcount;

#ifndef NDEBUG
void grpc_stream_ref_init(grpc_stream_refcount* refcount, int initial_refs,
                          grpc_iomgr_cb_func cb, void* cb_arg,
                          const char* object_type);
#define GRPC_STREAM_REF_INIT(rc, ir, cb, cb_arg, objtype) \
  grpc_stream_ref_init(rc, ir, cb, cb_arg, objtype)
#else
void grpc_stream_ref_init(grpc_stream_refcount* refcount, int initial_refs,
                          grpc_iomgr_cb_func cb, void* cb_arg);
#define GRPC_STREAM_REF_INIT(rc, ir, cb, cb_arg, objtype) \
  do {                                                    \
    grpc_stream_ref_init(rc, ir, cb, cb_arg);             \
    (void)(objtype);                                      \
  } while (0)
#endif

#ifndef NDEBUG
inline void grpc_stream_ref(grpc_stream_refcount* refcount,
                            const char* reason) {
  GRPC_TRACE_VLOG(stream_refcount, 2)
      << refcount->object_type << " " << refcount << ":"
      << refcount->destroy.cb_arg << " REF " << reason;
  refcount->refs.RefNonZero(DEBUG_LOCATION, reason);
}
#else
inline void grpc_stream_ref(grpc_stream_refcount* refcount) {
  refcount->refs.RefNonZero();
}
#endif

void grpc_stream_destroy(grpc_stream_refcount* refcount);

#ifndef NDEBUG
inline void grpc_stream_unref(grpc_stream_refcount* refcount,
                              const char* reason) {
  GRPC_TRACE_VLOG(stream_refcount, 2)
      << refcount->object_type << " " << refcount << ":"
      << refcount->destroy.cb_arg << " UNREF " << reason;
  if (GPR_UNLIKELY(refcount->refs.Unref(DEBUG_LOCATION, reason))) {
    grpc_stream_destroy(refcount);
  }
}
#else
inline void grpc_stream_unref(grpc_stream_refcount* refcount) {
  if (GPR_UNLIKELY(refcount->refs.Unref())) {
    grpc_stream_destroy(refcount);
  }
}
#endif

// Wrap a buffer that is owned by some stream object into a slice that shares
// the same refcount
grpc_slice grpc_slice_from_stream_owned_buffer(grpc_stream_refcount* refcount,
                                               void* buffer, size_t length);

// This struct (which is present in both grpc_transport_stream_op_batch
// and grpc_transport_op_batch) is a convenience to allow filters or
// transports to schedule a closure related to a particular batch without
// having to allocate memory.  The general pattern is to initialize the
// closure with the callback arg set to the batch and extra_arg set to
// whatever state is associated with the handler (e.g., the call element
// or the transport stream object).
//
// Note that this can only be used by the current handler of a given
// batch on the way down the stack (i.e., whichever filter or transport is
// currently handling the batch).  Once a filter or transport passes control
// of the batch to the next handler, it cannot depend on the contents of
// this struct anymore, because the next handler may reuse it.
struct grpc_handler_private_op_data {
  void* extra_arg = nullptr;
  grpc_closure closure;
  grpc_handler_private_op_data() { memset(&closure, 0, sizeof(closure)); }
};

typedef struct grpc_transport_stream_op_batch_payload
    grpc_transport_stream_op_batch_payload;

// Transport stream op: a set of operations to perform on a transport
// against a single stream
struct grpc_transport_stream_op_batch {
  grpc_transport_stream_op_batch()
      : send_initial_metadata(false),
        send_trailing_metadata(false),
        send_message(false),
        recv_initial_metadata(false),
        recv_message(false),
        recv_trailing_metadata(false),
        cancel_stream(false),
        is_traced(false) {}

  /// Should be scheduled when all of the non-recv operations in the batch
  /// are complete.

  /// The recv ops (recv_initial_metadata, recv_message, and
  /// recv_trailing_metadata) each have their own callbacks.  If a batch
  /// contains both recv ops and non-recv ops, on_complete should be
  /// scheduled as soon as the non-recv ops are complete, regardless of
  /// whether or not the recv ops are complete.  If a batch contains
  /// only recv ops, on_complete can be null.
  grpc_closure* on_complete = nullptr;

  /// Values for the stream op (fields set are determined by flags above)
  grpc_transport_stream_op_batch_payload* payload = nullptr;

  /// Send initial metadata to the peer, from the provided metadata batch.
  bool send_initial_metadata : 1;

  /// Send trailing metadata to the peer, from the provided metadata batch.
  bool send_trailing_metadata : 1;

  /// Send message data to the peer, from the provided byte stream.
  bool send_message : 1;

  /// Receive initial metadata from the stream, into provided metadata batch.
  bool recv_initial_metadata : 1;

  /// Receive message data from the stream, into provided byte stream.
  bool recv_message : 1;

  /// Receive trailing metadata from the stream, into provided metadata batch.
  ///
  bool recv_trailing_metadata : 1;

  /// Cancel this stream with the provided error
  bool cancel_stream : 1;

  /// Is this stream traced
  bool is_traced : 1;

  bool HasOp() const {
    return send_initial_metadata || send_trailing_metadata || send_message ||
           recv_initial_metadata || recv_message || recv_trailing_metadata ||
           cancel_stream;
  }

  //**************************************************************************
  // remaining fields are initialized and used at the discretion of the
  // current handler of the op

  grpc_handler_private_op_data handler_private;
};

struct grpc_transport_stream_op_batch_payload {
  struct {
    grpc_metadata_batch* send_initial_metadata = nullptr;
  } send_initial_metadata;

  struct {
    grpc_metadata_batch* send_trailing_metadata = nullptr;
    // Set by the transport to true if the stream successfully wrote the
    // trailing metadata. If this is not set but there was a send trailing
    // metadata op present, this can indicate that a server call can be marked
    // as  a cancellation (since the stream was write-closed before status could
    // be delivered).
    bool* sent = nullptr;
  } send_trailing_metadata;

  struct {
    // The transport (or a filter that decides to return a failure before
    // the op gets down to the transport) takes ownership.
    // The batch's on_complete will not be called until after the byte
    // stream is orphaned.
    grpc_core::SliceBuffer* send_message;
    uint32_t flags = 0;
    // Set by the transport if the stream has been closed for writes. If this
    // is set and send message op is present, we set the operation to be a
    // failure without sending a cancel OP down the stack. This is so that the
    // status of the call does not get overwritten by the Cancel OP, which would
    // be especially problematic if we had received a valid status from the
    // server.
    // For send_initial_metadata, it is fine for the status to be overwritten
    // because at that point, the client will not have received a status.
    // For send_trailing_metadata, we might overwrite the status if we have
    // non-zero metadata to send. This is fine because the API does not allow
    // the client to send trailing metadata.
    bool stream_write_closed = false;
  } send_message;

  struct {
    grpc_metadata_batch* recv_initial_metadata = nullptr;
    /// Should be enqueued when initial metadata is ready to be processed.
    grpc_closure* recv_initial_metadata_ready = nullptr;
    // If not NULL, will be set to true if trailing metadata is
    // immediately available. This may be a signal that we received a
    // Trailers-Only response. The retry filter checks this to know whether to
    // defer the decision to commit the call or not. The C++ callback API also
    // uses this to set the success flag of OnReadInitialMetadataDone()
    // callback.
    bool* trailing_metadata_available = nullptr;
  } recv_initial_metadata;

  struct {
    // Will be set by the transport to point to the byte stream containing a
    // received message. Will be nullopt if trailing metadata is received
    // instead of a message.
    std::optional<grpc_core::SliceBuffer>* recv_message = nullptr;
    uint32_t* flags = nullptr;
    // Was this recv_message failed for reasons other than a clean end-of-stream
    bool* call_failed_before_recv_message = nullptr;
    /// Should be enqueued when one message is ready to be processed.
    grpc_closure* recv_message_ready = nullptr;
  } recv_message;

  struct {
    grpc_metadata_batch* recv_trailing_metadata = nullptr;
    grpc_transport_stream_stats* collect_stats = nullptr;
    /// Should be enqueued when trailing metadata is ready to be processed.
    grpc_closure* recv_trailing_metadata_ready = nullptr;
  } recv_trailing_metadata;

  /// Forcefully close this stream.
  /// The HTTP2 semantics should be:
  /// - server side: if cancel_error has
  /// grpc_core::StatusIntProperty::kRpcStatus, and trailing metadata has not
  /// been sent, send trailing metadata with status and message from
  /// cancel_error (use grpc_error_get_status) followed by a RST_STREAM with
  /// error=GRPC_CHTTP2_NO_ERROR to force a full close
  /// - at all other times: use grpc_error_get_status to get a status code, and
  ///   convert to a HTTP2 error code using
  ///   grpc_chttp2_grpc_status_to_http2_error. Send a RST_STREAM with this
  ///   error.
  struct {
    // Error contract: the transport that gets this op must cause cancel_error
    //                 to be unref'ed after processing it
    grpc_error_handle cancel_error;
    // If true the transport should endeavor to delay sending the cancellation
    // notification for some small amount of time, in order to foil certain
    // exploits.
    // This should be set for cancellations that result from malformed client
    // initial metadata.
    bool tarpit = false;
    // Server-side only: If non-null, the transport sends this trailing
    // metadata to the client.
    // NOTE: This metadata bypasses subsequent filters and is sent directly
    // to the client. Ensure it contains only fields intended for the client.
    grpc_core::ServerMetadataHandle send_trailing_metadata = nullptr;
  } cancel_stream;
};

/// Transport op: a set of operations to perform on a transport as a whole
typedef struct grpc_transport_op {
  /// Called when processing of this op is done.
  grpc_closure* on_consumed = nullptr;
  /// connectivity monitoring - set connectivity_state to NULL to unsubscribe
  grpc_core::OrphanablePtr<grpc_core::ConnectivityStateWatcherInterface>
      start_connectivity_watch;
  grpc_core::ConnectivityStateWatcherInterface* stop_connectivity_watch =
      nullptr;
  /// should the transport be disconnected
  grpc_error_handle disconnect_with_error;
  /// Start a graceful goaway with the specified error message. (The error code
  /// is ignored since graceful GOAWAYs use a NO_ERROR error code.) Use
  /// disconnect_with_error if graceful shutdown is not needed.
  grpc_error_handle goaway_error;
  void (*set_accept_stream_fn)(void* user_data, grpc_core::Transport* transport,
                               const void* server_data) = nullptr;
  void (*set_registered_method_matcher_fn)(
      void* user_data, grpc_core::ServerMetadata* metadata) = nullptr;
  void* set_accept_stream_user_data = nullptr;
  void (*set_make_promise_fn)(void* user_data, grpc_core::Transport* transport,
                              const void* server_data) = nullptr;
  void* set_make_promise_user_data = nullptr;
  /// add this transport to a pollset
  grpc_pollset* bind_pollset = nullptr;
  /// add this transport to a pollset_set
  grpc_pollset_set* bind_pollset_set = nullptr;
  /// send a ping, if either on_initiate or on_ack is not NULL
  struct {
    /// Ping may be delayed by the transport, on_initiate callback will be
    /// called when the ping is actually being sent.
    grpc_closure* on_initiate = nullptr;
    /// Called when the ping ack is received
    grpc_closure* on_ack = nullptr;
  } send_ping;
  grpc_connectivity_state start_connectivity_watch_state = GRPC_CHANNEL_IDLE;
  // If true, will reset the channel's connection backoff.
  bool reset_connect_backoff = false;

  /// set the callback for accepting new streams;
  /// this is a permanent callback, unlike the other one-shot closures.
  /// If true, the callback is set to set_accept_stream_fn, with its
  /// user_data argument set to set_accept_stream_user_data.
  /// `set_registered_method_matcher_fn` is also set with its user_data argument
  /// set to set_accept_stream_user_data. The transport should invoke
  /// `set_registered_method_matcher_fn` after initial metadata is received but
  /// before recv_initial_metadata_ready callback is invoked. If the transport
  /// detects an error in the stream, invoking
  /// `set_registered_method_matcher_fn` can be skipped.
  bool set_accept_stream = false;

  /// set the callback for accepting new streams based upon promises;
  /// this is a permanent callback, unlike the other one-shot closures.
  /// If true, the callback is set to set_make_promise_fn, with its
  /// user_data argument set to set_make_promise_data
  bool set_make_promise = false;

  //**************************************************************************
  // remaining fields are initialized and used at the discretion of the
  // transport implementation

  grpc_handler_private_op_data handler_private;
} grpc_transport_op;

// Allocate a grpc_transport_op, and preconfigure the on_complete closure to
// \a on_complete and then delete the returned transport op
grpc_transport_op* grpc_make_transport_op(grpc_closure* on_complete);
// Allocate a grpc_transport_stream_op_batch, and preconfigure the on_complete
// closure
// to \a on_complete and then delete the returned transport op
grpc_transport_stream_op_batch* grpc_make_transport_stream_op(
    grpc_closure* on_complete);

void grpc_transport_stream_op_batch_finish_with_failure(
    grpc_transport_stream_op_batch* batch, grpc_error_handle error,
    grpc_core::CallCombiner* call_combiner);
void grpc_transport_stream_op_batch_queue_finish_with_failure(
    grpc_transport_stream_op_batch* batch, grpc_error_handle error,
    grpc_core::CallCombinerClosureList* closures);
// Fail a batch from within the transport (i.e. without the activity lock/call
// combiner taken).
void grpc_transport_stream_op_batch_finish_with_failure_from_transport(
    grpc_transport_stream_op_batch* batch, grpc_error_handle error);

std::string grpc_transport_stream_op_batch_string(
    grpc_transport_stream_op_batch* op, bool truncate);
std::string grpc_transport_op_string(grpc_transport_op* op);

namespace grpc_core {

class FilterStackTransport;
class ClientTransport;
class ServerTransport;

class Transport : public InternallyRefCounted<Transport> {
 public:
  // An interface used by channels or servers to watch the transport's state.
  class StateWatcher : public RefCounted<StateWatcher> {
   public:
    ~StateWatcher() override = default;

    // The list of reasons is defined in
    // https://github.com/grpc/proposal/blob/master/A94-subchannel-otel-metrics.md.
    // Note that we do not include the "subchannel shutdown" reason
    // here, since that reason is not generated by the transport.
    enum DisconnectReason {
      kUnknown,
      kGoaway,
      kConnectionReset,
      kConnectionTimedOut,
      kConnectionAborted,
      kSocketError,
    };

    struct DisconnectInfo {
      DisconnectReason reason = kUnknown;
      std::optional<http2::Http2ErrorCode> http2_error_code;
      std::optional<Duration> keepalive_time;
    };

    // Called on disconnection or GOAWAY.  The channel or server must
    // stop sending traffic to this transport.  The transport will
    // automatically stop the watch after this.
    virtual void OnDisconnect(absl::Status status,
                              DisconnectInfo disconnect_info) = 0;

    // A handle passed to the subchannel by the transport via
    // OnPeerMaxConcurrentStreamsUpdate().  The subchannel must delete
    // this handle when it has finished processing the update.
    class MaxConcurrentStreamsUpdateDoneHandle {
     public:
      virtual ~MaxConcurrentStreamsUpdateDoneHandle() = default;
    };

    // Used on client transports only.
    // Will be called once as soon as the watch is started to indicate
    // the current value of the peer's MAX_CONCURRENT_STREAMS setting.
    // Will then be called again whenever the peer changes this setting.
    // The on_done handle must be deleted when the implementation is
    // done processing the update.
    virtual void OnPeerMaxConcurrentStreamsUpdate(
        uint32_t max_concurrent_streams,
        std::unique_ptr<MaxConcurrentStreamsUpdateDoneHandle> on_done) = 0;

    // TODO(roth): Remove this as part of the EventEngine migration.
    virtual grpc_pollset_set* interested_parties() const = 0;
  };

  struct RawPointerChannelArgTag {};
  static absl::string_view ChannelArgName() { return GRPC_ARG_TRANSPORT; }

  using InternallyRefCounted<Transport>::InternallyRefCounted;

  // Though internally ref counted transports expose their "Ref" method to
  // create a RefCountedPtr to themselves. The OrphanablePtr owner is the
  // singleton decision maker on whether the transport should be destroyed or
  // not.
  // TODO(ctiller): consider moving to a DualRefCounted model (with the
  // disadvantage that we would accidentally have many strong owners which is
  // unnecessary for this type).
  RefCountedPtr<Transport> Ref() {
    return InternallyRefCounted<Transport>::Ref();
  }
  template <typename T>
  RefCountedPtr<T> RefAsSubclass() {
    return InternallyRefCounted<Transport>::RefAsSubclass<T>();
  }

  virtual FilterStackTransport* filter_stack_transport() = 0;
  virtual ClientTransport* client_transport() = 0;
  virtual ServerTransport* server_transport() = 0;

  // name of this transport implementation
  virtual absl::string_view GetTransportName() const = 0;

  // implementation of grpc_transport_set_pollset
  virtual void SetPollset(grpc_stream* stream, grpc_pollset* pollset) = 0;

  // implementation of grpc_transport_set_pollset
  virtual void SetPollsetSet(grpc_stream* stream,
                             grpc_pollset_set* pollset_set) = 0;

  void SetPollingEntity(grpc_stream* stream,
                        grpc_polling_entity* pollset_or_pollset_set);

  // implementation of grpc_transport_perform_op
  virtual void PerformOp(grpc_transport_op* op) = 0;

  // TODO(roth, ctiller): Migrate all callers to the new StartWatch()
  // API and remove this.
  void StartConnectivityWatch(
      OrphanablePtr<ConnectivityStateWatcherInterface> watcher) {
    grpc_transport_op* op = grpc_make_transport_op(nullptr);
    op->start_connectivity_watch = std::move(watcher);
    PerformOp(op);
  }

  void DisconnectWithError(grpc_error_handle error) {
    CHECK(!error.ok()) << error;
    grpc_transport_op* op = grpc_make_transport_op(nullptr);
    op->disconnect_with_error = error;
    PerformOp(op);
  }

  virtual void StartWatch(RefCountedPtr<StateWatcher> watcher) = 0;
  virtual void StopWatch(RefCountedPtr<StateWatcher> watcher) = 0;

  virtual RefCountedPtr<channelz::SocketNode> GetSocketNode() const = 0;
};

class FilterStackTransport : public Transport {
 public:
  // Memory required for a single stream element - this is allocated by upper
  // layers and initialized by the transport
  virtual size_t SizeOfStream() const = 0;

  // Initialize transport data for a stream.
  // Returns 0 on success, any other (transport-defined) value for failure.
  // May assume that stream contains all-zeros.
  // Arguments:
  //   stream      - a pointer to uninitialized memory to initialize
  //   server_data - either NULL for a client initiated stream, or a pointer
  //                 supplied from the accept_stream callback function
  virtual void InitStream(grpc_stream* stream, grpc_stream_refcount* refcount,
                          const void* server_data, Arena* arena) = 0;

  // HACK: inproc does not handle stream op batch callbacks correctly (receive
  // ops are required to complete prior to on_complete triggering).
  // This flag is used to disable coalescing of batches in connected_channel for
  // that specific transport.
  // TODO(ctiller): This ought not be necessary once we have promises complete.
  virtual bool HackyDisableStreamOpBatchCoalescingInConnectedChannel()
      const = 0;

  virtual void PerformStreamOp(grpc_stream* stream,
                               grpc_transport_stream_op_batch* op) = 0;

  // Destroy transport data for a stream.
  // Requires: a recv_batch with final_state == GRPC_STREAM_CLOSED has been
  // received by the up-layer. Must not be called in the same call stack as
  // recv_frame.
  // Arguments:
  //   stream    - the grpc_stream to destroy (memory is still owned by the
  //               caller, but any child memory must be cleaned up)
  virtual void DestroyStream(grpc_stream* stream,
                             grpc_closure* then_schedule_closure) = 0;

 protected:
  ~FilterStackTransport() override = default;
};

class ClientTransport : public Transport {
 public:
  using Transport::Transport;
  virtual void StartCall(CallHandler call_handler) = 0;

 protected:
  ~ClientTransport() override = default;
};

class ServerTransport : public Transport {
 public:
  using Transport::Transport;
  // Called once slightly after transport setup to register the accept function.
  virtual void SetCallDestination(
      RefCountedPtr<UnstartedCallDestination> unstarted_call_handler) = 0;

 protected:
  ~ServerTransport() override = default;
};

}  // namespace grpc_core

namespace grpc_core {
// This is the key to be used for loading/storing keepalive_throttling in the
// absl::Status object.
constexpr const char* kKeepaliveThrottlingKey =
    "grpc.internal.keepalive_throttling";
}  // namespace grpc_core

#endif  // GRPC_SRC_CORE_LIB_TRANSPORT_TRANSPORT_H
