src/core/lib/iomgr/exec_ctx.h

//
//
// 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_IOMGR_EXEC_CTX_H
#define GRPC_SRC_CORE_LIB_IOMGR_EXEC_CTX_H

#include <grpc/support/port_platform.h>

#include <limits>

#include <grpc/impl/grpc_types.h>
#include <grpc/support/atm.h>
#include <grpc/support/cpu.h>
#include <grpc/support/log.h>
#include <grpc/support/time.h>

#include "src/core/lib/gpr/time_precise.h"
#include "src/core/lib/gprpp/crash.h"
#include "src/core/lib/gprpp/debug_location.h"
#include "src/core/lib/gprpp/fork.h"
#include "src/core/lib/gprpp/time.h"
#include "src/core/lib/iomgr/closure.h"

/// A combiner represents a list of work to be executed later.
/// Forward declared here to avoid a circular dependency with combiner.h.
typedef struct grpc_combiner grpc_combiner;

// This exec_ctx is ready to return: either pre-populated, or cached as soon as
// the finish_check returns true
#define GRPC_EXEC_CTX_FLAG_IS_FINISHED 1
// The exec_ctx's thread is (potentially) owned by a call or channel: care
// should be given to not delete said call/channel from this exec_ctx
#define GRPC_EXEC_CTX_FLAG_THREAD_RESOURCE_LOOP 2
// This exec ctx was initialized by an internal thread, and should not
// be counted by fork handlers
#define GRPC_EXEC_CTX_FLAG_IS_INTERNAL_THREAD 4

// This application callback exec ctx was initialized by an internal thread, and
// should not be counted by fork handlers
#define GRPC_APP_CALLBACK_EXEC_CTX_FLAG_IS_INTERNAL_THREAD 1

namespace grpc_core {
class Combiner;
/// Execution context.
/// A bag of data that collects information along a callstack.
/// It is created on the stack at core entry points (public API or iomgr), and
/// stored internally as a thread-local variable.
///
/// Generally, to create an exec_ctx instance, add the following line at the top
/// of the public API entry point or at the start of a thread's work function :
///
/// ExecCtx exec_ctx;
///
/// Access the created ExecCtx instance using :
/// ExecCtx::Get()
///
/// Specific responsibilities (this may grow in the future):
/// - track a list of core work that needs to be delayed until the base of the
///   call stack (this provides a convenient mechanism to run callbacks
///   without worrying about locking issues)
/// - provide a decision maker (via IsReadyToFinish) that provides a
///   signal as to whether a borrowed thread should continue to do work or
///   should actively try to finish up and get this thread back to its owner
///
/// CONVENTIONS:
/// - Instance of this must ALWAYS be constructed on the stack, never
///   heap allocated.
/// - Do not pass exec_ctx as a parameter to a function. Always access it using
///   ExecCtx::Get().
/// - NOTE: In the future, the convention is likely to change to allow only one
///         ExecCtx on a thread's stack at the same time. The TODO below
///         discusses this plan in more detail.
///
/// TODO(yashykt): Only allow one "active" ExecCtx on a thread at the same time.
///               Stage 1: If a new one is created on the stack, it should just
///               pass-through to the underlying ExecCtx deeper in the thread's
///               stack.
///               Stage 2: Assert if a 2nd one is ever created on the stack
///               since that implies a core re-entry outside of application
///               callbacks.
///
class ExecCtx {
 public:
  /// Default Constructor

  ExecCtx() : flags_(GRPC_EXEC_CTX_FLAG_IS_FINISHED) {
    Fork::IncExecCtxCount();
    Set(this);
  }

  /// Parameterised Constructor
  explicit ExecCtx(uintptr_t fl) : flags_(fl) {
    if (!(GRPC_EXEC_CTX_FLAG_IS_INTERNAL_THREAD & flags_)) {
      Fork::IncExecCtxCount();
    }
    Set(this);
  }

  /// Destructor
  virtual ~ExecCtx() {
    flags_ |= GRPC_EXEC_CTX_FLAG_IS_FINISHED;
    Flush();
    Set(last_exec_ctx_);
    if (!(GRPC_EXEC_CTX_FLAG_IS_INTERNAL_THREAD & flags_)) {
      Fork::DecExecCtxCount();
    }
  }

  /// Disallow copy and assignment operators
  ExecCtx(const ExecCtx&) = delete;
  ExecCtx& operator=(const ExecCtx&) = delete;

  unsigned starting_cpu() {
    if (starting_cpu_ == std::numeric_limits<unsigned>::max()) {
      starting_cpu_ = gpr_cpu_current_cpu();
    }
    return starting_cpu_;
  }

  struct CombinerData {
    // currently active combiner: updated only via combiner.c
    Combiner* active_combiner;
    // last active combiner in the active combiner list
    Combiner* last_combiner;
  };

  /// Only to be used by grpc-combiner code
  CombinerData* combiner_data() { return &combiner_data_; }

  /// Return pointer to grpc_closure_list
  grpc_closure_list* closure_list() { return &closure_list_; }

  /// Return flags
  uintptr_t flags() { return flags_; }

  /// Checks if there is work to be done
  bool HasWork() {
    return combiner_data_.active_combiner != nullptr ||
           !grpc_closure_list_empty(closure_list_);
  }

  /// Flush any work that has been enqueued onto this grpc_exec_ctx.
  /// Caller must guarantee that no interfering locks are held.
  /// Returns true if work was performed, false otherwise.
  ///
  bool Flush();

  /// Returns true if we'd like to leave this execution context as soon as
  /// possible: useful for deciding whether to do something more or not
  /// depending on outside context.
  ///
  bool IsReadyToFinish() {
    if ((flags_ & GRPC_EXEC_CTX_FLAG_IS_FINISHED) == 0) {
      if (CheckReadyToFinish()) {
        flags_ |= GRPC_EXEC_CTX_FLAG_IS_FINISHED;
        return true;
      }
      return false;
    } else {
      return true;
    }
  }

  Timestamp Now() { return Timestamp::Now(); }
  void InvalidateNow() { time_cache_.InvalidateCache(); }
  void SetNowIomgrShutdown() {
    // We get to do a test only set now on this path just because iomgr
    // is getting removed and no point adding more interfaces for it.
    time_cache_.TestOnlySetNow(Timestamp::InfFuture());
  }
  void TestOnlySetNow(Timestamp now) { time_cache_.TestOnlySetNow(now); }

  /// Gets pointer to current exec_ctx.
  static ExecCtx* Get() { return exec_ctx_; }

  static void Run(const DebugLocation& location, grpc_closure* closure,
                  grpc_error_handle error);

  static void RunList(const DebugLocation& location, grpc_closure_list* list);

 protected:
  /// Check if ready to finish.
  virtual bool CheckReadyToFinish() { return false; }

  /// Disallow delete on ExecCtx.
  static void operator delete(void* /* p */) { abort(); }

 private:
  /// Set exec_ctx_ to exec_ctx.
  static void Set(ExecCtx* exec_ctx) { exec_ctx_ = exec_ctx; }

  grpc_closure_list closure_list_ = GRPC_CLOSURE_LIST_INIT;
  CombinerData combiner_data_ = {nullptr, nullptr};
  uintptr_t flags_;

  unsigned starting_cpu_ = std::numeric_limits<unsigned>::max();

  ScopedTimeCache time_cache_;
  static thread_local ExecCtx* exec_ctx_;
  ExecCtx* last_exec_ctx_ = Get();
};

/// Application-callback execution context.
/// A bag of data that collects information along a callstack.
/// It is created on the stack at core entry points, and stored internally
/// as a thread-local variable.
///
/// There are three key differences between this structure and ExecCtx:
///   1. ApplicationCallbackExecCtx builds a list of application-level
///      callbacks, but ExecCtx builds a list of internal callbacks to invoke.
///   2. ApplicationCallbackExecCtx invokes its callbacks only at destruction;
///      there is no explicit Flush method.
///   3. If more than one ApplicationCallbackExecCtx is created on the thread's
///      stack, only the one closest to the base of the stack is actually
///      active and this is the only one that enqueues application callbacks.
///      (Unlike ExecCtx, it is not feasible to prevent multiple of these on the
///      stack since the executing application callback may itself enter core.
///      However, the new one created will just pass callbacks through to the
///      base one and those will not be executed until the return to the
///      destructor of the base one, preventing unlimited stack growth.)
///
/// This structure exists because application callbacks may themselves cause a
/// core re-entry (e.g., through a public API call) and if that call in turn
/// causes another application-callback, there could be arbitrarily growing
/// stacks of core re-entries. Instead, any application callbacks instead should
/// not be invoked until other core work is done and other application callbacks
/// have completed. To accomplish this, any application callback should be
/// enqueued using ApplicationCallbackExecCtx::Enqueue .
///
/// CONVENTIONS:
/// - Instances of this must ALWAYS be constructed on the stack, never
///   heap allocated.
/// - Instances of this are generally constructed before ExecCtx when needed.
///   The only exception is for ExecCtx's that are explicitly flushed and
///   that survive beyond the scope of the function that can cause application
///   callbacks to be invoked (e.g., in the timer thread).
///
/// Generally, core entry points that may trigger application-level callbacks
/// will have the following declarations:
///
/// ApplicationCallbackExecCtx callback_exec_ctx;
/// ExecCtx exec_ctx;
///
/// This ordering is important to make sure that the ApplicationCallbackExecCtx
/// is destroyed after the ExecCtx (to prevent the re-entry problem described
/// above, as well as making sure that ExecCtx core callbacks are invoked first)
///
///

class ApplicationCallbackExecCtx {
 public:
  /// Default Constructor
  ApplicationCallbackExecCtx() { Set(this, flags_); }

  /// Parameterised Constructor
  explicit ApplicationCallbackExecCtx(uintptr_t fl) : flags_(fl) {
    Set(this, flags_);
  }

  ~ApplicationCallbackExecCtx() {
    if (Get() == this) {
      while (head_ != nullptr) {
        auto* f = head_;
        head_ = f->internal_next;
        if (f->internal_next == nullptr) {
          tail_ = nullptr;
        }
        (*f->functor_run)(f, f->internal_success);
      }
      callback_exec_ctx_ = nullptr;
      if (!(GRPC_APP_CALLBACK_EXEC_CTX_FLAG_IS_INTERNAL_THREAD & flags_)) {
        Fork::DecExecCtxCount();
      }
    } else {
      GPR_DEBUG_ASSERT(head_ == nullptr);
      GPR_DEBUG_ASSERT(tail_ == nullptr);
    }
  }

  uintptr_t Flags() { return flags_; }

  static ApplicationCallbackExecCtx* Get() { return callback_exec_ctx_; }

  static void Set(ApplicationCallbackExecCtx* exec_ctx, uintptr_t flags) {
    if (Get() == nullptr) {
      if (!(GRPC_APP_CALLBACK_EXEC_CTX_FLAG_IS_INTERNAL_THREAD & flags)) {
        Fork::IncExecCtxCount();
      }
      callback_exec_ctx_ = exec_ctx;
    }
  }

  static void Enqueue(grpc_completion_queue_functor* functor, int is_success) {
    functor->internal_success = is_success;
    functor->internal_next = nullptr;

    ApplicationCallbackExecCtx* ctx = Get();

    if (ctx->head_ == nullptr) {
      ctx->head_ = functor;
    }
    if (ctx->tail_ != nullptr) {
      ctx->tail_->internal_next = functor;
    }
    ctx->tail_ = functor;
  }

  static bool Available() { return Get() != nullptr; }

 private:
  uintptr_t flags_{0u};
  grpc_completion_queue_functor* head_{nullptr};
  grpc_completion_queue_functor* tail_{nullptr};
  static thread_local ApplicationCallbackExecCtx* callback_exec_ctx_;
};

template <typename F>
void EnsureRunInExecCtx(F f) {
  if (ExecCtx::Get() == nullptr) {
    ApplicationCallbackExecCtx app_ctx;
    ExecCtx exec_ctx;
    f();
  } else {
    f();
  }
}

}  // namespace grpc_core

#endif  // GRPC_SRC_CORE_LIB_IOMGR_EXEC_CTX_H