/*
 *
 * Copyright 2017 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/gprpp/fork.h"

#include <string.h>

#include <grpc/support/alloc.h>
#include <grpc/support/sync.h>
#include <grpc/support/time.h>

#include "src/core/lib/gpr/useful.h"
#include "src/core/lib/gprpp/global_config.h"
#include "src/core/lib/gprpp/memory.h"

/*
 * NOTE: FORKING IS NOT GENERALLY SUPPORTED, THIS IS ONLY INTENDED TO WORK
 *       AROUND VERY SPECIFIC USE CASES.
 */

#ifdef GRPC_ENABLE_FORK_SUPPORT
#define GRPC_ENABLE_FORK_SUPPORT_DEFAULT true
#else
#define GRPC_ENABLE_FORK_SUPPORT_DEFAULT false
#endif  // GRPC_ENABLE_FORK_SUPPORT

GPR_GLOBAL_CONFIG_DEFINE_BOOL(grpc_enable_fork_support,
                              GRPC_ENABLE_FORK_SUPPORT_DEFAULT,
                              "Enable folk support");

namespace grpc_core {
namespace internal {
// The exec_ctx_count has 2 modes, blocked and unblocked.
// When unblocked, the count is 2-indexed; exec_ctx_count=2 indicates
// 0 active ExecCtxs, exex_ctx_count=3 indicates 1 active ExecCtxs...

// When blocked, the exec_ctx_count is 0-indexed.  Note that ExecCtx
// creation can only be blocked if there is exactly 1 outstanding ExecCtx,
// meaning that BLOCKED and UNBLOCKED counts partition the integers
#define UNBLOCKED(n) (n + 2)
#define BLOCKED(n) (n)

class ExecCtxState {
 public:
  ExecCtxState() : fork_complete_(true) {
    gpr_mu_init(&mu_);
    gpr_cv_init(&cv_);
    gpr_atm_no_barrier_store(&count_, UNBLOCKED(0));
  }

  void IncExecCtxCount() {
    gpr_atm count = gpr_atm_no_barrier_load(&count_);
    while (true) {
      if (count <= BLOCKED(1)) {
        // This only occurs if we are trying to fork.  Wait until the fork()
        // operation completes before allowing new ExecCtxs.
        gpr_mu_lock(&mu_);
        if (gpr_atm_no_barrier_load(&count_) <= BLOCKED(1)) {
          while (!fork_complete_) {
            gpr_cv_wait(&cv_, &mu_, gpr_inf_future(GPR_CLOCK_REALTIME));
          }
        }
        gpr_mu_unlock(&mu_);
      } else if (gpr_atm_no_barrier_cas(&count_, count, count + 1)) {
        break;
      }
      count = gpr_atm_no_barrier_load(&count_);
    }
  }

  void DecExecCtxCount() { gpr_atm_no_barrier_fetch_add(&count_, -1); }

  bool BlockExecCtx() {
    // Assumes there is an active ExecCtx when this function is called
    if (gpr_atm_no_barrier_cas(&count_, UNBLOCKED(1), BLOCKED(1))) {
      gpr_mu_lock(&mu_);
      fork_complete_ = false;
      gpr_mu_unlock(&mu_);
      return true;
    }
    return false;
  }

  void AllowExecCtx() {
    gpr_mu_lock(&mu_);
    gpr_atm_no_barrier_store(&count_, UNBLOCKED(0));
    fork_complete_ = true;
    gpr_cv_broadcast(&cv_);
    gpr_mu_unlock(&mu_);
  }

  ~ExecCtxState() {
    gpr_mu_destroy(&mu_);
    gpr_cv_destroy(&cv_);
  }

 private:
  bool fork_complete_;
  gpr_mu mu_;
  gpr_cv cv_;
  gpr_atm count_;
};

class ThreadState {
 public:
  ThreadState() : awaiting_threads_(false), threads_done_(false), count_(0) {
    gpr_mu_init(&mu_);
    gpr_cv_init(&cv_);
  }

  void IncThreadCount() {
    gpr_mu_lock(&mu_);
    count_++;
    gpr_mu_unlock(&mu_);
  }

  void DecThreadCount() {
    gpr_mu_lock(&mu_);
    count_--;
    if (awaiting_threads_ && count_ == 0) {
      threads_done_ = true;
      gpr_cv_signal(&cv_);
    }
    gpr_mu_unlock(&mu_);
  }
  void AwaitThreads() {
    gpr_mu_lock(&mu_);
    awaiting_threads_ = true;
    threads_done_ = (count_ == 0);
    while (!threads_done_) {
      gpr_cv_wait(&cv_, &mu_, gpr_inf_future(GPR_CLOCK_REALTIME));
    }
    awaiting_threads_ = true;
    gpr_mu_unlock(&mu_);
  }

  ~ThreadState() {
    gpr_mu_destroy(&mu_);
    gpr_cv_destroy(&cv_);
  }

 private:
  bool awaiting_threads_;
  bool threads_done_;
  gpr_mu mu_;
  gpr_cv cv_;
  int count_;
};

}  // namespace

void Fork::GlobalInit() {
  if (!override_enabled_) {
    support_enabled_.Store(GPR_GLOBAL_CONFIG_GET(grpc_enable_fork_support),
                           MemoryOrder::RELAXED);
  }
  if (support_enabled_.Load(MemoryOrder::RELAXED)) {
    exec_ctx_state_ = grpc_core::New<internal::ExecCtxState>();
    thread_state_ = grpc_core::New<internal::ThreadState>();
  }
}

void Fork::GlobalShutdown() {
  if (support_enabled_.Load(MemoryOrder::RELAXED)) {
    grpc_core::Delete(exec_ctx_state_);
    grpc_core::Delete(thread_state_);
  }
}

bool Fork::Enabled() { return support_enabled_.Load(MemoryOrder::RELAXED); }

// Testing Only
void Fork::Enable(bool enable) {
  override_enabled_ = true;
  support_enabled_.Store(enable, MemoryOrder::RELAXED);
}

void Fork::DoIncExecCtxCount() { exec_ctx_state_->IncExecCtxCount(); }

void Fork::DoDecExecCtxCount() { exec_ctx_state_->DecExecCtxCount(); }

void Fork::SetResetChildPollingEngineFunc(
    Fork::child_postfork_func reset_child_polling_engine) {
  reset_child_polling_engine_ = reset_child_polling_engine;
}
Fork::child_postfork_func Fork::GetResetChildPollingEngineFunc() {
  return reset_child_polling_engine_;
}

bool Fork::BlockExecCtx() {
  if (support_enabled_.Load(MemoryOrder::RELAXED)) {
    return exec_ctx_state_->BlockExecCtx();
  }
  return false;
}

void Fork::AllowExecCtx() {
  if (support_enabled_.Load(MemoryOrder::RELAXED)) {
    exec_ctx_state_->AllowExecCtx();
  }
}

void Fork::IncThreadCount() {
  if (support_enabled_.Load(MemoryOrder::RELAXED)) {
    thread_state_->IncThreadCount();
  }
}

void Fork::DecThreadCount() {
  if (support_enabled_.Load(MemoryOrder::RELAXED)) {
    thread_state_->DecThreadCount();
  }
}
void Fork::AwaitThreads() {
  if (support_enabled_.Load(MemoryOrder::RELAXED)) {
    thread_state_->AwaitThreads();
  }
}

internal::ExecCtxState* Fork::exec_ctx_state_ = nullptr;
internal::ThreadState* Fork::thread_state_ = nullptr;
Atomic<bool> Fork::support_enabled_(false);
bool Fork::override_enabled_ = false;
Fork::child_postfork_func Fork::reset_child_polling_engine_ = nullptr;
}  // namespace grpc_core