// Copyright (C) 2019-2021 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

#ifndef MULTI_THREADING_MGR_H
#define MULTI_THREADING_MGR_H

#include <util/thread_pool.h>
#include <functional>
#include <list>

#include <boost/noncopyable.hpp>

#include <stdint.h>

namespace isc {
namespace util {


/// @brief Embodies a named set of CriticalSection callbacks.
///
/// This class associates with a name, a set of callbacks, one to be invoked
/// before CriticalSection entry and exit callbacks to check current thread
/// permissions to perform such actions, one to be invoked upon CriticalSection
/// entry and one to be invoked upon CriticalSection exit.
/// The name allows the set to be uniquely identified such that they can be
/// added and removed as needed.
/// The check current thread permissions callback needs to throw
/// @ref MultiThreadingInvalidOperation if the thread is not allowed to perform
/// CriticalSection entry and exit. Any other exception thrown by the check
/// permission callbacks will be silently ignored.
/// The CriticalSection entry and exit callbacks exceptions will be silently
/// ignored.
struct CSCallbackSet {
    /// @brief Defines a callback as a simple void() functor.
    typedef std::function<void()> Callback;

    /// @brief Constructor
    ///
    /// @param name Name by which the callbacks can be found.
    /// @param check_cb Callback to check current thread permissions to call
    /// the CriticalSection entry and exit callbacks.
    /// @param entry_cb Callback to invoke upon CriticalSection entry.
    /// @param exit_cb Callback to invoke upon CriticalSection exit.
    CSCallbackSet(const std::string& name, const Callback& check_cb,
                  const Callback& entry_cb, const Callback& exit_cb)
        : name_(name), check_cb_(check_cb), entry_cb_(entry_cb),
          exit_cb_(exit_cb) {}

    /// @brief Name by which the callback can be found.
    std::string name_;

    /// @brief Check permissions callback associated with name.
    Callback check_cb_;

    /// @brief Entry point callback associated with name.
    Callback entry_cb_;

    /// @brief Exit point callback associated with name.
    Callback exit_cb_;
};

/// @brief Maintains list of unique CSCallbackSets.
///
/// The list emphasizes iteration order and speed over
/// retrieval by name. When iterating over the list of
/// callback sets, they are returned in the order they were
/// added, not by name.
class CSCallbackSetList {
public:
    /// @brief Constructor.
    CSCallbackSetList() {}

    /// @brief Adds a callback set to the list.
    ///
    /// @param name Name of the callback to add.
    /// @param check_cb The check permissions callback to add.
    /// @param entry_cb The CriticalSection entry callback to add.
    /// @param exit_cb The CriticalSection exit callback to add.
    ///
    /// @throw BadValue if the name is already in the list,
    /// the name is blank, or either callback is empty.
    void addCallbackSet(const std::string& name,
                        const CSCallbackSet::Callback& check_cb,
                        const CSCallbackSet::Callback& entry_cb,
                        const CSCallbackSet::Callback& exit_cb);

    /// @brief Removes a callback set from the list.
    ///
    /// @param name Name of the callback to remove.
    /// If no such callback exists, it simply returns.
    void removeCallbackSet(const std::string& name);

    /// @brief Removes all callbacks from the list.
    void removeAll();

    /// @brief Fetches the list of callback sets.
    const std::list<CSCallbackSet>& getCallbackSets();

private:
    /// @brief The list of callback sets.
    std::list<CSCallbackSet> cb_sets_;
};

/// @brief Multi Threading Manager.
///
/// This singleton class holds the multi-threading mode.
///
/// The standard way to use it is:
/// @code
/// if (MultiThreadingMgr::instance().getMode()) {
///     multi-threaded code
/// } else {
///     single-threaded code
/// }
/// @endcode
///
/// For instance for a class protected by its mutex:
/// @code
/// namespace locked {
///     void foo() { ... }
/// } // end of locked namespace
///
/// void foo() {
///     if (MultiThreadingMgr::instance().getMode()) {
///         lock_guard<mutex> lock(mutex_);
///         locked::foo();
///     } else {
///         locked::foo();
///     }
/// }
/// @endcode
class MultiThreadingMgr : public boost::noncopyable {
public:
    /// @brief Returns a single instance of Multi Threading Manager.
    ///
    /// MultiThreadingMgr is a singleton and this method is the only way
    /// of accessing it.
    ///
    /// @return the single instance.
    static MultiThreadingMgr& instance();

    /// @brief Get the multi-threading mode.
    ///
    /// @return The current multi-threading mode: true if multi-threading is
    /// enabled, false otherwise.
    bool getMode() const;

    /// @brief Set the multi-threading mode.
    ///
    /// @param enabled The new mode.
    void setMode(bool enabled);

    /// @brief Enter critical section.
    ///
    /// When entering @ref MultiThreadingCriticalSection, increment internal
    /// counter so that any configuration change that might start the packet
    /// thread pool is delayed until exiting the respective section.
    /// If the internal counter is 0, then stop the thread pool.
    ///
    /// Invokes all CriticalSection entry callbacks. Has no effect in
    /// single-threaded mode.
    ///
    /// @note This function swallows exceptions thrown by all entry callbacks
    /// without logging to avoid breaking the CS chain.
    void enterCriticalSection();

    /// @brief Exit critical section.
    ///
    /// When exiting @ref MultiThreadingCriticalSection, decrement internal
    /// counter so that the dhcp thread pool can be started according to the
    /// new configuration.
    /// If the internal counter is 0, then start the thread pool.
    ///
    /// Invokes all CriticalSection exit callbacks. Has no effect in
    /// single-threaded mode.
    ///
    /// @note This function swallows exceptions thrown by all exit callbacks
    /// without logging to avoid breaking the CS chain.
    void exitCriticalSection();

    /// @brief Is in critical section flag.
    ///
    /// @return The critical section flag.
    bool isInCriticalSection();

    /// @brief Get the dhcp thread pool.
    ///
    /// @return The dhcp thread pool.
    ThreadPool<std::function<void()>>& getThreadPool();

    /// @brief Get the configured dhcp thread pool size.
    ///
    /// @return The dhcp thread pool size.
    uint32_t getThreadPoolSize() const;

    /// @brief Set the configured dhcp thread pool size.
    ///
    /// @param size The dhcp thread pool size.
    void setThreadPoolSize(uint32_t size);

    /// @brief Get the configured dhcp packet queue size.
    ///
    /// @return The dhcp packet queue size.
    uint32_t getPacketQueueSize();

    /// @brief Set the configured dhcp packet queue size.
    ///
    /// @param size The dhcp packet queue size.
    void setPacketQueueSize(uint32_t size);

    /// @brief The system current detected hardware concurrency thread count.
    ///
    /// This function will return 0 if the value can not be determined.
    ///
    /// @return The thread count.
    static uint32_t detectThreadCount();

    /// @brief Apply the multi-threading related settings.
    ///
    /// This function should usually be called after constructing a
    /// @ref MultiThreadingCriticalSection so that all thread pool parameters
    /// can be safely applied.
    ///
    /// @param enabled The enabled flag: true if multi-threading is enabled,
    /// false otherwise.
    /// @param thread_count The desired number of threads: non 0 if explicitly
    /// configured, 0 if auto scaling is desired
    /// @param queue_size The desired thread queue size: non 0 if explicitly
    /// configured, 0 for unlimited size
    void apply(bool enabled, uint32_t thread_count, uint32_t queue_size);

    /// @brief Adds a set of callbacks to the list of CriticalSection callbacks.
    ///
    /// @note Callbacks must be exception-safe, handling any errors themselves.
    ///
    /// @param name Name of the set of callbacks. This value is used by the
    /// callback owner to add and remove them. Duplicates are not allowed.
    /// @param check_cb Callback to check current thread permissions to call
    /// the CriticalSection entry and exit callbacks.
    /// @param entry_cb Callback to invoke upon CriticalSection entry. Cannot be
    /// empty.
    /// @param exit_cb Callback to invoke upon CriticalSection exit. Cannot be
    /// empty.
    void addCriticalSectionCallbacks(const std::string& name,
                                     const CSCallbackSet::Callback& check_cb,
                                     const CSCallbackSet::Callback& entry_cb,
                                     const CSCallbackSet::Callback& exit_cb);

    /// @brief Removes the set of callbacks associated with a given name
    /// from the list of CriticalSection callbacks.
    ///
    /// If the name is not found in the list, it simply returns, otherwise
    /// both callbacks registered under the name are removed.
    ///
    /// @param name Name of the set of callbacks to remove.
    void removeCriticalSectionCallbacks(const std::string& name);

    /// @brief Removes all callbacks in the list of CriticalSection callbacks.
    void removeAllCriticalSectionCallbacks();

protected:

    /// @brief Constructor.
    MultiThreadingMgr();

    /// @brief Destructor.
    virtual ~MultiThreadingMgr();

private:

    /// @brief Class method tests if current thread is allowed to enter the
    /// @ref MultiThreadingCriticalSection and to invoke the entry and exit
    /// callbacks.
    ///
    /// Has no effect in single-threaded mode.
    ///
    /// @note This function swallows exceptions thrown by all check permission
    /// callbacks without logging to avoid breaking the CS chain, except for the
    /// @ref MultiThreadingInvalidOperation which needs to be propagated to the
    /// scope of the @ref MultiThreadingCriticalSection constructor.
    /// @throw MultiThreadingInvalidOperation if current thread has no
    /// permission to enter CriticalSection.
    void checkCallbacksPermissions();

    /// @brief Class method which invokes CriticalSection entry callbacks.
    ///
    /// Has no effect in single-threaded mode.
    ///
    /// @note This function swallows exceptions thrown by all entry callbacks
    /// without logging to avoid breaking the CS chain.
    void callEntryCallbacks();

    /// @brief Class method which invokes CriticalSection exit callbacks.
    ///
    /// Has no effect in single-threaded mode.
    ///
    /// @note This function swallows exceptions thrown by all exit callbacks
    /// without logging to avoid breaking the CS chain.
    void callExitCallbacks();

    /// @brief The current multi-threading mode.
    ///
    /// The multi-threading flag: true if multi-threading is enabled, false
    /// otherwise.
    bool enabled_;

    /// @brief The critical section count.
    ///
    /// In case the configuration is applied within a
    /// @ref MultiThreadingCriticalSection, the thread pool should not be
    /// started until leaving the respective section.
    /// This handles multiple interleaved sections.
    uint32_t critical_section_count_;

    /// @brief The configured size of the dhcp thread pool.
    uint32_t thread_pool_size_;

    /// @brief Packet processing thread pool.
    ThreadPool<std::function<void()>> thread_pool_;

    /// @brief List of CriticalSection entry and exit callback sets.
    CSCallbackSetList cs_callbacks_;
};

/// @note: everything here MUST be used ONLY from the main thread.
/// When called from a thread of the pool it can deadlock.

/// @brief RAII class creating a critical section.
///
/// @note: the multi-threading mode MUST NOT be changed in the RAII
/// @c MultiThreadingCriticalSection body.
/// @note: starting and stopping the dhcp thread pool should be handled
/// in the main thread, if done on one of the processing threads will cause a
/// deadlock.
/// This is mainly useful in hook commands which handle configuration
/// changes.
class MultiThreadingCriticalSection : public boost::noncopyable {
public:

    /// @brief Constructor.
    ///
    /// Entering the critical section. The dhcp thread pool instance will be
    /// stopped so that all configuration changes can be safely applied.
    MultiThreadingCriticalSection();

    /// @brief Destructor.
    ///
    /// Leaving the critical section. The dhcp thread pool instance will be
    /// started according to the new configuration.
    virtual ~MultiThreadingCriticalSection();
};

}  // namespace util
}  // namespace isc

#endif // MULTI_THREADING_MGR_H
