callstack.h

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  it under the terms of the GNU General Public License, version 2.0,
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*/
 
#ifndef MYSQL_HARNESS_NET_TS_IMPL_CALLSTACK_H_
#define MYSQL_HARNESS_NET_TS_IMPL_CALLSTACK_H_
 
namespace net::impl {
 
/**
 * callstack of a thread.
 *
 * use-cases:
 * - track debuginfo of call chains
 * - check if the function calls itself
 *
 * Each new stackframe links to the previous stackframe and sets itself as
 * stacktop. As callstacks are per-thread, no locking is needed.
 *
 * # Usage
 *
 * @code
 * struct Frame {
 *   const char *filename;
 *   int line;
 *   const char *func;
 *
 *   Frame(const char *f, int l, const char *fun):
 *     filename(f), line(l), func(fun) {}
 * };
 * void a() {
 *   Frame frame(__FILE__, __LINE__, __func__);
 *   Callstack<Frame>::Context stackframe(&frame);
 * }
 *
 * void b() {
 *   // store a frame on the functions stack
 *   Frame frame(__FILE__, __LINE__, __func__);
 *   // link the 'frame' to the callstack
 *   Callstack<Frame>::Context stackframe(&frame);
 *
 *   a();
 * }
 * @endcode
 *
 * @tparam Key   key-type
 * @tparam Value value-type to assign to the key
 */
template <class Key, class Value = unsigned char>
class Callstack {
 public:
  class Context;
  class Iterator;
 
  using value_type = Context *;
  using iterator = Iterator;
  using const_iterator = Iterator;
 
  class Context {
   public:
    /**
     * construct a stackframe.
     *
     * sets top of stack to this frame.
     */
    explicit Context(const Key *k)
        : Context(k, reinterpret_cast<Value &>(*this)) {}
 
    Context(const Key *k, Value &v)
        : key_{k}, value_{&v}, next_{Callstack<Key, Value>::stack_top_} {
      Callstack<Key, Value>::stack_top_ = this;
    }
 
    /**
     * destruct a stackframe.
     *
     * sets top of stack to the previous stackframe.
     */
    ~Context() { Callstack<Key, Value>::stack_top_ = next_; }
 
    // disable copy construct and copy-assign
    Context(const Context &) = delete;
    Context &operator=(const Context &) = delete;
 
    const Key *key() const { return key_; }
    Value *value() const { return value_; }
 
   private:
    friend class Callstack<Key, Value>;
    friend class Callstack<Key, Value>::Iterator;
 
    const Key *key_;
    Value *value_;
 
    Context *next_; /**!< next stackframe */
  };
 
  /**
   * forward-iterator over stack frames.
   *
   * just enough to implement for-range loops
   */
  class Iterator {
   public:
    Iterator(Context *ctx) : ctx_{ctx} {}
 
    Iterator &operator++() {
      if (ctx_ != nullptr) ctx_ = ctx_->next_;
      return *this;
    }
 
    bool operator!=(const Iterator &other) const { return ctx_ != other.ctx_; }
 
    Context *operator*() { return ctx_; }
 
   private:
    Context *ctx_;
  };
 
  /**
   * check if a callstack contains a pointer already.
   *
   * walks the stack from the top the last element and checks if a frames key
   * makes k
   *
   * @param k key to search for in the callstack
   * @returns stored value if key is found
   * @retval nullptr if not found.
   */
  static constexpr Value *contains(const Key *k) {
    for (auto *e : Callstack<Key, Value>()) {
      if (e->key() == k) return e->value();
    }
 
    return nullptr;
  }
 
  /** begin() iterator */
  static Iterator begin() { return Iterator(stack_top_); }
  /** end() iterator */
  static Iterator end() { return Iterator(nullptr); }
 
 private:
  static thread_local Context *stack_top_;
};
 
// define thread-local initial stack-top
template <class Key, class Value>
thread_local
    typename Callstack<Key, Value>::Context *Callstack<Key, Value>::stack_top_ =
        nullptr;
}  // namespace net::impl
 
#endif