buf0block_hint.h

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#ifndef buf0block_hint_h
#define buf0block_hint_h
#include "buf0types.h"
 
namespace buf {
class Block_hint {
 public:
  /** Stores the pointer to the block, which is currently buffer-fixed.
  @param[in]  block   a pointer to a buffer-fixed block to be stored */
  void store(buf_block_t *block);
 
  /** Clears currently stored pointer. */
  void clear();
 
  /** Executes given function with the block pointer which was previously stored
  or with nullptr if the pointer is no longer valid, was cleared or not stored.
  @param[in]  f   The function to be executed. It will be passed the pointer.
                  If you wish to use the block pointer subsequently, you need to
                  ensure you buffer-fix it before returning from f.
  @return the return value of f
  */
  template <typename F>
  auto run_with_hint(F &&f) {
    buffer_fix_block_if_still_valid();
    /* m_block could be changed during f() call, so we use local variable to
    remember which block we need to unfix */
    buf_block_t *buffer_fixed_block = m_block;
    auto res = f(buffer_fixed_block);
    buffer_unfix_block_if_needed(buffer_fixed_block);
    return res;
  }
 
 private:
  /** The block pointer stored by store(). */
  buf_block_t *m_block{nullptr};
  /** If m_block is non-null, the m_block->page.id at time it was stored. */
  page_id_t m_page_id{0, 0};
 
  /** A helper function which checks if m_block is not a dangling pointer and
  still points to block with page with m_page_id and if so, buffer-fixes it,
  otherwise clear()s it */
  void buffer_fix_block_if_still_valid();
 
  /** A helper function which decrements block->buf_fix_count if it's non-null
  @param[in]  block   A pointer to a block or nullptr */
  static void buffer_unfix_block_if_needed(buf_block_t *block);
};
 
}  // namespace buf
#endif /* buf0hint_h*/