read0types.h

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/** @file include/read0types.h
 Cursor read
 
 Created 2/16/1997 Heikki Tuuri
 *******************************************************/
 
#ifndef read0types_h
#define read0types_h
 
#include <algorithm>
#include "dict0mem.h"
#include "read0read_view_interface.h"
#include "trx0types.h"
#include "ut0cpu_cache.h"
 
// Friend declaration
class MVCC;
 
/** Read view lists the trx ids of those transactions for which a consistent
read should not see the modifications to the database. */
 
class ReadView : public Read_view_interface {
  /** This is similar to a std::vector but it is not a drop
  in replacement. It is specific to ReadView. */
  class ids_t {
    typedef trx_ids_t::value_type value_type;
 
    /**
    Constructor */
    ids_t() : m_ptr(), m_size(), m_reserved() {}
 
    /**
    Destructor */
    ~ids_t() { ut::delete_arr(m_ptr); }
 
    /** Try and increase the size of the array. Old elements are copied across.
    It is a no-op if n is < current size.
    @param n            Make space for n elements */
    void reserve(ulint n);
 
    /**
    Resize the array, sets the current element count.
    @param n            new size of the array, in elements */
    void resize(ulint n) {
      ut_ad(n <= capacity());
 
      m_size = n;
    }
 
    /**
    Reset the size to 0 */
    void clear() { resize(0); }
 
    /**
    @return the capacity of the array in elements */
    ulint capacity() const { return (m_reserved); }
 
    /**
    Copy and overwrite this array contents
 
    @param start            Source array
    @param end              Pointer to end of array */
    void assign(const value_type *start, const value_type *end);
 
    /**
    Insert the value in the correct slot, preserving the order.
    Doesn't check for duplicates. */
    void insert(value_type value);
 
    /**
    @return the value of the first element in the array */
    value_type front() const {
      ut_ad(!empty());
 
      return (m_ptr[0]);
    }
 
    /**
    @return the value of the last element in the array */
    value_type back() const {
      ut_ad(!empty());
 
      return (m_ptr[m_size - 1]);
    }
 
    /**
    Append a value to the array.
    @param value                the value to append */
    void push_back(value_type value);
 
    /**
    @return a pointer to the start of the array */
    trx_id_t *data() { return (m_ptr); }
 
    /**
    @return a const pointer to the start of the array */
    const trx_id_t *data() const { return (m_ptr); }
 
    /**
    @return the number of elements in the array */
    ulint size() const { return (m_size); }
 
    /**
    @return true if size() == 0 */
    bool empty() const { return (size() == 0); }
 
   private:
    // Prevent copying
    ids_t(const ids_t &);
    ids_t &operator=(const ids_t &);
 
   private:
    /** Memory for the array */
    value_type *m_ptr;
 
    /** Number of active elements in the array */
    ulint m_size;
 
    /** Size of m_ptr in elements */
    ulint m_reserved;
 
    friend class ReadView;
  };
 
 public:
  ReadView();
  ~ReadView() override;
 
  /** Check whether the changes by id are visible.
  @param[in]    id      transaction id to check against the view
  @return whether the view sees the modifications of id. */
  [[nodiscard]] bool changes_visible(trx_id_t id) const override {
    ut_ad(id > 0);
 
    if (id < m_up_limit_id || id == m_creator_trx_id) {
      return true;
    }
    if (id >= m_low_limit_id) {
      return false;
    }
    if (m_ids.empty()) {
      return true;
    }
 
    const ids_t::value_type *p = m_ids.data();
 
    return !std::binary_search(p, p + m_ids.size(), id);
  }
 
  [[nodiscard]] bool sees_all_trxs_with_id_smaller_or_equal_to(
      trx_id_t id) const override {
    return id < m_up_limit_id;
  }
 
  /**
  @return true if the view is closed */
  [[nodiscard]] bool is_closed() const { return m_closed.load(); }
 
  void print(FILE *file) const override {
    fprintf(file,
            "Trx read view will not see trx with"
            " id >= " TRX_ID_FMT ", sees < " TRX_ID_FMT "\n",
            m_low_limit_id, m_up_limit_id);
  }
 
  [[nodiscard]] trx_id_t get_lowest_needed_trx_no() const override {
    return m_low_limit_no;
  }
 
  /**
  @return true if there are no transaction ids in the snapshot */
  [[nodiscard]] bool empty() const { return (m_ids.empty()); }
 
#ifdef UNIV_DEBUG
  /**
  @param rhs            view to compare with
  @return true if this view is less than or equal rhs */
  [[nodiscard]] bool le(const ReadView *rhs) const {
    return (m_low_limit_no <= rhs->m_low_limit_no);
  }
#endif /* UNIV_DEBUG */
 private:
  /**
  Copy the transaction ids from the source vector */
  inline void copy_trx_ids(const trx_ids_t &trx_ids);
 
  /**
  Opens a read view where exactly the transactions serialized before this
  point in time are seen in the view.
  @param id             Creator transaction id */
  inline void prepare(trx_id_t id);
 
  /**
  Copy state from another view. Must call copy_complete() to finish.
  @param other          view to copy from */
  inline void copy_prepare(const ReadView &other);
 
  /**
  Complete the copy, insert the creator transaction id into the
  m_trx_ids too and adjust the m_up_limit_id *, if required */
  inline void copy_complete();
 
  /**
  Set the creator transaction id, existing id must be 0 */
  void creator_trx_id(trx_id_t id) {
    ut_ad(m_creator_trx_id == 0);
    m_creator_trx_id = id;
  }
 
  friend class MVCC;
 
 private:
  // Disable copying
  ReadView(const ReadView &);
  ReadView &operator=(const ReadView &);
 
 private:
  /** The read should not see any transaction with trx id >= this
  value. In other words, this is the "high water mark". */
  trx_id_t m_low_limit_id;
 
  /** The read should see all trx ids which are strictly
  smaller (<) than this value.  In other words, this is the
  low water mark". */
  trx_id_t m_up_limit_id;
 
  /** If the view is open, then this is a trx->id of the transaction which has
  created this view, used to let this view see the changes of this transaction.
  Note that a transaction might have no trx->id assigned in which case this
  will be 0. A transaction may also get trx->id assigned after it has already
  created a read view, in which case it should call set_view_creator_trx_id to
  update this field.
  It is 0 for read views cloned by clone_oldest_view.
  Otherwise its value doesn't matter. */
  trx_id_t m_creator_trx_id;
 
  /** Set of RW transactions that was active when this snapshot
  was taken */
  ids_t m_ids;
 
  /** The view does not need to see the undo logs for transactions
  whose transaction number is strictly smaller (<) than this value:
  they can be removed in purge if not needed by other views */
  trx_id_t m_low_limit_no;
 
  /** False iff this view is in use by a transaction at the moment (is open).*/
  std::atomic_bool m_closed{true};
 
  typedef UT_LIST_NODE_T(ReadView) node_t;
 
  /** List of read views in trx_sys */
  byte pad1[ut::INNODB_CACHE_LINE_SIZE];
  node_t m_view_list{};
};
 
#endif