xa.cc

/*
   Copyright (c) 2000, 2016, Oracle and/or its affiliates.
   Copyright (c) 2009, 2019, MariaDB Corporation.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; version 2 of the License.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
*/

#include "mariadb.h"
#include "sql_class.h"
#include "transaction.h"


/***************************************************************************
  Handling of XA id cacheing
***************************************************************************/
class XID_cache_element
{
  /*
    m_state is used to prevent elements from being deleted while XA RECOVER
    iterates xid cache and to prevent recovered elments from being acquired by
    multiple threads.

    bits 1..29 are reference counter
    bit 30 is RECOVERED flag
    bit 31 is ACQUIRED flag (thread owns this xid)
    bit 32 is unused

    Newly allocated and deleted elements have m_state set to 0.

    On lock() m_state is atomically incremented. It also creates load-ACQUIRE
    memory barrier to make sure m_state is actually updated before furhter
    memory accesses. Attempting to lock an element that has neither ACQUIRED
    nor RECOVERED flag set returns failure and further accesses to element
    memory are forbidden.

    On unlock() m_state is decremented. It also creates store-RELEASE memory
    barrier to make sure m_state is actually updated after preceding memory
    accesses.

    ACQUIRED flag is set when thread registers it's xid or when thread acquires
    recovered xid.

    RECOVERED flag is set for elements found during crash recovery.

    ACQUIRED and RECOVERED flags are cleared before element is deleted from
    hash in a spin loop, after last reference is released.
  */
  std::atomic<int32_t> m_state;
public:
  static const int32 ACQUIRED= 1 << 30;
  static const int32 RECOVERED= 1 << 29;
  XID_STATE *m_xid_state;
  /* Error reported by the Resource Manager (RM) to the Transaction Manager. */
  uint rm_error;
  bool is_set(int32_t flag)
  { return m_state.load(std::memory_order_relaxed) & flag; }
  void set(int32_t flag)
  {
    DBUG_ASSERT(!is_set(ACQUIRED | RECOVERED));
    m_state.fetch_add(flag, std::memory_order_relaxed);
  }
  bool lock()
  {
    int32_t old= m_state.fetch_add(1, std::memory_order_acquire);
    if (old & (ACQUIRED | RECOVERED))
      return true;
    unlock();
    return false;
  }
  void unlock()
  { m_state.fetch_sub(1, std::memory_order_release); }
  void mark_uninitialized()
  {
    int32_t old= ACQUIRED;
    while (!m_state.compare_exchange_weak(old, 0,
                                          std::memory_order_relaxed,
                                          std::memory_order_relaxed))
    {
      old&= ACQUIRED | RECOVERED;
      (void) LF_BACKOFF();
    }
  }
  bool acquire_recovered()
  {
    int32_t old= RECOVERED;
    while (!m_state.compare_exchange_weak(old, ACQUIRED | RECOVERED,
                                          std::memory_order_relaxed,
                                          std::memory_order_relaxed))
    {
      if (!(old & RECOVERED) || (old & ACQUIRED))
        return false;
      old= RECOVERED;
      (void) LF_BACKOFF();
    }
    return true;
  }
  static void lf_hash_initializer(LF_HASH *hash __attribute__((unused)),
                                  XID_cache_element *element,
                                  XID_STATE *xid_state)
  {
    DBUG_ASSERT(!element->is_set(ACQUIRED | RECOVERED));
    element->rm_error= 0;
    element->m_xid_state= xid_state;
    xid_state->xid_cache_element= element;
  }
  static void lf_alloc_constructor(uchar *ptr)
  {
    XID_cache_element *element= (XID_cache_element*) (ptr + LF_HASH_OVERHEAD);
    element->m_state= 0;
  }
  static void lf_alloc_destructor(uchar *ptr)
  {
    XID_cache_element *element= (XID_cache_element*) (ptr + LF_HASH_OVERHEAD);
    DBUG_ASSERT(!element->is_set(ACQUIRED));
    if (element->is_set(RECOVERED))
      my_free(element->m_xid_state);
  }
  static uchar *key(const XID_cache_element *element, size_t *length,
                    my_bool not_used __attribute__((unused)))
  {
    *length= element->m_xid_state->xid.key_length();
    return element->m_xid_state->xid.key();
  }
};


static LF_HASH xid_cache;
static bool xid_cache_inited;
const char *xa_state_names[]= {
  "NON-EXISTING", "ACTIVE", "IDLE", "PREPARED", "ROLLBACK ONLY"
};


bool THD::fix_xid_hash_pins()
{
  if (!xid_hash_pins)
    xid_hash_pins= lf_hash_get_pins(&xid_cache);
  return !xid_hash_pins;
}


void XID_STATE::set_error(uint error)
{
  if (is_explicit_XA())
    xid_cache_element->rm_error= error;
}


void xid_cache_init()
{
  xid_cache_inited= true;
  lf_hash_init(&xid_cache, sizeof(XID_cache_element), LF_HASH_UNIQUE, 0, 0,
               (my_hash_get_key) XID_cache_element::key, &my_charset_bin);
  xid_cache.alloc.constructor= XID_cache_element::lf_alloc_constructor;
  xid_cache.alloc.destructor= XID_cache_element::lf_alloc_destructor;
  xid_cache.initializer=
    (lf_hash_initializer) XID_cache_element::lf_hash_initializer;
}


void xid_cache_free()
{
  if (xid_cache_inited)
  {
    lf_hash_destroy(&xid_cache);
    xid_cache_inited= false;
  }
}


/**
  Find recovered XA transaction by XID.
*/

static XID_STATE *xid_cache_search(THD *thd, XID *xid)
{
  XID_STATE *xs= 0;
  DBUG_ASSERT(thd->xid_hash_pins);
  XID_cache_element *element=
    (XID_cache_element*) lf_hash_search(&xid_cache, thd->xid_hash_pins,
                                        xid->key(), xid->key_length());
  if (element)
  {
    if (element->acquire_recovered())
      xs= element->m_xid_state;
    lf_hash_search_unpin(thd->xid_hash_pins);
    DEBUG_SYNC(thd, "xa_after_search");
  }
  return xs;
}


bool xid_cache_insert(XID *xid, enum xa_states xa_state)
{
  XID_STATE *xs;
  LF_PINS *pins;
  int res= 1;

  if (!(pins= lf_hash_get_pins(&xid_cache)))
    return true;

  if ((xs= (XID_STATE*) my_malloc(sizeof(*xs), MYF(MY_WME))))
  {
    xs->xa_state=xa_state;
    xs->xid.set(xid);

    if ((res= lf_hash_insert(&xid_cache, pins, xs)))
      my_free(xs);
    else
      xs->xid_cache_element->set(XID_cache_element::RECOVERED);
    if (res == 1)
      res= 0;
  }
  lf_hash_put_pins(pins);
  return res;
}


bool xid_cache_insert(THD *thd, XID_STATE *xid_state)
{
  if (thd->fix_xid_hash_pins())
    return true;

  int res= lf_hash_insert(&xid_cache, thd->xid_hash_pins, xid_state);
  switch (res)
  {
  case 0:
    xid_state->xid_cache_element->set(XID_cache_element::ACQUIRED);
    break;
  case 1:
    my_error(ER_XAER_DUPID, MYF(0));
    /* fall through */
  default:
    xid_state->xid_cache_element= 0;
  }
  return res;
}


void xid_cache_delete(THD *thd, XID_STATE *xid_state)
{
  if (xid_state->xid_cache_element)
  {
    bool recovered= xid_state->xid_cache_element->is_set(XID_cache_element::RECOVERED);
    DBUG_ASSERT(thd->xid_hash_pins);
    xid_state->xid_cache_element->mark_uninitialized();
    lf_hash_delete(&xid_cache, thd->xid_hash_pins,
                   xid_state->xid.key(), xid_state->xid.key_length());
    if (recovered)
      my_free(xid_state);
    else
    {
      xid_state->xid_cache_element= 0;
      xid_state->xid.null();
    }
  }
}


struct xid_cache_iterate_arg
{
  my_hash_walk_action action;
  void *argument;
};

static my_bool xid_cache_iterate_callback(XID_cache_element *element,
                                          xid_cache_iterate_arg *arg)
{
  my_bool res= FALSE;
  if (element->lock())
  {
    res= arg->action(element->m_xid_state, arg->argument);
    element->unlock();
  }
  return res;
}

static int xid_cache_iterate(THD *thd, my_hash_walk_action action, void *arg)
{
  xid_cache_iterate_arg argument= { action, arg };
  return thd->fix_xid_hash_pins() ? -1 :
         lf_hash_iterate(&xid_cache, thd->xid_hash_pins,
                         (my_hash_walk_action) xid_cache_iterate_callback,
                         &argument);
}


/**
  Mark a XA transaction as rollback-only if the RM unilaterally
  rolled back the transaction branch.

  @note If a rollback was requested by the RM, this function sets
        the appropriate rollback error code and transits the state
        to XA_ROLLBACK_ONLY.

  @return TRUE if transaction was rolled back or if the transaction
          state is XA_ROLLBACK_ONLY. FALSE otherwise.
*/
static bool xa_trans_rolled_back(XID_STATE *xid_state)
{
  DBUG_ASSERT(xid_state->is_explicit_XA());
  if (xid_state->xid_cache_element->rm_error)
  {
    switch (xid_state->xid_cache_element->rm_error) {
    case ER_LOCK_WAIT_TIMEOUT:
      my_error(ER_XA_RBTIMEOUT, MYF(0));
      break;
    case ER_LOCK_DEADLOCK:
      my_error(ER_XA_RBDEADLOCK, MYF(0));
      break;
    default:
      my_error(ER_XA_RBROLLBACK, MYF(0));
    }
    xid_state->xa_state= XA_ROLLBACK_ONLY;
  }

  return (xid_state->xa_state == XA_ROLLBACK_ONLY);
}


/**
  Rollback the active XA transaction.

  @return TRUE if the rollback failed, FALSE otherwise.
*/

static bool xa_trans_force_rollback(THD *thd)
{
  if (ha_rollback_trans(thd, true))
  {
    my_error(ER_XAER_RMERR, MYF(0));
    return true;
  }
  return false;
}


/**
  Starts an XA transaction with the given xid value.

  @param thd    Current thread

  @retval FALSE  Success
  @retval TRUE   Failure
*/

bool trans_xa_start(THD *thd)
{
  enum xa_states xa_state= thd->transaction.xid_state.xa_state;
  DBUG_ENTER("trans_xa_start");

  if (xa_state == XA_IDLE && thd->lex->xa_opt == XA_RESUME)
  {
    bool not_equal= !thd->transaction.xid_state.xid.eq(thd->lex->xid);
    if (not_equal)
      my_error(ER_XAER_NOTA, MYF(0));
    else
      thd->transaction.xid_state.xa_state= XA_ACTIVE;
    DBUG_RETURN(not_equal);
  }

  /* TODO: JOIN is not supported yet. */
  if (thd->lex->xa_opt != XA_NONE)
    my_error(ER_XAER_INVAL, MYF(0));
  else if (xa_state != XA_NOTR)
    my_error(ER_XAER_RMFAIL, MYF(0), xa_state_names[xa_state]);
  else if (thd->locked_tables_mode || thd->in_active_multi_stmt_transaction())
    my_error(ER_XAER_OUTSIDE, MYF(0));
  else if (!trans_begin(thd))
  {
    DBUG_ASSERT(thd->transaction.xid_state.xid.is_null());
    thd->transaction.xid_state.xa_state= XA_ACTIVE;
    thd->transaction.xid_state.xid.set(thd->lex->xid);
    if (xid_cache_insert(thd, &thd->transaction.xid_state))
    {
      thd->transaction.xid_state.xa_state= XA_NOTR;
      thd->transaction.xid_state.xid.null();
      trans_rollback(thd);
      DBUG_RETURN(true);
    }
    DBUG_RETURN(FALSE);
  }

  DBUG_RETURN(TRUE);
}


/**
  Put a XA transaction in the IDLE state.

  @param thd    Current thread

  @retval FALSE  Success
  @retval TRUE   Failure
*/

bool trans_xa_end(THD *thd)
{
  DBUG_ENTER("trans_xa_end");

  /* TODO: SUSPEND and FOR MIGRATE are not supported yet. */
  if (thd->lex->xa_opt != XA_NONE)
    my_error(ER_XAER_INVAL, MYF(0));
  else if (thd->transaction.xid_state.xa_state != XA_ACTIVE)
    my_error(ER_XAER_RMFAIL, MYF(0),
             xa_state_names[thd->transaction.xid_state.xa_state]);
  else if (!thd->transaction.xid_state.xid.eq(thd->lex->xid))
    my_error(ER_XAER_NOTA, MYF(0));
  else if (!xa_trans_rolled_back(&thd->transaction.xid_state))
    thd->transaction.xid_state.xa_state= XA_IDLE;

  DBUG_RETURN(thd->is_error() ||
              thd->transaction.xid_state.xa_state != XA_IDLE);
}


/**
  Put a XA transaction in the PREPARED state.

  @param thd    Current thread

  @retval FALSE  Success
  @retval TRUE   Failure
*/

bool trans_xa_prepare(THD *thd)
{
  DBUG_ENTER("trans_xa_prepare");

  if (thd->transaction.xid_state.xa_state != XA_IDLE)
    my_error(ER_XAER_RMFAIL, MYF(0),
             xa_state_names[thd->transaction.xid_state.xa_state]);
  else if (!thd->transaction.xid_state.xid.eq(thd->lex->xid))
    my_error(ER_XAER_NOTA, MYF(0));
  else if (ha_prepare(thd))
  {
    xid_cache_delete(thd, &thd->transaction.xid_state);
    thd->transaction.xid_state.xa_state= XA_NOTR;
    my_error(ER_XA_RBROLLBACK, MYF(0));
  }
  else
    thd->transaction.xid_state.xa_state= XA_PREPARED;

  DBUG_RETURN(thd->is_error() ||
              thd->transaction.xid_state.xa_state != XA_PREPARED);
}


/**
  Commit and terminate the a XA transaction.

  @param thd    Current thread

  @retval FALSE  Success
  @retval TRUE   Failure
*/

bool trans_xa_commit(THD *thd)
{
  bool res= TRUE;
  enum xa_states xa_state= thd->transaction.xid_state.xa_state;
  DBUG_ENTER("trans_xa_commit");

  if (!thd->transaction.xid_state.xid.eq(thd->lex->xid))
  {
    if (thd->fix_xid_hash_pins())
    {
      my_error(ER_OUT_OF_RESOURCES, MYF(0));
      DBUG_RETURN(TRUE);
    }

    XID_STATE *xs= xid_cache_search(thd, thd->lex->xid);
    res= !xs;
    if (res)
      my_error(ER_XAER_NOTA, MYF(0));
    else
    {
      res= xa_trans_rolled_back(xs);
      ha_commit_or_rollback_by_xid(thd->lex->xid, !res);
      xid_cache_delete(thd, xs);
    }
    DBUG_RETURN(res);
  }

  if (xa_trans_rolled_back(&thd->transaction.xid_state))
  {
    xa_trans_force_rollback(thd);
    res= thd->is_error();
  }
  else if (xa_state == XA_IDLE && thd->lex->xa_opt == XA_ONE_PHASE)
  {
    int r= ha_commit_trans(thd, TRUE);
    if ((res= MY_TEST(r)))
      my_error(r == 1 ? ER_XA_RBROLLBACK : ER_XAER_RMERR, MYF(0));
  }
  else if (xa_state == XA_PREPARED && thd->lex->xa_opt == XA_NONE)
  {
    MDL_request mdl_request;

    /*
      Acquire metadata lock which will ensure that COMMIT is blocked
      by active FLUSH TABLES WITH READ LOCK (and vice versa COMMIT in
      progress blocks FTWRL).

      We allow FLUSHer to COMMIT; we assume FLUSHer knows what it does.
    */
    mdl_request.init(MDL_key::BACKUP, "", "", MDL_BACKUP_COMMIT,
                     MDL_TRANSACTION);

    if (thd->mdl_context.acquire_lock(&mdl_request,
                                      thd->variables.lock_wait_timeout))
    {
      ha_rollback_trans(thd, TRUE);
      my_error(ER_XAER_RMERR, MYF(0));
    }
    else
    {
      DEBUG_SYNC(thd, "trans_xa_commit_after_acquire_commit_lock");

      res= MY_TEST(ha_commit_one_phase(thd, 1));
      if (res)
        my_error(ER_XAER_RMERR, MYF(0));
    }
  }
  else
  {
    my_error(ER_XAER_RMFAIL, MYF(0), xa_state_names[xa_state]);
    DBUG_RETURN(TRUE);
  }

  thd->variables.option_bits&= ~(OPTION_BEGIN | OPTION_KEEP_LOG);
  thd->transaction.all.reset();
  thd->server_status&=
    ~(SERVER_STATUS_IN_TRANS | SERVER_STATUS_IN_TRANS_READONLY);
  DBUG_PRINT("info", ("clearing SERVER_STATUS_IN_TRANS"));
  xid_cache_delete(thd, &thd->transaction.xid_state);
  thd->transaction.xid_state.xa_state= XA_NOTR;

  trans_track_end_trx(thd);

  DBUG_RETURN(res);
}


/**
  Roll back and terminate a XA transaction.

  @param thd    Current thread

  @retval FALSE  Success
  @retval TRUE   Failure
*/

bool trans_xa_rollback(THD *thd)
{
  bool res= TRUE;
  enum xa_states xa_state= thd->transaction.xid_state.xa_state;
  DBUG_ENTER("trans_xa_rollback");

  if (!thd->transaction.xid_state.xid.eq(thd->lex->xid))
  {
    if (thd->fix_xid_hash_pins())
    {
      my_error(ER_OUT_OF_RESOURCES, MYF(0));
      DBUG_RETURN(TRUE);
    }

    XID_STATE *xs= xid_cache_search(thd, thd->lex->xid);
    if (!xs)
      my_error(ER_XAER_NOTA, MYF(0));
    else
    {
      xa_trans_rolled_back(xs);
      ha_commit_or_rollback_by_xid(thd->lex->xid, 0);
      xid_cache_delete(thd, xs);
    }
    DBUG_RETURN(thd->get_stmt_da()->is_error());
  }

  if (xa_state != XA_IDLE && xa_state != XA_PREPARED && xa_state != XA_ROLLBACK_ONLY)
  {
    my_error(ER_XAER_RMFAIL, MYF(0), xa_state_names[xa_state]);
    DBUG_RETURN(TRUE);
  }

  res= xa_trans_force_rollback(thd);

  thd->variables.option_bits&= ~(OPTION_BEGIN | OPTION_KEEP_LOG);
  thd->transaction.all.reset();
  thd->server_status&=
    ~(SERVER_STATUS_IN_TRANS | SERVER_STATUS_IN_TRANS_READONLY);
  DBUG_PRINT("info", ("clearing SERVER_STATUS_IN_TRANS"));
  xid_cache_delete(thd, &thd->transaction.xid_state);
  thd->transaction.xid_state.xa_state= XA_NOTR;

  trans_track_end_trx(thd);

  DBUG_RETURN(res);
}


/**
  return the XID as it appears in the SQL function's arguments.
  So this string can be passed to XA START, XA PREPARE etc...

  @note
    the 'buf' has to have space for at least SQL_XIDSIZE bytes.
*/


/*
  'a'..'z' 'A'..'Z', '0'..'9'
  and '-' '_' ' ' symbols don't have to be
  converted.
*/

static const char xid_needs_conv[128]=
{
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
  0,1,1,1,1,1,1,1,1,1,1,1,1,0,1,1,
  0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,1,
  1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
  0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,0,
  1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
  0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1
};

/*
  The size of XID string representation in the form
  'gtrid', 'bqual', formatID
  see xid_t::get_sql_string() for details.
*/
#define SQL_XIDSIZE (XIDDATASIZE * 2 + 8 + MY_INT64_NUM_DECIMAL_DIGITS)

/* The 'buf' has to have space for at least SQL_XIDSIZE bytes. */
static uint get_sql_xid(XID *xid, char *buf)
{
  int tot_len= xid->gtrid_length + xid->bqual_length;
  int i;
  const char *orig_buf= buf;

  for (i=0; i<tot_len; i++)
  {
    uchar c= ((uchar *) xid->data)[i];
    if (c >= 128 || xid_needs_conv[c])
      break;
  }

  if (i >= tot_len)
  {
    /* No need to convert characters to hexadecimals. */
    *buf++= '\'';
    memcpy(buf, xid->data, xid->gtrid_length);
    buf+= xid->gtrid_length;
    *buf++= '\'';
    if (xid->bqual_length > 0 || xid->formatID != 1)
    {
      *buf++= ',';
      *buf++= '\'';
      memcpy(buf, xid->data+xid->gtrid_length, xid->bqual_length);
      buf+= xid->bqual_length;
      *buf++= '\'';
    }
  }
  else
  {
    *buf++= 'X';
    *buf++= '\'';
    for (i= 0; i < xid->gtrid_length; i++)
    {
      *buf++=_dig_vec_lower[((uchar*) xid->data)[i] >> 4];
      *buf++=_dig_vec_lower[((uchar*) xid->data)[i] & 0x0f];
    }
    *buf++= '\'';
    if (xid->bqual_length > 0 || xid->formatID != 1)
    {
      *buf++= ',';
      *buf++= 'X';
      *buf++= '\'';
      for (; i < tot_len; i++)
      {
        *buf++=_dig_vec_lower[((uchar*) xid->data)[i] >> 4];
        *buf++=_dig_vec_lower[((uchar*) xid->data)[i] & 0x0f];
      }
      *buf++= '\'';
    }
  }

  if (xid->formatID != 1)
  {
    *buf++= ',';
    buf+= my_longlong10_to_str_8bit(&my_charset_bin, buf,
            MY_INT64_NUM_DECIMAL_DIGITS, -10, xid->formatID);
  }

  return (uint)(buf - orig_buf);
}


/**
  return the list of XID's to a client, the same way SHOW commands do.

  @note
    I didn't find in XA specs that an RM cannot return the same XID twice,
    so mysql_xa_recover does not filter XID's to ensure uniqueness.
    It can be easily fixed later, if necessary.
*/

static my_bool xa_recover_callback(XID_STATE *xs, Protocol *protocol,
                  char *data, uint data_len, CHARSET_INFO *data_cs)
{
  if (xs->xa_state == XA_PREPARED)
  {
    protocol->prepare_for_resend();
    protocol->store_longlong((longlong) xs->xid.formatID, FALSE);
    protocol->store_longlong((longlong) xs->xid.gtrid_length, FALSE);
    protocol->store_longlong((longlong) xs->xid.bqual_length, FALSE);
    protocol->store(data, data_len, data_cs);
    if (protocol->write())
      return TRUE;
  }
  return FALSE;
}


static my_bool xa_recover_callback_short(XID_STATE *xs, Protocol *protocol)
{
  return xa_recover_callback(xs, protocol, xs->xid.data,
      xs->xid.gtrid_length + xs->xid.bqual_length, &my_charset_bin);
}


static my_bool xa_recover_callback_verbose(XID_STATE *xs, Protocol *protocol)
{
  char buf[SQL_XIDSIZE];
  uint len= get_sql_xid(&xs->xid, buf);
  return xa_recover_callback(xs, protocol, buf, len,
                             &my_charset_utf8_general_ci);
}


bool mysql_xa_recover(THD *thd)
{
  List<Item> field_list;
  Protocol *protocol= thd->protocol;
  MEM_ROOT *mem_root= thd->mem_root;
  my_hash_walk_action action;
  DBUG_ENTER("mysql_xa_recover");

  field_list.push_back(new (mem_root)
                       Item_int(thd, "formatID", 0,
                                MY_INT32_NUM_DECIMAL_DIGITS), mem_root);
  field_list.push_back(new (mem_root)
                       Item_int(thd, "gtrid_length", 0,
                                MY_INT32_NUM_DECIMAL_DIGITS), mem_root);
  field_list.push_back(new (mem_root)
                       Item_int(thd, "bqual_length", 0,
                                MY_INT32_NUM_DECIMAL_DIGITS), mem_root);
  {
    uint len;
    CHARSET_INFO *cs;

    if (thd->lex->verbose)
    {
      len= SQL_XIDSIZE;
      cs= &my_charset_utf8_general_ci;
      action= (my_hash_walk_action) xa_recover_callback_verbose;
    }
    else
    {
      len= XIDDATASIZE;
      cs= &my_charset_bin;
      action= (my_hash_walk_action) xa_recover_callback_short;
    }

    field_list.push_back(new (mem_root)
                         Item_empty_string(thd, "data", len, cs), mem_root);
  }

  if (protocol->send_result_set_metadata(&field_list,
                            Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF))
    DBUG_RETURN(1);

  if (xid_cache_iterate(thd, action, protocol))
    DBUG_RETURN(1);
  my_eof(thd);
  DBUG_RETURN(0);
}