lock.cc

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/* Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

   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,
   51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA */


#include "sql_priv.h"
#include "debug_sync.h"
#include "unireg.h"                    // REQUIRED: for other includes
#include "lock.h"
#include "sql_base.h"                       // close_tables_for_reopen
#include "sql_parse.h"                     // is_log_table_write_query
#include "sql_acl.h"                       // SUPER_ACL
#include <hash.h>
#include <assert.h>

extern HASH open_cache;

/* flags for get_lock_data */
#define GET_LOCK_UNLOCK         1
#define GET_LOCK_STORE_LOCKS    2

static MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table_ptr, uint count,
                                 uint flags);
static int lock_external(THD *thd, TABLE **table,uint count);
static int unlock_external(THD *thd, TABLE **table,uint count);
static void print_lock_error(int error, const char *);

/* Map the return value of thr_lock to an error from errmsg.txt */
static int thr_lock_errno_to_mysql[]=
{ 0, ER_LOCK_ABORTED, ER_LOCK_WAIT_TIMEOUT, ER_LOCK_DEADLOCK };

static int
lock_tables_check(THD *thd, TABLE **tables, uint count, uint flags)
{
  uint system_count= 0, i= 0;
  bool is_superuser= false;
  /*
    Identifies if the executed sql command can updated either a log
    or rpl info table.
  */
  bool log_table_write_query= false;

  DBUG_ENTER("lock_tables_check");

  is_superuser= thd->security_ctx->master_access & SUPER_ACL;
  log_table_write_query=
     is_log_table_write_query(thd->lex->sql_command);

  for (i=0 ; i<count; i++)
  {
    TABLE *t= tables[i];

    /* Protect against 'fake' partially initialized TABLE_SHARE */
    DBUG_ASSERT(t->s->table_category != TABLE_UNKNOWN_CATEGORY);

    /*
      Table I/O to performance schema tables is performed
      only internally by the server implementation.
      When a user is requesting a lock, the following
      constraints are enforced:
    */
    if (t->s->table_category == TABLE_CATEGORY_LOG &&
        (flags & MYSQL_LOCK_LOG_TABLE) == 0 &&
        !log_table_write_query)
    {
      /*
        A user should not be able to prevent writes,
        or hold any type of lock in a session,
        since this would be a DOS attack.
      */
      if (t->reginfo.lock_type >= TL_READ_NO_INSERT ||
          thd->lex->sql_command == SQLCOM_LOCK_TABLES)
      {
        my_error(ER_CANT_LOCK_LOG_TABLE, MYF(0));
        DBUG_RETURN(1);
      }
    }

    if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE)
    {
      if (t->s->table_category == TABLE_CATEGORY_SYSTEM)
        system_count++;

      if (t->db_stat & HA_READ_ONLY)
      {
        my_error(ER_OPEN_AS_READONLY, MYF(0), t->alias);
        DBUG_RETURN(1);
      }
    }

    /*
      If we are going to lock a non-temporary table we must own metadata
      lock of appropriate type on it (I.e. for table to be locked for
      write we must own metadata lock of MDL_SHARED_WRITE or stronger
      type. For table to be locked for read we must own metadata lock
      of MDL_SHARED_READ or stronger type).
      The only exception are HANDLER statements which are allowed to
      lock table for read while having only MDL_SHARED lock on it.
    */
    DBUG_ASSERT(t->s->tmp_table ||
                thd->mdl_context.is_lock_owner(MDL_key::TABLE,
                                 t->s->db.str, t->s->table_name.str,
                                 t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE ?
                                 MDL_SHARED_WRITE : MDL_SHARED_READ) ||
                (t->open_by_handler &&
                 thd->mdl_context.is_lock_owner(MDL_key::TABLE,
                                  t->s->db.str, t->s->table_name.str,
                                  MDL_SHARED)));

    /*
      Prevent modifications to base tables if READ_ONLY is activated.
      In any case, read only does not apply to temporary tables.
    */
    if (!(flags & MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY) && !t->s->tmp_table)
    {
      if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&
          !is_superuser && opt_readonly && !thd->slave_thread)
      {
        my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0), "--read-only");
        DBUG_RETURN(1);
      }
    }
  }

  /*
    Locking of system tables is restricted:
    locking a mix of system and non-system tables in the same lock
    is prohibited, to prevent contention.
  */
  if ((system_count > 0) && (system_count < count))
  {
    my_error(ER_WRONG_LOCK_OF_SYSTEM_TABLE, MYF(0));
    DBUG_RETURN(1);
  }

  DBUG_RETURN(0);
}

static void reset_lock_data(MYSQL_LOCK *sql_lock)
{
  THR_LOCK_DATA **ldata, **ldata_end;
  DBUG_ENTER("reset_lock_data");

  /* Clear the lock type of all lock data to avoid reusage. */
  for (ldata= sql_lock->locks, ldata_end= ldata + sql_lock->lock_count;
       ldata < ldata_end;
       ldata++)
  {
    /* Reset lock type. */
    (*ldata)->type= TL_UNLOCK;
  }
  DBUG_VOID_RETURN;
}


static void reset_lock_data_and_free(MYSQL_LOCK **mysql_lock)
{
  reset_lock_data(*mysql_lock);
  my_free(*mysql_lock);
  *mysql_lock= 0;
}


MYSQL_LOCK *mysql_lock_tables(THD *thd, TABLE **tables, uint count, uint flags)
{
  int rc;
  MYSQL_LOCK *sql_lock;
  ulong timeout= (flags & MYSQL_LOCK_IGNORE_TIMEOUT) ?
    LONG_TIMEOUT : thd->variables.lock_wait_timeout;

  DBUG_ENTER("mysql_lock_tables");

  if (lock_tables_check(thd, tables, count, flags))
    DBUG_RETURN(NULL);

  if (! (sql_lock= get_lock_data(thd, tables, count, GET_LOCK_STORE_LOCKS)))
    DBUG_RETURN(NULL);

  THD_STAGE_INFO(thd, stage_system_lock);
  DBUG_PRINT("info", ("thd->proc_info %s", thd->proc_info));
  if (sql_lock->table_count && lock_external(thd, sql_lock->table,
                                             sql_lock->table_count))
  {
    /* Clear the lock type of all lock data to avoid reusage. */
    reset_lock_data_and_free(&sql_lock);
    goto end;
  }

  /* Copy the lock data array. thr_multi_lock() reorders its contents. */
  memcpy(sql_lock->locks + sql_lock->lock_count, sql_lock->locks,
         sql_lock->lock_count * sizeof(*sql_lock->locks));
  /* Lock on the copied half of the lock data array. */
  rc= thr_lock_errno_to_mysql[(int) thr_multi_lock(sql_lock->locks +
                                                   sql_lock->lock_count,
                                                   sql_lock->lock_count,
                                                   &thd->lock_info, timeout)];
  if (rc)
  {
    if (sql_lock->table_count)
      (void) unlock_external(thd, sql_lock->table, sql_lock->table_count);
    reset_lock_data_and_free(&sql_lock);
    if (! thd->killed)
      my_error(rc, MYF(0));
  }
end:
  if (!(flags & MYSQL_OPEN_IGNORE_KILLED) && thd->killed)
  {
    thd->send_kill_message();
    if (sql_lock)
    {
      mysql_unlock_tables(thd, sql_lock);
      sql_lock= 0;
    }
  }

  thd->set_time_after_lock();
  DBUG_RETURN(sql_lock);
}


static int lock_external(THD *thd, TABLE **tables, uint count)
{
  reg1 uint i;
  int lock_type,error;
  DBUG_ENTER("lock_external");

  DBUG_PRINT("info", ("count %d", count));
  for (i=1 ; i <= count ; i++, tables++)
  {
    DBUG_ASSERT((*tables)->reginfo.lock_type >= TL_READ);
    lock_type=F_WRLCK;                          /* Lock exclusive */
    if ((*tables)->db_stat & HA_READ_ONLY ||
        ((*tables)->reginfo.lock_type >= TL_READ &&
         (*tables)->reginfo.lock_type <= TL_READ_NO_INSERT))
      lock_type=F_RDLCK;

    if ((error=(*tables)->file->ha_external_lock(thd,lock_type)))
    {
      print_lock_error(error, (*tables)->file->table_type());
      while (--i)
      {
        tables--;
        (*tables)->file->ha_external_lock(thd, F_UNLCK);
        (*tables)->current_lock=F_UNLCK;
      }
      DBUG_RETURN(error);
    }
    else
    {
      (*tables)->db_stat &= ~ HA_BLOCK_LOCK;
      (*tables)->current_lock= lock_type;
    }
  }
  DBUG_RETURN(0);
}


void mysql_unlock_tables(THD *thd, MYSQL_LOCK *sql_lock)
{
  DBUG_ENTER("mysql_unlock_tables");
  if (sql_lock->lock_count)
    thr_multi_unlock(sql_lock->locks,sql_lock->lock_count);
  if (sql_lock->table_count)
    (void) unlock_external(thd,sql_lock->table,sql_lock->table_count);
  my_free(sql_lock);
  DBUG_VOID_RETURN;
}

void mysql_unlock_some_tables(THD *thd, TABLE **table,uint count)
{
  MYSQL_LOCK *sql_lock;
  if ((sql_lock= get_lock_data(thd, table, count, GET_LOCK_UNLOCK)))
    mysql_unlock_tables(thd, sql_lock);
}


void mysql_unlock_read_tables(THD *thd, MYSQL_LOCK *sql_lock)
{
  uint i,found;
  DBUG_ENTER("mysql_unlock_read_tables");

  /* Move all write locks first */
  THR_LOCK_DATA **lock=sql_lock->locks;
  for (i=found=0 ; i < sql_lock->lock_count ; i++)
  {
    if (sql_lock->locks[i]->type > TL_WRITE_ALLOW_WRITE)
    {
      swap_variables(THR_LOCK_DATA *, *lock, sql_lock->locks[i]);
      lock++;
      found++;
    }
  }
  /* unlock the read locked tables */
  if (i != found)
  {
    thr_multi_unlock(lock,i-found);
    sql_lock->lock_count= found;
  }

  /* Then do the same for the external locks */
  /* Move all write locked tables first */
  TABLE **table=sql_lock->table;
  for (i=found=0 ; i < sql_lock->table_count ; i++)
  {
    DBUG_ASSERT(sql_lock->table[i]->lock_position == i);
    if ((uint) sql_lock->table[i]->reginfo.lock_type > TL_WRITE_ALLOW_WRITE)
    {
      swap_variables(TABLE *, *table, sql_lock->table[i]);
      table++;
      found++;
    }
  }
  /* Unlock all read locked tables */
  if (i != found)
  {
    (void) unlock_external(thd,table,i-found);
    sql_lock->table_count=found;
  }
  /* Fix the lock positions in TABLE */
  table= sql_lock->table;
  found= 0;
  for (i= 0; i < sql_lock->table_count; i++)
  {
    TABLE *tbl= *table;
    tbl->lock_position= (uint) (table - sql_lock->table);
    tbl->lock_data_start= found;
    found+= tbl->lock_count;
    table++;
  }
  DBUG_VOID_RETURN;
}


void mysql_lock_remove(THD *thd, MYSQL_LOCK *locked,TABLE *table)
{
  if (locked)
  {
    reg1 uint i;
    for (i=0; i < locked->table_count; i++)
    {
      if (locked->table[i] == table)
      {
        uint  j, removed_locks, old_tables;
        TABLE *tbl;
        uint lock_data_end;

        DBUG_ASSERT(table->lock_position == i);

        /* Unlock the table. */
        mysql_unlock_some_tables(thd, &table, /* table count */ 1);

        /* Decrement table_count in advance, making below expressions easier */
        old_tables= --locked->table_count;

        /* The table has 'removed_locks' lock data elements in locked->locks */
        removed_locks= table->lock_count;

        /* Move down all table pointers above 'i'. */
        bmove((char*) (locked->table+i),
              (char*) (locked->table+i+1),
              (old_tables - i) * sizeof(TABLE*));

        lock_data_end= table->lock_data_start + table->lock_count;
        /* Move down all lock data pointers above 'table->lock_data_end-1' */
        bmove((char*) (locked->locks + table->lock_data_start),
              (char*) (locked->locks + lock_data_end),
              (locked->lock_count - lock_data_end) *
              sizeof(THR_LOCK_DATA*));

        /*
          Fix moved table elements.
          lock_position is the index in the 'locked->table' array,
          it must be fixed by one.
          table->lock_data_start is pointer to the lock data for this table
          in the 'locked->locks' array, they must be fixed by 'removed_locks',
          the lock data count of the removed table.
        */
        for (j= i ; j < old_tables; j++)
        {
          tbl= locked->table[j];
          tbl->lock_position--;
          DBUG_ASSERT(tbl->lock_position == j);
          tbl->lock_data_start-= removed_locks;
        }

        /* Finally adjust lock_count. */
        locked->lock_count-= removed_locks;
        break;
      }
    }
  }
}


void mysql_lock_abort(THD *thd, TABLE *table, bool upgrade_lock)
{
  MYSQL_LOCK *locked;
  DBUG_ENTER("mysql_lock_abort");

  if ((locked= get_lock_data(thd, &table, 1, GET_LOCK_UNLOCK)))
  {
    for (uint i=0; i < locked->lock_count; i++)
      thr_abort_locks(locked->locks[i]->lock, upgrade_lock);
    my_free(locked);
  }
  DBUG_VOID_RETURN;
}


bool mysql_lock_abort_for_thread(THD *thd, TABLE *table)
{
  MYSQL_LOCK *locked;
  bool result= FALSE;
  DBUG_ENTER("mysql_lock_abort_for_thread");

  if ((locked= get_lock_data(thd, &table, 1, GET_LOCK_UNLOCK)))
  {
    for (uint i=0; i < locked->lock_count; i++)
    {
      if (thr_abort_locks_for_thread(locked->locks[i]->lock,
                                     table->in_use->thread_id))
        result= TRUE;
    }
    my_free(locked);
  }
  DBUG_RETURN(result);
}


MYSQL_LOCK *mysql_lock_merge(MYSQL_LOCK *a,MYSQL_LOCK *b)
{
  MYSQL_LOCK *sql_lock;
  TABLE **table, **end_table;
  DBUG_ENTER("mysql_lock_merge");

  if (!(sql_lock= (MYSQL_LOCK*)
        my_malloc(sizeof(*sql_lock)+
                  sizeof(THR_LOCK_DATA*)*(a->lock_count+b->lock_count)+
                  sizeof(TABLE*)*(a->table_count+b->table_count),MYF(MY_WME))))
    DBUG_RETURN(0);                             // Fatal error
  sql_lock->lock_count=a->lock_count+b->lock_count;
  sql_lock->table_count=a->table_count+b->table_count;
  sql_lock->locks=(THR_LOCK_DATA**) (sql_lock+1);
  sql_lock->table=(TABLE**) (sql_lock->locks+sql_lock->lock_count);
  memcpy(sql_lock->locks,a->locks,a->lock_count*sizeof(*a->locks));
  memcpy(sql_lock->locks+a->lock_count,b->locks,
         b->lock_count*sizeof(*b->locks));
  memcpy(sql_lock->table,a->table,a->table_count*sizeof(*a->table));
  memcpy(sql_lock->table+a->table_count,b->table,
         b->table_count*sizeof(*b->table));

  /*
    Now adjust lock_position and lock_data_start for all objects that was
    moved in 'b' (as there is now all objects in 'a' before these).
  */
  for (table= sql_lock->table + a->table_count,
         end_table= table + b->table_count;
       table < end_table;
       table++)
  {
    (*table)->lock_position+=   a->table_count;
    (*table)->lock_data_start+= a->lock_count;
  }

  /* Delete old, not needed locks */
  my_free(a);
  my_free(b);

  thr_lock_merge_status(sql_lock->locks, sql_lock->lock_count);
  DBUG_RETURN(sql_lock);
}


static int unlock_external(THD *thd, TABLE **table,uint count)
{
  int error,error_code;
  DBUG_ENTER("unlock_external");

  error_code=0;
  do
  {
    if ((*table)->current_lock != F_UNLCK)
    {
      (*table)->current_lock = F_UNLCK;
      if ((error=(*table)->file->ha_external_lock(thd, F_UNLCK)))
      {
        error_code=error;
        print_lock_error(error_code, (*table)->file->table_type());
      }
    }
    table++;
  } while (--count);
  DBUG_RETURN(error_code);
}


static MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table_ptr, uint count,
                                 uint flags)
{
  uint i,tables,lock_count;
  MYSQL_LOCK *sql_lock;
  THR_LOCK_DATA **locks, **locks_buf, **locks_start;
  TABLE **to, **table_buf;
  DBUG_ENTER("get_lock_data");

  DBUG_ASSERT((flags == GET_LOCK_UNLOCK) || (flags == GET_LOCK_STORE_LOCKS));
  DBUG_PRINT("info", ("count %d", count));

  for (i=tables=lock_count=0 ; i < count ; i++)
  {
    TABLE *t= table_ptr[i];

    if (t->s->tmp_table != NON_TRANSACTIONAL_TMP_TABLE)
    {
      tables+= t->file->lock_count();
      lock_count++;
    }
  }

  /*
    Allocating twice the number of pointers for lock data for use in
    thr_mulit_lock(). This function reorders the lock data, but cannot
    update the table values. So the second part of the array is copied
    from the first part immediately before calling thr_multi_lock().
  */
  if (!(sql_lock= (MYSQL_LOCK*)
        my_malloc(sizeof(*sql_lock) +
                  sizeof(THR_LOCK_DATA*) * tables * 2 +
                  sizeof(table_ptr) * lock_count,
                  MYF(0))))
    DBUG_RETURN(0);
  locks= locks_buf= sql_lock->locks= (THR_LOCK_DATA**) (sql_lock + 1);
  to= table_buf= sql_lock->table= (TABLE**) (locks + tables * 2);
  sql_lock->table_count=lock_count;

  for (i=0 ; i < count ; i++)
  {
    TABLE *table;
    enum thr_lock_type lock_type;
    THR_LOCK_DATA **org_locks = locks;

    if ((table=table_ptr[i])->s->tmp_table == NON_TRANSACTIONAL_TMP_TABLE)
      continue;
    lock_type= table->reginfo.lock_type;
    DBUG_ASSERT(lock_type != TL_WRITE_DEFAULT && lock_type != TL_READ_DEFAULT);
    locks_start= locks;
    locks= table->file->store_lock(thd, locks,
                                   (flags & GET_LOCK_UNLOCK) ? TL_IGNORE :
                                   lock_type);
    if (flags & GET_LOCK_STORE_LOCKS)
    {
      table->lock_position=   (uint) (to - table_buf);
      table->lock_data_start= (uint) (locks_start - locks_buf);
      table->lock_count=      (uint) (locks - locks_start);
    }
    *to++= table;
    if (locks)
    {
      for ( ; org_locks != locks ; org_locks++)
      {
        (*org_locks)->debug_print_param= (void *) table;
        (*org_locks)->m_psi= table->file->m_psi;
      }
    }
  }
  /*
    We do not use 'tables', because there are cases where store_lock()
    returns less locks than lock_count() claimed. This can happen when
    a FLUSH TABLES tries to abort locks from a MERGE table of another
    thread. When that thread has just opened the table, but not yet
    attached its children, it cannot return the locks. lock_count()
    always returns the number of locks that an attached table has.
    This is done to avoid the reverse situation: If lock_count() would
    return 0 for a non-attached MERGE table, and that table becomes
    attached between the calls to lock_count() and store_lock(), then
    we would have allocated too little memory for the lock data. Now
    we may allocate too much, but better safe than memory overrun.
    And in the FLUSH case, the memory is released quickly anyway.
  */
  sql_lock->lock_count= locks - locks_buf;
  DBUG_PRINT("info", ("sql_lock->table_count %d sql_lock->lock_count %d",
                      sql_lock->table_count, sql_lock->lock_count));
  DBUG_RETURN(sql_lock);
}


bool lock_schema_name(THD *thd, const char *db)
{
  MDL_request_list mdl_requests;
  MDL_request global_request;
  MDL_request mdl_request;

  if (thd->locked_tables_mode)
  {
    my_message(ER_LOCK_OR_ACTIVE_TRANSACTION,
               ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
    return TRUE;
  }

  if (thd->global_read_lock.can_acquire_protection())
    return TRUE;
  global_request.init(MDL_key::GLOBAL, "", "", MDL_INTENTION_EXCLUSIVE,
                      MDL_STATEMENT);
  mdl_request.init(MDL_key::SCHEMA, db, "", MDL_EXCLUSIVE, MDL_TRANSACTION);

  mdl_requests.push_front(&mdl_request);
  mdl_requests.push_front(&global_request);

  if (thd->mdl_context.acquire_locks(&mdl_requests,
                                     thd->variables.lock_wait_timeout))
    return TRUE;

  DEBUG_SYNC(thd, "after_wait_locked_schema_name");
  return FALSE;
}


bool lock_object_name(THD *thd, MDL_key::enum_mdl_namespace mdl_type,
                       const char *db, const char *name)
{
  MDL_request_list mdl_requests;
  MDL_request global_request;
  MDL_request schema_request;
  MDL_request mdl_request;

  if (thd->locked_tables_mode)
  {
    my_message(ER_LOCK_OR_ACTIVE_TRANSACTION,
               ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
    return TRUE;
  }

  DBUG_ASSERT(name);
  DEBUG_SYNC(thd, "before_wait_locked_pname");

  if (thd->global_read_lock.can_acquire_protection())
    return TRUE;
  global_request.init(MDL_key::GLOBAL, "", "", MDL_INTENTION_EXCLUSIVE,
                      MDL_STATEMENT);
  schema_request.init(MDL_key::SCHEMA, db, "", MDL_INTENTION_EXCLUSIVE,
                      MDL_TRANSACTION);
  mdl_request.init(mdl_type, db, name, MDL_EXCLUSIVE, MDL_TRANSACTION);

  mdl_requests.push_front(&mdl_request);
  mdl_requests.push_front(&schema_request);
  mdl_requests.push_front(&global_request);

  if (thd->mdl_context.acquire_locks(&mdl_requests,
                                     thd->variables.lock_wait_timeout))
    return TRUE;

  DEBUG_SYNC(thd, "after_wait_locked_pname");
  return FALSE;
}


static void print_lock_error(int error, const char *table)
{
  DBUG_ENTER("print_lock_error");

  switch (error) {
  case HA_ERR_LOCK_WAIT_TIMEOUT:
    my_error(ER_LOCK_WAIT_TIMEOUT, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG), error);
    break;
  case HA_ERR_READ_ONLY_TRANSACTION:
    my_error(ER_READ_ONLY_TRANSACTION, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG),
             error);
    break;
  case HA_ERR_LOCK_DEADLOCK:
    my_error(ER_LOCK_DEADLOCK, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG), error);
    break;
  case HA_ERR_WRONG_COMMAND:
    my_error(ER_ILLEGAL_HA, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG), table);
    break;
  default:
    {
      char errbuf[MYSYS_STRERROR_SIZE];
      my_error(ER_CANT_LOCK, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG),
               error, my_strerror(errbuf, sizeof(errbuf), error));
    }
    break;
  }

  DBUG_VOID_RETURN;
}


/****************************************************************************
  Handling of global read locks

  Global read lock is implemented using metadata lock infrastructure.

  Taking the global read lock is TWO steps (2nd step is optional; without
  it, COMMIT of existing transactions will be allowed):
  lock_global_read_lock() THEN make_global_read_lock_block_commit().

  How blocking of threads by global read lock is achieved: that's
  semi-automatic. We assume that any statement which should be blocked
  by global read lock will either open and acquires write-lock on tables
  or acquires metadata locks on objects it is going to modify. For any
  such statement global IX metadata lock is automatically acquired for
  its duration (in case of LOCK TABLES until end of LOCK TABLES mode).
  And lock_global_read_lock() simply acquires global S metadata lock
  and thus prohibits execution of statements which modify data (unless
  they modify only temporary tables). If deadlock happens it is detected
  by MDL subsystem and resolved in the standard fashion (by backing-off
  metadata locks acquired so far and restarting open tables process
  if possible).

  Why does FLUSH TABLES WITH READ LOCK need to block COMMIT: because it's used
  to read a non-moving SHOW MASTER STATUS, and a COMMIT writes to the binary
  log.

  Why getting the global read lock is two steps and not one. Because FLUSH
  TABLES WITH READ LOCK needs to insert one other step between the two:
  flushing tables. So the order is
  1) lock_global_read_lock() (prevents any new table write locks, i.e. stalls
  all new updates)
  2) close_cached_tables() (the FLUSH TABLES), which will wait for tables
  currently opened and being updated to close (so it's possible that there is
  a moment where all new updates of server are stalled *and* FLUSH TABLES WITH
  READ LOCK is, too).
  3) make_global_read_lock_block_commit().
  If we have merged 1) and 3) into 1), we would have had this deadlock:
  imagine thread 1 and 2, in non-autocommit mode, thread 3, and an InnoDB
  table t.
  thd1: SELECT * FROM t FOR UPDATE;
  thd2: UPDATE t SET a=1; # blocked by row-level locks of thd1
  thd3: FLUSH TABLES WITH READ LOCK; # blocked in close_cached_tables() by the
  table instance of thd2
  thd1: COMMIT; # blocked by thd3.
  thd1 blocks thd2 which blocks thd3 which blocks thd1: deadlock.

  Note that we need to support that one thread does
  FLUSH TABLES WITH READ LOCK; and then COMMIT;
  (that's what innobackup does, for some good reason).
  So in this exceptional case the COMMIT should not be blocked by the FLUSH
  TABLES WITH READ LOCK.

****************************************************************************/

bool Global_read_lock::lock_global_read_lock(THD *thd)
{
  DBUG_ENTER("lock_global_read_lock");

  if (!m_state)
  {
    MDL_request mdl_request;

    DBUG_ASSERT(! thd->mdl_context.is_lock_owner(MDL_key::GLOBAL, "", "",
                                                 MDL_SHARED));
    mdl_request.init(MDL_key::GLOBAL, "", "", MDL_SHARED, MDL_EXPLICIT);

    if (thd->mdl_context.acquire_lock(&mdl_request,
                                      thd->variables.lock_wait_timeout))
      DBUG_RETURN(1);

    m_mdl_global_shared_lock= mdl_request.ticket;
    m_state= GRL_ACQUIRED;
  }
  /*
    We DON'T set global_read_lock_blocks_commit now, it will be set after
    tables are flushed (as the present function serves for FLUSH TABLES WITH
    READ LOCK only). Doing things in this order is necessary to avoid
    deadlocks (we must allow COMMIT until all tables are closed; we should not
    forbid it before, or we can have a 3-thread deadlock if 2 do SELECT FOR
    UPDATE and one does FLUSH TABLES WITH READ LOCK).
  */
  DBUG_RETURN(0);
}


void Global_read_lock::unlock_global_read_lock(THD *thd)
{
  DBUG_ENTER("unlock_global_read_lock");

  DBUG_ASSERT(m_mdl_global_shared_lock && m_state);

  if (m_mdl_blocks_commits_lock)
  {
    thd->mdl_context.release_lock(m_mdl_blocks_commits_lock);
    m_mdl_blocks_commits_lock= NULL;
  }
  thd->mdl_context.release_lock(m_mdl_global_shared_lock);
  m_mdl_global_shared_lock= NULL;
  m_state= GRL_NONE;

  DBUG_VOID_RETURN;
}


bool Global_read_lock::make_global_read_lock_block_commit(THD *thd)
{
  MDL_request mdl_request;
  DBUG_ENTER("make_global_read_lock_block_commit");
  /*
    If we didn't succeed lock_global_read_lock(), or if we already suceeded
    make_global_read_lock_block_commit(), do nothing.
  */
  if (m_state != GRL_ACQUIRED)
    DBUG_RETURN(0);

  mdl_request.init(MDL_key::COMMIT, "", "", MDL_SHARED, MDL_EXPLICIT);

  if (thd->mdl_context.acquire_lock(&mdl_request,
                                    thd->variables.lock_wait_timeout))
    DBUG_RETURN(TRUE);

  m_mdl_blocks_commits_lock= mdl_request.ticket;
  m_state= GRL_ACQUIRED_AND_BLOCKS_COMMIT;

  DBUG_RETURN(FALSE);
}


void Global_read_lock::set_explicit_lock_duration(THD *thd)
{
  if (m_mdl_global_shared_lock)
    thd->mdl_context.set_lock_duration(m_mdl_global_shared_lock, MDL_EXPLICIT);
  if (m_mdl_blocks_commits_lock)
    thd->mdl_context.set_lock_duration(m_mdl_blocks_commits_lock, MDL_EXPLICIT);
}