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A single-row table is read once during optimization and its row kept in `record[0]` for the rest of the statement. `JOIN::optimize_stage2()` then releases the lock on every const table, on the premise stated in its own comment: *"It's safe to ignore result code as all tables where opened for read only."* That premise assumes a read leaves a **copy** of the row behind. MEMORY with a blob does not. `hp_read_blobs()` answers the read by pointing `record[0]` at the blob data inside `HP_SHARE` rather than copying it, so from the moment the lock is dropped another connection is free to overwrite, free or recycle those bytes -- and the statement goes on reading them. The result is a const table whose value changes in the middle of the statement using it, and a read of freed memory. Let a caller that keeps reading a row after the unlock ask for such tables to be left alone. `GET_LOCK_SKIP_ZERO_COPY_ROWS` drops them from the lock set `get_lock_data()` builds, exactly as `GET_LOCK_SKIP_SEQUENCES` already does, and the const-table unlock in `JOIN::optimize_stage2()` passes it. Which tables those are is for the engine to say rather than for the lock layer to infer. The 64-bit `table_flags()` space is full, so a second word `table_flags2()` carries the first such property, `HA2_CANNOT_ACCESS_ROWDATA_AFTER_UNLOCK`, and `ha_heap::open()` raises it for any table that has a blob. The skip has to be opt-in rather than a rule. `mysql_lock_remove()` also reaches `mysql_unlock_some_tables()`, and there the unlock is permanent and must not be skipped. A MEMORY blob const table now stays read-locked for the whole statement and blocks writers, which is the price every non-const MEMORY table already pays. The regression test parks the reader with `GET_LOCK()` rather than with a stored function. A stored function puts the statement into prelocked mode, and the const-table unlock is skipped entirely in that mode, so the code path under test would never run. The gate that parks it is taken with `--disable_ps2_protocol` in force. `--ps-protocol` executes every complete `SELECT` twice and compares the two result sets, and `GET_LOCK()` is recursive, so a doubled acquisition would outlive the single `RELEASE_LOCK()` that opens the gate again.
56 lines
2.2 KiB
C++
56 lines
2.2 KiB
C++
/* Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; version 2 of the License.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335 USA */
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#ifndef LOCK_INCLUDED
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#define LOCK_INCLUDED
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#include "thr_lock.h" /* thr_lock_type */
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#include "mdl.h"
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// Forward declarations
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struct TABLE;
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struct TABLE_LIST;
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class THD;
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typedef struct st_mysql_lock MYSQL_LOCK;
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MYSQL_LOCK *mysql_lock_tables(THD *thd, TABLE **table, uint count, uint flags);
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bool mysql_lock_tables(THD *thd, MYSQL_LOCK *sql_lock, uint flags);
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int mysql_unlock_tables(THD *thd, MYSQL_LOCK *sql_lock, bool free_lock);
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int mysql_unlock_tables(THD *thd, MYSQL_LOCK *sql_lock);
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int mysql_unlock_read_tables(THD *thd, MYSQL_LOCK *sql_lock);
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int mysql_unlock_some_tables(THD *thd, TABLE **table,uint count, uint flag);
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int mysql_lock_remove(THD *thd, MYSQL_LOCK *locked,TABLE *table);
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bool mysql_lock_abort_for_thread(THD *thd, TABLE *table);
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MYSQL_LOCK *mysql_lock_merge(MYSQL_LOCK *a, MYSQL_LOCK *b, THD *thd= NULL);
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/* Lock based on name */
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bool lock_schema_name(THD *thd, const Lex_ident_db_normalized &db);
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/* Lock based on stored routine name */
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bool lock_object_name(THD *thd, MDL_key::enum_mdl_namespace mdl_type,
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const LEX_CSTRING &db, const LEX_CSTRING &name);
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/* flags for get_lock_data */
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#define GET_LOCK_UNLOCK 0
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#define GET_LOCK_STORE_LOCKS 1
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#define GET_LOCK_ACTION_MASK 1
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#define GET_LOCK_ON_THD (1 << 1)
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#define GET_LOCK_SKIP_SEQUENCES (1 << 2)
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#define GET_LOCK_SKIP_ZERO_COPY_ROWS (1 << 3)
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MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table_ptr, uint count, uint flags);
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void reset_lock_data(MYSQL_LOCK *sql_lock, bool unlock);
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#endif /* LOCK_INCLUDED */
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