Bug#18544 - LOCK TABLES timeout causes MyISAM table corruption

After a locking error the open table(s) were not fully
cleaned up for reuse. But they were put into the open table
cache even before the lock was tried. The next statement
reused the table(s) with a wrong lock type set up. This
tricked MyISAM into believing that it don't need to update
the table statistics. Hence CHECK TABLE reported a mismatch
of record count and table size.

Fortunately nothing worse has been detected yet. The effect
of the test case was that the insert worked on a read locked
table. (!)

I added a new function that clears the lock type from all
tables that were prepared for a lock. I call this function
when a lock failes.

No test case. One test would add 50 seconds to the
test suite. Another test requires file mode modifications.
I added a test script to the bug report. It contains three
cases for failing locks. All could reproduce a table
corruption. All are fixed by this patch.

This bug was not lock timeout specific.


sql/lock.cc:
  Bug#18544 - LOCK TABLES timeout causes MyISAM table corruption
  Resetting the lock type in the open table(s) lock data
  after a locking error.

sql/lock.cc

@@ -78,6 +78,7 @@ extern HASH open_cache;
 
 static MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table,uint count,
 				 uint flags, TABLE **write_locked);
+static void reset_lock_data(MYSQL_LOCK *sql_lock);
 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);
@@ -120,12 +121,16 @@ MYSQL_LOCK *mysql_lock_tables(THD *thd, TABLE **tables, uint count, uint flags)
       */
       if (wait_if_global_read_lock(thd, 1, 1))
       {
+        /* Clear the lock type of all lock data to avoid reusage. */
+        reset_lock_data(sql_lock);
 	my_free((gptr) sql_lock,MYF(0));
 	sql_lock=0;
 	break;
-      }	
+      }
       if (thd->version != refresh_version)
       {
+        /* Clear the lock type of all lock data to avoid reusage. */
+        reset_lock_data(sql_lock);
 	my_free((gptr) sql_lock,MYF(0));
 	goto retry;
       }
@@ -134,6 +139,8 @@ MYSQL_LOCK *mysql_lock_tables(THD *thd, TABLE **tables, uint count, uint flags)
     thd->proc_info="System lock";
     if (lock_external(thd, tables, count))
     {
+      /* Clear the lock type of all lock data to avoid reusage. */
+      reset_lock_data(sql_lock);
       my_free((gptr) sql_lock,MYF(0));
       sql_lock=0;
       thd->proc_info=0;
@@ -639,6 +646,9 @@ static MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table_ptr, uint count,
       if (table->db_stat & HA_READ_ONLY)
       {
 	my_error(ER_OPEN_AS_READONLY,MYF(0),table->table_name);
+        /* Clear the lock type of the lock data that are stored already. */
+        sql_lock->lock_count= locks - sql_lock->locks;
+        reset_lock_data(sql_lock);
 	my_free((gptr) sql_lock,MYF(0));
 	DBUG_RETURN(0);
       }
@@ -663,6 +673,48 @@ static MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table_ptr, uint count,
 }
 
 
+/*
+  Reset lock type in lock data.
+
+  SYNOPSIS
+    reset_lock_data()
+      sql_lock                  The MySQL lock.
+
+  DESCRIPTION
+
+    After a locking error we want to quit the locking of the table(s).
+    The test case in the bug report for Bug #18544 has the following
+    cases: 1. Locking error in lock_external() due to InnoDB timeout.
+    2. Locking error in get_lock_data() due to missing write permission.
+    3. Locking error in wait_if_global_read_lock() due to lock conflict.
+
+    In all these cases we have already set the lock type into the lock
+    data of the open table(s). If the table(s) are in the open table
+    cache, they could be reused with the non-zero lock type set. This
+    could lead to ignoring a different lock type with the next lock.
+
+    Clear the lock type of all lock data. This ensures that the next
+    lock request will set its lock type properly.
+
+  RETURN
+    void
+*/
+
+static void reset_lock_data(MYSQL_LOCK *sql_lock)
+{
+  THR_LOCK_DATA **ldata;
+  THR_LOCK_DATA **ldata_end;
+
+  for (ldata= sql_lock->locks, ldata_end= ldata + sql_lock->lock_count;
+       ldata < ldata_end;
+       ldata++)
+  {
+    /* Reset lock type. */
+    (*ldata)->type= TL_UNLOCK;
+  }
+}
+
+
 /*****************************************************************************
   Lock table based on the name.
   This is used when we need total access to a closed, not open table