The origin of “Assertion Failed” errors in MySQL

Recently, we have discussed BUGs with our local community members. Where the half off people said that, they have seen “Assertion Failure” errors in error log. And the question was what is this at all?

In general “assert” is a code portion put by developer to handle certain things.

Well, i am not a core MySQL developer and my C/C++ knowledge is limited to simple calculator 🙂 But i will try to explain where all these “Assertion Failure” errors come up.
The exact reason of failing Assertion/crash should be known by core developers. I know how to crash, they know how to fix. You help developers to reproduce a crash filling a bug report, by uploading test cases or maybe the core dump, backtrace etc.
Say, for eg, if you try to create a view after getting “full disk” error you will crash MySQL. See related BUG report -> #76827

While testing full disk conditions, if we try to import huge dump, we will see:

2015-04-24 09:07:52 7faf8bcab700 InnoDB: Error: Write to file ./sales/sales.ibd failed at offset 247463936.
InnoDB: 1048576 bytes should have been written, only 299008 were written.
InnoDB: Operating system error number 11.
InnoDB: Check that your OS and file system support files of this size.
InnoDB: Check also that the disk is not full or a disk quota exceeded.
InnoDB: Error number 11 means 'Resource temporarily unavailable'.
InnoDB: Some operating system error numbers are described at
2015-04-24 09:07:52 28213 [ERROR] /opt/mysql/bin/mysqld: The table 'sales' is full
2015-04-24 09:08:13 28213 [ERROR] /opt/mysql/bin/mysqld: The table 'sales' is full
2015-04-24 09:08:27 28213 [ERROR] /opt/mysql/bin/mysqld: The table 'sales' is full

After detecting Full Disk error, if you try to create view:

mysql> create view f as select * from sales;
ERROR 2013 (HY000): Lost connection to MySQL server during query

From error log:

mysqld: /root/mysql-5.6.24/mysys/mf_iocache.c:1799: my_b_flush_io_cache: Assertion `info->end_of_file == inline_mysql_file_tell("/root/mysql-5.6.24/mysys/mf_iocache.c", 1799, info->file, (myf) (0))' failed.
13:13:48 UTC - mysqld got signal 6 ;

So that’s all, from error log it obvious that there is a file named: /root/mysql-5.6.24/mysys/mf_iocache.c and on line 1799 there is an assert inside my_b_flush_io_cache function.
If you go ahead and open up this line you will see something like:

       DBUG_ASSERT(info->end_of_file == mysql_file_tell(info->file, MYF(0)));

For now let’s pause here and introduce new things, such as what is “Optimized”, “Debug” and “Valgrind” builds of MySQL. Please watch this wonderfull video recorded by QA expert Roel Van de Paar after you will learn about newly intoduced topics. -> MySQL QA Episode 11

In general the “Optimized” MySQL is a GA version of MySQL released by vendors. It is production ready and it is working as fast as possible. So there is no “unnecessary” codes inside this build.
The “Debug” build is for debugging purpose and there is a DEBUG instrumentation code portions inside source code.
in “Valgrind” build, there are “Debug” + “Valgrind” instrumentation codes inside source code.

So above we saw DBUG_ASSERT(info->end_of_file == mysql_file_tell(info->file, MYF(0))); -> It means that this “assert” code will be shown only with “Debug” build. You will not see this code in “Optimized” MySQL source code.

Ok, let’s go on. As we have mentioned “assert” code is written by developer to handle several conditions. It might be for eg, developer decides that, if variable named num will be equal to NULL something weird is happened, terminate the program at that point.

Let’s write a simple code with our very own “assert”. Here is our assert_test.c file:

#include <stdio.h>      /* printf */
#include <assert.h>     /* assert */

void check_number(int myInt) {
  assert (myInt!=NULL);
  printf ("%dn",myInt);

int main ()
  int a=10;
  int b = NULL;

  check_number (a);
  check_number (b);

  return 0;

We have put an “assert” ensuring that myInt will never be “NULL”.
Compile source code:

    gcc assert_test.c -o assert_test

And run:

sh@shrzayev:~$ ./assert_test
assert_test: assert_test.c:5: check_number: Assertion `myInt!=((void *)0)' failed.
Aborted (core dumped)

So as you see, the same error comes up with our little code.
We have assert_test.c file and inside check_number function at line 5 there is an “assert” code which is failed.

I Hope have explained this point. Thank you for reading.

Author: Shahriyar Rzayev

Azerbaijan MySQL User Group leader.

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