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Re: Review request for a fix for MDEV-5506


Hi Sergei,

On 01/27/2014 05:52 PM, Sergei Golubchik wrote:
Hi, Alexander!

On Jan 27, Alexander Barkov wrote:
On 01/26/2014 11:23 PM, Sergei Golubchik wrote:

Okay, I understand what you did. But I don't understand why.
What was the problem? Why there was a crash?

- Item_func_make_set::val_str() returns &my_empty_string
After the fix my_empty_string:
- remembers that it is null terminated and does not call realloc() from
- remembers that it is read-only and protects itself from realloc() by

I'd rather fix Item_func_make_set instead. Now it seems to be the only
Item that uses it (there's also SELECT ... OUTFILE, but that's

And Item_func_make_set has String tmp_str property, which is already
used as an internal storage.

So, I'd remove my_empty_string usage from Item_func_make_set whatsoever.
Simply with

-   String *result=&my_empty_string;
+   String *result=&tmp_str;
+   tmp_str.length(0);

In fact, perhaps Item_func_make_set::tmp_str is not needed at all, and
Item:str_value can be used instead:

-   String *result=&my_empty_string;
+   String *result=make_empty_result();

These types of bug are annoying. We've fixed a lot of them.
I hoped to extend the new code gradually for non-static strings as well:

- to fix String::c_ptr*() not to use realloc if constructor knows that
  the value is null terminated.
- protect use of c_ptr*() is unsafe contexts.

and also put String::thread_specific into String::flags when
merging to 10.0.

But this all can be done in 10.0 only. No need to do it in 5.3.

I will fix 5.3 simply to use make_empty_result().


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