maria-developers team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, Monty!

On Mar 31, Michael Widenius wrote:
> > > > --- a/sql/sql_string.h
> > > > +++ b/sql/sql_string.h
> > > > @@ -600,25 +600,34 @@ class Binary_string: public Static_binary_string
> > > >
> > > >    inline char *c_ptr()
> > > >    {
> > > > -    DBUG_ASSERT(!alloced || !Ptr || !Alloced_length ||
> > > > -                (Alloced_length >= (str_length + 1)));
> > > > -
> > > > -    if (!Ptr || Ptr[str_length])              // Should be safe
> > > > -      (void) realloc(str_length);
> > > > +    if (unlikely(!Ptr))
> > > > +      return (char*) "";
> > > > +    /*
> > > > +      Here we assume that any buffer used to initalize String has
> > > > +      an end \0 or have at least an accessable character at end.
> > > > +      This is to handle the case of String("Hello",5) efficently.
> > > > +    */
> > > > +    if (unlikely(!alloced && !Ptr[str_length]))
> > > > +      return Ptr;
> > >
> > > No, this is not good. Note the difference between
> > >
> > >   String a("Hello", 5)
> > >
> > > and
> > >
> > >   char hello[5];
> > >   String a(buf, 5);
> > >
> > > Your assumption should only apply to the first case, not to the second.
> > > In  the first case alloced=Alloced_length=0, in the second case only
> > > alloced=0 and Alloced_length=5. So in the if() above you need to look
> > > at Alloced_length:
> > >
> > >   if (!Alloced_length && !Ptr[str_length])
> > >     return Ptr;
> 
> Unfortunately this does not work good with our code.
> 
> We have a lot of code that does things like this:
>  String *new_str= new (thd->mem_root) String((const char*) name.str,
>                                               name.length,
>                                               system_charset_info)
> Where string is 0 terminated.
> With your proposed change, all of these will require a full realloc
> when using c_ptr().

Hmm, I don't understand. The difference between

  String a("Hello", 5);

and

  char hello[5];
  String a(buf, 5);

is that in the first case the argument is const char*. And in that case
Alloced_length=0 and in that case you can expect the string to be \0
terminated.

In your example that we do "a lot", the first argument is const char*,
so String will have Alloced_length==0 and my suggestion will work
exactly as planned, it'll use a \0 terminated fast path.

Regards,
Sergei
VP of MariaDB Server Engineering
and security@xxxxxxxxxxx