Re: [xml] Per parser xmlGcMemSetup

[Rob Richards <rrichards ctindustries net>]

Hi Joachim,

Joachim Zobel wrote:
> Hi.
>
> I have an apache module that uses libxml2. I have found a (potential, no
> reports yet) problem with its usage of xmlGcMemSetup. I need to call
> this for my module to work correctly. If there are other modules using
> libxml2 (mod_php does) and they call xmlGcMemSetup, one of the modules
> will stop working.
>   
PHP does not change the memory handling routines (although this had been 
a request early on but turned down). Doing so would start screwing 
around with other modules that use libxml2. I urge you not to do this if 
your module is for more than just some specific implementation and going 
to be distributed publicly.
> The problem is not threads, the problem is that other parts of apache
> may be using libxml2 globals independently.
>   
There is pretty much a gentleman's agreement with the other module 
developers using libxml2 that I know not to abuse the globals. This 
allows for all the modules to co-exist peacefully. When problems are 
identified in other modules, everyone, so far at least, is amenable to 
altering their code to make sure they all play nice together.

- During module initialization, don't alter globals (as that impacts all 
other modules and also can be problematic depending upon order of module 
initialization). For example, I limit my call to xmlInitParser to insure 
it gets initialized in main thread.

- Try not to use globals. Many functions have been added over the years 
that allow settings/options to be passed directly eliminating the need 
to use globals.

- If having to use globals, make sure they do not persists over the 
request. Any changes to globals should be reset to previous state as 
soon as possible.
> I have just found globals.c, but do not understand it yet. Are there any
> examples on how to use this?
>   
The globals are really not that difficult. Here is a small example of 
disabling empty tags I use when supporting older libxml2 versions (prior 
to being able to use the  XML_SAVE_NO_EMPTY option). Notice that it is 
reset back to original value right after use.
/* get curent setting and then set to 1 */
int saveempty = xmlSaveNoEmptyTags;
xmlSaveNoEmptyTags = 1;

... /* your code here */

/* reset global to previous value */
xmlSaveNoEmptyTags = saveempty;

Rob