Bug 554903

http://xmlsoft.org/html/libxml-parser.html#xmlCleanupParser

"This function name is somewhat misleading. It does not clean up parser state, it cleans up memory allocated by the library itself. It is a cleanup function for the XML library. It tries to reclaim all related global memory allocated for the library processing. It doesn't deallocate any document related memory. One should call xmlCleanupParser() only when the process has finished using the library and all XML/HTML documents built with it. See also xmlInitParser() which has the opposite function of preparing the library for operations. WARNING: if your application is multithreaded or has plugin support calling this may crash the application if another thread or a plugin is still using libxml2. It's sometimes very hard to guess if libxml2 is in use in the application, some libraries or plugins may use it without notice. In case of doubt abstain from calling this function or do it just before calling exit() to avoid leak reports from valgrind !"

How is that not clear enough ?

Daniel

Note that an application could perfectly call xmlCleanupParser() multiple
time, if it controls the fact that it's the single user of the library
and uses libxml2 at very specific moments. So I'm still unclear about the
warning there, though for the majority of use it's likely to be a real problem
but you can't garantee it.

Daniel

(In reply to comment #1)
> http://xmlsoft.org/html/libxml-parser.html#xmlCleanupParser
> 
> How is that not clear enough ?

It's a simple fact that nobody has read that, even if they should. They probably just did a copy/paste from some other code which already did it wrong.

A google code search or a debian source code grep shows that really about nobody got this right. empathy, abiword, inkscape, dia, you name it.

There is no doubt that that those apps are broken and need to be fixed. The problem is that it is really hard to track them all down. It would make things a lot easier if libxml2 would by itself warn about this misuse. It's a trivial patch and would make it easy to fix the apps in question. This bug is not about something to fix in libxml2, it's just about something that would make it easier to track down those who misuse it.

Also, in times where people use gcc destructors and threaded code I believe it is practically impossible to call xmlCleanupParser() properly, in everything but the most trivial code. However, if you want to allow calling it, wouldn't it be possible to at least makeit idempotent? Or count how often it was called and decrease the counter if the library is reinitialized again?

(In reply to comment #2)
> Note that an application could perfectly call xmlCleanupParser() multiple
> time, if it controls the fact that it's the single user of the library
> and uses libxml2 at very specific moments.

Daniel,
you are wrong here. Even if the program is the only user of libxml2, it can still get into trouble by calling xmlCleanupParser() multiple times. I'll attach a trivial reproducer.

Created attachment 403444 [details]
reproducer to show xmlCleanupParser() is never safe to call twice

Build it with:
gcc -Wall -g -lpthread `pkg-config --libs --cflags libxml-2.0` -o xml2test xml2test.c

$ ./xml2test 
Failed to set the thread-specific value: Invalid argument

Daniel, any comments about the single-threaded reproducer in comment 5?

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