Bug #63380: Use PHP's allocator for libxml

Allocation via libxml does not use PHP's per-request allocator. So any memory used by libxml will not be accounted against memory_get_usage() or memory_limit.

At Wikimedia we use libxml DOM trees to store wikitext parse trees, because they are more compact in memory than the equivalent pure-PHP data structures. When these parse trees are cached, the memory requirements can become excessive, and the memory is typically not returned to the system after request termination. Using xmlMemSetup() to set hook functions which use PHP's per-request allocation functions will allow us to more effectively monitor and limit the use of libxml in production.

The task is somewhat complicated by the fact that libxml does not use a different hook for persistent and request-local allocations. So we use the persistent allocator during MINIT, and take extra care to ensure all libxml globals are initialised during MINIT, rather than lazy-initialised on the first call to the relevant module.

The typical consequences of the initialisation of a global pointer during request execution would be a dangling pointer after deactivate, which is exploitable for memory corruption during subsequent requests. I used gdb's "info variables" command to audit all global variables in libxml, as configured in the Ubuntu package.

I tested the code with make test, and with server-tests.php against Apache with "worker" MPM. I also tested it with and without LIBXML_THREAD_ALLOC_ENABLED. There was a bug in libxml2 which prevented it from working with LIBXML_THREAD_ALLOC_ENABLED. I submitted a patch:

https://bugzilla.gnome.org/show_bug.cgi?id=687084

That configuration option appears to be rarely used.