Re: Segfault in the attr stack

> Junio C Hamano <gitster@pobox.com> writes:
>
>> Gaah, of course.
>>
>> This is coming from the cache preload codepath, where multiple threads
>> try to run ce_path_match().
>> It used to be OK because pathspec magic never looked at attributes,
>> but now it does, and attribute system is not thread-safe.
>
> The symlink check code has dealt with a similar issue in the past in
> the codepath.  threaded_has_symlink_leading_path() is called with
> per-thread data structure called "cache", because the leading
> symbolic link check uses a cache that is shared across different
> invocations of the check.
>
> We need something similar to duplicate the attribute stack per
> thread.  git_check_attrs() currently uses the singleton instance of
> attr_stack chain, that is rooted at the file scope global
> "attr_stack", and no wonder that would be clobbered when multiple
> threads try to use it.  The result of attribute collection is also
> accumulated in a file scope global, which should probably be moved
> to the "struct git_attr_check" introduced in the jc/attr topic.
>
> We need to teach the callchain that includes prepare_attr_stack()
> and bootstrap_attr_stack() to take a pointer to the attr_stack root,
> give git_check_attr_threaded() that takes such root so that threaded
> code can use per-thread attr stack, and for non-threaded
> applications use &the_default_attr just like the file scope global
> "default_cache" is used in symlinks.c.  Then a threaded attribute
> lookup can maintain its own attr_stack when running more than one
> instance of lookup.