Race in git_packfile_unpack #2906
Description
I'm trying to generate a bunch of diffs in parallel with git_diff_tree_to_tree. When I run it on a largish repo (like git.git) it crashes after a while. Address sanitizer points to a sort of race condition in git_packfile_unpack related to caching. I'll include the address sanitizer output below.
As far as I can tell, the problem starts at the call to cache_add around line 702 in pack.c. A cache entry gets added without incrementing its refcount. Then the underlying data gets used in the subsequent call to git__delta_apply. The issue is that another thread can come along and evict that cache entry if the cache is full. The underlying data gets destroyed since the refcount is zero.
I have a fairly trivial patch to increment the refcount on cache add. I'll got ahead and submit it for review unless someone more familiar with that code wants to look at it.