In some cases pygit2.Repository.diff() returns no difference between trees that are actually different

[Robie Basak]

Steps to reproduce:

git clone https://git.launchpad.net/~racb/+git/pygit2-tree-compare-bug
cd pygit2-tree-compare-bug
python3
import pygit2
r = pygit2.Repository('.')
tag1 = r.get('12f94a0b8488844522310b9eda9cbf183e24d7ab')
tag2 = r.get('bee50a71086cf4ae8f74fe9f0690404caf280d5e')
len(r.diff(tag1, tag2))

Expected result: non-zero
Actual result: zero (0)

This differs from reality. "git diff 12f94a0b8488844522310b9eda9cbf183e24d7ab bee50a71086cf4ae8f74fe9f0690404caf280d5e" does produce results. If I peel the Tag objects to Tree objects, these present the same (different) ids as <hash>^{tree} on the git command line. If I fetch the tree objects by their hashes directly using r.get(), the problem does not appear to occur.

I'm using python3-pygit2 0.24.0-0ubuntu2 on Ubuntu 16.04.

[Robie Basak (racb)]

Also affects python3-pygit2 0.24.2-2build1 on Artful.

Does not affect pygit2 built from upstream master (commit c6305a062ba190fa4b968614911f0def752cc264) against libgit2 upstream git tag v0.25.1 (commit 2fcb8705e584ca61f6c4657525c9d2713f6a39d2). Current libgit2 upstream git master builds some v0.26 soname against which current pygit2 master refuses to build.

[Robie Basak (racb)]

I'm leaving this bug for now. It's valid, but we have a trivial workaround for what we need (compare the hashes directly) and it looks like this doesn't affect today's upstream so it should get fixed in the Ubuntu development release in the end with no specific action.