Re: [PATCH 0/2] Sparse Index: fix a checkout bug with deep sparse-checkout patterns

On Fri, Dec 3, 2021 at 6:55 PM Derrick Stolee via GitGitGadget
[off-list ref] wrote:

This week, we rolled out the sparse index to a large internal monorepo. We
got two very similar bug reports that dealt with a strange error that
involved the same set of paths. One was during git pull (pull was a red
herring) and the other was git checkout. The git checkout case gave enough
of a reproduction to debug deep into unpack-trees.c and find the problem.

This bug dates back to 523506d (unpack-trees: unpack sparse directory
entries, 2021-07-14). The reason we didn't hit this before is because it
requires the following:

 1. The sparse-checkout definition needs to have recursive inclusion of deep
    folders (depth 3 or more).
 2. Adjacent to those deep folders, we need a deep sparse directory entry
    that receives changes.
 3. In this particular repo, deep directories are only added to the
    sparse-checkout in rare occasions and those adjacent folders are rarely
    updated. They happened to update this week and hit our sparse index
    dogfooders in surprising ways.

The first patch adds a test that fails without the fix. It requires
modifying our test data to make adjacent, deep sparse directory entries
possible. It's a rather simple test after we have that data change.

The second patch includes the actual fix. It's really just an error of not
understanding the difference between the name and traverse_path members of
the struct traverse_info structure. name only stores a single tree entry
while traverse_path actually includes the full name from root. The method we
are editing also has an additional struct name_entry that fills in the tree
entry on top of the traverse_path, which explains how this worked to depth
two, but not depth three.

Thanks for the detailed explanation.  I looked around for similar
potential problems elsewhere, but only noted that the comment at the
top of the function is also wrong and should be updated (as I
commented on Patch 2).  After you fix the comment similarly, feel free
to add my

Reviewed-by: Elijah Newren <redacted>