[PATCH 0/6] commit-graph: use 'struct object_directory *' everywhere

Hi,

While working on deploying layered commit-graphs at GitHub, I noticed
that I was able to trigger the mechanism to prevent merging
cross-alternate commit-graph layers in funny ways, like claiming that
"./objects" and "objects" were in different directories.

This resulted in removing all of the places in the commit-graph.c code
that use 'char *object_dir's with 'struct object_directory *'s instead.
This allows us to replace brittle string-based path comparisons with raw
pointer comparisons to check whether two commit-graphs belong to the
same object store or not.

This has a pleasant side effect in PATCH 5/6 of fixing an uninitialized
read as described in [1].

This series became a little bit longer than I was expecting it to be, so
here is the high-level structure:

  - 1/6 fixes a bug in a test that would cause a subsequent failure if
    unaddressed.

  - 2/6 does the first half of the removal by using 'struct
    object_directory *'s within the 'commit_graph' structure.

  - 4/6 does the second half by removing 'char *object_dir' usage in the
    'write_commit_graph_context' structure.

  - 5/6 ties 2/6 and 4/6 together by removing all path normalization
    completely, fixing the uninitialized read bug.

  - And 6/6 cleans up.

We've been running a version of this series based on our fork (which is
in turn based on 2.24) for a few hours without issue.

Thanks in advance for your review.

[1]: https://lore.kernel.org/git/20191027042116.GA5801@sigill.intra.peff.net/ (local)

Taylor Blau (6):
  t5318: don't pass non-object directory to '--object-dir'
  commit-graph.h: store object directory in 'struct commit_graph'
  builtin/commit-graph.c: die() with unknown '--object-dir'
  commit-graph.h: store an odb in 'struct write_commit_graph_context'
  commit-graph.c: remove path normalization, comparison
  commit-graph.h: use odb in 'load_commit_graph_one_fd_st'

 Documentation/git-commit-graph.txt |   5 +-
 builtin/commit-graph.c             |  25 ++++--
 builtin/commit.c                   |   3 +-
 builtin/fetch.c                    |   2 +-
 builtin/gc.c                       |   2 +-
 commit-graph.c                     | 135 +++++++++++++++--------------
 commit-graph.h                     |  17 ++--
 t/helper/test-read-graph.c         |   8 +-
 t/t5318-commit-graph.sh            |   4 +-
 9 files changed, 114 insertions(+), 87 deletions(-)

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2.25.0.dirty