As discussed [1], there is some concern around binary file formats requiring
the context of the repository config in order to infer hash lengths. Two
formats that were designed with the hash transition in mind (commit-graph
and multi-pack-index) have bytes available to indicate the hash algorithm
used. Let's actually update these formats to be more self-contained with the
two hash algorithms being available.

[1]
https://lore.kernel.org/git/CAN0heSp024=Kyy7gdQ2VSetk_5iVhj_qdT8CMVPcry_AwWrhHQ@mail.gmail.com/

This merges cleanly with tb/bloom-improvements, but both that branch and
this patch series have merge conflicts with the corrected commit date patch
series [2].

[2]
https://lore.kernel.org/git/pull.676.v2.git.1596941624.gitgitgadget@gmail.com/

In particular, the following conflict can be resolved in the "obvioius" way:

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< HEAD
    header: 43475048 1 $(test_oid oid_version) 3 $NUM_BASE
================================
    header: 43475048 1 1 4 $NUM_BASE
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> abhishek/corrected_commit_date

Instead use:

    header: 43475048 1 $(test_oid oid_version) 4 $NUM_BASE

But, it also needs the following fix to actually work with this series:

diff --git a/t/t5324-split-commit-graph.sh b/t/t5324-split-commit-graph.sh
index 211ec625d2..09f133792c 100755
--- a/t/t5324-split-commit-graph.sh
+++ b/t/t5324-split-commit-graph.sh
@@ -464,7 +464,7 @@ test_expect_success 'setup repo for mixed generation commit-graph-chain' '
        GIT_TEST_COMMIT_GRAPH_NO_GDAT=1 git commit-graph write --reachable --split=no-merge &&
        test-tool read-graph >output &&
        cat >expect <<-EOF &&
-       header: 43475048 1 1 4 1
+       header: 43475048 1 $(test_oid oid_version) 4 1
        num_commits: 2
        chunks: oid_fanout oid_lookup commit_metadata
        EOF
@@ -482,7 +482,7 @@ test_expect_success 'does not write generation data chunk if not present on exis
        git commit-graph write --reachable --split=no-merge &&
        test-tool read-graph >output &&
        cat >expect <<-EOF &&
-       header: 43475048 1 1 4 2
+       header: 43475048 1 $(test_oid oid_version) 4 2
        num_commits: 3
        chunks: oid_fanout oid_lookup commit_metadata
        EOF

If this is the way we want to go with the formats, then I'll assist
coordinating these textual and semantic merge conflicts.

UPDATES IN V2
=============

 * Documentation is updated, thanks to Eric's suggestion.
 * The implementation of oid_version() and the way we access it in the test
   scripts is improved, thanks to Brian's suggestion.
 * I use "mv" instead of "cp" in the cross-version tests because of a
   subtlety on macOS when overwriting these files.

Thanks, -Stolee

Derrick Stolee (3):
  t/README: document GIT_TEST_DEFAULT_HASH
  commit-graph: use the "hash version" byte
  multi-pack-index: use hash version byte

 .../technical/commit-graph-format.txt         |  9 +++-
 Documentation/technical/pack-format.txt       |  7 ++-
 commit-graph.c                                |  9 +++-
 midx.c                                        | 35 ++++++++++++---
 t/README                                      |  4 ++
 t/helper/test-read-midx.c                     |  8 +++-
 t/t4216-log-bloom.sh                          |  9 +++-
 t/t5318-commit-graph.sh                       | 38 +++++++++++++++-
 t/t5319-multi-pack-index.sh                   | 43 +++++++++++++++++--
 t/t5324-split-commit-graph.sh                 |  5 ++-
 10 files changed, 146 insertions(+), 21 deletions(-)

base-commit: 878e727637ec5815ccb3301eb994a54df95b21b8

Submitted-As: https://lore.kernel.org/git/pull.703.v2.git.1597673089.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.703.git.1597428440.gitgitgadget@gmail.com