This version is now ready for review.

The commit-graph is a valuable performance feature for repos with large
commit histories, but suffers from the same problem as git repack: it
rewrites the entire file every time. This can be slow when there are
millions of commits, especially after we stopped reading from the
commit-graph file during a write in 43d3561 (commit-graph write: don't die
if the existing graph is corrupt).

Instead, create a "chain" of commit-graphs in the
.git/objects/info/commit-graphs folder with name graph-{hash}.graph. The
list of hashes is given by the commit-graph-chain file, and also in a "base
graph chunk" in the commit-graph format. As we read a chain, we can verify
that the hashes match the trailing hash of each commit-graph we read along
the way and each hash below a level is expected by that graph file.

When writing, we don't always want to add a new level to the stack. This
would eventually result in performance degradation, especially when
searching for a commit (before we know its graph position). We decide to
merge levels of the stack when the new commits we will write is less than
half of the commits in the level above. This can be tweaked by the
--size-multiple and --max-commits options.

The performance is necessarily amortized across multiple writes, so I tested
by writing commit-graphs from the (non-rc) tags in the Linux repo. My test
included 72 tags, and wrote everything reachable from the tag using
--stdin-commits. Here are the overall perf numbers:

write --stdin-commits:         8m 12s
write --stdin-commits --split:    28s
write --split && verify --shallow: 60s

Updates in V3:

 * git commit-graph verify now works on commit-graph chains. We do a simple
   test to check the behavior of a new --shallow option.

 * When someone writes a flat commit-graph, we now expire the old chain
   according to the expire time.

 * The "max commits" limit is no longer enabled by default, but instead is
   enabled by a --max-commits=<n> option. Ignored if n=0.

Updates in V4:

Johannes pointed out some test failures on the Windows platform. We found
that the tests were not running on Windows in the gitgitgadget PR builds,
which is now resolved.

 * We need to close commit-graphs recursively down the chain. This prevented
   an unlink() from working because of an open handle.

 * Creating the alternates file used a path-specification that didn't work
   on Windows.

 * Renaming a file to the same name failed, but is probably related to the
   unlink() error mentioned above.

Updates in V5:

 * Responding to multiple items of feedback. Thanks Philip, Junio, and
   Ramsay!

 * Used the test coverage report to find holes in the test coverage. While
   adding tests, I found a bug in octopus merges. The fix is in the rewrite
   of "deduplicate_commits()" as "sort_and_scan_merged_commits()" and
   covered by the new tests.

Updates in V6:

 * Rebased onto ds/close-object-store and resolved conflicts around
   close_commit_graph().

 * Updated path normalization to be resilient to double-slashes and trailing
   slashes.

 * Added a prepare_alt_odb() call in load_commit_graph_one() for
   cross-alternate graph loads during 'verify' subcommands.

Thanks, -Stolee

[1]
https://github.com/git/git/commit/43d356180556180b4ef6ac232a14498a5bb2b446
commit-graph write: don't die if the existing graph is corrupt

Derrick Stolee (18):
  commit-graph: document commit-graph chains
  commit-graph: prepare for commit-graph chains
  commit-graph: rename commit_compare to oid_compare
  commit-graph: load commit-graph chains
  commit-graph: add base graphs chunk
  commit-graph: rearrange chunk count logic
  commit-graph: write commit-graph chains
  commit-graph: add --split option to builtin
  commit-graph: merge commit-graph chains
  commit-graph: allow cross-alternate chains
  commit-graph: expire commit-graph files
  commit-graph: create options for split files
  commit-graph: verify chains with --shallow mode
  commit-graph: clean up chains after flattened write
  commit-graph: test octopus merges with --split
  commit-graph: test --split across alternate without --split
  commit-graph: normalize commit-graph filenames
  commit-graph: test verify across alternates

 Documentation/git-commit-graph.txt            |  26 +-
 .../technical/commit-graph-format.txt         |  11 +-
 Documentation/technical/commit-graph.txt      | 195 +++++
 builtin/commit-graph.c                        |  58 +-
 builtin/commit.c                              |   2 +-
 builtin/gc.c                                  |   3 +-
 commit-graph.c                                | 823 ++++++++++++++++--
 commit-graph.h                                |  25 +-
 t/t5318-commit-graph.sh                       |   2 +-
 t/t5324-split-commit-graph.sh                 | 343 ++++++++
 10 files changed, 1414 insertions(+), 74 deletions(-)
 create mode 100755 t/t5324-split-commit-graph.sh

base-commit: 2d511cfc0bfe1d2b98ba8b272ddd9ba83e84e5f8

Submitted-As: https://public-inbox.org/git/pull.184.v6.git.gitgitgadget@gmail.com
In-Reply-To: https://public-inbox.org/git/pull.184.git.gitgitgadget@gmail.com
In-Reply-To: https://public-inbox.org/git/pull.184.v2.git.gitgitgadget@gmail.com
In-Reply-To: https://public-inbox.org/git/pull.184.v3.git.gitgitgadget@gmail.com
In-Reply-To: https://public-inbox.org/git/pull.184.v4.git.gitgitgadget@gmail.com
In-Reply-To: https://public-inbox.org/git/pull.184.v5.git.gitgitgadget@gmail.com