Re: [BUG REPORT] split-index behavior during interactive rebase

On Wed, Sep 15, 2021 at 10:50:57PM -0700, Matt Roper wrote:

What did you do before the bug happened? (Steps to reproduce your issue)

  I activated split index mode on a repo ("git config core.splitIndex
  true"), performed an interactive rebase, modified a commit earlier in
  the history.

  The steps can be reproduced via a sequence of:
      $ mkdir tmp && cd tmp && git init
      $ git config core.splitIndex true
      $ for x in `seq 20`; do echo $x >> count; git add count; git commit -m "Commit $x"; done

It's important to note that this test repository has only a single
tracked file in it.

      $ git rebase -i HEAD~10
      
      ## Add "x git commit --amend --no-edit" as the first command of
      ## the todo list.

What did you expect to happen? (Expected behavior)

  My expectation was that there would still only be a single shared index
  file in the .git directory upon completion of the rebase.

What happened instead? (Actual behavior)

  A large number of distinct sharedindex.* files were generated in the .git
  directory during the rebase.

I think this works as intended.

A new shared index is written when the number of index entries that
would be writen to '.git/index' is higher than a given percentage of
the total number of index entries.  This percentage can be specified
with the 'splitIndex.maxPercentChange' configuration variable and it
defaults to 20%.  In your test repository above there is only a single
file and it is modified in every commit, so when switching from one
commit to the other 100% of the index entries would be written to
'.git/index', resulting in a new shared index file written for each
rebase step.

What's different between what you expected and what actually happened?

  Rather than a single shared index file, I wound up with a huge number of
  large shared index files.  The real repository I was working with (a Linux
  kernel source tree) had a shared index file size of about 7MB, and I was
  modifying a commit several hundred back in history (in case it
  matters, these were all linear commits, no merges), so the resulting
  collection of shared index files consumed a surprising amount of disk
  space.

The last commit in my somewhat outdated linux repo contains ~71k
files, the 20% of that is ~14k.  Does that linear string of "several
hundred" commits modify that many files?

Anything else you want to add:

  As an experiment, I tried setting splitIndex.sharedIndexExpire=now

I would advise against that, it's potentially dangerous, because it
can remove shared index files that are still in use by other git
processes.

  to see
  if it would avoid the explosion of shared index files, but it appears the
  stale index files are still not being removed during the rebase, and I
  still wind up with a huge number at the end of the rebase.  If I manually
  run "git update-index --split-index" after the rebase completes it will
  properly delete all of the stale ones at that point.

  Rebases that do not actually modify the history do _not_ trigger the
  explosion of shared index files (e.g., "git rebase -i HEAD~10 --exec 'echo
  foo'").

  If I do not set the core.splitIndex setting on the repository, but only
  activate split index manually via "git update-index --split-index" there
  is only one shared index file at the end of the rebase, but based on the
  file size it appears the repository is no longer operating in split index
  mode.

  Before:
  $ ll .git | grep index
  -rw-rw-r--   1 mdroper mdroper   149165 Sep 15 22:21 index
  -rw-rw-r--   1 mdroper mdroper  7296080 Sep 15 22:21 sharedindex.f916dd59ccc22ca34298f557a4659aca2767dae4

  After (just amending HEAD~1 in this case):
  $ ls -l .git | grep index
  -rw-rw-r--   1 mdroper mdroper  7445145 Sep 15 22:22 index
  -rw-rw-r--   1 mdroper mdroper  7296080 Sep 15 22:22 sharedindex.f916dd59ccc22ca34298f557a4659aca2767dae4

git version 2.33.0

I could reproduce all this with v2.33.0 (except that I saw the split
index being turned off even with core.splitIndex enabled), but was
unable to do so with current master.

I think that this is a bug in the interaction between the split index
feature and 'git rebase' when using the recursive merge strategy and
when a couple of other, more subtle conditions are met.  It seems that
with the right conditions rebase only writes regular index files, and
by not entering the split index code paths it doesn't look for old
shared index files to expire.

After v2.33.0 we switched the default merge strategy from recursive to
'ort', and with that these cases appear to work as intended, i.e. old
shared index files are expired and the split index feature doesn't get
turned off.  Since the 'ort' strategy is in many ways better (faster,
more correct, etc.) than the recursive, I don't think it's worth the
effort to try to fix this issue with split index, rebase and the
recursive strategy.