[PATCH v6] subtree: fix split processing with multiple subtrees present

From: Zach FettersMoore <zach.fetters@apollographql.com>

When there are multiple subtrees present in a repository and they are all using 'git subtree split', the 'split' command can take a significant (and constantly growing) amount of time to run even when using the '--rejoin' flag. This is due to the fact that when processing commits to determine the last known split to start from when looking for changes, if there has been a split/merge done from another subtree there will be 2 split commits, one mainline and one subtree, for the second subtree that are part of the processing. The non-mainline subtree split commit will cause the processing to always need to search the entire history of the given subtree as part of its processing even though those commits are totally irrelevant to the current subtree split being run.

To see this in practice you can use the open source GitHub repo 'apollo-ios-dev' and do the following in order:

-Make a changes to a file in 'apollo-ios' and 'apollo-ios-codegen'
 directories
-Create a commit containing these changes
-Do a split on apollo-ios-codegen
   - Do a fetch on the subtree repo
      - git fetch git@github.com:apollographql/apollo-ios-codegen.git
   - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
   - Depending on the current state of the 'apollo-ios-dev' repo
     you may see the issue at this point if the last split was on
     apollo-ios
-Do a split on apollo-ios
   - Do a fetch on the subtree repo
      - git fetch git@github.com:apollographql/apollo-ios.git
   - git subtree split --prefix=apollo-ios --squash --rejoin
-Make changes to a file in apollo-ios-codegen
-Create a commit containing the change(s)
-Do a split on apollo-ios-codegen
   - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
-To see that the patch fixes the issue you can use the custom subtree
 script in the repo so following the same steps as above, except
 instead of using 'git subtree ...' for the commands use
 'git-subtree.sh ...' for the commands

You will see that the final split is looking for the last split on apollo-ios-codegen to use as it's starting point to process commits. Since there is a split commit from apollo-ios in between the 2 splits run on apollo-ios-codegen, the processing ends up traversing the entire history of apollo-ios which increases the time it takes to do a split based on how long of a history apollo-ios has, while none of these commits are relevant to the split being done on apollo-ios-codegen.

So this commit makes a change to the processing of commits for the split command in order to ignore non-mainline commits from other subtrees such as apollo-ios in the above breakdown by adding a new function 'should_ignore_subtree_commit' which is called during 'process_split_commit'. This allows the split/rejoin processing to still function as expected but removes all of the unnecessary processing that takes place currently which greatly inflates the processing time. In the above example, previously the final split would take ~10-12 minutes, while after this fix it takes seconds.

Added a test to validate that the proposed fix solves the issue.

The test accomplishes this by checking the output of the split command to ensure the output from the progress of 'process_split_commit' function that represents the 'extracount' of commits processed remains at 0, meaning none of the commits from the second subtree were processed.

This was tested against the original functionality to show the test failed, and then with this fix to show the test passes.

This illustrated that when using multiple subtrees, A and B, when doing a split on subtree B, the processing does not traverse the entire history of subtree A which is unnecessary and would cause the 'extracount' of processed commits to climb based on the number of commits in the history of subtree A.

Signed-off-by: Zach FettersMoore <zach.fetters@apollographql.com>
---
    subtree: fix split processing with multiple subtrees present
    
    When there are multiple subtrees in a repo and git subtree split
    --rejoin is being used for the subtrees, the processing of commits for a
    new split can take a significant (and constantly growing) amount of time
    because the split commits from other subtrees cause the processing to
    have to scan the entire history of the other subtree(s). This patch
    filters out the other subtree split commits that are unnecessary for the
    split commit processing.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1587%2FBobaFetters%2Fzf%2Fmulti-subtree-processing-v6
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1587/BobaFetters/zf/multi-subtree-processing-v6
Pull-Request: https://github.com/gitgitgadget/git/pull/1587

Range-diff vs v5:

 1:  e7445a95f30 ! 1:  2a65ec0e4df subtree: fix split processing with multiple subtrees present
     @@ Commit message
          To see this in practice you can use the open source GitHub repo
          'apollo-ios-dev' and do the following in order:
      
     -    -Make a changes to a file in 'apollo-ios'A and 'apollo-ios-codegen'
     +    -Make a changes to a file in 'apollo-ios' and 'apollo-ios-codegen'
           directories
          -Create a commit containing these changes
          -Do a split on apollo-ios-codegen
     +       - Do a fetch on the subtree repo
     +          - git fetch git@github.com:apollographql/apollo-ios-codegen.git
             - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
     +       - Depending on the current state of the 'apollo-ios-dev' repo
     +         you may see the issue at this point if the last split was on
     +         apollo-ios
          -Do a split on apollo-ios
     +       - Do a fetch on the subtree repo
     +          - git fetch git@github.com:apollographql/apollo-ios.git
             - git subtree split --prefix=apollo-ios --squash --rejoin
          -Make changes to a file in apollo-ios-codegen
          -Create a commit containing the change(s)
          -Do a split on apollo-ios-codegen
             - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
     +    -To see that the patch fixes the issue you can use the custom subtree
     +     script in the repo so following the same steps as above, except
     +     instead of using 'git subtree ...' for the commands use
     +     'git-subtree.sh ...' for the commands
      
          You will see that the final split is looking for the last split
          on apollo-ios-codegen to use as it's starting point to process

 contrib/subtree/git-subtree.sh     | 30 +++++++++++++++++++++-
 contrib/subtree/t/t7900-subtree.sh | 40 ++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 1 deletion(-)