[PATCH v5] 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'A and 'apollo-ios-codegen'
 directories
-Create a commit containing these changes
-Do a split on apollo-ios-codegen
   - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
-Do a split on apollo-ios
   - 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

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-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1587/BobaFetters/zf/multi-subtree-processing-v5
Pull-Request: https://github.com/gitgitgadget/git/pull/1587

Range-diff vs v4:

 1:  353152910eb ! 1:  e7445a95f30 subtree: fix split processing with multiple subtrees present
     @@ Commit message
          though those commits are totally irrelevant to the current subtree
          split being run.
      
     -    In the diagram below, 'M' represents the mainline repo branch, 'A'
     -    represents one subtree, and 'B' represents another. M3 and B1 represent
     -    a split commit for subtree B that was created from commit M4. M2 and A1
     -    represent a split commit made from subtree A that was also created
     -    based on changes back to and including M4. M1 represents new changes to
     -    the repo, in this scenario if you try to run a 'git subtree split
     -    --rejoin' for subtree B, commits M1, M2, and A1, will be included in
     -    the processing of changes for the new split commit since the last
     -    split/rejoin for subtree B was at M3. The issue is that by having A1
     -    included in this processing the command ends up needing to processing
     -    every commit down tree A even though none of that is needed or relevant
     -    to the current command and result.
     +    To see this in practice you can use the open source GitHub repo
     +    'apollo-ios-dev' and do the following in order:
      
     -    M1
     -     |        \       \
     -    M2         |       |
     -     |        A1       |
     -    M3         |       |
     -     |         |      B1
     -    M4         |       |
     +    -Make a changes to a file in 'apollo-ios'A and 'apollo-ios-codegen'
     +     directories
     +    -Create a commit containing these changes
     +    -Do a split on apollo-ios-codegen
     +       - git subtree split --prefix=apollo-ios-codegen --squash --rejoin
     +    -Do a split on apollo-ios
     +       - 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
      
     -    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 A1 in the diagram 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.
     +    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.
     @@ Commit message
          of the split command to ensure the output from
          the progress of 'process_split_commit' function
          that represents the 'extracount' of commits
     -    processed does not increment.
     +    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
     @@ contrib/subtree/git-subtree.sh: ensure_valid_ref_format () {
      +# Usage: check if a commit from another subtree should be
      +# ignored from processing for splits
      +should_ignore_subtree_split_commit () {
     -+  if test -n "$(git log -1 --grep="git-subtree-dir:" $1)"
     -+  then
     -+    if test -z "$(git log -1 --grep="git-subtree-mainline:" $1)" &&
     -+			test -z "$(git log -1 --grep="git-subtree-dir: $arg_prefix$" $1)"
     -+    then
     -+      return 0
     -+    fi
     -+  fi
     -+  return 1
     ++	assert test $# = 1
     ++	local rev="$1"
     ++	if test -n "$(git log -1 --grep="git-subtree-dir:" $rev)"
     ++	then
     ++		if test -z "$(git log -1 --grep="git-subtree-mainline:" $rev)" &&
     ++			test -z "$(git log -1 --grep="git-subtree-dir: $arg_prefix$" $rev)"
     ++		then
     ++			return 0
     ++		fi
     ++	fi
     ++	return 1
      +}
      +
       # Usage: process_split_commit REV PARENTS
     @@ contrib/subtree/git-subtree.sh: cmd_split () {
      +		then
      +			continue
      +		fi
     -+		parsedParents=''
     ++		parsedparents=''
      +		for parent in $parents
      +		do
     -+			should_ignore_subtree_split_commit "$parent"
     -+			if test $? -eq 1
     ++			if ! should_ignore_subtree_split_commit "$parent"
      +			then
     -+				parsedParents+="$parent "
     ++				parsedparents="$parsedparents$parent "
      +			fi
      +		done
     -+		process_split_commit "$rev" "$parsedParents"
     ++		process_split_commit "$rev" "$parsedparents"
       	done || exit $?
       
       	latest_new=$(cache_get latest_new) || exit $?
     @@ contrib/subtree/t/t7900-subtree.sh: test_expect_success 'split sub dir/ with --r
       	)
       '
       
     ++# Tests that commits from other subtrees are not processed as
     ++# part of a split.
     ++#
     ++# This test performs the following:
     ++# - Creates Repo with subtrees 'subA' and 'subB'
     ++# - Creates commits in the repo including changes to subtrees
     ++# - Runs the following 'split' and commit' commands in order:
     ++# 	- Perform 'split' on subtree A
     ++# 	- Perform 'split' on subtree B
     ++# 	- Create new commits with changes to subtree A and B
     ++# 	- Perform split on subtree A
     ++# 	- Check that the commits in subtree B are not processed
     ++#			as part of the subtree A split
      +test_expect_success 'split with multiple subtrees' '
      +	subtree_test_create_repo "$test_count" &&
      +	subtree_test_create_repo "$test_count/subA" &&
     @@ contrib/subtree/t/t7900-subtree.sh: test_expect_success 'split sub dir/ with --r
      +	test_create_commit "$test_count/subA" subA2 &&
      +	test_create_commit "$test_count/subA" subA3 &&
      +	test_create_commit "$test_count/subB" subB1 &&
     -+	(
     -+		cd "$test_count" &&
     -+		git fetch ./subA HEAD &&
     -+		git subtree add --prefix=subADir FETCH_HEAD
     -+	) &&
     -+	(
     -+		cd "$test_count" &&
     -+		git fetch ./subB HEAD &&
     -+		git subtree add --prefix=subBDir FETCH_HEAD
     -+	) &&
     ++	git -C "$test_count" fetch ./subA HEAD &&
     ++	git -C "$test_count" subtree add --prefix=subADir FETCH_HEAD &&
     ++	git -C "$test_count" fetch ./subB HEAD &&
     ++	git -C "$test_count" subtree add --prefix=subBDir FETCH_HEAD &&
      +	test_create_commit "$test_count" subADir/main-subA1 &&
      +	test_create_commit "$test_count" subBDir/main-subB1 &&
     -+	(
     -+		cd "$test_count" &&
     -+		git subtree split --prefix=subADir --squash --rejoin -m "Sub A Split 1"
     -+	) &&
     -+	(
     -+		cd "$test_count" &&
     -+		git subtree split --prefix=subBDir --squash --rejoin -m "Sub B Split 1"
     -+	) &&
     ++	git -C "$test_count" subtree split --prefix=subADir \
     ++		--squash --rejoin -m "Sub A Split 1" &&
     ++	git -C "$test_count" subtree split --prefix=subBDir \
     ++		--squash --rejoin -m "Sub B Split 1" &&
      +	test_create_commit "$test_count" subADir/main-subA2 &&
      +	test_create_commit "$test_count" subBDir/main-subB2 &&
     -+	(
     -+		cd "$test_count" &&
     -+		git subtree split --prefix=subADir --squash --rejoin -m "Sub A Split 2"
     -+	) &&
     -+	(
     -+		cd "$test_count" &&
     -+		test "$(git subtree split --prefix=subBDir --squash --rejoin \
     -+		 -d -m "Sub B Split 1" 2>&1 | grep -w "\[1\]")" = ""
     -+	)
     ++	git -C "$test_count" subtree split --prefix=subADir \
     ++		--squash --rejoin -m "Sub A Split 2" &&
     ++	test "$(git -C "$test_count" subtree split --prefix=subBDir \
     ++		--squash --rejoin -d -m "Sub B Split 1" 2>&1 | grep -w "\[1\]")" = ""
      +'
      +
       test_expect_success 'split sub dir/ with --rejoin from scratch' '

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