From 28a7e27cff717e5ef91f7445e6a418068608082d Mon Sep 17 00:00:00 2001 From: Colin Stagner Date: Fri, 9 Jan 2026 19:18:11 -0600 Subject: [PATCH] contrib/subtree: detect rewritten subtree commits MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit git subtree split --prefix P detects splits that are outside of path prefix `P` and prunes them from history graph processing. This improves the performance of repeated `split --rejoin` with many different prefixes. Both before and after 83f9dad7d6 (contrib/subtree: fix split with squashed subtrees, 2025-09-09), the pruning logic does not detect **rebased** or **cherry-picked** git-subtree commits. If `split` encounters any of these commits, the split output may have incomplete history. All commits authored by git subtree merge [--squash] --prefix Q have a first or second parent that has *only* subtree commits as ancestors. When splitting a completely different path `P/`, it is safe to ignore: 1. the merged tree 2. the subtree parent 3. *all* of that parent's ancestry, which applies only to path `Q/` and not `P/`. But this relationship no longer holds if the git-subtree commit is rebased or otherwise reauthored. After a rebase, the former git-subtree commit will have other unrelated commits as ancestors. Ignoring these commits may exclude the history of `P/`, leading to incomplete `subtree split` output. The pruning logic relies solely on the `git-subtree-*:` trailers to detect git-subtree commits, which it blindly accepts without further validation. The split logic also takes its time about being wrong: `cmd_split()` execs a `git show` for *every* commit in the split range… twice. This is inefficient in a shell script. Add a "reality check" to ignore rebased or rewritten commits: * Rewrites of non-merge commits cannot be detected, so the new detector no longer looks for them. * Merges carry a `git-subtree-mainline:` trailer with the hash of the **first parent**. If this hash differs, or if the "merge" commit no longer has multiple parents, a rewrite has occurred. To increase speed, package this logic in a new method, `find_other_splits()`. Perform the check up-front by iterating over a single `git log`. Add ignored subtrees to: 1. the `notree` cache, which excludes them from the `split` history 2. a `prune` negative refs list. The negative refs prevent recursing into other subtrees. Since there are potentially a *lot* of these, cache them on disk and use rev-list's `--stdin` mode. Reported-by: George Signed-off-by: Colin Stagner Signed-off-by: Junio C Hamano --- contrib/subtree/git-subtree.sh | 141 +++++++++++++++++++---------- contrib/subtree/t/t7900-subtree.sh | 83 +++++++++++++++-- 2 files changed, 169 insertions(+), 55 deletions(-) diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh index 17106d1a72..3ebe88cbea 100755 --- a/contrib/subtree/git-subtree.sh +++ b/contrib/subtree/git-subtree.sh @@ -325,6 +325,12 @@ check_parents () { done } +# Usage: get_notree REV +get_notree () { + assert test $# = 1 + test -r "$cachedir/notree/$1" +} + # Usage: set_notree REV set_notree () { assert test $# = 1 @@ -511,6 +517,71 @@ find_existing_splits () { done || exit $? } +# Usage: find_other_splits DIR REV UNREVS... +# +# Scan history in REV UNREVS for other `git subtree split --rejoin` +# merge commits belonging to prefixes outside of DIR. These +# "other splits" don't contribute to DIR and can be ignored. +# +# If any such rejoins are found, +# +# * emit their second-parent as an UNREV, avoiding a +# potentially costly history traversal +# +# * mark the merge commit as "notree" to ignore it +find_other_splits () { + assert test $# -ge 2 + dir="${1%/}" + rev="$2" + shift 2 + debug "Looking for other splits with dir != $dir..." + + git log \ + --grep '^git-subtree-mainline:' \ + --no-patch \ + --no-show-signature \ + --format='hash: %H%nparents: %P%n%(trailers:key=git-subtree-dir,key=git-subtree-mainline,key=git-subtree-split)%nEND' \ + "$rev" ${@:+"$@"} | + while read -r key val + do + case "$key" in + hash:) + commit_hash="${val}" + commit_parents= + subtree_dir= + subtree_mainline= + subtree_split= + ;; + parents:) + commit_parents="${val}" ;; + git-subtree-dir:) + subtree_dir="${val%/}/" ;; + git-subtree-mainline:) + subtree_mainline="${val}" ;; + git-subtree-split:) + subtree_split="${val}" ;; + END) + # verify: + # * all git-subtree-* trailers are present + # * this subtree is outside of $dir + # * the first parent is the git-subtree-mainline: + # * the commit has at least two parents + if test -n "${subtree_dir}" && + test -n "${subtree_split}" && + test -n "${subtree_mainline}" && + test "${subtree_dir}" = "${subtree_dir#"${dir}/"}" && + test "${commit_parents}" != "${commit_parents#"$subtree_mainline "}" && + rev_exists "${commit_hash}^2" + then + debug "find_other_splits excluding dir=$subtree_dir merged in ${commit_hash}" + echo "^${commit_hash}^2" + set_notree "${commit_hash}" + fi + ;; + esac + done +} + # Usage: copy_commit REV TREE FLAGS_STR copy_commit () { assert test $# = 3 @@ -785,42 +856,6 @@ ensure_valid_ref_format () { die "fatal: '$1' does not look like a ref" } -# Usage: should_ignore_subtree_split_commit REV -# -# Check if REV is a commit from another subtree and should be -# ignored from processing for splits -should_ignore_subtree_split_commit () { - assert test $# = 1 - - git show \ - --no-patch \ - --no-show-signature \ - --format='%(trailers:key=git-subtree-dir,key=git-subtree-mainline)' \ - "$1" | - ( - have_mainline= - subtree_dir= - - while read -r trailer val - do - case "$trailer" in - git-subtree-dir:) - subtree_dir="${val%/}" ;; - git-subtree-mainline:) - have_mainline=y ;; - esac - done - - if test -n "${subtree_dir}" && - test -z "${have_mainline}" && - test "${subtree_dir}" != "$arg_prefix" - then - return 0 - fi - return 1 - ) -} - # Usage: process_split_commit REV PARENTS process_split_commit () { assert test $# = 2 @@ -994,31 +1029,39 @@ cmd_split () { fi unrevs="$(find_existing_splits "$dir" "$rev" "$repository")" || exit $? + (find_other_splits >"$cachedir/prune" "$dir" "$rev" $unrevs) || exit $? # We can't restrict rev-list to only $dir here, because some of our # parents have the $dir contents the root, and those won't match. # (and rev-list --follow doesn't seem to solve this) - grl='git rev-list --topo-order --reverse --parents $rev $unrevs' - revmax=$(eval "$grl" | wc -l) + revmax="$(git rev-list \ + <"$cachedir/prune" \ + --topo-order \ + --reverse \ + --parents \ + --stdin \ + --count \ + "$rev" \ + $unrevs + )" revcount=0 createcount=0 extracount=0 - eval "$grl" | + git rev-list \ + <"$cachedir/prune" \ + --topo-order \ + --reverse \ + --parents \ + --stdin \ + "$rev" \ + $unrevs | while read rev parents do - if should_ignore_subtree_split_commit "$rev" + if get_notree "$rev" then continue fi - parsedparents='' - for parent in $parents - do - if ! should_ignore_subtree_split_commit "$parent" - then - parsedparents="$parsedparents$parent " - fi - done - process_split_commit "$rev" "$parsedparents" + process_split_commit "$rev" "$parents" done || exit $? latest_new=$(cache_get latest_new) || exit $? diff --git a/contrib/subtree/t/t7900-subtree.sh b/contrib/subtree/t/t7900-subtree.sh index 316dc5269e..4db3a6eff3 100755 --- a/contrib/subtree/t/t7900-subtree.sh +++ b/contrib/subtree/t/t7900-subtree.sh @@ -411,8 +411,9 @@ test_expect_success 'split sub dir/ with --rejoin' ' git fetch ./"sub proj" HEAD && git subtree merge --prefix="sub dir" FETCH_HEAD && split_hash=$(git subtree split --prefix="sub dir" --annotate="*") && - git subtree split --prefix="sub dir" --annotate="*" --rejoin && - test "$(last_commit_subject)" = "Split '\''sub dir/'\'' into commit '\''$split_hash'\''" + git subtree split --prefix="sub dir" --annotate="*" -b spl --rejoin && + test "$(last_commit_subject)" = "Split '\''sub dir/'\'' into commit '\''$split_hash'\''" && + test "$(git rev-list --count spl)" -eq 5 ) ' @@ -442,18 +443,25 @@ test_expect_success 'split with multiple subtrees' ' git -C "$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 "$(git -C "$test_count" rev-list --count main)" -eq 7 && test_create_commit "$test_count" subADir/main-subA1 && test_create_commit "$test_count" subBDir/main-subB1 && git -C "$test_count" subtree split --prefix=subADir \ - --squash --rejoin -m "Sub A Split 1" && + --squash --rejoin -m "Sub A Split 1" -b a1 && + test "$(git -C "$test_count" rev-list --count main..a1)" -eq 1 && git -C "$test_count" subtree split --prefix=subBDir \ - --squash --rejoin -m "Sub B Split 1" && + --squash --rejoin -m "Sub B Split 1" -b b1 && + test "$(git -C "$test_count" rev-list --count main..b1)" -eq 1 && test_create_commit "$test_count" subADir/main-subA2 && test_create_commit "$test_count" subBDir/main-subB2 && git -C "$test_count" subtree split --prefix=subADir \ - --squash --rejoin -m "Sub A Split 2" && + --squash --rejoin -m "Sub A Split 2" -b a2 && + test "$(git -C "$test_count" rev-list --count main..a2)" -eq 2 && + test "$(git -C "$test_count" rev-list --count a1..a2)" -eq 1 && test "$(git -C "$test_count" subtree split --prefix=subBDir \ - --squash --rejoin -d -m "Sub B Split 1" 2>&1 | grep -w "\[1\]")" = "" + --squash --rejoin -d -m "Sub B Split 1" -b b2 2>&1 | grep -w "\[1\]")" = "" && + test "$(git -C "$test_count" rev-list --count main..b2)" -eq 2 && + test "$(git -C "$test_count" rev-list --count b1..b2)" -eq 1 ' # When subtree split-ing a directory that has other subtree @@ -477,6 +485,7 @@ do test_path_is_file subA/file1.t && test_path_is_file subA/subB/file2.t && git subtree split --prefix=subA --branch=bsplit && + test "$(git rev-list --count bsplit)" -eq 2 && git checkout bsplit && test_path_is_file file1.t && test_path_is_file subB/file2.t && @@ -489,6 +498,7 @@ do --prefix=subA/subB mksubtree && test_path_is_file subA/subB/file3.t && git subtree split --prefix=subA --branch=bsplit && + test "$(git rev-list --count bsplit)" -eq 3 && git checkout bsplit && test_path_is_file file1.t && test_path_is_file subB/file2.t && @@ -497,6 +507,67 @@ do ' done +# Usually, +# +# git subtree merge -P subA --squash f00... +# +# makes two commits, in this order: +# +# 1. Squashed 'subA/' content from commit f00... +# 2. Merge commit (1) as 'subA' +# +# Commit 1 updates the subtree but does *not* rewrite paths. +# Commit 2 rewrites all trees to start with `subA/` +# +# Commit 1 either has no parents or depends only on other +# "Squashed 'subA/' content" commits. +# +# For merge without --squash, subtree produces just one commit: +# a merge commit with git-subtree trailers. +# +# In either case, if the user rebases these commits, they will +# still have the git-subtree-* trailers… but will NOT have +# the layout described above. +# +# Test that subsequent `git subtree split` are not confused by this. +test_expect_success 'split with rebased subtree commit' ' + subtree_test_create_repo "$test_count" && + ( + cd "$test_count" && + test_commit file0 && + test_create_subtree_add \ + . mksubtree subA file1 --squash && + test_path_is_file subA/file1.t && + mkdir subB && + test_commit subB/bfile && + git commit --amend -F - <<'EOF' && +Squashed '\''subB/'\'' content from commit '\''badf00da911bbe895347b4b236f5461d55dc9877'\'' + +Simulate a cherry-picked or rebased subtree commit. + +git-subtree-dir: subB +git-subtree-split: badf00da911bbe895347b4b236f5461d55dc9877 +EOF + test_commit subA/file2 && + test_commit subB/bfile2 && + git commit --amend -F - <<'EOF' && +Split '\''subB/'\'' into commit '\''badf00da911bbe895347b4b236f5461d55dc9877'\'' + +Simulate a cherry-picked or rebased subtree commit. + +git-subtree-dir: subB +git-subtree-mainline: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +git-subtree-split: badf00da911bbe895347b4b236f5461d55dc9877 +EOF + git subtree split --prefix=subA --branch=bsplit && + git checkout bsplit && + test_path_is_file file1.t && + test_path_is_file file2.t && + test "$(last_commit_subject)" = "subA/file2" && + test "$(git rev-list --count bsplit)" -eq 2 + ) +' + test_expect_success 'split sub dir/ with --rejoin from scratch' ' subtree_test_create_repo "$test_count" && test_create_commit "$test_count" main1 &&