# [PATCH 2/3] contrib/subtree: functionalize split traversal 4 messages from 2026-02-15 to 2026-02-15. Participants: Colin Stagner. Thread: https://gitlist.dev/t/65000 ## Colin Stagner, 2026-02-15 20:17 Subject: [PATCH 2/3] contrib/subtree: functionalize split traversal Message-ID: <20260215201748.889866-3-ask+git@howdoi.land> URL: https://gitlist.dev/e/20260215201748.889866-3-ask%2Bgit%40howdoi.land In-Reply-To: <20260215201748.889866-1-ask+git@howdoi.land> ``` `git subtree split` requires an ancestor-first history traversal. Refactor the existing rev-list traversal into its own function, `find_commits_to_split`. Pass unrevs via stdin to avoid limits on the maximum length of command-line arguments. Also remove an unnecessary `eval`. Signed-off-by: Colin Stagner --- contrib/subtree/git-subtree.sh | 30 +++++++++++++++++++++++++++--- 1 file changed, 27 insertions(+), 3 deletions(-) diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh index 1cdf39a481..7a62ef7504 100755 --- a/contrib/subtree/git-subtree.sh +++ b/contrib/subtree/git-subtree.sh @@ -516,6 +516,31 @@ find_existing_splits () { done || exit $? } +# Usage: find_commits_to_split REV UNREVS [ARGS...] +# +# List each commit to split, with its parents. +# +# Specify the starting REV for the split, which is usually +# a branch tip. Populate UNREVS with the last --rejoin for +# this prefix, if any. Typically, `subtree split` ignores +# history prior to the last --rejoin... unless and if it +# becomes necessary to consider it. `find_existing_splits` is +# a convenient source of UNREVS. +# +# Remaining arguments are passed to rev-list. +# +# Outputs commits in ancestor-first order, one per line, with +# parent information. Outputs all parents before any child. +find_commits_to_split() { + assert test $# -ge 2 + rev="$1" + unrevs="$2" + shift 2 + + echo "$unrevs" | + git rev-list --topo-order --reverse --parents --stdin "$rev" "$@" +} + # Usage: copy_commit REV TREE FLAGS_STR copy_commit () { assert test $# = 3 @@ -1003,12 +1028,11 @@ cmd_split () { # 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 "$grl" | wc -l) + revmax="$(find_commits_to_split "$rev" "$unrevs" --count)" revcount=0 createcount=0 extracount=0 - eval "$grl" | + find_commits_to_split "$rev" "$unrevs" | while read rev parents do if should_ignore_subtree_split_commit "$rev" -- 2.43.0 ``` ## Colin Stagner, 2026-02-15 20:17 Subject: [PATCH 1/3] contrib/subtree: reduce function side-effects Message-ID: <20260215201748.889866-2-ask+git@howdoi.land> URL: https://gitlist.dev/e/20260215201748.889866-2-ask%2Bgit%40howdoi.land In-Reply-To: <20260215201748.889866-1-ask+git@howdoi.land> ``` `process_subtree_split_trailer()` communicates its return value to the caller by setting a variable (`sub`) that is also defined by the calling function. This is both unclear and encourages side-effects. Invoke this function in a sub-shell instead. Signed-off-by: Colin Stagner --- contrib/subtree/git-subtree.sh | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh index 17106d1a72..1cdf39a481 100755 --- a/contrib/subtree/git-subtree.sh +++ b/contrib/subtree/git-subtree.sh @@ -370,6 +370,10 @@ try_remove_previous () { } # Usage: process_subtree_split_trailer SPLIT_HASH MAIN_HASH [REPOSITORY] +# +# Parse SPLIT_HASH as a commit. If the commit is not found, fetches +# REPOSITORY and tries again. If found, prints full commit hash. +# Otherwise, dies. process_subtree_split_trailer () { assert test $# -ge 2 assert test $# -le 3 @@ -397,6 +401,7 @@ process_subtree_split_trailer () { die "$fail_msg" fi fi + echo "${sub}" } # Usage: find_latest_squash DIR [REPOSITORY] @@ -429,7 +434,7 @@ find_latest_squash () { main="$b" ;; git-subtree-split:) - process_subtree_split_trailer "$b" "$sq" "$repository" + sub="$(process_subtree_split_trailer "$b" "$sq" "$repository")" || exit 1 ;; END) if test -n "$sub" @@ -486,7 +491,7 @@ find_existing_splits () { main="$b" ;; git-subtree-split:) - process_subtree_split_trailer "$b" "$sq" "$repository" + sub="$(process_subtree_split_trailer "$b" "$sq" "$repository")" || exit 1 ;; END) debug "Main is: '$main'" -- 2.43.0 ``` ## Colin Stagner, 2026-02-15 20:17 Subject: [PATCH 0/3] contrib/subtree: reduce recursion during split Message-ID: <20260215201748.889866-1-ask+git@howdoi.land> URL: https://gitlist.dev/e/20260215201748.889866-1-ask%2Bgit%40howdoi.land ``` * cs/subtree-split-recursion: when processing large history graphs on Debian or Ubuntu, "git subtree" can die with a "recursion depth reached" error. Reduce recursion. On Debian's POSIX sh, shell recursion is artificially limited to 1000 calls. You can check if your sh has limited recursion with: #!/bin/sh recurse() { r=$(( r + 1 )) test "$r" -le 1000 || { echo OK; exit; } recurse } && r=0 && recurse Depending on the history graph, subtree split can recurse deeply enough to encounter this limit. Rewrite the rejoin-deepening algorithm to reduce recursive calls. Colin Stagner (3): contrib/subtree: reduce function side-effects contrib/subtree: functionalize split traversal contrib/subtree: reduce recursion during split contrib/subtree/git-subtree.sh | 95 +++++++++++++++++++++++++++++++--- 1 file changed, 88 insertions(+), 7 deletions(-) base-commit: 852829b3dd2fe4e7c7fc4d8badde644cf1b66c74 -- 2.43.0 ``` ## Colin Stagner, 2026-02-15 20:17 Subject: [PATCH 3/3] contrib/subtree: reduce recursion during split Message-ID: <20260215201748.889866-4-ask+git@howdoi.land> URL: https://gitlist.dev/e/20260215201748.889866-4-ask%2Bgit%40howdoi.land In-Reply-To: <20260215201748.889866-1-ask+git@howdoi.land> ``` On Debian-alikes, POSIX sh has a hardcoded recursion depth of 1000. This limit operates like bash's `$FUNCNEST` [1], but it does not actually respect `$FUNCNEST`. This is non-standard behavior. On other distros, the sh recursion depth is limited only by the available stack size. With certain history graphs, subtree splits are recursive—with one recursion per commit. Attempting to split complex repos that have thousands of commits, like [2], may fail on these distros. Reduce the amount of recursion required by eagerly discovering the complete range of commits to process. The recursion is a side-effect of the rejoin-finder in `find_existing_splits`. Rejoin mode, as in git subtree split --rejoin -b hax main ... improves the speed of later splits by merging the split history back into `main`. This gives the splitting algorithm a stopping point. The rejoin maps one commit on `main` to one split commit on `hax`. If we encounter this commit, we know that it maps to `hax`. But this is only a single point in the history. Many splits require history from before the rejoin. See patch content for examples. If pre-rejoin history is required, `check_parents` recursively discovers each individual parent, with one recursion per commit. The recursion deepens the entire tree, even if an older rejoin is available. This quickly overwhelms the Debian sh stack. Instead of recursively processing each commit, process *all* the commits back to the next obvious starting point: i.e., either the next-oldest --rejoin or the beginning of history. This is where the recursion is likely to stop anyway. While this still requires recursion, it is *considerably* less recursive. [1]: https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-FUNCNEST [2]: https://github.com/christian-heusel/aur.git Signed-off-by: Colin Stagner --- contrib/subtree/git-subtree.sh | 56 ++++++++++++++++++++++++++++++++-- 1 file changed, 54 insertions(+), 2 deletions(-) diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh index 7a62ef7504..54d7151a50 100755 --- a/contrib/subtree/git-subtree.sh +++ b/contrib/subtree/git-subtree.sh @@ -312,6 +312,46 @@ cache_miss () { } # Usage: check_parents [REVS...] +# +# During a split, check that every commit in REVS has already been +# processed via `process_split_commit`. If not, deepen the history +# until it is. +# +# Commits authored by `subtree split` have to be created in the +# same order as every other git commit: ancestor-first, with new +# commits building on old commits. The traversal order normally +# ensures this is the case, but it also excludes --rejoins commits +# by default. +# +# The --rejoin tells us, "this mainline commit is equivalent to +# this split commit." The relationship is only known for that +# exact commit---and not before or after it. Frequently, commits +# prior to a rejoin are not needed... but, just as often, they +# are! Consider this history graph: +# +# --D--- +# / \ +# A--B--C--R--X--Y main +# / / +# a--b--c / split +# \ / +# --e--/ +# +# The main branch has commits A, B, and C. main is split into +# commits a, b, and c. The split history is rejoined at R. +# +# There are at least two cases where we might need the A-B-C +# history that is prior to R: +# +# 1. Commit D is based on history prior to R, but +# it isn't merged into mainline until after R. +# +# 2. Commit e is based on old split history. It is merged +# back into mainline with a subtree merge. Again, this +# happens after R. +# +# check_parents detects these cases and deepens the history +# to the next available rejoin. check_parents () { missed=$(cache_miss "$@") || exit $? local indent=$(($indent + 1)) @@ -319,8 +359,20 @@ check_parents () { do if ! test -r "$cachedir/notree/$miss" then - debug "incorrect order: $miss" - process_split_commit "$miss" "" + debug "found commit excluded by --rejoin: $miss. skipping to the next --rejoin..." + unrevs="$(find_existing_splits "$dir" "$miss" "$repository")" || exit 1 + + find_commits_to_split "$miss" "$unrevs" | + while read -r rev parents + do + process_split_commit "$rev" "$parents" + done + + if ! test -r "$cachedir/$miss" && + ! test -r "$cachedir/notree/$miss" + then + die "failed to deepen history at $miss" + fi fi done } -- 2.43.0 ```