[PATCH v2] shallow: set borders which are all reachable after clone shallow since

From: =?UTF-8?q?Samo=20Poga=C4=8Dnik?= <samo_pogacnik@t-2.net>

When shallow cloning based on a date, it happens that not all shallow border commits are reachable.

Original implementation of a generic shallow boundary finder based on rev-list sets a commit (from the initial list of border commit candidates) to be the border commit as soon as it finds one of its parentis that wasn't on the list of initial candidates. This results in a successful shallow clone, where some of its declared border commits may not be reachable and they would not actually exist in the cloned repository. Thus the result may contradict existing comment in the code, which correctly states that such commmit should not be considered border.

One can inspect such case by running the added test scenario:
- 'clone shallow since all borders reachable'

The modified implementation of a generic shallow boundary finder based on rev-list ensures that all shallow border commits are reachable also after being grafted. This is achieved by inspecting all parents of each initial border commit candidate. The border commit candidate is set border only when all its parents wern't on the initial list of candidates. Otherwise the border commit candidate is not set as border however its parents that weren't on the list of candidates are set as borders.

Signed-off-by: Samo Pogačnik <samo_pogacnik@t-2.net>
---
    Fixed --shallow-since generating descendant borders
    
    When shallow cloning based on a date, it happens that a list of commits
    is received, where some of the list border commits actually descend one
    from another. In such cases borders need to be expanded by additional
    parents and excluding the child as border.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2107%2Fspog%2Ffix-shallow-since-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2107/spog/fix-shallow-since-v2
Pull-Request: https://github.com/git/git/pull/2107

Range-diff vs v1:

 1:  aaeff44f83 ! 1:  479692c386 Fixed --shallow-since generating descendant borders
     @@ Metadata
      Author: Samo Pogačnik <samo_pogacnik@t-2.net>
      
       ## Commit message ##
     -    Fixed --shallow-since generating descendant borders
     +    shallow: set borders which are all reachable after clone shallow since
      
     -    When shallow cloning based on a date, it happens that a list
     -    of commits is received, where some of the list border commits
     -    actually descend one from another. In such cases borders need
     -    to be expanded by additional parents and excluding the child
     -    as border.
     +    When shallow cloning based on a date, it happens that not all
     +    shallow border commits are reachable.
     +
     +    Original implementation of a generic shallow boundary finder
     +    based on rev-list sets a commit (from the initial list of border
     +    commit candidates) to be the border commit as soon as it finds one
     +    of its parentis that wasn't on the list of initial candidates. This
     +    results in a successful shallow clone, where some of its declared
     +    border commits may not be reachable and they would not actually exist
     +    in the cloned repository. Thus the result may contradict existing
     +    comment in the code, which correctly states that such commmit should
     +    not be considered border.
     +
     +    One can inspect such case by running the added test scenario:
     +    - 'clone shallow since all borders reachable'
     +
     +    The modified implementation of a generic shallow boundary finder
     +    based on rev-list ensures that all shallow border commits are reachable
     +    also after being grafted. This is achieved by inspecting all parents
     +    of each initial border commit candidate. The border commit candidate
     +    is set border only when all its parents wern't on the initial list of
     +    candidates. Otherwise the border commit candidate is not set as border
     +    however its parents that weren't on the list of candidates are set as
     +    borders.
      
          Signed-off-by: Samo Pogačnik <samo_pogacnik@t-2.net>
      
     @@ shallow.c: struct commit_list *get_shallow_commits_by_rev_list(struct strvec *ar
       	 * commit A is processed first, then commit B, whose parent is
       	 * A, later. If NOT_SHALLOW on A is cleared at step 1, B
       	 * itself is considered border at step 2, which is incorrect.
     -+	 * We must also consider that B has multiple parents, some of
     -+	 * them not being in the not_shallow_list, but must be added
     -+	 * as border commits to the result.
     ++	 *
     ++	 * We must also consider that B has multiple parents which may
     ++	 * not all be marked NOT_SHALLOW (as they weren't traversed into
     ++	 * the not_shallow_list from revs in the first place). Because of
     ++	 * that an additional step is required to reconsider B as border.
     ++	 * A commit from the not_shallow_list is considered border only
     ++	 * when ALL its parents weren't on the not_shallow_list.
     ++	 * When one or more parents of a commit from the not_shellow_list
     ++	 * also come from that list, the commit is not considered border,
     ++	 * but its non-listed parents are considered border commits.
      +	 *
      +	 * The general processing goes like this:
     -+	 * 1. Above we've coloured the whole not_shallow_list of commits
     -+	 *    with 'not_shallow'.
     ++	 * 1. Above we've painted the whole not_shallow_list of commits
     ++	 *    NOT_SHALLOW.
      +	 * 2. For each commit from the not_shallow_list (the code below)
     -+	 *    we colour 'shallow' the commit and its parents, which are not
     -+	 *    already coloured 'not_shallow'.
     -+	 * 3. Commits with all parents being coloured only 'shallow' remain
     ++	 *    we paint SHALLOW this commit and its parent for all its
     ++	 *    parents that had not yet been painted NOT_SHALLOW.
     ++	 * 3. Commits with all parents being painted only SHALLOW remain
      +	 *    shallow and are being added to result list.
     -+	 * 4. Commits without all parents being coloured only 'shallow' are
     -+	 *    being excluded as borders, however their parents coloured only
     -+	 *    'shallow' are being added to the result borders list.
     ++	 * 4. Commits without all parents being painted only SHALLOW are
     ++	 *    being excluded as borders, however their parents painted only
     ++	 *    SHALLOW are being added to the result borders list.
       	 */
       	for (p = not_shallow_list; p; p = p->next) {
       		struct commit *c = p->item;
     @@ shallow.c: struct commit_list *get_shallow_commits_by_rev_list(struct strvec *ar
       	}
       	free_commit_list(not_shallow_list);
       
     +
     + ## t/t5500-fetch-pack.sh ##
     +@@ t/t5500-fetch-pack.sh: test_expect_success 'shallow since with commit graph and already-seen commit' '
     + 	)
     + '
     + 
     ++test_expect_success 'clone shallow since all borders reachable' '
     ++	test_create_repo shallow-since-all-borders-reachable &&
     ++	(
     ++	rm -rf shallow123 &&
     ++	cd shallow-since-all-borders-reachable &&
     ++	GIT_COMMITTER_DATE="2025-08-19 12:34:56" git commit --allow-empty -m one &&
     ++	GIT_COMMITTER_DATE="2025-08-20 12:34:56" git switch -c branch &&
     ++	GIT_COMMITTER_DATE="2025-08-21 12:34:56" git commit --allow-empty -m two &&
     ++	GIT_COMMITTER_DATE="2025-08-22 12:34:56" git commit --allow-empty -m three &&
     ++	GIT_COMMITTER_DATE="2025-08-23 12:34:56" git switch main &&
     ++	GIT_COMMITTER_DATE="2025-08-24 12:34:56" git merge branch --no-ff &&
     ++	GIT_COMMITTER_DATE="2025-08-26 12:34:56" git clone --shallow-since "2025-08-21 12:34:56" "file://$(pwd)/." ../shallow123 &&
     ++	cd ../shallow123 &&
     ++	echo "Shallow borders:" &&
     ++	cat .git/shallow &&
     ++	$(for commit in $(cat .git/shallow); do git rev-list $commit 1>/dev/null || exit 1; done)
     ++	)
     ++'
     ++
     + test_expect_success 'shallow clone exclude tag two' '
     + 	test_create_repo shallow-exclude &&
     + 	(

 shallow.c             | 42 +++++++++++++++++++++++++++++++++++++++---
 t/t5500-fetch-pack.sh | 19 +++++++++++++++++++
 2 files changed, 58 insertions(+), 3 deletions(-)