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Add focused tests using the 'test-tool reach' command. In repositories that use pull requests (or merge requests) to advance one or more "protected" branches, the history of that reference can be recovered by following the first-parent history in most cases. Most are completed using no-fast-forward merges, though squash merges are quite common. Less common is rebase-and-merge, which still validates this assumption. Finally, the case that breaks this assumption is the fast-forward update (with potential rebasing). Even in this case, the previous commit commonly appears in the first-parent history of the branch. Similar assumptions can be made for a topic branch created by a single user with the intention to merge back into another branch. Using 'git commit', 'git merge', and 'git cherry-pick' from HEAD will default to having the first-parent commit be the previous commit at HEAD. This history changes only with commands such as 'git reset' or 'git rebase', where the command names also imply that the branch is starting from a new location. With this movement of branches in mind, the following heuristic is proposed as a way to determine the base branch for a given source branch: Among a list of candidate base branches, select the candidate that minimizes the number of commits in the first-parent history of the source that are not in the first-parent history of the candidate. Prior third-party solutions to this problem have used this optimization criteria, but have relied upon extracting the first-parent history and comparing those lists as tables instead of using commit-graph walks. Given current command-line interface options, this optimization criteria is not easy to detect directly. Even using the command git rev-list --count --first-parent .. does not measure this count, as it uses full reachability from to determine which commits to remove from the range '..'. This may lead to one asking if we should instead be using the full reachability of the candidate and only the first-parent history of the source. This, unfortunately, does not work for repositories that use long-lived branches and automation to merge across those branches. In extremely large repositories, merging into a single trunk may not be feasible. This is usually due to the desired frequency of updates (thousands of engineers doing daily work) combined with the time required to perform a validation build. These factors combine to create significant risk of semantic merge conflicts, leading to build breaks on the trunk. In response, repository maintainers can create a single Level Zero (L0) trunk and multiple Level One (L1) branches. By partitioning the engineers by organization, these engineers may see lower risk of semantic merge conflicts as well as be protected against build breaks in other L1 branches. The key to making this system work is a semi-automated process of merging L1 branches into the L0 trunk and vice-versa. In a large enough organization, these L1 branches may further split into L2 or L3 branches, but the same principles apply for merging across deeper levels. If these automated merges use a typical merge with the second parent bringing in the "new" content, then each L0 and L1 branch can track its previous positions by following first-parent history, which appear as parallel paths (until reaching the first place where the branches diverged). If we also walk to second parents, then the histories overlap significantly and cannot be distinguished except for very-recent changes. For this reason, the first-parent condition should be symmetrical across the base and source branches. Another common case for desiring the result of this optimization method is the use of release branches. When releasing a version of a repository, a branch can be used to track that release. Any updates that are worth fixing in that release can be merged to the release branch and shipped with only the necessary fixes without any new features introduced in the trunk branch. The 'maint-2.' branches represent this pattern in the Git project. The microsoft/git fork uses 'vfs-2..' branches to track the changes that are custom to that fork on top of each upstream Git release 2... This application doesn't need the symmetrical first-parent condition, but the use of first-parent histories does not change the results for these branches. To determine the base branch from a list of candidates, create a new method in commit-reach.c that performs a single* commit-graph walk. The core concept is to walk first-parents starting at the candidate bases and the source, tracking the "best" base to reach a given commit. Use generation numbers to ensure that a commit is walked at most once and all children have been explored before visiting it. When reaching a commit that is reachable from both a base and the source, we will then have a guarantee that this is the closest intersection of first-parent histories. Track the best base to reach that commit and return it as a result. In rare cases involving multiple root commits, the first-parent history of the source may never intersect any of the candidates and thus a null result is returned. * There are up to two walks, since we require all commits to have a computed generation number in order to avoid incorrect results. This is similar to the need for computed generation numbers in ahead_behind() as implemented in fd67d149bde (commit-reach: implement ahead_behind() logic, 2023-03-20). In order to track the "best" base, use a new commit slab that stores an integer. This value defaults to zero upon initialization, so use -1 to track that the source commit can reach this commit and use 'i + 1' to track that the ith base can reach this commit. When multiple bases can reach a commit, minimize the index to break ties. This allows the caller to specify an order to the bases that determines some amount of preference when the heuristic does not result in a unique result. The trickiest part of the integer slab is what happens when reaching a collision among the histories of the bases and the history of the source. This is noticed when viewing the first parent and seeing that it has a slab value that differs in sign (negative or positive). In this case, the collision commit is stored in the method variable 'branch_point' and its slab value is set to -1. The index of the best base (so far) is stored in the method variable 'best_index'. It is possible that there are multiple commits that have the branch_point as its first parent, leading to multiple updates of best_index. The result is determined when 'branch_point' is visited in the commit walk, giving the guarantee that all commits that could reach 'branch_point' were visited. Several interesting cases of collisions and different results are tested in the t6600-test-reach.sh script. Recall that this script also tests the algorithm in three possible states involving the commit-graph file and how many commits are written in the file. This provides some coverage of the need (and lack of need) for the ensure_generations_valid() method. Signed-off-by: Derrick Stolee --- commit-reach.c | 126 ++++++++++++++++++++++++++++++++++++++++++ commit-reach.h | 17 ++++++ t/helper/test-reach.c | 2 + t/t6600-test-reach.sh | 61 ++++++++++++++++++++ 4 files changed, 206 insertions(+) diff --git a/commit-reach.c b/commit-reach.c index 8f9b008f876..4753197ec88 100644 --- a/commit-reach.c +++ b/commit-reach.c @@ -1222,3 +1222,129 @@ done: free(commits); repo_clear_commit_marks(r, SEEN); } + +/* + * This slab initializes integers to zero, so use "-1" for "tip is best" and + * "i + 1" for "bases[i] is best". + */ +define_commit_slab(best_branch_base, int); +static struct best_branch_base best_branch_base; +#define get_best(c) (*best_branch_base_at(&best_branch_base, (c))) +#define set_best(c,v) (*best_branch_base_at(&best_branch_base, (c)) = (v)) + +int get_branch_base_for_tip(struct repository *r, + struct commit *tip, + struct commit **bases, + size_t bases_nr) +{ + int best_index = -1; + struct commit *branch_point = NULL; + struct prio_queue queue = { compare_commits_by_gen_then_commit_date }; + int found_missing_gen = 0; + + if (!bases_nr) + return -1; + + repo_parse_commit(r, tip); + if (commit_graph_generation(tip) == GENERATION_NUMBER_INFINITY) + found_missing_gen = 1; + + /* Check for missing generation numbers. */ + for (size_t i = 0; i < bases_nr; i++) { + struct commit *c = bases[i]; + repo_parse_commit(r, c); + if (commit_graph_generation(c) == GENERATION_NUMBER_INFINITY) + found_missing_gen = 1; + } + + if (found_missing_gen) { + struct commit **commits; + size_t commits_nr = bases_nr + 1; + + CALLOC_ARRAY(commits, commits_nr); + COPY_ARRAY(commits, bases, bases_nr); + commits[bases_nr] = tip; + ensure_generations_valid(r, commits, commits_nr); + free(commits); + } + + /* Initialize queue and slab now that generations are guaranteed. */ + init_best_branch_base(&best_branch_base); + set_best(tip, -1); + prio_queue_put(&queue, tip); + + for (size_t i = 0; i < bases_nr; i++) { + struct commit *c = bases[i]; + int best = get_best(c); + + /* Has this already been marked as best by another commit? */ + if (best) { + if (best == -1) { + /* We agree at this position. Stop now. */ + best_index = i + 1; + goto cleanup; + } + continue; + } + + set_best(c, i + 1); + prio_queue_put(&queue, c); + } + + while (queue.nr) { + struct commit *c = prio_queue_get(&queue); + int best_for_c = get_best(c); + int best_for_p, positive; + struct commit *parent; + + /* Have we reached a known branch point? It's optimal. */ + if (c == branch_point) + break; + + repo_parse_commit(r, c); + if (!c->parents) + continue; + + parent = c->parents->item; + repo_parse_commit(r, parent); + best_for_p = get_best(parent); + + if (!best_for_p) { + /* 'parent' is new, so pass along best_for_c. */ + set_best(parent, best_for_c); + prio_queue_put(&queue, parent); + continue; + } + + if (best_for_p > 0 && best_for_c > 0) { + /* Collision among bases. Minimize. */ + if (best_for_c < best_for_p) + set_best(parent, best_for_c); + continue; + } + + /* + * At this point, we have reached a commit that is reachable + * from the tip, either from 'c' or from an earlier commit to + * have 'parent' as its first parent. + * + * Update 'best_index' to match the minimum of all base indices + * to reach 'parent'. + */ + + /* Exactly one is positive due to initial conditions. */ + positive = (best_for_c < 0) ? best_for_p : best_for_c; + + if (best_index < 0 || positive < best_index) + best_index = positive; + + /* No matter what, track that the parent is reachable from tip. */ + set_best(parent, -1); + branch_point = parent; + } + +cleanup: + clear_best_branch_base(&best_branch_base); + clear_prio_queue(&queue); + return best_index > 0 ? best_index - 1 : -1; +} diff --git a/commit-reach.h b/commit-reach.h index bf63cc468fd..9a745b7e176 100644 --- a/commit-reach.h +++ b/commit-reach.h @@ -139,4 +139,21 @@ void tips_reachable_from_bases(struct repository *r, struct commit **tips, size_t tips_nr, int mark); +/* + * Given a 'tip' commit and a list potential 'bases', return the index 'i' that + * minimizes the number of commits in the first-parent history of 'tip' and not + * in the first-parent history of 'bases[i]'. + * + * Among a list of long-lived branches that are updated only by merges (with the + * first parent being the previous position of the branch), this would inform + * which branch was used to create the tip reference. + * + * Returns -1 if no common point is found in first-parent histories, which is + * rare, but possible with multiple root commits. + */ +int get_branch_base_for_tip(struct repository *r, + struct commit *tip, + struct commit **bases, + size_t bases_nr); + #endif diff --git a/t/helper/test-reach.c b/t/helper/test-reach.c index 1e3b431e3e7..8579b607aa5 100644 --- a/t/helper/test-reach.c +++ b/t/helper/test-reach.c @@ -114,6 +114,8 @@ int cmd__reach(int ac, const char **av) repo_in_merge_bases_many(the_repository, A, X_nr, X_array, 0)); else if (!strcmp(av[1], "is_descendant_of")) printf("%s(A,X):%d\n", av[1], repo_is_descendant_of(r, A, X)); + else if (!strcmp(av[1], "get_branch_base_for_tip")) + printf("%s(A,X):%d\n", av[1], get_branch_base_for_tip(r, A, X_array, X_nr)); else if (!strcmp(av[1], "get_merge_bases_many")) { struct commit_list *list = NULL; if (repo_get_merge_bases_many(the_repository, diff --git a/t/t6600-test-reach.sh b/t/t6600-test-reach.sh index b330945f497..e789a4720c1 100755 --- a/t/t6600-test-reach.sh +++ b/t/t6600-test-reach.sh @@ -612,4 +612,65 @@ test_expect_success 'for-each-ref merged:none' ' --format="%(refname)" --stdin ' +# For get_branch_base_for_tip, we only care about +# first-parent history. Here is the test graph with +# second parents removed: +# +# (10,10) +# / +# (10,9) (9,10) +# / / +# (10,8) (9,9) (8,10) +# / / / +# ( continued...) +# \ / / / +# (3,1) (2,2) (1,3) +# \ / / +# (2,1) (1,2) +# \ / +# (1,1) +# +# In short, for a commit (i,j), the first-parent history +# walks all commits (i, k) with k from j to 1, then the +# commits (l, 1) with l from i to 1. + +test_expect_success 'get_branch_base_for_tip: none reach' ' + # (2,3) branched from the first tip (i,4) in X with i > 2 + cat >input <<-\EOF && + A:commit-2-3 + X:commit-1-2 + X:commit-1-4 + X:commit-4-4 + X:commit-8-4 + X:commit-10-4 + EOF + echo "get_branch_base_for_tip(A,X):2" >expect && + test_all_modes get_branch_base_for_tip +' + +test_expect_success 'get_branch_base_for_tip: equal to tip' ' + # (2,3) branched from the first tip (i,4) in X with i > 2 + cat >input <<-\EOF && + A:commit-8-4 + X:commit-1-2 + X:commit-1-4 + X:commit-4-4 + X:commit-8-4 + X:commit-10-4 + EOF + echo "get_branch_base_for_tip(A,X):3" >expect && + test_all_modes get_branch_base_for_tip +' + +test_expect_success 'get_branch_base_for_tip: all reach tip' ' + # (2,3) branched from the first tip (i,4) in X with i > 2 + cat >input <<-\EOF && + A:commit-4-1 + X:commit-4-2 + X:commit-5-1 + EOF + echo "get_branch_base_for_tip(A,X):0" >expect && + test_all_modes get_branch_base_for_tip +' + test_done