[PATCH 04/12] Prepare `paint_down_to_common()` for handling shallow commits

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When `git fetch --update-shallow` needs to test for commit ancestry, it
can naturally run into a missing object (e.g. if it is a parent of a
shallow commit). To accommodate, the merge base logic will need to be
able to handle this situation gracefully.

Currently, that logic pretends that a missing parent commit is
equivalent to a a missing parent commit, and for the purpose of
`--update-shallow` that is exactly what we need it to do.

Therefore, let's introduce a flag to indicate when that is precisely the
logic we want.

We need a flag, and cannot rely on `is_repository_shallow()` to indicate
that situation, because that function would return 0: There may not
actually be a `shallow` file, as demonstrated e.g. by t5537.10 ("add new
shallow root with receive.updateshallow on") and t5538.4 ("add new
shallow root with receive.updateshallow on").

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 commit-reach.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/commit-reach.c b/commit-reach.c
index 248a0f2b39d..1d1d8c989de 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -53,7 +53,8 @@ static int queue_has_nonstale(struct prio_queue *queue)
 static struct commit_list *paint_down_to_common(struct repository *r,
 						struct commit *one, int n,
 						struct commit **twos,
-						timestamp_t min_generation)
+						timestamp_t min_generation,
+						int ignore_missing_commits)
 {
 	struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
 	struct commit_list *result = NULL;
@@ -106,6 +107,13 @@ static struct commit_list *paint_down_to_common(struct repository *r,
 			if ((p->object.flags & flags) == flags)
 				continue;
 			if (repo_parse_commit(r, p)) {
+				/*
+				 * At this stage, we know that the commit is
+				 * missing: `repo_parse_commit()` uses
+				 * `OBJECT_INFO_DIE_IF_CORRUPT` and therefore
+				 * corrupt commits would already have been
+				 * dispatched with a `die()`.
+				 */
 				free_commit_list(result);
 				return NULL;
 			}
@@ -142,7 +150,7 @@ static struct commit_list *merge_bases_many(struct repository *r,
 			return NULL;
 	}
 
-	list = paint_down_to_common(r, one, n, twos, 0);
+	list = paint_down_to_common(r, one, n, twos, 0, 0);
 
 	while (list) {
 		struct commit *commit = pop_commit(&list);
@@ -213,7 +221,7 @@ static int remove_redundant_no_gen(struct repository *r,
 				min_generation = curr_generation;
 		}
 		common = paint_down_to_common(r, array[i], filled,
-					      work, min_generation);
+					      work, min_generation, 0);
 		if (array[i]->object.flags & PARENT2)
 			redundant[i] = 1;
 		for (j = 0; j < filled; j++)
@@ -503,7 +511,7 @@ int repo_in_merge_bases_many(struct repository *r, struct commit *commit,
 
 	bases = paint_down_to_common(r, commit,
 				     nr_reference, reference,
-				     generation);
+				     generation, ignore_missing_commits);
 	if (commit->object.flags & PARENT2)
 		ret = 1;
 	clear_commit_marks(commit, all_flags);
-- 
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