[PATCH v2 01/13] merge-ort: use STABLE_QSORT instead of QSORT where required

["Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>]

From: Elijah Newren <newren@gmail.com>

rename/rename conflict handling depends on the fact that if both sides
renamed the same path, that the one on the MERGE_SIDE1 will appear first
in the combined diff_queue_struct passed to process_renames().  Since we
add all pairs from MERGE_SIDE1 to combined first, and then all pairs
from MERGE_SIDE2, and then sort based on filename, this will only be
true if the sort used is stable.  This was found due to the fact that
Mac, unlike Linux, apparently has a system-defined qsort that is not
stable.

While we are at it, review the other callers of QSORT and add comments
about why they can remain as calls to QSORT instead of being modified
to call STABLE_QSORT.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 merge-ort.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/merge-ort.c b/merge-ort.c
index 7f5750ce6ab0..34a91c435737 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -2405,7 +2405,7 @@ static int detect_and_process_renames(struct merge_options *opt,
 	clean &= collect_renames(opt, &combined, MERGE_SIDE2,
 				 &renames->dir_renames[1],
 				 &renames->dir_renames[2]);
-	QSORT(combined.queue, combined.nr, compare_pairs);
+	STABLE_QSORT(combined.queue, combined.nr, compare_pairs);
 	trace2_region_leave("merge", "directory renames", opt->repo);
 
 	trace2_region_enter("merge", "process renames", opt->repo);
@@ -2550,6 +2550,7 @@ static void write_tree(struct object_id *result_oid,
 	 */
 	relevant_entries.items = versions->items + offset;
 	relevant_entries.nr = versions->nr - offset;
+	/* No need for STABLE_QSORT -- filenames must be unique */
 	QSORT(relevant_entries.items, relevant_entries.nr, tree_entry_order);
 
 	/* Pre-allocate some space in buf */
@@ -3325,6 +3326,11 @@ static int record_conflicted_index_entries(struct merge_options *opt,
 	 * entries we added to the end into their right locations.
 	 */
 	remove_marked_cache_entries(index, 1);
+	/*
+	 * No need for STABLE_QSORT -- cmp_cache_name_compare sorts primarily
+	 * on filename and secondarily on stage, and (name, stage #) are a
+	 * unique tuple.
+	 */
 	QSORT(index->cache, index->cache_nr, cmp_cache_name_compare);
 
 	return errs;
-- 
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