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[PATCH 1/2] merge-ort: add a new mergeability_only option

From
Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>
Date
May 10, 2025, 22:02 UTC
Message-ID
<09292804cffc41d15f1156e0b4bba4cba7ec7ca4.1746914561.git.gitgitgadget@gmail.com>
In-Reply-To
<pull.1920.git.1746914561.gitgitgadget@gmail.com>
From: Elijah Newren <newren@gmail.com>
Git Forges may be interested in whether two branches can be merged while
not being interested in what the resulting merge tree is nor which files
conflicted.  For such cases, add a new mergeability_only option.  This
option allows the merge machinery to, in the outer layer of the merge:
  * exit upon first conflict
  * avoid writing merged blobs/trees to the object store

Note that since the recursive merge of merge bases (corresponding to call_depth > 0) can conflict without the outer final merge (corresponding to call_depth == 0) conflicting, we can't short-circuit nor avoid writing merges blobs/trees to the object store during those inner merges.

There is a further potential micro-optimization here. rename/rename conflicts have the potential for nested conflicts even without recursive merges; because of that, handle_content_merge() can be called multiple times and is done via different paths. Currently, we only exit early in process_entries(), which is where the final handle_content_merge() is invoked. Since rename/rename conflicts have an additional earlier handle_content_merge() call that can be invoked from detect_and_process_renames() (via process_renames()), we could potentially exit earlier at that call point. However, rename/rename conflicts are exceptionally rare, and feeding the extra logic through didn't seem worth it. (And, if we don't exit early at that point, then any resulting blobs need to be written to the store so that subsequent handle_content_merge() calls trying to use the blob don't throw exceptions.)

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 merge-ort.c | 38 +++++++++++++++++++++++++++++++-------
 merge-ort.h |  1 +
 2 files changed, 32 insertions(+), 7 deletions(-)
diff --git a/merge-ort.c b/merge-ort.c
index 77310a4a52c9..47b3d1730ece 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -2127,6 +2127,7 @@ static int handle_content_merge(struct merge_options *opt,
 				const struct version_info *b,
 				const char *pathnames[3],
 				const int extra_marker_size,
+				const int record_object,
 				struct version_info *result)
 {
 	/*
@@ -2214,7 +2215,7 @@ static int handle_content_merge(struct merge_options *opt,
 			ret = -1;
 		}
 
-		if (!ret &&
+		if (!ret && record_object &&
 		    write_object_file(result_buf.ptr, result_buf.size,
 				      OBJ_BLOB, &result->oid)) {
 			path_msg(opt, ERROR_OBJECT_WRITE_FAILED, 0,
@@ -2897,6 +2898,7 @@ static int process_renames(struct merge_options *opt,
 			struct version_info merged;
 			struct conflict_info *base, *side1, *side2;
 			unsigned was_binary_blob = 0;
+			const int record_object = true;
 
 			pathnames[0] = oldpath;
 			pathnames[1] = newpath;
@@ -2947,6 +2949,7 @@ static int process_renames(struct merge_options *opt,
 							   &side2->stages[2],
 							   pathnames,
 							   1 + 2 * opt->priv->call_depth,
+							   record_object,
 							   &merged);
 			if (clean_merge < 0)
 				return -1;
@@ -3061,6 +3064,7 @@ static int process_renames(struct merge_options *opt,
 
 			struct conflict_info *base, *side1, *side2;
 			int clean;
+			const int record_object = true;
 
 			pathnames[0] = oldpath;
 			pathnames[other_source_index] = oldpath;
@@ -3080,6 +3084,7 @@ static int process_renames(struct merge_options *opt,
 						     &side2->stages[2],
 						     pathnames,
 						     1 + 2 * opt->priv->call_depth,
+						     record_object,
 						     &merged);
 			if (clean < 0)
 				return -1;
@@ -3931,9 +3936,12 @@ static int write_completed_directory(struct merge_options *opt,
 		 * Write out the tree to the git object directory, and also
 		 * record the mode and oid in dir_info->result.
 		 */
+		int record_tree = (!opt->mergeability_only ||
+				   opt->priv->call_depth);
 		dir_info->is_null = 0;
 		dir_info->result.mode = S_IFDIR;
-		if (write_tree(&dir_info->result.oid, &info->versions, offset,
+		if (record_tree &&
+		    write_tree(&dir_info->result.oid, &info->versions, offset,
 			       opt->repo->hash_algo->rawsz) < 0)
 			ret = -1;
 	}
@@ -4231,10 +4239,13 @@ static int process_entry(struct merge_options *opt,
 		struct version_info *o = &ci->stages[0];
 		struct version_info *a = &ci->stages[1];
 		struct version_info *b = &ci->stages[2];
+		int record_object = (!opt->mergeability_only ||
+				     opt->priv->call_depth);
 
 		clean_merge = handle_content_merge(opt, path, o, a, b,
 						   ci->pathnames,
 						   opt->priv->call_depth * 2,
+						   record_object,
 						   &merged_file);
 		if (clean_merge < 0)
 			return -1;
@@ -4395,6 +4406,8 @@ static int process_entries(struct merge_options *opt,
 						   STRING_LIST_INIT_NODUP,
 						   NULL, 0 };
 	int ret = 0;
+	const int record_tree = (!opt->mergeability_only ||
+				 opt->priv->call_depth);
 
 	trace2_region_enter("merge", "process_entries setup", opt->repo);
 	if (strmap_empty(&opt->priv->paths)) {
@@ -4454,6 +4467,12 @@ static int process_entries(struct merge_options *opt,
 				ret = -1;
 				goto cleanup;
 			};
+			if (!ci->merged.clean && opt->mergeability_only &&
+			    !opt->priv->call_depth) {
+				ret = 0;
+				goto cleanup;
+			}
+
 		}
 	}
 	trace2_region_leave("merge", "processing", opt->repo);
@@ -4468,7 +4487,8 @@ static int process_entries(struct merge_options *opt,
 		fflush(stdout);
 		BUG("dir_metadata accounting completely off; shouldn't happen");
 	}
-	if (write_tree(result_oid, &dir_metadata.versions, 0,
+	if (record_tree &&
+	    write_tree(result_oid, &dir_metadata.versions, 0,
 		       opt->repo->hash_algo->rawsz) < 0)
 		ret = -1;
 cleanup:
@@ -4715,6 +4735,8 @@ void merge_display_update_messages(struct merge_options *opt,
 
 	if (opt->record_conflict_msgs_as_headers)
 		BUG("Either display conflict messages or record them as headers, not both");
+	if (opt->mergeability_only)
+		BUG("Displaying conflict messages incompatible with mergeability-only checks");
 
 	trace2_region_enter("merge", "display messages", opt->repo);
 
@@ -5171,10 +5193,12 @@ redo:
 	result->path_messages = &opt->priv->conflicts;
 
 	if (result->clean >= 0) {
-		result->tree = parse_tree_indirect(&working_tree_oid);
-		if (!result->tree)
-			die(_("unable to read tree (%s)"),
-			    oid_to_hex(&working_tree_oid));
+		if (!opt->mergeability_only) {
+			result->tree = parse_tree_indirect(&working_tree_oid);
+			if (!result->tree)
+				die(_("unable to read tree (%s)"),
+				    oid_to_hex(&working_tree_oid));
+		}
 		/* existence of conflicted entries implies unclean */
 		result->clean &= strmap_empty(&opt->priv->conflicted);
 	}
diff --git a/merge-ort.h b/merge-ort.h
index 30750c03962f..6045579825da 100644
--- a/merge-ort.h
+++ b/merge-ort.h
@@ -83,6 +83,7 @@ struct merge_options {
 	/* miscellaneous control options */
 	const char *subtree_shift;
 	unsigned renormalize : 1;
+	unsigned mergeability_only : 1; /* exit early, write fewer objects */
 	unsigned record_conflict_msgs_as_headers : 1;
 	const char *msg_header_prefix;
 

-- 
gitgitgadget
Previous: Elijah Newren via GitGitGadgetNext: Elijah Newren via GitGitGadget
Message 2 of 29 in “merge-tree: add new --mergeability-only option”
  1. 0/2 merge-tree: add new --mergeability-only optionElijah Newren via GitGitGadget, May 10, 2025
  2. 1/2 merge-ort: add a new mergeability_only optionElijah Newren via GitGitGadget, May 10, 2025
  3. 2/2 merge-tree: add a new --mergeability-only flagElijah Newren via GitGitGadget, May 10, 2025
  4. Junio C HamanoMay 12, 2025
  5. Elijah NewrenMay 12, 2025
  6. Junio C HamanoMay 12, 2025
  7. Elijah NewrenMay 12, 2025
  8. 0/2 merge-tree: add new --dry-run optionElijah Newren via GitGitGadget, May 12, 2025
  9. 1/2 merge-ort: add a new mergeability_only optionElijah Newren via GitGitGadget, May 12, 2025
  10. 2/2 merge-tree: add a new --dry-run flagElijah Newren via GitGitGadget, May 12, 2025
  11. Kristoffer HaugsbakkMay 13, 2025
  12. Elijah NewrenMay 13, 2025
  13. Junio C HamanoMay 13, 2025
  14. Elijah NewrenMay 13, 2025
  15. Junio C HamanoMay 14, 2025
  16. 0/2 merge-tree: add new --dry-run optionElijah Newren via GitGitGadget, May 14, 2025
  17. 1/2 merge-ort: add a new mergeability_only optionElijah Newren via GitGitGadget, May 14, 2025
  18. 2/2 merge-tree: add a new --dry-run flagElijah Newren via GitGitGadget, May 14, 2025
  19. Junio C HamanoMay 15, 2025
  20. Phillip WoodMay 16, 2025
  21. Elijah NewrenMay 16, 2025
  22. Kristoffer HaugsbakkMay 14, 2025
  23. 0/2 merge-tree: add new --quiet optionElijah Newren via GitGitGadget, May 16, 2025
  24. 1/2 merge-ort: add a new mergeability_only optionElijah Newren via GitGitGadget, May 16, 2025
  25. 2/2 merge-tree: add a new --quiet flagElijah Newren via GitGitGadget, May 16, 2025
  26. Kristoffer HaugsbakkMay 17, 2025
  27. Kristoffer HaugsbakkMay 17, 2025
  28. Phillip WoodMay 19, 2025
  29. Junio C HamanoMay 19, 2025

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