From 4757c4810d3421b6c41723da837cab14824239e2 Mon Sep 17 00:00:00 2001 From: Elijah Newren Date: Thu, 8 May 2025 12:41:10 -0700 Subject: [PATCH 1/2] merge-ort: add a new mergeability_only option 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[-ish] conflict * avoid (not prevent) writing merged blobs/trees to the object store I have a number of qualifiers there, so let me explain each: "outer layer": 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 merged blobs/trees to the object store during those inner merges. "first-ish conflict": The current patch only exits early from process_entries() on the first conflict it detects, but conflicts could have been detected in a previous function call, namely detect_and_process_renames(). However: * conflicts detected by detect_and_process_renames() are quite rare conflict types * the detection would still come after regular rename detection (which is the expensive part of detect_and_process_renames()), so it is not saving us much in computation time given that process_entries() directly follows detect_and_process_renames() * [this overlaps with the next bullet point] process_entries() is the place where virtually all object writing occurs (object writing is sometimes more of a concern for Forges than computation time), so exiting early here isn't saving us much in object writes either * the code changes needed to handle an earlier exit are slightly more invasive in detect_and_process_renames() than for process_entries(). Given the rareness of the even earlier conflicts, the limited savings we'd get from exiting even earlier, and in an attempt to keep this patch simpler, we don't guarantee that we actually exit on the first conflict detected. We can always revisit this decision later if we decide that a further micro-optimization to exit slightly earlier in rare cases is worthwhile. "avoid (not prevent) writing objects": The detect_and_process_renames() call can also write objects to the object store, when rename/rename conflicts involve one (or more) files that have also been modified on both sides. Because of this alternate call path leading to handle_content_merges(), our "early exit" does not prevent writing objects entirely, even within the "outer layer" (i.e. even within call_depth == 0). I figure that's fine though, since we're already writing objects for the inner merges (i.e. for call_depth > 0), which are likely going to represent vastly more objects than files involved in rename/rename+modify/modify cases in the outer merge, on average. Signed-off-by: Elijah Newren --- 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 77310a4a52c972..47b3d1730ece36 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 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt, 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 30750c03962f2c..6045579825da8b 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; From 7c40d3c921610576dc7947bc4c517055717dd94e Mon Sep 17 00:00:00 2001 From: Elijah Newren Date: Thu, 8 May 2025 16:08:21 -0700 Subject: [PATCH 2/2] merge-tree: add a new --quiet flag 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 --quiet flag which will make use of the new mergeability_only flag added to merge-ort in the previous commit. This option allows the merge machinery to, in the outer layer of the merge: * exit early when a conflict is detected * avoid writing (most) merged blobs/trees to the object store Signed-off-by: Elijah Newren --- Documentation/git-merge-tree.adoc | 6 +++++ builtin/merge-tree.c | 18 +++++++++++++++ t/t4301-merge-tree-write-tree.sh | 38 +++++++++++++++++++++++++++++++ 3 files changed, 62 insertions(+) diff --git a/Documentation/git-merge-tree.adoc b/Documentation/git-merge-tree.adoc index cf0578f9b5e86d..f824eea61f1e06 100644 --- a/Documentation/git-merge-tree.adoc +++ b/Documentation/git-merge-tree.adoc @@ -65,6 +65,12 @@ OPTIONS default is to include these messages if there are merge conflicts, and to omit them otherwise. +--quiet:: + Disable all output from the program. Useful when you are only + interested in the exit status. Allows merge-tree to exit + early when it finds a conflict, and allows it to avoid writing + most objects created by merges. + --allow-unrelated-histories:: merge-tree will by default error out if the two branches specified share no common history. This flag can be given to override that diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c index 4aafa73c61559e..7f41665dfd7e67 100644 --- a/builtin/merge-tree.c +++ b/builtin/merge-tree.c @@ -490,6 +490,9 @@ static int real_merge(struct merge_tree_options *o, if (result.clean < 0) die(_("failure to merge")); + if (o->merge_options.mergeability_only) + goto cleanup; + if (show_messages == -1) show_messages = !result.clean; @@ -522,6 +525,8 @@ static int real_merge(struct merge_tree_options *o, } if (o->use_stdin) putchar(line_termination); + +cleanup: merge_finalize(&opt, &result); clear_merge_options(&opt); return !result.clean; /* result.clean < 0 handled above */ @@ -538,6 +543,7 @@ int cmd_merge_tree(int argc, int original_argc; const char *merge_base = NULL; int ret; + int quiet = 0; const char * const merge_tree_usage[] = { N_("git merge-tree [--write-tree] [] "), @@ -552,6 +558,10 @@ int cmd_merge_tree(int argc, N_("do a trivial merge only"), MODE_TRIVIAL), OPT_BOOL(0, "messages", &o.show_messages, N_("also show informational/conflict messages")), + OPT_BOOL_F(0, "quiet", + &quiet, + N_("suppress all output; only exit status wanted"), + PARSE_OPT_NONEG), OPT_SET_INT('z', NULL, &line_termination, N_("separate paths with the NUL character"), '\0'), OPT_BOOL_F(0, "name-only", @@ -583,6 +593,14 @@ int cmd_merge_tree(int argc, argc = parse_options(argc, argv, prefix, mt_options, merge_tree_usage, PARSE_OPT_STOP_AT_NON_OPTION); + if (quiet && o.show_messages == -1) + o.show_messages = 0; + o.merge_options.mergeability_only = quiet; + die_for_incompatible_opt2(quiet, "--quiet", o.show_messages, "--messages"); + die_for_incompatible_opt2(quiet, "--quiet", o.name_only, "--name-only"); + die_for_incompatible_opt2(quiet, "--quiet", o.use_stdin, "--stdin"); + die_for_incompatible_opt2(quiet, "--quiet", !line_termination, "-z"); + if (xopts.nr && o.mode == MODE_TRIVIAL) die(_("--trivial-merge is incompatible with all other options")); for (size_t x = 0; x < xopts.nr; x++) diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh index f9c5883a7f7cd6..6e117ee93c8b5b 100755 --- a/t/t4301-merge-tree-write-tree.sh +++ b/t/t4301-merge-tree-write-tree.sh @@ -54,6 +54,25 @@ test_expect_success setup ' git commit -m first-commit ' +test_expect_success '--quiet on clean merge' ' + # Get rid of loose objects to start with + git gc && + echo "0 objects, 0 kilobytes" >expect && + git count-objects >actual && + test_cmp expect actual && + + # Ensure merge is successful (exit code of 0) + git merge-tree --write-tree --quiet side1 side3 >output && + + # Ensure there is no output + test_must_be_empty output && + + # Ensure no loose objects written (all new objects written would have + # been in "outer layer" of the merge) + git count-objects >actual && + test_cmp expect actual +' + test_expect_success 'Clean merge' ' TREE_OID=$(git merge-tree --write-tree side1 side3) && q_to_tab <<-EOF >expect && @@ -72,6 +91,25 @@ test_expect_success 'Failed merge without rename detection' ' grep "CONFLICT (modify/delete): numbers deleted" out ' +test_expect_success '--quiet on conflicted merge' ' + # Get rid of loose objects to start with + git gc && + echo "0 objects, 0 kilobytes" >expect && + git count-objects >actual && + test_cmp expect actual && + + # Ensure merge has conflict + test_expect_code 1 git merge-tree --write-tree --quiet side1 side2 >output && + + # Ensure there is no output + test_must_be_empty output && + + # Ensure no loose objects written (all new objects written would have + # been in "outer layer" of the merge) + git count-objects >actual && + test_cmp expect actual +' + test_expect_success 'Content merge and a few conflicts' ' git checkout side1^0 && test_must_fail git merge side2 &&