merge-recursive: honor diff.algorithm

The documentation claims that "recursive defaults to the diff.algorithm
config setting", but this is currently not the case. This fixes it,
ensuring that diff.algorithm is used when -Xdiff-algorithm is not
supplied. This affects the following porcelain commands: "merge",
"rebase", "cherry-pick", "pull", "stash", "log", "am" and "checkout".
It also affects the "merge-tree" ancillary interrogator.

This change refactors the initialization of merge options to introduce
two functions, "init_merge_ui_options" and "init_merge_basic_options"
instead of just one "init_merge_options". This design follows the
approach used in diff.c, providing initialization methods for
porcelain and plumbing commands respectively. Thanks to that, the
"replay" and "merge-recursive" plumbing commands remain unaffected by
diff.algorithm.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1630,7 +1630,7 @@ static int fall_back_threeway(const struct am_state *state, const char *index_pa
 	 * changes.
 	 */
 
-	init_merge_options(&o, the_repository);
+	init_ui_merge_options(&o, the_repository);
 
 	o.branch1 = "HEAD";
 	their_tree_name = xstrfmt("%.*s", linelen(state->msg), state->msg);

builtin/checkout.c

@@ -884,7 +884,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 
 			add_files_to_cache(the_repository, NULL, NULL, NULL, 0,
 					   0);
-			init_merge_options(&o, the_repository);
+			init_ui_merge_options(&o, the_repository);
 			o.verbosity = 0;
 			work = write_in_core_index_as_tree(the_repository);
 

builtin/merge-recursive.c

@@ -31,7 +31,7 @@ int cmd_merge_recursive(int argc, const char **argv, const char *prefix UNUSED)
 	char *better1, *better2;
 	struct commit *result;
 
-	init_merge_options(&o, the_repository);
+	init_basic_merge_options(&o, the_repository);
 	if (argv[0] && ends_with(argv[0], "-subtree"))
 		o.subtree_shift = "";
 

builtin/merge-tree.c

@@ -571,7 +571,7 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 	};
 
 	/* Init merge options */
-	init_merge_options(&o.merge_options, the_repository);
+	init_ui_merge_options(&o.merge_options, the_repository);
 
 	/* Parse arguments */
 	original_argc = argc - 1; /* ignoring argv[0] */

builtin/merge.c

@@ -724,7 +724,7 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 			return 2;
 		}
 
-		init_merge_options(&o, the_repository);
+		init_ui_merge_options(&o, the_repository);
 		if (!strcmp(strategy, "subtree"))
 			o.subtree_shift = "";
 

builtin/replay.c

@@ -377,7 +377,7 @@ int cmd_replay(int argc, const char **argv, const char *prefix)
 		goto cleanup;
 	}
 
-	init_merge_options(&merge_opt, the_repository);
+	init_basic_merge_options(&merge_opt, the_repository);
 	memset(&result, 0, sizeof(result));
 	merge_opt.show_rename_progress = 0;
 	last_commit = onto;

builtin/stash.c

@@ -574,7 +574,7 @@ static int do_apply_stash(const char *prefix, struct stash_info *info,
 		}
 	}
 
-	init_merge_options(&o, the_repository);
+	init_ui_merge_options(&o, the_repository);
 
 	o.branch1 = "Updated upstream";
 	o.branch2 = "Stashed changes";

log-tree.c

@@ -1025,7 +1025,7 @@ static int do_remerge_diff(struct rev_info *opt,
 	struct strbuf parent2_desc = STRBUF_INIT;
 
 	/* Setup merge options */
-	init_merge_options(&o, the_repository);
+	init_ui_merge_options(&o, the_repository);
 	o.show_rename_progress = 0;
 	o.record_conflict_msgs_as_headers = 1;
 	o.msg_header_prefix = "remerge";

merge-recursive.c

@@ -3921,7 +3921,7 @@ int merge_recursive_generic(struct merge_options *opt,
 	return clean ? 0 : 1;
 }
 
-static void merge_recursive_config(struct merge_options *opt)
+static void merge_recursive_config(struct merge_options *opt, int ui)
 {
 	char *value = NULL;
 	int renormalize = 0;
@@ -3950,11 +3950,20 @@ static void merge_recursive_config(struct merge_options *opt)
 		} /* avoid erroring on values from future versions of git */
 		free(value);
 	}
+	if (ui) {
+		if (!git_config_get_string("diff.algorithm", &value)) {
+			long diff_algorithm = parse_algorithm_value(value);
+			if (diff_algorithm < 0)
+				die(_("unknown value for config '%s': %s"), "diff.algorithm", value);
+			opt->xdl_opts = (opt->xdl_opts & ~XDF_DIFF_ALGORITHM_MASK) | diff_algorithm;
+			free(value);
+		}
+	}
 	git_config(git_xmerge_config, NULL);
 }
 
-void init_merge_options(struct merge_options *opt,
-			struct repository *repo)
+static void init_merge_options(struct merge_options *opt,
+			struct repository *repo, int ui)
 {
 	const char *merge_verbosity;
 	memset(opt, 0, sizeof(struct merge_options));
@@ -3973,7 +3982,7 @@ void init_merge_options(struct merge_options *opt,
 
 	opt->conflict_style = -1;
 
-	merge_recursive_config(opt);
+	merge_recursive_config(opt, ui);
 	merge_verbosity = getenv("GIT_MERGE_VERBOSITY");
 	if (merge_verbosity)
 		opt->verbosity = strtol(merge_verbosity, NULL, 10);
@@ -3981,6 +3990,18 @@ void init_merge_options(struct merge_options *opt,
 		opt->buffer_output = 0;
 }
 
+void init_ui_merge_options(struct merge_options *opt,
+			struct repository *repo)
+{
+	init_merge_options(opt, repo, 1);
+}
+
+void init_basic_merge_options(struct merge_options *opt,
+			struct repository *repo)
+{
+	init_merge_options(opt, repo, 0);
+}
+
 /*
  * For now, members of merge_options do not need deep copying, but
  * it may change in the future, in which case we would need to update

merge-recursive.h

@@ -54,7 +54,10 @@ struct merge_options {
 	struct merge_options_internal *priv;
 };
 
-void init_merge_options(struct merge_options *opt, struct repository *repo);
+/* for use by porcelain commands */
+void init_ui_merge_options(struct merge_options *opt, struct repository *repo);
+/* for use by plumbing commands */
+void init_basic_merge_options(struct merge_options *opt, struct repository *repo);
 
 void copy_merge_options(struct merge_options *dst, struct merge_options *src);
 void clear_merge_options(struct merge_options *opt);

sequencer.c

@@ -762,7 +762,7 @@ static int do_recursive_merge(struct repository *r,
 
 	repo_read_index(r);
 
-	init_merge_options(&o, r);
+	init_ui_merge_options(&o, r);
 	o.ancestor = base ? base_label : "(empty tree)";
 	o.branch1 = "HEAD";
 	o.branch2 = next ? next_label : "(empty tree)";
@@ -4309,7 +4309,7 @@ static int do_merge(struct repository *r,
 	bases = reverse_commit_list(bases);
 
 	repo_read_index(r);
-	init_merge_options(&o, r);
+	init_ui_merge_options(&o, r);
 	o.branch1 = "HEAD";
 	o.branch2 = ref_name.buf;
 	o.buffer_output = 2;

t/t7615-diff-algo-with-mergy-operations.sh

@@ -0,0 +1,60 @@
+#!/bin/sh
+
+test_description='git merge and other operations that rely on merge
+
+Testing the influence of the diff algorithm on the merge output.'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	cp "$TEST_DIRECTORY"/t7615/base.c file.c &&
+	git add file.c &&
+	git commit -m c0 &&
+	git tag c0 &&
+	cp "$TEST_DIRECTORY"/t7615/ours.c file.c &&
+	git add file.c &&
+	git commit -m c1 &&
+	git tag c1 &&
+	git reset --hard c0 &&
+	cp "$TEST_DIRECTORY"/t7615/theirs.c file.c &&
+	git add file.c &&
+	git commit -m c2 &&
+	git tag c2
+'
+
+GIT_TEST_MERGE_ALGORITHM=recursive
+
+test_expect_success 'merge c2 to c1 with recursive merge strategy fails with the current default myers diff algorithm' '
+	git reset --hard c1 &&
+	test_must_fail git merge -s recursive c2
+'
+
+test_expect_success 'merge c2 to c1 with recursive merge strategy succeeds with -Xdiff-algorithm=histogram' '
+	git reset --hard c1 &&
+	git merge --strategy recursive -Xdiff-algorithm=histogram c2
+'
+
+test_expect_success 'merge c2 to c1 with recursive merge strategy succeeds with diff.algorithm = histogram' '
+	git reset --hard c1 &&
+	git config diff.algorithm histogram &&
+	git merge --strategy recursive c2
+'
+
+test_expect_success 'cherry-pick c2 to c1 with recursive merge strategy fails with the current default myers diff algorithm' '
+	git reset --hard c1 &&
+	test_must_fail git cherry-pick -s recursive c2
+'
+
+test_expect_success 'cherry-pick c2 to c1 with recursive merge strategy succeeds with -Xdiff-algorithm=histogram' '
+	git reset --hard c1 &&
+	git cherry-pick --strategy recursive -Xdiff-algorithm=histogram c2
+'
+
+test_expect_success 'cherry-pick c2 to c1 with recursive merge strategy succeeds with diff.algorithm = histogram' '
+	git reset --hard c1 &&
+	git config diff.algorithm histogram &&
+	git cherry-pick --strategy recursive c2
+'
+
+test_done

t/t7615/base.c

@@ -0,0 +1,17 @@
+int f(int x, int y)
+{
+        if (x == 0)
+        {
+                return y;
+        }
+        return x;
+}
+
+int g(size_t u)
+{
+        while (u < 30)
+        {
+                u++;
+        }
+        return u;
+}

t/t7615/ours.c

@@ -0,0 +1,17 @@
+int g(size_t u)
+{
+        while (u < 30)
+        {
+                u++;
+        }
+        return u;
+}
+
+int h(int x, int y, int z)
+{
+        if (z == 0)
+        {
+                return x;
+        }
+        return y;
+}

t/t7615/theirs.c

@@ -0,0 +1,17 @@
+int f(int x, int y)
+{
+        if (x == 0)
+        {
+                return y;
+        }
+        return x;
+}
+
+int g(size_t u)
+{
+        while (u > 34)
+        {
+                u--;
+        }
+        return u;
+}