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));
@@ -3973,14 +3982,26 @@ 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);
  if (opt->verbosity >= 5)
  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;
+}