Merge branch 'maint'
[gitweb.git] / submodule.c
index 7d70c4f7bfe2749953726fecb27144a9588a326f..5294cef641ef74ec525d8d747c82faf0bef7352a 100644 (file)
@@ -5,14 +5,30 @@
 #include "commit.h"
 #include "revision.h"
 #include "run-command.h"
+#include "diffcore.h"
+#include "refs.h"
+#include "string-list.h"
+
+static struct string_list config_name_for_path;
+static struct string_list config_fetch_recurse_submodules_for_name;
+static struct string_list config_ignore_for_name;
+static int config_fetch_recurse_submodules = RECURSE_SUBMODULES_ON_DEMAND;
+static struct string_list changed_submodule_paths;
 
 static int add_submodule_odb(const char *path)
 {
        struct strbuf objects_directory = STRBUF_INIT;
        struct alternate_object_database *alt_odb;
        int ret = 0;
+       const char *git_dir;
 
-       strbuf_addf(&objects_directory, "%s/.git/objects/", path);
+       strbuf_addf(&objects_directory, "%s/.git", path);
+       git_dir = read_gitfile_gently(objects_directory.buf);
+       if (git_dir) {
+               strbuf_reset(&objects_directory);
+               strbuf_addstr(&objects_directory, git_dir);
+       }
+       strbuf_addstr(&objects_directory, "/objects/");
        if (!is_directory(objects_directory.buf)) {
                ret = -1;
                goto done;
@@ -38,17 +54,182 @@ static int add_submodule_odb(const char *path)
        return ret;
 }
 
+void set_diffopt_flags_from_submodule_config(struct diff_options *diffopt,
+                                            const char *path)
+{
+       struct string_list_item *path_option, *ignore_option;
+       path_option = unsorted_string_list_lookup(&config_name_for_path, path);
+       if (path_option) {
+               ignore_option = unsorted_string_list_lookup(&config_ignore_for_name, path_option->util);
+               if (ignore_option)
+                       handle_ignore_submodules_arg(diffopt, ignore_option->util);
+       }
+}
+
+int submodule_config(const char *var, const char *value, void *cb)
+{
+       if (!prefixcmp(var, "submodule."))
+               return parse_submodule_config_option(var, value);
+       else if (!strcmp(var, "fetch.recursesubmodules")) {
+               config_fetch_recurse_submodules = parse_fetch_recurse_submodules_arg(var, value);
+               return 0;
+       }
+       return 0;
+}
+
+void gitmodules_config(void)
+{
+       const char *work_tree = get_git_work_tree();
+       if (work_tree) {
+               struct strbuf gitmodules_path = STRBUF_INIT;
+               strbuf_addstr(&gitmodules_path, work_tree);
+               strbuf_addstr(&gitmodules_path, "/.gitmodules");
+               git_config_from_file(submodule_config, gitmodules_path.buf, NULL);
+               strbuf_release(&gitmodules_path);
+       }
+}
+
+int parse_submodule_config_option(const char *var, const char *value)
+{
+       int len;
+       struct string_list_item *config;
+       struct strbuf submodname = STRBUF_INIT;
+
+       var += 10;              /* Skip "submodule." */
+
+       len = strlen(var);
+       if ((len > 5) && !strcmp(var + len - 5, ".path")) {
+               strbuf_add(&submodname, var, len - 5);
+               config = unsorted_string_list_lookup(&config_name_for_path, value);
+               if (config)
+                       free(config->util);
+               else
+                       config = string_list_append(&config_name_for_path, xstrdup(value));
+               config->util = strbuf_detach(&submodname, NULL);
+               strbuf_release(&submodname);
+       } else if ((len > 23) && !strcmp(var + len - 23, ".fetchrecursesubmodules")) {
+               strbuf_add(&submodname, var, len - 23);
+               config = unsorted_string_list_lookup(&config_fetch_recurse_submodules_for_name, submodname.buf);
+               if (!config)
+                       config = string_list_append(&config_fetch_recurse_submodules_for_name,
+                                                   strbuf_detach(&submodname, NULL));
+               config->util = (void *)(intptr_t)parse_fetch_recurse_submodules_arg(var, value);
+               strbuf_release(&submodname);
+       } else if ((len > 7) && !strcmp(var + len - 7, ".ignore")) {
+               if (strcmp(value, "untracked") && strcmp(value, "dirty") &&
+                   strcmp(value, "all") && strcmp(value, "none")) {
+                       warning("Invalid parameter \"%s\" for config option \"submodule.%s.ignore\"", value, var);
+                       return 0;
+               }
+
+               strbuf_add(&submodname, var, len - 7);
+               config = unsorted_string_list_lookup(&config_ignore_for_name, submodname.buf);
+               if (config)
+                       free(config->util);
+               else
+                       config = string_list_append(&config_ignore_for_name,
+                                                   strbuf_detach(&submodname, NULL));
+               strbuf_release(&submodname);
+               config->util = xstrdup(value);
+               return 0;
+       }
+       return 0;
+}
+
+void handle_ignore_submodules_arg(struct diff_options *diffopt,
+                                 const char *arg)
+{
+       DIFF_OPT_CLR(diffopt, IGNORE_SUBMODULES);
+       DIFF_OPT_CLR(diffopt, IGNORE_UNTRACKED_IN_SUBMODULES);
+       DIFF_OPT_CLR(diffopt, IGNORE_DIRTY_SUBMODULES);
+
+       if (!strcmp(arg, "all"))
+               DIFF_OPT_SET(diffopt, IGNORE_SUBMODULES);
+       else if (!strcmp(arg, "untracked"))
+               DIFF_OPT_SET(diffopt, IGNORE_UNTRACKED_IN_SUBMODULES);
+       else if (!strcmp(arg, "dirty"))
+               DIFF_OPT_SET(diffopt, IGNORE_DIRTY_SUBMODULES);
+       else if (strcmp(arg, "none"))
+               die("bad --ignore-submodules argument: %s", arg);
+}
+
+static int prepare_submodule_summary(struct rev_info *rev, const char *path,
+               struct commit *left, struct commit *right,
+               int *fast_forward, int *fast_backward)
+{
+       struct commit_list *merge_bases, *list;
+
+       init_revisions(rev, NULL);
+       setup_revisions(0, NULL, rev, NULL);
+       rev->left_right = 1;
+       rev->first_parent_only = 1;
+       left->object.flags |= SYMMETRIC_LEFT;
+       add_pending_object(rev, &left->object, path);
+       add_pending_object(rev, &right->object, path);
+       merge_bases = get_merge_bases(left, right, 1);
+       if (merge_bases) {
+               if (merge_bases->item == left)
+                       *fast_forward = 1;
+               else if (merge_bases->item == right)
+                       *fast_backward = 1;
+       }
+       for (list = merge_bases; list; list = list->next) {
+               list->item->object.flags |= UNINTERESTING;
+               add_pending_object(rev, &list->item->object,
+                       sha1_to_hex(list->item->object.sha1));
+       }
+       return prepare_revision_walk(rev);
+}
+
+static void print_submodule_summary(struct rev_info *rev, FILE *f,
+               const char *del, const char *add, const char *reset)
+{
+       static const char format[] = "  %m %s";
+       struct strbuf sb = STRBUF_INIT;
+       struct commit *commit;
+
+       while ((commit = get_revision(rev))) {
+               struct pretty_print_context ctx = {0};
+               ctx.date_mode = rev->date_mode;
+               strbuf_setlen(&sb, 0);
+               if (commit->object.flags & SYMMETRIC_LEFT) {
+                       if (del)
+                               strbuf_addstr(&sb, del);
+               }
+               else if (add)
+                       strbuf_addstr(&sb, add);
+               format_commit_message(commit, format, &sb, &ctx);
+               if (reset)
+                       strbuf_addstr(&sb, reset);
+               strbuf_addch(&sb, '\n');
+               fprintf(f, "%s", sb.buf);
+       }
+       strbuf_release(&sb);
+}
+
+int parse_fetch_recurse_submodules_arg(const char *opt, const char *arg)
+{
+       switch (git_config_maybe_bool(opt, arg)) {
+       case 1:
+               return RECURSE_SUBMODULES_ON;
+       case 0:
+               return RECURSE_SUBMODULES_OFF;
+       default:
+               if (!strcmp(arg, "on-demand"))
+                       return RECURSE_SUBMODULES_ON_DEMAND;
+               die("bad %s argument: %s", opt, arg);
+       }
+}
+
 void show_submodule_summary(FILE *f, const char *path,
                unsigned char one[20], unsigned char two[20],
                unsigned dirty_submodule,
                const char *del, const char *add, const char *reset)
 {
        struct rev_info rev;
-       struct commit *commit, *left = left, *right = right;
-       struct commit_list *merge_bases, *list;
+       struct commit *left = left, *right = right;
        const char *message = NULL;
        struct strbuf sb = STRBUF_INIT;
-       static const char *format = "  %m %s";
        int fast_forward = 0, fast_backward = 0;
 
        if (is_null_sha1(two))
@@ -61,28 +242,19 @@ void show_submodule_summary(FILE *f, const char *path,
                 !(right = lookup_commit_reference(two)))
                message = "(commits not present)";
 
-       if (!message) {
-               init_revisions(&rev, NULL);
-               setup_revisions(0, NULL, &rev, NULL);
-               rev.left_right = 1;
-               rev.first_parent_only = 1;
-               left->object.flags |= SYMMETRIC_LEFT;
-               add_pending_object(&rev, &left->object, path);
-               add_pending_object(&rev, &right->object, path);
-               merge_bases = get_merge_bases(left, right, 1);
-               if (merge_bases) {
-                       if (merge_bases->item == left)
-                               fast_forward = 1;
-                       else if (merge_bases->item == right)
-                               fast_backward = 1;
-               }
-               for (list = merge_bases; list; list = list->next) {
-                       list->item->object.flags |= UNINTERESTING;
-                       add_pending_object(&rev, &list->item->object,
-                               sha1_to_hex(list->item->object.sha1));
-               }
-               if (prepare_revision_walk(&rev))
-                       message = "(revision walker failed)";
+       if (!message &&
+           prepare_submodule_summary(&rev, path, left, right,
+                                       &fast_forward, &fast_backward))
+               message = "(revision walker failed)";
+
+       if (dirty_submodule & DIRTY_SUBMODULE_UNTRACKED)
+               fprintf(f, "Submodule %s contains untracked content\n", path);
+       if (dirty_submodule & DIRTY_SUBMODULE_MODIFIED)
+               fprintf(f, "Submodule %s contains modified content\n", path);
+
+       if (!hashcmp(one, two)) {
+               strbuf_release(&sb);
+               return;
        }
 
        strbuf_addf(&sb, "Submodule %s %s..", path,
@@ -90,8 +262,6 @@ void show_submodule_summary(FILE *f, const char *path,
        if (!fast_backward && !fast_forward)
                strbuf_addch(&sb, '.');
        strbuf_addf(&sb, "%s", find_unique_abbrev(two, DEFAULT_ABBREV));
-       if (dirty_submodule)
-               strbuf_add(&sb, "-dirty", 6);
        if (message)
                strbuf_addf(&sb, " %s\n", message);
        else
@@ -99,42 +269,230 @@ void show_submodule_summary(FILE *f, const char *path,
        fwrite(sb.buf, sb.len, 1, f);
 
        if (!message) {
-               while ((commit = get_revision(&rev))) {
-                       struct pretty_print_context ctx = {0};
-                       ctx.date_mode = rev.date_mode;
-                       strbuf_setlen(&sb, 0);
-                       if (commit->object.flags & SYMMETRIC_LEFT) {
-                               if (del)
-                                       strbuf_addstr(&sb, del);
-                       }
-                       else if (add)
-                               strbuf_addstr(&sb, add);
-                       format_commit_message(commit, format, &sb, &ctx);
-                       if (reset)
-                               strbuf_addstr(&sb, reset);
-                       strbuf_addch(&sb, '\n');
-                       fprintf(f, "%s", sb.buf);
-               }
+               print_submodule_summary(&rev, f, del, add, reset);
                clear_commit_marks(left, ~0);
                clear_commit_marks(right, ~0);
        }
+
        strbuf_release(&sb);
 }
 
-int is_submodule_modified(const char *path)
+void set_config_fetch_recurse_submodules(int value)
 {
-       int len;
+       config_fetch_recurse_submodules = value;
+}
+
+static int is_submodule_commit_present(const char *path, unsigned char sha1[20])
+{
+       int is_present = 0;
+       if (!add_submodule_odb(path) && lookup_commit_reference(sha1)) {
+               /* Even if the submodule is checked out and the commit is
+                * present, make sure it is reachable from a ref. */
+               struct child_process cp;
+               const char *argv[] = {"rev-list", "-n", "1", NULL, "--not", "--all", NULL};
+               struct strbuf buf = STRBUF_INIT;
+
+               argv[3] = sha1_to_hex(sha1);
+               memset(&cp, 0, sizeof(cp));
+               cp.argv = argv;
+               cp.env = local_repo_env;
+               cp.git_cmd = 1;
+               cp.no_stdin = 1;
+               cp.out = -1;
+               cp.dir = path;
+               if (!run_command(&cp) && !strbuf_read(&buf, cp.out, 1024))
+                       is_present = 1;
+
+               close(cp.out);
+               strbuf_release(&buf);
+       }
+       return is_present;
+}
+
+static void submodule_collect_changed_cb(struct diff_queue_struct *q,
+                                        struct diff_options *options,
+                                        void *data)
+{
+       int i;
+       for (i = 0; i < q->nr; i++) {
+               struct diff_filepair *p = q->queue[i];
+               if (!S_ISGITLINK(p->two->mode))
+                       continue;
+
+               if (S_ISGITLINK(p->one->mode)) {
+                       /* NEEDSWORK: We should honor the name configured in
+                        * the .gitmodules file of the commit we are examining
+                        * here to be able to correctly follow submodules
+                        * being moved around. */
+                       struct string_list_item *path;
+                       path = unsorted_string_list_lookup(&changed_submodule_paths, p->two->path);
+                       if (!path && !is_submodule_commit_present(p->two->path, p->two->sha1))
+                               string_list_append(&changed_submodule_paths, xstrdup(p->two->path));
+               } else {
+                       /* Submodule is new or was moved here */
+                       /* NEEDSWORK: When the .git directories of submodules
+                        * live inside the superprojects .git directory some
+                        * day we should fetch new submodules directly into
+                        * that location too when config or options request
+                        * that so they can be checked out from there. */
+                       continue;
+               }
+       }
+}
+
+void check_for_new_submodule_commits(unsigned char new_sha1[20])
+{
+       struct rev_info rev;
+       struct commit *commit;
+       const char *argv[] = {NULL, NULL, "--not", "--all", NULL};
+       int argc = ARRAY_SIZE(argv) - 1;
+
+       init_revisions(&rev, NULL);
+       argv[1] = xstrdup(sha1_to_hex(new_sha1));
+       setup_revisions(argc, argv, &rev, NULL);
+       if (prepare_revision_walk(&rev))
+               die("revision walk setup failed");
+
+       /*
+        * Collect all submodules (whether checked out or not) for which new
+        * commits have been recorded upstream in "changed_submodule_paths".
+        */
+       while ((commit = get_revision(&rev))) {
+               struct commit_list *parent = commit->parents;
+               while (parent) {
+                       struct diff_options diff_opts;
+                       diff_setup(&diff_opts);
+                       diff_opts.output_format |= DIFF_FORMAT_CALLBACK;
+                       diff_opts.format_callback = submodule_collect_changed_cb;
+                       if (diff_setup_done(&diff_opts) < 0)
+                               die("diff_setup_done failed");
+                       diff_tree_sha1(parent->item->object.sha1, commit->object.sha1, "", &diff_opts);
+                       diffcore_std(&diff_opts);
+                       diff_flush(&diff_opts);
+                       parent = parent->next;
+               }
+       }
+       free((char *)argv[1]);
+}
+
+int fetch_populated_submodules(int num_options, const char **options,
+                              const char *prefix, int command_line_option,
+                              int quiet)
+{
+       int i, result = 0, argc = 0, default_argc;
+       struct child_process cp;
+       const char **argv;
+       struct string_list_item *name_for_path;
+       const char *work_tree = get_git_work_tree();
+       if (!work_tree)
+               goto out;
+
+       if (!the_index.initialized)
+               if (read_cache() < 0)
+                       die("index file corrupt");
+
+       /* 6: "fetch" (options) --recurse-submodules-default default "--submodule-prefix" prefix NULL */
+       argv = xcalloc(num_options + 6, sizeof(const char *));
+       argv[argc++] = "fetch";
+       for (i = 0; i < num_options; i++)
+               argv[argc++] = options[i];
+       argv[argc++] = "--recurse-submodules-default";
+       default_argc = argc++;
+       argv[argc++] = "--submodule-prefix";
+
+       memset(&cp, 0, sizeof(cp));
+       cp.argv = argv;
+       cp.env = local_repo_env;
+       cp.git_cmd = 1;
+       cp.no_stdin = 1;
+
+       for (i = 0; i < active_nr; i++) {
+               struct strbuf submodule_path = STRBUF_INIT;
+               struct strbuf submodule_git_dir = STRBUF_INIT;
+               struct strbuf submodule_prefix = STRBUF_INIT;
+               struct cache_entry *ce = active_cache[i];
+               const char *git_dir, *name, *default_argv;
+
+               if (!S_ISGITLINK(ce->ce_mode))
+                       continue;
+
+               name = ce->name;
+               name_for_path = unsorted_string_list_lookup(&config_name_for_path, ce->name);
+               if (name_for_path)
+                       name = name_for_path->util;
+
+               default_argv = "yes";
+               if (command_line_option == RECURSE_SUBMODULES_DEFAULT) {
+                       struct string_list_item *fetch_recurse_submodules_option;
+                       fetch_recurse_submodules_option = unsorted_string_list_lookup(&config_fetch_recurse_submodules_for_name, name);
+                       if (fetch_recurse_submodules_option) {
+                               if ((intptr_t)fetch_recurse_submodules_option->util == RECURSE_SUBMODULES_OFF)
+                                       continue;
+                               if ((intptr_t)fetch_recurse_submodules_option->util == RECURSE_SUBMODULES_ON_DEMAND) {
+                                       if (!unsorted_string_list_lookup(&changed_submodule_paths, ce->name))
+                                               continue;
+                                       default_argv = "on-demand";
+                               }
+                       } else {
+                               if (config_fetch_recurse_submodules == RECURSE_SUBMODULES_OFF)
+                                       continue;
+                               if (config_fetch_recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND) {
+                                       if (!unsorted_string_list_lookup(&changed_submodule_paths, ce->name))
+                                               continue;
+                                       default_argv = "on-demand";
+                               }
+                       }
+               } else if (command_line_option == RECURSE_SUBMODULES_ON_DEMAND) {
+                       if (!unsorted_string_list_lookup(&changed_submodule_paths, ce->name))
+                               continue;
+                       default_argv = "on-demand";
+               }
+
+               strbuf_addf(&submodule_path, "%s/%s", work_tree, ce->name);
+               strbuf_addf(&submodule_git_dir, "%s/.git", submodule_path.buf);
+               strbuf_addf(&submodule_prefix, "%s%s/", prefix, ce->name);
+               git_dir = read_gitfile_gently(submodule_git_dir.buf);
+               if (!git_dir)
+                       git_dir = submodule_git_dir.buf;
+               if (is_directory(git_dir)) {
+                       if (!quiet)
+                               printf("Fetching submodule %s%s\n", prefix, ce->name);
+                       cp.dir = submodule_path.buf;
+                       argv[default_argc] = default_argv;
+                       argv[argc] = submodule_prefix.buf;
+                       if (run_command(&cp))
+                               result = 1;
+               }
+               strbuf_release(&submodule_path);
+               strbuf_release(&submodule_git_dir);
+               strbuf_release(&submodule_prefix);
+       }
+       free(argv);
+out:
+       string_list_clear(&changed_submodule_paths, 1);
+       return result;
+}
+
+unsigned is_submodule_modified(const char *path, int ignore_untracked)
+{
+       ssize_t len;
        struct child_process cp;
        const char *argv[] = {
                "status",
                "--porcelain",
                NULL,
+               NULL,
        };
-       char *env[4];
        struct strbuf buf = STRBUF_INIT;
+       unsigned dirty_submodule = 0;
+       const char *line, *next_line;
+       const char *git_dir;
 
-       strbuf_addf(&buf, "%s/.git/", path);
-       if (!is_directory(buf.buf)) {
+       strbuf_addf(&buf, "%s/.git", path);
+       git_dir = read_gitfile_gently(buf.buf);
+       if (!git_dir)
+               git_dir = buf.buf;
+       if (!is_directory(git_dir)) {
                strbuf_release(&buf);
                /* The submodule is not checked out, so it is not modified */
                return 0;
@@ -142,32 +500,204 @@ int is_submodule_modified(const char *path)
        }
        strbuf_reset(&buf);
 
-       strbuf_addf(&buf, "GIT_WORK_TREE=%s", path);
-       env[0] = strbuf_detach(&buf, NULL);
-       strbuf_addf(&buf, "GIT_DIR=%s/.git", path);
-       env[1] = strbuf_detach(&buf, NULL);
-       strbuf_addf(&buf, "GIT_INDEX_FILE");
-       env[2] = strbuf_detach(&buf, NULL);
-       env[3] = NULL;
+       if (ignore_untracked)
+               argv[2] = "-uno";
 
        memset(&cp, 0, sizeof(cp));
        cp.argv = argv;
-       cp.env = (const char *const *)env;
+       cp.env = local_repo_env;
        cp.git_cmd = 1;
        cp.no_stdin = 1;
        cp.out = -1;
+       cp.dir = path;
        if (start_command(&cp))
                die("Could not run git status --porcelain");
 
        len = strbuf_read(&buf, cp.out, 1024);
+       line = buf.buf;
+       while (len > 2) {
+               if ((line[0] == '?') && (line[1] == '?')) {
+                       dirty_submodule |= DIRTY_SUBMODULE_UNTRACKED;
+                       if (dirty_submodule & DIRTY_SUBMODULE_MODIFIED)
+                               break;
+               } else {
+                       dirty_submodule |= DIRTY_SUBMODULE_MODIFIED;
+                       if (ignore_untracked ||
+                           (dirty_submodule & DIRTY_SUBMODULE_UNTRACKED))
+                               break;
+               }
+               next_line = strchr(line, '\n');
+               if (!next_line)
+                       break;
+               next_line++;
+               len -= (next_line - line);
+               line = next_line;
+       }
        close(cp.out);
 
        if (finish_command(&cp))
                die("git status --porcelain failed");
 
-       free(env[0]);
-       free(env[1]);
-       free(env[2]);
        strbuf_release(&buf);
-       return len != 0;
+       return dirty_submodule;
+}
+
+static int find_first_merges(struct object_array *result, const char *path,
+               struct commit *a, struct commit *b)
+{
+       int i, j;
+       struct object_array merges;
+       struct commit *commit;
+       int contains_another;
+
+       char merged_revision[42];
+       const char *rev_args[] = { "rev-list", "--merges", "--ancestry-path",
+                                  "--all", merged_revision, NULL };
+       struct rev_info revs;
+       struct setup_revision_opt rev_opts;
+
+       memset(&merges, 0, sizeof(merges));
+       memset(result, 0, sizeof(struct object_array));
+       memset(&rev_opts, 0, sizeof(rev_opts));
+
+       /* get all revisions that merge commit a */
+       snprintf(merged_revision, sizeof(merged_revision), "^%s",
+                       sha1_to_hex(a->object.sha1));
+       init_revisions(&revs, NULL);
+       rev_opts.submodule = path;
+       setup_revisions(sizeof(rev_args)/sizeof(char *)-1, rev_args, &revs, &rev_opts);
+
+       /* save all revisions from the above list that contain b */
+       if (prepare_revision_walk(&revs))
+               die("revision walk setup failed");
+       while ((commit = get_revision(&revs)) != NULL) {
+               struct object *o = &(commit->object);
+               if (in_merge_bases(b, &commit, 1))
+                       add_object_array(o, NULL, &merges);
+       }
+
+       /* Now we've got all merges that contain a and b. Prune all
+        * merges that contain another found merge and save them in
+        * result.
+        */
+       for (i = 0; i < merges.nr; i++) {
+               struct commit *m1 = (struct commit *) merges.objects[i].item;
+
+               contains_another = 0;
+               for (j = 0; j < merges.nr; j++) {
+                       struct commit *m2 = (struct commit *) merges.objects[j].item;
+                       if (i != j && in_merge_bases(m2, &m1, 1)) {
+                               contains_another = 1;
+                               break;
+                       }
+               }
+
+               if (!contains_another)
+                       add_object_array(merges.objects[i].item,
+                                        merges.objects[i].name, result);
+       }
+
+       free(merges.objects);
+       return result->nr;
+}
+
+static void print_commit(struct commit *commit)
+{
+       struct strbuf sb = STRBUF_INIT;
+       struct pretty_print_context ctx = {0};
+       ctx.date_mode = DATE_NORMAL;
+       format_commit_message(commit, " %h: %m %s", &sb, &ctx);
+       fprintf(stderr, "%s\n", sb.buf);
+       strbuf_release(&sb);
+}
+
+#define MERGE_WARNING(path, msg) \
+       warning("Failed to merge submodule %s (%s)", path, msg);
+
+int merge_submodule(unsigned char result[20], const char *path,
+                   const unsigned char base[20], const unsigned char a[20],
+                   const unsigned char b[20])
+{
+       struct commit *commit_base, *commit_a, *commit_b;
+       int parent_count;
+       struct object_array merges;
+
+       int i;
+
+       /* store a in result in case we fail */
+       hashcpy(result, a);
+
+       /* we can not handle deletion conflicts */
+       if (is_null_sha1(base))
+               return 0;
+       if (is_null_sha1(a))
+               return 0;
+       if (is_null_sha1(b))
+               return 0;
+
+       if (add_submodule_odb(path)) {
+               MERGE_WARNING(path, "not checked out");
+               return 0;
+       }
+
+       if (!(commit_base = lookup_commit_reference(base)) ||
+           !(commit_a = lookup_commit_reference(a)) ||
+           !(commit_b = lookup_commit_reference(b))) {
+               MERGE_WARNING(path, "commits not present");
+               return 0;
+       }
+
+       /* check whether both changes are forward */
+       if (!in_merge_bases(commit_base, &commit_a, 1) ||
+           !in_merge_bases(commit_base, &commit_b, 1)) {
+               MERGE_WARNING(path, "commits don't follow merge-base");
+               return 0;
+       }
+
+       /* Case #1: a is contained in b or vice versa */
+       if (in_merge_bases(commit_a, &commit_b, 1)) {
+               hashcpy(result, b);
+               return 1;
+       }
+       if (in_merge_bases(commit_b, &commit_a, 1)) {
+               hashcpy(result, a);
+               return 1;
+       }
+
+       /*
+        * Case #2: There are one or more merges that contain a and b in
+        * the submodule. If there is only one, then present it as a
+        * suggestion to the user, but leave it marked unmerged so the
+        * user needs to confirm the resolution.
+        */
+
+       /* find commit which merges them */
+       parent_count = find_first_merges(&merges, path, commit_a, commit_b);
+       switch (parent_count) {
+       case 0:
+               MERGE_WARNING(path, "merge following commits not found");
+               break;
+
+       case 1:
+               MERGE_WARNING(path, "not fast-forward");
+               fprintf(stderr, "Found a possible merge resolution "
+                               "for the submodule:\n");
+               print_commit((struct commit *) merges.objects[0].item);
+               fprintf(stderr,
+                       "If this is correct simply add it to the index "
+                       "for example\n"
+                       "by using:\n\n"
+                       "  git update-index --cacheinfo 160000 %s \"%s\"\n\n"
+                       "which will accept this suggestion.\n",
+                       sha1_to_hex(merges.objects[0].item->sha1), path);
+               break;
+
+       default:
+               MERGE_WARNING(path, "multiple merges found");
+               for (i = 0; i < merges.nr; i++)
+                       print_commit((struct commit *) merges.objects[i].item);
+       }
+
+       free(merges.objects);
+       return 0;
 }