Add the configuration option skipFetchAll
[gitweb.git] / builtin-merge.c
index 370d0034ca188724344f2085c32e4150de7f9792..b6b84286b26a4317dfd5185ae83fd861c6f9fa7d 100644 (file)
@@ -22,6 +22,8 @@
 #include "log-tree.h"
 #include "color.h"
 #include "rerere.h"
+#include "help.h"
+#include "merge-recursive.h"
 
 #define DEFAULT_TWOHEAD (1<<0)
 #define DEFAULT_OCTOPUS (1<<1)
@@ -34,8 +36,8 @@ struct strategy {
 };
 
 static const char * const builtin_merge_usage[] = {
-       "git-merge [options] <remote>...",
-       "git-merge [options] <msg> HEAD <remote>",
+       "git merge [options] <remote>...",
+       "git merge [options] <msg> HEAD <remote>",
        NULL
 };
 
@@ -48,6 +50,7 @@ static unsigned char head[20], stash[20];
 static struct strategy **use_strategies;
 static size_t use_strategies_nr, use_strategies_alloc;
 static const char *branch;
+static int verbosity;
 
 static struct strategy all_strategy[] = {
        { "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
@@ -77,7 +80,9 @@ static int option_parse_message(const struct option *opt,
 static struct strategy *get_strategy(const char *name)
 {
        int i;
-       struct strbuf err;
+       struct strategy *ret;
+       static struct cmdnames main_cmds, other_cmds;
+       static int loaded;
 
        if (!name)
                return NULL;
@@ -86,12 +91,42 @@ static struct strategy *get_strategy(const char *name)
                if (!strcmp(name, all_strategy[i].name))
                        return &all_strategy[i];
 
-       strbuf_init(&err, 0);
-       for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
-               strbuf_addf(&err, " %s", all_strategy[i].name);
-       fprintf(stderr, "Could not find merge strategy '%s'.\n", name);
-       fprintf(stderr, "Available strategies are:%s.\n", err.buf);
-       exit(1);
+       if (!loaded) {
+               struct cmdnames not_strategies;
+               loaded = 1;
+
+               memset(&not_strategies, 0, sizeof(struct cmdnames));
+               load_command_list("git-merge-", &main_cmds, &other_cmds);
+               for (i = 0; i < main_cmds.cnt; i++) {
+                       int j, found = 0;
+                       struct cmdname *ent = main_cmds.names[i];
+                       for (j = 0; j < ARRAY_SIZE(all_strategy); j++)
+                               if (!strncmp(ent->name, all_strategy[j].name, ent->len)
+                                               && !all_strategy[j].name[ent->len])
+                                       found = 1;
+                       if (!found)
+                               add_cmdname(&not_strategies, ent->name, ent->len);
+                       exclude_cmds(&main_cmds, &not_strategies);
+               }
+       }
+       if (!is_in_cmdlist(&main_cmds, name) && !is_in_cmdlist(&other_cmds, name)) {
+               fprintf(stderr, "Could not find merge strategy '%s'.\n", name);
+               fprintf(stderr, "Available strategies are:");
+               for (i = 0; i < main_cmds.cnt; i++)
+                       fprintf(stderr, " %s", main_cmds.names[i]->name);
+               fprintf(stderr, ".\n");
+               if (other_cmds.cnt) {
+                       fprintf(stderr, "Available custom strategies are:");
+                       for (i = 0; i < other_cmds.cnt; i++)
+                               fprintf(stderr, " %s", other_cmds.names[i]->name);
+                       fprintf(stderr, ".\n");
+               }
+               exit(1);
+       }
+
+       ret = xcalloc(1, sizeof(struct strategy));
+       ret->name = xstrdup(name);
+       return ret;
 }
 
 static void append_strategy(struct strategy *s)
@@ -137,6 +172,7 @@ static struct option builtin_merge_options[] = {
        OPT_CALLBACK('m', "message", &merge_msg, "message",
                "message to be used for the merge commit (if any)",
                option_parse_message),
+       OPT__VERBOSITY(&verbosity),
        OPT_END()
 };
 
@@ -145,6 +181,7 @@ static void drop_save(void)
 {
        unlink(git_path("MERGE_HEAD"));
        unlink(git_path("MERGE_MSG"));
+       unlink(git_path("MERGE_MODE"));
 }
 
 static void save_state(void)
@@ -192,7 +229,7 @@ static void reset_hard(unsigned const char *sha1, int verbose)
 
 static void restore_state(void)
 {
-       struct strbuf sb;
+       struct strbuf sb = STRBUF_INIT;
        const char *args[] = { "stash", "apply", NULL, NULL };
 
        if (is_null_sha1(stash))
@@ -200,7 +237,6 @@ static void restore_state(void)
 
        reset_hard(head, 1);
 
-       strbuf_init(&sb, 0);
        args[2] = sha1_to_hex(stash);
 
        /*
@@ -216,7 +252,8 @@ static void restore_state(void)
 /* This is called when no merge was necessary. */
 static void finish_up_to_date(const char *msg)
 {
-       printf("%s%s\n", squash ? " (nothing to squash)" : "", msg);
+       if (verbosity >= 0)
+               printf("%s%s\n", squash ? " (nothing to squash)" : "", msg);
        drop_save();
 }
 
@@ -224,14 +261,14 @@ static void squash_message(void)
 {
        struct rev_info rev;
        struct commit *commit;
-       struct strbuf out;
+       struct strbuf out = STRBUF_INIT;
        struct commit_list *j;
        int fd;
 
        printf("Squash commit -- not updating HEAD\n");
        fd = open(git_path("SQUASH_MSG"), O_WRONLY | O_CREAT, 0666);
        if (fd < 0)
-               die("Could not write to %s", git_path("SQUASH_MSG"));
+               die_errno("Could not write to '%s'", git_path("SQUASH_MSG"));
 
        init_revisions(&rev, NULL);
        rev.ignore_merges = 1;
@@ -248,7 +285,6 @@ static void squash_message(void)
        if (prepare_revision_walk(&rev))
                die("revision walk setup failed");
 
-       strbuf_init(&out, 0);
        strbuf_addstr(&out, "Squashed commit of the following:\n");
        while ((commit = get_revision(&rev)) != NULL) {
                strbuf_addch(&out, '\n');
@@ -257,56 +293,29 @@ static void squash_message(void)
                pretty_print_commit(rev.commit_format, commit, &out, rev.abbrev,
                        NULL, NULL, rev.date_mode, 0);
        }
-       write(fd, out.buf, out.len);
-       close(fd);
+       if (write(fd, out.buf, out.len) < 0)
+               die_errno("Writing SQUASH_MSG");
+       if (close(fd))
+               die_errno("Finishing SQUASH_MSG");
        strbuf_release(&out);
 }
 
-static int run_hook(const char *name)
-{
-       struct child_process hook;
-       const char *argv[3], *env[2];
-       char index[PATH_MAX];
-
-       argv[0] = git_path("hooks/%s", name);
-       if (access(argv[0], X_OK) < 0)
-               return 0;
-
-       snprintf(index, sizeof(index), "GIT_INDEX_FILE=%s", get_index_file());
-       env[0] = index;
-       env[1] = NULL;
-
-       if (squash)
-               argv[1] = "1";
-       else
-               argv[1] = "0";
-       argv[2] = NULL;
-
-       memset(&hook, 0, sizeof(hook));
-       hook.argv = argv;
-       hook.no_stdin = 1;
-       hook.stdout_to_stderr = 1;
-       hook.env = env;
-
-       return run_command(&hook);
-}
-
 static void finish(const unsigned char *new_head, const char *msg)
 {
-       struct strbuf reflog_message;
+       struct strbuf reflog_message = STRBUF_INIT;
 
-       strbuf_init(&reflog_message, 0);
        if (!msg)
                strbuf_addstr(&reflog_message, getenv("GIT_REFLOG_ACTION"));
        else {
-               printf("%s\n", msg);
+               if (verbosity >= 0)
+                       printf("%s\n", msg);
                strbuf_addf(&reflog_message, "%s: %s",
                        getenv("GIT_REFLOG_ACTION"), msg);
        }
        if (squash) {
                squash_message();
        } else {
-               if (!merge_msg.len)
+               if (verbosity >= 0 && !merge_msg.len)
                        printf("No merge message -- not updating HEAD\n");
                else {
                        const char *argv_gc_auto[] = { "gc", "--auto", NULL };
@@ -336,7 +345,7 @@ static void finish(const unsigned char *new_head, const char *msg)
        }
 
        /* Run a post-merge hook */
-       run_hook("post-merge");
+       run_hook(NULL, "post-merge", squash ? "1" : "0", NULL);
 
        strbuf_release(&reflog_message);
 }
@@ -346,24 +355,31 @@ static void merge_name(const char *remote, struct strbuf *msg)
 {
        struct object *remote_head;
        unsigned char branch_head[20], buf_sha[20];
-       struct strbuf buf;
+       struct strbuf buf = STRBUF_INIT;
+       struct strbuf bname = STRBUF_INIT;
        const char *ptr;
+       char *found_ref;
        int len, early;
 
+       strbuf_branchname(&bname, remote);
+       remote = bname.buf;
+
        memset(branch_head, 0, sizeof(branch_head));
        remote_head = peel_to_type(remote, 0, NULL, OBJ_COMMIT);
        if (!remote_head)
                die("'%s' does not point to a commit", remote);
 
-       strbuf_init(&buf, 0);
-       strbuf_addstr(&buf, "refs/heads/");
-       strbuf_addstr(&buf, remote);
-       resolve_ref(buf.buf, branch_head, 0, 0);
-
-       if (!hashcmp(remote_head->sha1, branch_head)) {
-               strbuf_addf(msg, "%s\t\tbranch '%s' of .\n",
-                       sha1_to_hex(branch_head), remote);
-               return;
+       if (dwim_ref(remote, strlen(remote), branch_head, &found_ref) > 0) {
+               if (!prefixcmp(found_ref, "refs/heads/")) {
+                       strbuf_addf(msg, "%s\t\tbranch '%s' of .\n",
+                                   sha1_to_hex(branch_head), remote);
+                       goto cleanup;
+               }
+               if (!prefixcmp(found_ref, "refs/remotes/")) {
+                       strbuf_addf(msg, "%s\t\tremote branch '%s' of .\n",
+                                   sha1_to_hex(branch_head), remote);
+                       goto cleanup;
+               }
        }
 
        /* See if remote matches <name>^^^.. or <name>~<number> */
@@ -397,27 +413,27 @@ static void merge_name(const char *remote, struct strbuf *msg)
                strbuf_addstr(&truname, "refs/heads/");
                strbuf_addstr(&truname, remote);
                strbuf_setlen(&truname, truname.len - len);
-               if (resolve_ref(truname.buf, buf_sha, 0, 0)) {
+               if (resolve_ref(truname.buf, buf_sha, 0, NULL)) {
                        strbuf_addf(msg,
                                    "%s\t\tbranch '%s'%s of .\n",
                                    sha1_to_hex(remote_head->sha1),
                                    truname.buf + 11,
                                    (early ? " (early part)" : ""));
-                       return;
+                       strbuf_release(&truname);
+                       goto cleanup;
                }
        }
 
        if (!strcmp(remote, "FETCH_HEAD") &&
                        !access(git_path("FETCH_HEAD"), R_OK)) {
                FILE *fp;
-               struct strbuf line;
+               struct strbuf line = STRBUF_INIT;
                char *ptr;
 
-               strbuf_init(&line, 0);
                fp = fopen(git_path("FETCH_HEAD"), "r");
                if (!fp)
-                       die("could not open %s for reading: %s",
-                               git_path("FETCH_HEAD"), strerror(errno));
+                       die_errno("could not open '%s' for reading",
+                                 git_path("FETCH_HEAD"));
                strbuf_getline(&line, fp, '\n');
                fclose(fp);
                ptr = strstr(line.buf, "\tnot-for-merge\t");
@@ -425,10 +441,13 @@ static void merge_name(const char *remote, struct strbuf *msg)
                        strbuf_remove(&line, ptr-line.buf+1, 13);
                strbuf_addbuf(msg, &line);
                strbuf_release(&line);
-               return;
+               goto cleanup;
        }
        strbuf_addf(msg, "%s\t\tcommit '%s'\n",
                sha1_to_hex(remote_head->sha1), remote);
+cleanup:
+       strbuf_release(&buf);
+       strbuf_release(&bname);
 }
 
 static int git_merge_config(const char *k, const char *v, void *cb)
@@ -447,7 +466,7 @@ static int git_merge_config(const char *k, const char *v, void *cb)
                argv = xrealloc(argv, sizeof(*argv) * (argc + 2));
                memmove(argv + 1, argv, sizeof(*argv) * (argc + 1));
                argc++;
-               parse_options(argc, argv, builtin_merge_options,
+               parse_options(argc, argv, NULL, builtin_merge_options,
                              builtin_merge_usage, 0);
                free(buf);
        }
@@ -511,30 +530,76 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
        const char **args;
        int i = 0, ret;
        struct commit_list *j;
-       struct strbuf buf;
-
-       args = xmalloc((4 + commit_list_count(common) +
-                       commit_list_count(remoteheads)) * sizeof(char *));
-       strbuf_init(&buf, 0);
-       strbuf_addf(&buf, "merge-%s", strategy);
-       args[i++] = buf.buf;
-       for (j = common; j; j = j->next)
-               args[i++] = xstrdup(sha1_to_hex(j->item->object.sha1));
-       args[i++] = "--";
-       args[i++] = head_arg;
-       for (j = remoteheads; j; j = j->next)
-               args[i++] = xstrdup(sha1_to_hex(j->item->object.sha1));
-       args[i] = NULL;
-       ret = run_command_v_opt(args, RUN_GIT_CMD);
-       strbuf_release(&buf);
-       i = 1;
-       for (j = common; j; j = j->next)
-               free((void *)args[i++]);
-       i += 2;
-       for (j = remoteheads; j; j = j->next)
-               free((void *)args[i++]);
-       free(args);
-       return -ret;
+       struct strbuf buf = STRBUF_INIT;
+       int index_fd;
+       struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
+
+       index_fd = hold_locked_index(lock, 1);
+       refresh_cache(REFRESH_QUIET);
+       if (active_cache_changed &&
+                       (write_cache(index_fd, active_cache, active_nr) ||
+                        commit_locked_index(lock)))
+               return error("Unable to write index.");
+       rollback_lock_file(lock);
+
+       if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree")) {
+               int clean;
+               struct commit *result;
+               struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
+               int index_fd;
+               struct commit_list *reversed = NULL;
+               struct merge_options o;
+
+               if (remoteheads->next) {
+                       error("Not handling anything other than two heads merge.");
+                       return 2;
+               }
+
+               init_merge_options(&o);
+               if (!strcmp(strategy, "subtree"))
+                       o.subtree_merge = 1;
+
+               o.branch1 = head_arg;
+               o.branch2 = remoteheads->item->util;
+
+               for (j = common; j; j = j->next)
+                       commit_list_insert(j->item, &reversed);
+
+               index_fd = hold_locked_index(lock, 1);
+               clean = merge_recursive(&o, lookup_commit(head),
+                               remoteheads->item, reversed, &result);
+               if (active_cache_changed &&
+                               (write_cache(index_fd, active_cache, active_nr) ||
+                                commit_locked_index(lock)))
+                       die ("unable to write %s", get_index_file());
+               rollback_lock_file(lock);
+               return clean ? 0 : 1;
+       } else {
+               args = xmalloc((4 + commit_list_count(common) +
+                                       commit_list_count(remoteheads)) * sizeof(char *));
+               strbuf_addf(&buf, "merge-%s", strategy);
+               args[i++] = buf.buf;
+               for (j = common; j; j = j->next)
+                       args[i++] = xstrdup(sha1_to_hex(j->item->object.sha1));
+               args[i++] = "--";
+               args[i++] = head_arg;
+               for (j = remoteheads; j; j = j->next)
+                       args[i++] = xstrdup(sha1_to_hex(j->item->object.sha1));
+               args[i] = NULL;
+               ret = run_command_v_opt(args, RUN_GIT_CMD);
+               strbuf_release(&buf);
+               i = 1;
+               for (j = common; j; j = j->next)
+                       free((void *)args[i++]);
+               i += 2;
+               for (j = remoteheads; j; j = j->next)
+                       free((void *)args[i++]);
+               free(args);
+               discard_cache();
+               if (read_cache() < 0)
+                       die("failed to read the cache");
+               return ret;
+       }
 }
 
 static void count_diff_files(struct diff_queue_struct *q,
@@ -574,7 +639,7 @@ static int checkout_fast_forward(unsigned char *head, unsigned char *remote)
        memset(&opts, 0, sizeof(opts));
        memset(&t, 0, sizeof(t));
        memset(&dir, 0, sizeof(dir));
-       dir.show_ignored = 1;
+       dir.flags |= DIR_SHOW_IGNORED;
        dir.exclude_per_dir = ".gitignore";
        opts.dir = &dir;
 
@@ -659,7 +724,7 @@ static int merge_trivial(void)
        parent->next = xmalloc(sizeof(*parent->next));
        parent->next->item = remoteheads->item;
        parent->next->next = NULL;
-       commit_tree(merge_msg.buf, result_tree, parent, result_commit);
+       commit_tree(merge_msg.buf, result_tree, parent, result_commit, NULL);
        finish(result_commit, "In-index merge");
        drop_save();
        return 0;
@@ -688,7 +753,7 @@ static int finish_automerge(struct commit_list *common,
        }
        free_commit_list(remoteheads);
        strbuf_addch(&merge_msg, '\n');
-       commit_tree(merge_msg.buf, result_tree, parents, result_commit);
+       commit_tree(merge_msg.buf, result_tree, parents, result_commit, NULL);
        strbuf_addf(&buf, "Merge made by %s.", wt_strategy);
        finish(result_commit, buf.buf);
        strbuf_release(&buf);
@@ -703,7 +768,8 @@ static int suggest_conflicts(void)
 
        fp = fopen(git_path("MERGE_MSG"), "a");
        if (!fp)
-               die("Could not open %s for writing", git_path("MERGE_MSG"));
+               die_errno("Could not open '%s' for writing",
+                         git_path("MERGE_MSG"));
        fprintf(fp, "\nConflicts:\n");
        for (pos = 0; pos < active_nr; pos++) {
                struct cache_entry *ce = active_cache[pos];
@@ -745,10 +811,6 @@ static int evaluate_result(void)
        int cnt = 0;
        struct rev_info rev;
 
-       discard_cache();
-       if (read_cache() < 0)
-               die("failed to read the cache");
-
        /* Check how many files differ. */
        init_revisions(&rev, "");
        setup_revisions(0, NULL, &rev, NULL);
@@ -770,7 +832,7 @@ static int evaluate_result(void)
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
        unsigned char result_tree[20];
-       struct strbuf buf;
+       struct strbuf buf = STRBUF_INIT;
        const char *head_arg;
        int flag, head_invalid = 0, i;
        int best_cnt = -1, merge_was_ok = 0, automerge_was_ok = 0;
@@ -779,8 +841,11 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
        struct commit_list **remotes = &remoteheads;
 
        setup_work_tree();
+       if (file_exists(git_path("MERGE_HEAD")))
+               die("You have not concluded your merge. (MERGE_HEAD exists)");
        if (read_cache_unmerged())
-               die("You are in the middle of a conflicted merge.");
+               die("You are in the middle of a conflicted merge."
+                               " (index unmerged)");
 
        /*
         * Check if we are _not_ on a detached HEAD, i.e. if there is a
@@ -798,8 +863,10 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
        if (diff_use_color_default == -1)
                diff_use_color_default = git_use_color_default;
 
-       argc = parse_options(argc, argv, builtin_merge_options,
+       argc = parse_options(argc, argv, prefix, builtin_merge_options,
                        builtin_merge_usage, 0);
+       if (verbosity < 0)
+               show_diffstat = 0;
 
        if (squash) {
                if (!allow_fast_forward)
@@ -819,7 +886,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
         * Traditional format never would have "-m" so it is an
         * additional safety measure to check for it.
         */
-       strbuf_init(&buf, 0);
 
        if (!have_message && is_old_style_invocation(argc, argv)) {
                strbuf_addstr(&merge_msg, argv[0]);
@@ -836,6 +902,11 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
                if (argc != 1)
                        die("Can merge only exactly one commit into "
                                "empty head");
+               if (squash)
+                       die("Squash commit into empty head not supported yet");
+               if (!allow_fast_forward)
+                       die("Non-fast-forward commit does not make sense into "
+                           "an empty head");
                remote_head = peel_to_type(argv[0], 0, NULL, OBJ_COMMIT);
                if (!remote_head)
                        die("%s - not something we can merge", argv[0]);
@@ -844,7 +915,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
                reset_hard(remote_head->sha1, 0);
                return 0;
        } else {
-               struct strbuf msg;
+               struct strbuf msg = STRBUF_INIT;
 
                /* We are invoked directly as the first-class UI. */
                head_arg = "HEAD";
@@ -857,7 +928,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
                 * codepath so we discard the error in this
                 * loop.
                 */
-               strbuf_init(&msg, 0);
                for (i = 0; i < argc; i++)
                        merge_name(argv[i], &msg);
                fmt_merge_msg(option_log, &msg, &merge_msg);
@@ -877,12 +947,14 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 
        for (i = 0; i < argc; i++) {
                struct object *o;
+               struct commit *commit;
 
                o = peel_to_type(argv[i], 0, NULL, OBJ_COMMIT);
                if (!o)
                        die("%s - not something we can merge", argv[i]);
-               remotes = &commit_list_insert(lookup_commit(o->sha1),
-                       remotes)->next;
+               commit = lookup_commit(o->sha1);
+               commit->util = (void *)argv[i];
+               remotes = &commit_list_insert(commit, remotes)->next;
 
                strbuf_addf(&buf, "GITHEAD_%s", sha1_to_hex(o->sha1));
                setenv(buf.buf, argv[i], 1);
@@ -930,17 +1002,17 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
                        !common->next &&
                        !hashcmp(common->item->object.sha1, head)) {
                /* Again the most common case of merging one remote. */
-               struct strbuf msg;
+               struct strbuf msg = STRBUF_INIT;
                struct object *o;
                char hex[41];
 
                strcpy(hex, find_unique_abbrev(head, DEFAULT_ABBREV));
 
-               printf("Updating %s..%s\n",
-                       hex,
-                       find_unique_abbrev(remoteheads->item->object.sha1,
-                       DEFAULT_ABBREV));
-               strbuf_init(&msg, 0);
+               if (verbosity >= 0)
+                       printf("Updating %s..%s\n",
+                               hex,
+                               find_unique_abbrev(remoteheads->item->object.sha1,
+                               DEFAULT_ABBREV));
                strbuf_addstr(&msg, "Fast forward");
                if (have_message)
                        strbuf_addstr(&msg,
@@ -1076,7 +1148,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
                }
 
                /* Automerge succeeded. */
-               discard_cache();
                write_tree_trivial(result_tree);
                automerge_was_ok = 1;
                break;
@@ -1123,18 +1194,29 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
                                sha1_to_hex(j->item->object.sha1));
                fd = open(git_path("MERGE_HEAD"), O_WRONLY | O_CREAT, 0666);
                if (fd < 0)
-                       die("Could open %s for writing",
-                               git_path("MERGE_HEAD"));
+                       die_errno("Could not open '%s' for writing",
+                                 git_path("MERGE_HEAD"));
                if (write_in_full(fd, buf.buf, buf.len) != buf.len)
-                       die("Could not write to %s", git_path("MERGE_HEAD"));
+                       die_errno("Could not write to '%s'", git_path("MERGE_HEAD"));
                close(fd);
                strbuf_addch(&merge_msg, '\n');
                fd = open(git_path("MERGE_MSG"), O_WRONLY | O_CREAT, 0666);
                if (fd < 0)
-                       die("Could open %s for writing", git_path("MERGE_MSG"));
+                       die_errno("Could not open '%s' for writing",
+                                 git_path("MERGE_MSG"));
                if (write_in_full(fd, merge_msg.buf, merge_msg.len) !=
                        merge_msg.len)
-                       die("Could not write to %s", git_path("MERGE_MSG"));
+                       die_errno("Could not write to '%s'", git_path("MERGE_MSG"));
+               close(fd);
+               fd = open(git_path("MERGE_MODE"), O_WRONLY | O_CREAT | O_TRUNC, 0666);
+               if (fd < 0)
+                       die_errno("Could not open '%s' for writing",
+                                 git_path("MERGE_MODE"));
+               strbuf_reset(&buf);
+               if (!allow_fast_forward)
+                       strbuf_addf(&buf, "no-ff");
+               if (write_in_full(fd, buf.buf, buf.len) != buf.len)
+                       die_errno("Could not write to '%s'", git_path("MERGE_MODE"));
                close(fd);
        }