Andrew's git / gitweb.git / read-tree.c
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read-tree.con commit documentation: add 'see also' sections to git-rm and git-add (872d001)
   1/*
   2 * GIT - The information manager from hell
   3 *
   4 * Copyright (C) Linus Torvalds, 2005
   5 */
   6#define DBRT_DEBUG 1
   7
   8#include "cache.h"
   9
  10#include "object.h"
  11#include "tree.h"
  12#include <sys/time.h>
  13#include <signal.h>
  14
  15static int merge = 0;
  16static int update = 0;
  17static int index_only = 0;
  18static int nontrivial_merge = 0;
  19static int trivial_merges_only = 0;
  20static int aggressive = 0;
  21static int verbose_update = 0;
  22static volatile int progress_update = 0;
  23
  24static int head_idx = -1;
  25static int merge_size = 0;
  26
  27static struct object_list *trees = NULL;
  28
  29static struct cache_entry df_conflict_entry = { 
  30};
  31
  32static struct tree_entry_list df_conflict_list = {
  33        .name = NULL,
  34        .next = &df_conflict_list
  35};
  36
  37typedef int (*merge_fn_t)(struct cache_entry **src);
  38
  39static int entcmp(char *name1, int dir1, char *name2, int dir2)
  40{
  41        int len1 = strlen(name1);
  42        int len2 = strlen(name2);
  43        int len = len1 < len2 ? len1 : len2;
  44        int ret = memcmp(name1, name2, len);
  45        unsigned char c1, c2;
  46        if (ret)
  47                return ret;
  48        c1 = name1[len];
  49        c2 = name2[len];
  50        if (!c1 && dir1)
  51                c1 = '/';
  52        if (!c2 && dir2)
  53                c2 = '/';
  54        ret = (c1 < c2) ? -1 : (c1 > c2) ? 1 : 0;
  55        if (c1 && c2 && !ret)
  56                ret = len1 - len2;
  57        return ret;
  58}
  59
  60static int unpack_trees_rec(struct tree_entry_list **posns, int len,
  61                            const char *base, merge_fn_t fn, int *indpos)
  62{
  63        int baselen = strlen(base);
  64        int src_size = len + 1;
  65        do {
  66                int i;
  67                char *first;
  68                int firstdir = 0;
  69                int pathlen;
  70                unsigned ce_size;
  71                struct tree_entry_list **subposns;
  72                struct cache_entry **src;
  73                int any_files = 0;
  74                int any_dirs = 0;
  75                char *cache_name;
  76                int ce_stage;
  77
  78                /* Find the first name in the input. */
  79
  80                first = NULL;
  81                cache_name = NULL;
  82
  83                /* Check the cache */
  84                if (merge && *indpos < active_nr) {
  85                        /* This is a bit tricky: */
  86                        /* If the index has a subdirectory (with
  87                         * contents) as the first name, it'll get a
  88                         * filename like "foo/bar". But that's after
  89                         * "foo", so the entry in trees will get
  90                         * handled first, at which point we'll go into
  91                         * "foo", and deal with "bar" from the index,
  92                         * because the base will be "foo/". The only
  93                         * way we can actually have "foo/bar" first of
  94                         * all the things is if the trees don't
  95                         * contain "foo" at all, in which case we'll
  96                         * handle "foo/bar" without going into the
  97                         * directory, but that's fine (and will return
  98                         * an error anyway, with the added unknown
  99                         * file case.
 100                         */
 101
 102                        cache_name = active_cache[*indpos]->name;
 103                        if (strlen(cache_name) > baselen &&
 104                            !memcmp(cache_name, base, baselen)) {
 105                                cache_name += baselen;
 106                                first = cache_name;
 107                        } else {
 108                                cache_name = NULL;
 109                        }
 110                }
 111
 112#if DBRT_DEBUG > 1
 113                if (first)
 114                        printf("index %s\n", first);
 115#endif
 116                for (i = 0; i < len; i++) {
 117                        if (!posns[i] || posns[i] == &df_conflict_list)
 118                                continue;
 119#if DBRT_DEBUG > 1
 120                        printf("%d %s\n", i + 1, posns[i]->name);
 121#endif
 122                        if (!first || entcmp(first, firstdir,
 123                                             posns[i]->name, 
 124                                             posns[i]->directory) > 0) {
 125                                first = posns[i]->name;
 126                                firstdir = posns[i]->directory;
 127                        }
 128                }
 129                /* No name means we're done */
 130                if (!first)
 131                        return 0;
 132
 133                pathlen = strlen(first);
 134                ce_size = cache_entry_size(baselen + pathlen);
 135
 136                src = xmalloc(sizeof(struct cache_entry *) * src_size);
 137                memset(src, 0, sizeof(struct cache_entry *) * src_size);
 138
 139                subposns = xmalloc(sizeof(struct tree_list_entry *) * len);
 140                memset(subposns, 0, sizeof(struct tree_list_entry *) * len);
 141
 142                if (cache_name && !strcmp(cache_name, first)) {
 143                        any_files = 1;
 144                        src[0] = active_cache[*indpos];
 145                        remove_cache_entry_at(*indpos);
 146                }
 147
 148                for (i = 0; i < len; i++) {
 149                        struct cache_entry *ce;
 150
 151                        if (!posns[i] ||
 152                            (posns[i] != &df_conflict_list &&
 153                             strcmp(first, posns[i]->name))) {
 154                                continue;
 155                        }
 156
 157                        if (posns[i] == &df_conflict_list) {
 158                                src[i + merge] = &df_conflict_entry;
 159                                continue;
 160                        }
 161
 162                        if (posns[i]->directory) {
 163                                any_dirs = 1;
 164                                parse_tree(posns[i]->item.tree);
 165                                subposns[i] = posns[i]->item.tree->entries;
 166                                posns[i] = posns[i]->next;
 167                                src[i + merge] = &df_conflict_entry;
 168                                continue;
 169                        }
 170
 171                        if (!merge)
 172                                ce_stage = 0;
 173                        else if (i + 1 < head_idx)
 174                                ce_stage = 1;
 175                        else if (i + 1 > head_idx)
 176                                ce_stage = 3;
 177                        else
 178                                ce_stage = 2;
 179
 180                        ce = xmalloc(ce_size);
 181                        memset(ce, 0, ce_size);
 182                        ce->ce_mode = create_ce_mode(posns[i]->mode);
 183                        ce->ce_flags = create_ce_flags(baselen + pathlen,
 184                                                       ce_stage);
 185                        memcpy(ce->name, base, baselen);
 186                        memcpy(ce->name + baselen, first, pathlen + 1);
 187
 188                        any_files = 1;
 189
 190                        memcpy(ce->sha1, posns[i]->item.any->sha1, 20);
 191                        src[i + merge] = ce;
 192                        subposns[i] = &df_conflict_list;
 193                        posns[i] = posns[i]->next;
 194                }
 195                if (any_files) {
 196                        if (merge) {
 197                                int ret;
 198
 199#if DBRT_DEBUG > 1
 200                                printf("%s:\n", first);
 201                                for (i = 0; i < src_size; i++) {
 202                                        printf(" %d ", i);
 203                                        if (src[i])
 204                                                printf("%s\n", sha1_to_hex(src[i]->sha1));
 205                                        else
 206                                                printf("\n");
 207                                }
 208#endif
 209                                ret = fn(src);
 210                                
 211#if DBRT_DEBUG > 1
 212                                printf("Added %d entries\n", ret);
 213#endif
 214                                *indpos += ret;
 215                        } else {
 216                                for (i = 0; i < src_size; i++) {
 217                                        if (src[i]) {
 218                                                add_cache_entry(src[i], ADD_CACHE_OK_TO_ADD|ADD_CACHE_SKIP_DFCHECK);
 219                                        }
 220                                }
 221                        }
 222                }
 223                if (any_dirs) {
 224                        char *newbase = xmalloc(baselen + 2 + pathlen);
 225                        memcpy(newbase, base, baselen);
 226                        memcpy(newbase + baselen, first, pathlen);
 227                        newbase[baselen + pathlen] = '/';
 228                        newbase[baselen + pathlen + 1] = '\0';
 229                        if (unpack_trees_rec(subposns, len, newbase, fn,
 230                                             indpos))
 231                                return -1;
 232                        free(newbase);
 233                }
 234                free(subposns);
 235                free(src);
 236        } while (1);
 237}
 238
 239static void reject_merge(struct cache_entry *ce)
 240{
 241        die("Entry '%s' would be overwritten by merge. Cannot merge.", 
 242            ce->name);
 243}
 244
 245/* Unlink the last component and attempt to remove leading
 246 * directories, in case this unlink is the removal of the
 247 * last entry in the directory -- empty directories are removed.
 248 */
 249static void unlink_entry(char *name)
 250{
 251        char *cp, *prev;
 252
 253        if (unlink(name))
 254                return;
 255        prev = NULL;
 256        while (1) {
 257                int status;
 258                cp = strrchr(name, '/');
 259                if (prev)
 260                        *prev = '/';
 261                if (!cp)
 262                        break;
 263
 264                *cp = 0;
 265                status = rmdir(name);
 266                if (status) {
 267                        *cp = '/';
 268                        break;
 269                }
 270                prev = cp;
 271        }
 272}
 273
 274static void progress_interval(int signum)
 275{
 276        signal(SIGALRM, progress_interval);
 277        progress_update = 1;
 278}
 279
 280static void check_updates(struct cache_entry **src, int nr)
 281{
 282        static struct checkout state = {
 283                .base_dir = "",
 284                .force = 1,
 285                .quiet = 1,
 286                .refresh_cache = 1,
 287        };
 288        unsigned short mask = htons(CE_UPDATE);
 289        unsigned last_percent = 200, cnt = 0, total = 0;
 290
 291        if (update && verbose_update) {
 292                struct itimerval v;
 293
 294                for (total = cnt = 0; cnt < nr; cnt++) {
 295                        struct cache_entry *ce = src[cnt];
 296                        if (!ce->ce_mode || ce->ce_flags & mask)
 297                                total++;
 298                }
 299
 300                /* Don't bother doing this for very small updates */
 301                if (total < 250)
 302                        total = 0;
 303
 304                if (total) {
 305                        v.it_interval.tv_sec = 1;
 306                        v.it_interval.tv_usec = 0;
 307                        v.it_value = v.it_interval;
 308                        signal(SIGALRM, progress_interval);
 309                        setitimer(ITIMER_REAL, &v, NULL);
 310                        fprintf(stderr, "Checking files out...\n");
 311                        progress_update = 1;
 312                }
 313                cnt = 0;
 314        }
 315
 316        while (nr--) {
 317                struct cache_entry *ce = *src++;
 318
 319                if (total) {
 320                        if (!ce->ce_mode || ce->ce_flags & mask) {
 321                                unsigned percent;
 322                                cnt++;
 323                                percent = (cnt * 100) / total;
 324                                if (percent != last_percent ||
 325                                    progress_update) {
 326                                        fprintf(stderr, "%4u%% (%u/%u) done\r",
 327                                                percent, cnt, total);
 328                                        last_percent = percent;
 329                                }
 330                        }
 331                }
 332                if (!ce->ce_mode) {
 333                        if (update)
 334                                unlink_entry(ce->name);
 335                        continue;
 336                }
 337                if (ce->ce_flags & mask) {
 338                        ce->ce_flags &= ~mask;
 339                        if (update)
 340                                checkout_entry(ce, &state);
 341                }
 342        }
 343        if (total) {
 344                fputc('\n', stderr);
 345                signal(SIGALRM, SIG_IGN);
 346        }
 347}
 348
 349static int unpack_trees(merge_fn_t fn)
 350{
 351        int indpos = 0;
 352        unsigned len = object_list_length(trees);
 353        struct tree_entry_list **posns;
 354        int i;
 355        struct object_list *posn = trees;
 356        merge_size = len;
 357
 358        if (len) {
 359                posns = xmalloc(len * sizeof(struct tree_entry_list *));
 360                for (i = 0; i < len; i++) {
 361                        posns[i] = ((struct tree *) posn->item)->entries;
 362                        posn = posn->next;
 363                }
 364                if (unpack_trees_rec(posns, len, "", fn, &indpos))
 365                        return -1;
 366        }
 367
 368        if (trivial_merges_only && nontrivial_merge)
 369                die("Merge requires file-level merging");
 370
 371        check_updates(active_cache, active_nr);
 372        return 0;
 373}
 374
 375static int list_tree(unsigned char *sha1)
 376{
 377        struct tree *tree = parse_tree_indirect(sha1);
 378        if (!tree)
 379                return -1;
 380        object_list_append(&tree->object, &trees);
 381        return 0;
 382}
 383
 384static int same(struct cache_entry *a, struct cache_entry *b)
 385{
 386        if (!!a != !!b)
 387                return 0;
 388        if (!a && !b)
 389                return 1;
 390        return a->ce_mode == b->ce_mode && 
 391                !memcmp(a->sha1, b->sha1, 20);
 392}
 393
 394
 395/*
 396 * When a CE gets turned into an unmerged entry, we
 397 * want it to be up-to-date
 398 */
 399static void verify_uptodate(struct cache_entry *ce)
 400{
 401        struct stat st;
 402
 403        if (index_only)
 404                return;
 405
 406        if (!lstat(ce->name, &st)) {
 407                unsigned changed = ce_match_stat(ce, &st, 1);
 408                if (!changed)
 409                        return;
 410                errno = 0;
 411        }
 412        if (errno == ENOENT)
 413                return;
 414        die("Entry '%s' not uptodate. Cannot merge.", ce->name);
 415}
 416
 417static int merged_entry(struct cache_entry *merge, struct cache_entry *old)
 418{
 419        merge->ce_flags |= htons(CE_UPDATE);
 420        if (old) {
 421                /*
 422                 * See if we can re-use the old CE directly?
 423                 * That way we get the uptodate stat info.
 424                 *
 425                 * This also removes the UPDATE flag on
 426                 * a match.
 427                 */
 428                if (same(old, merge)) {
 429                        *merge = *old;
 430                } else {
 431                        verify_uptodate(old);
 432                }
 433        }
 434        merge->ce_flags &= ~htons(CE_STAGEMASK);
 435        add_cache_entry(merge, ADD_CACHE_OK_TO_ADD);
 436        return 1;
 437}
 438
 439static int deleted_entry(struct cache_entry *ce, struct cache_entry *old)
 440{
 441        if (old)
 442                verify_uptodate(old);
 443        ce->ce_mode = 0;
 444        add_cache_entry(ce, ADD_CACHE_OK_TO_ADD);
 445        return 1;
 446}
 447
 448static int keep_entry(struct cache_entry *ce)
 449{
 450        add_cache_entry(ce, ADD_CACHE_OK_TO_ADD);
 451        return 1;
 452}
 453
 454#if DBRT_DEBUG
 455static void show_stage_entry(FILE *o,
 456                             const char *label, const struct cache_entry *ce)
 457{
 458        if (!ce)
 459                fprintf(o, "%s (missing)\n", label);
 460        else
 461                fprintf(o, "%s%06o %s %d\t%s\n",
 462                        label,
 463                        ntohl(ce->ce_mode),
 464                        sha1_to_hex(ce->sha1),
 465                        ce_stage(ce),
 466                        ce->name);
 467}
 468#endif
 469
 470static int threeway_merge(struct cache_entry **stages)
 471{
 472        struct cache_entry *index;
 473        struct cache_entry *head; 
 474        struct cache_entry *remote = stages[head_idx + 1];
 475        int count;
 476        int head_match = 0;
 477        int remote_match = 0;
 478
 479        int df_conflict_head = 0;
 480        int df_conflict_remote = 0;
 481
 482        int any_anc_missing = 0;
 483        int no_anc_exists = 1;
 484        int i;
 485
 486        for (i = 1; i < head_idx; i++) {
 487                if (!stages[i])
 488                        any_anc_missing = 1;
 489                else
 490                        no_anc_exists = 0;
 491        }
 492
 493        index = stages[0];
 494        head = stages[head_idx];
 495
 496        if (head == &df_conflict_entry) {
 497                df_conflict_head = 1;
 498                head = NULL;
 499        }
 500
 501        if (remote == &df_conflict_entry) {
 502                df_conflict_remote = 1;
 503                remote = NULL;
 504        }
 505
 506        /* First, if there's a #16 situation, note that to prevent #13
 507         * and #14. 
 508         */
 509        if (!same(remote, head)) {
 510                for (i = 1; i < head_idx; i++) {
 511                        if (same(stages[i], head)) {
 512                                head_match = i;
 513                        }
 514                        if (same(stages[i], remote)) {
 515                                remote_match = i;
 516                        }
 517                }
 518        }
 519
 520        /* We start with cases where the index is allowed to match
 521         * something other than the head: #14(ALT) and #2ALT, where it
 522         * is permitted to match the result instead.
 523         */
 524        /* #14, #14ALT, #2ALT */
 525        if (remote && !df_conflict_head && head_match && !remote_match) {
 526                if (index && !same(index, remote) && !same(index, head))
 527                        reject_merge(index);
 528                return merged_entry(remote, index);
 529        }
 530        /*
 531         * If we have an entry in the index cache, then we want to
 532         * make sure that it matches head.
 533         */
 534        if (index && !same(index, head)) {
 535                reject_merge(index);
 536        }
 537
 538        if (head) {
 539                /* #5ALT, #15 */
 540                if (same(head, remote))
 541                        return merged_entry(head, index);
 542                /* #13, #3ALT */
 543                if (!df_conflict_remote && remote_match && !head_match)
 544                        return merged_entry(head, index);
 545        }
 546
 547        /* #1 */
 548        if (!head && !remote && any_anc_missing)
 549                return 0;
 550
 551        /* Under the new "aggressive" rule, we resolve mostly trivial
 552         * cases that we historically had git-merge-one-file resolve.
 553         */
 554        if (aggressive) {
 555                int head_deleted = !head && !df_conflict_head;
 556                int remote_deleted = !remote && !df_conflict_remote;
 557                /*
 558                 * Deleted in both.
 559                 * Deleted in one and unchanged in the other.
 560                 */
 561                if ((head_deleted && remote_deleted) ||
 562                    (head_deleted && remote && remote_match) ||
 563                    (remote_deleted && head && head_match)) {
 564                        if (index)
 565                                return deleted_entry(index, index);
 566                        return 0;
 567                }
 568                /*
 569                 * Added in both, identically.
 570                 */
 571                if (no_anc_exists && head && remote && same(head, remote))
 572                        return merged_entry(head, index);
 573
 574        }
 575
 576        /* Below are "no merge" cases, which require that the index be
 577         * up-to-date to avoid the files getting overwritten with
 578         * conflict resolution files. 
 579         */
 580        if (index) {
 581                verify_uptodate(index);
 582        }
 583
 584        nontrivial_merge = 1;
 585
 586        /* #2, #3, #4, #6, #7, #9, #11. */
 587        count = 0;
 588        if (!head_match || !remote_match) {
 589                for (i = 1; i < head_idx; i++) {
 590                        if (stages[i]) {
 591                                keep_entry(stages[i]);
 592                                count++;
 593                                break;
 594                        }
 595                }
 596        }
 597#if DBRT_DEBUG
 598        else {
 599                fprintf(stderr, "read-tree: warning #16 detected\n");
 600                show_stage_entry(stderr, "head   ", stages[head_match]);
 601                show_stage_entry(stderr, "remote ", stages[remote_match]);
 602        }
 603#endif
 604        if (head) { count += keep_entry(head); }
 605        if (remote) { count += keep_entry(remote); }
 606        return count;
 607}
 608
 609/*
 610 * Two-way merge.
 611 *
 612 * The rule is to "carry forward" what is in the index without losing
 613 * information across a "fast forward", favoring a successful merge
 614 * over a merge failure when it makes sense.  For details of the
 615 * "carry forward" rule, please see <Documentation/git-read-tree.txt>.
 616 *
 617 */
 618static int twoway_merge(struct cache_entry **src)
 619{
 620        struct cache_entry *current = src[0];
 621        struct cache_entry *oldtree = src[1], *newtree = src[2];
 622
 623        if (merge_size != 2)
 624                return error("Cannot do a twoway merge of %d trees",
 625                             merge_size);
 626
 627        if (current) {
 628                if ((!oldtree && !newtree) || /* 4 and 5 */
 629                    (!oldtree && newtree &&
 630                     same(current, newtree)) || /* 6 and 7 */
 631                    (oldtree && newtree &&
 632                     same(oldtree, newtree)) || /* 14 and 15 */
 633                    (oldtree && newtree &&
 634                     !same(oldtree, newtree) && /* 18 and 19*/
 635                     same(current, newtree))) {
 636                        return keep_entry(current);
 637                }
 638                else if (oldtree && !newtree && same(current, oldtree)) {
 639                        /* 10 or 11 */
 640                        return deleted_entry(oldtree, current);
 641                }
 642                else if (oldtree && newtree &&
 643                         same(current, oldtree) && !same(current, newtree)) {
 644                        /* 20 or 21 */
 645                        return merged_entry(newtree, current);
 646                }
 647                else {
 648                        /* all other failures */
 649                        if (oldtree)
 650                                reject_merge(oldtree);
 651                        if (current)
 652                                reject_merge(current);
 653                        if (newtree)
 654                                reject_merge(newtree);
 655                        return -1;
 656                }
 657        }
 658        else if (newtree)
 659                return merged_entry(newtree, current);
 660        else
 661                return deleted_entry(oldtree, current);
 662}
 663
 664/*
 665 * One-way merge.
 666 *
 667 * The rule is:
 668 * - take the stat information from stage0, take the data from stage1
 669 */
 670static int oneway_merge(struct cache_entry **src)
 671{
 672        struct cache_entry *old = src[0];
 673        struct cache_entry *a = src[1];
 674
 675        if (merge_size != 1)
 676                return error("Cannot do a oneway merge of %d trees",
 677                             merge_size);
 678
 679        if (!a)
 680                return 0;
 681        if (old && same(old, a)) {
 682                return keep_entry(old);
 683        }
 684        return merged_entry(a, NULL);
 685}
 686
 687static int read_cache_unmerged(void)
 688{
 689        int i, deleted;
 690        struct cache_entry **dst;
 691
 692        read_cache();
 693        dst = active_cache;
 694        deleted = 0;
 695        for (i = 0; i < active_nr; i++) {
 696                struct cache_entry *ce = active_cache[i];
 697                if (ce_stage(ce)) {
 698                        deleted++;
 699                        continue;
 700                }
 701                if (deleted)
 702                        *dst = ce;
 703                dst++;
 704        }
 705        active_nr -= deleted;
 706        return deleted;
 707}
 708
 709static const char read_tree_usage[] = "git-read-tree (<sha> | -m [--aggressive] [-u | -i] <sha1> [<sha2> [<sha3>]])";
 710
 711static struct cache_file cache_file;
 712
 713int main(int argc, char **argv)
 714{
 715        int i, newfd, reset, stage = 0;
 716        unsigned char sha1[20];
 717        merge_fn_t fn = NULL;
 718
 719        setup_git_directory();
 720
 721        newfd = hold_index_file_for_update(&cache_file, get_index_file());
 722        if (newfd < 0)
 723                die("unable to create new cachefile");
 724
 725        git_config(git_default_config);
 726
 727        merge = 0;
 728        reset = 0;
 729        for (i = 1; i < argc; i++) {
 730                const char *arg = argv[i];
 731
 732                /* "-u" means "update", meaning that a merge will update
 733                 * the working tree.
 734                 */
 735                if (!strcmp(arg, "-u")) {
 736                        update = 1;
 737                        continue;
 738                }
 739
 740                if (!strcmp(arg, "-v")) {
 741                        verbose_update = 1;
 742                        continue;
 743                }
 744
 745                /* "-i" means "index only", meaning that a merge will
 746                 * not even look at the working tree.
 747                 */
 748                if (!strcmp(arg, "-i")) {
 749                        index_only = 1;
 750                        continue;
 751                }
 752
 753                /* This differs from "-m" in that we'll silently ignore unmerged entries */
 754                if (!strcmp(arg, "--reset")) {
 755                        if (stage || merge)
 756                                usage(read_tree_usage);
 757                        reset = 1;
 758                        merge = 1;
 759                        stage = 1;
 760                        read_cache_unmerged();
 761                        continue;
 762                }
 763
 764                if (!strcmp(arg, "--trivial")) {
 765                        trivial_merges_only = 1;
 766                        continue;
 767                }
 768
 769                if (!strcmp(arg, "--aggressive")) {
 770                        aggressive = 1;
 771                        continue;
 772                }
 773
 774                /* "-m" stands for "merge", meaning we start in stage 1 */
 775                if (!strcmp(arg, "-m")) {
 776                        if (stage || merge)
 777                                usage(read_tree_usage);
 778                        if (read_cache_unmerged())
 779                                die("you need to resolve your current index first");
 780                        stage = 1;
 781                        merge = 1;
 782                        continue;
 783                }
 784
 785                /* using -u and -i at the same time makes no sense */
 786                if (1 < index_only + update)
 787                        usage(read_tree_usage);
 788
 789                if (get_sha1(arg, sha1) < 0)
 790                        usage(read_tree_usage);
 791                if (list_tree(sha1) < 0)
 792                        die("failed to unpack tree object %s", arg);
 793                stage++;
 794        }
 795        if ((update||index_only) && !merge)
 796                usage(read_tree_usage);
 797
 798        if (merge) {
 799                if (stage < 2)
 800                        die("just how do you expect me to merge %d trees?", stage-1);
 801                switch (stage - 1) {
 802                case 1:
 803                        fn = oneway_merge;
 804                        break;
 805                case 2:
 806                        fn = twoway_merge;
 807                        break;
 808                case 3:
 809                        fn = threeway_merge;
 810                        break;
 811                default:
 812                        fn = threeway_merge;
 813                        break;
 814                }
 815
 816                if (stage - 1 >= 3)
 817                        head_idx = stage - 2;
 818                else
 819                        head_idx = 1;
 820        }
 821
 822        unpack_trees(fn);
 823        if (write_cache(newfd, active_cache, active_nr) ||
 824            commit_index_file(&cache_file))
 825                die("unable to write new index file");
 826        return 0;
 827}