commit.c

   1 #include "cache.h"
   2 #include "tag.h"
   3 #include "commit.h"
   4 #include "pkt-line.h"
   5 #include "utf8.h"
   6 #include "diff.h"
   7 #include "revision.h"
   8
   9 int save_commit_buffer = 1;
  10
  11 const char *commit_type = "commit";
  12
  13 static struct commit *check_commit(struct object *obj,
  14                                    const unsigned char *sha1,
  15                                    int quiet)
  16 {
  17         if (obj->type != OBJ_COMMIT) {
  18                 if (!quiet)
  19                         error("Object %s is a %s, not a commit",
  20                               sha1_to_hex(sha1), typename(obj->type));
  21                 return NULL;
  22         }
  23         return (struct commit *) obj;
  24 }
  25
  26 struct commit *lookup_commit_reference_gently(const unsigned char *sha1,
  27                                               int quiet)
  28 {
  29         struct object *obj = deref_tag(parse_object(sha1), NULL, 0);
  30
  31         if (!obj)
  32                 return NULL;
  33         return check_commit(obj, sha1, quiet);
  34 }
  35
  36 struct commit *lookup_commit_reference(const unsigned char *sha1)
  37 {
  38         return lookup_commit_reference_gently(sha1, 0);
  39 }
  40
  41 struct commit *lookup_commit(const unsigned char *sha1)
  42 {
  43         struct object *obj = lookup_object(sha1);
  44         if (!obj)
  45                 return create_object(sha1, OBJ_COMMIT, alloc_commit_node());
  46         if (!obj->type)
  47                 obj->type = OBJ_COMMIT;
  48         return check_commit(obj, sha1, 0);
  49 }
  50
  51 static unsigned long parse_commit_date(const char *buf, const char *tail)
  52 {
  53         const char *dateptr;
  54
  55         if (buf + 6 >= tail)
  56                 return 0;
  57         if (memcmp(buf, "author", 6))
  58                 return 0;
  59         while (buf < tail && *buf++ != '\n')
  60                 /* nada */;
  61         if (buf + 9 >= tail)
  62                 return 0;
  63         if (memcmp(buf, "committer", 9))
  64                 return 0;
  65         while (buf < tail && *buf++ != '>')
  66                 /* nada */;
  67         if (buf >= tail)
  68                 return 0;
  69         dateptr = buf;
  70         while (buf < tail && *buf++ != '\n')
  71                 /* nada */;
  72         if (buf >= tail)
  73                 return 0;
  74         /* dateptr < buf && buf[-1] == '\n', so strtoul will stop at buf-1 */
  75         return strtoul(dateptr, NULL, 10);
  76 }
  77
  78 static struct commit_graft **commit_graft;
  79 static int commit_graft_alloc, commit_graft_nr;
  80
  81 static int commit_graft_pos(const unsigned char *sha1)
  82 {
  83         int lo, hi;
  84         lo = 0;
  85         hi = commit_graft_nr;
  86         while (lo < hi) {
  87                 int mi = (lo + hi) / 2;
  88                 struct commit_graft *graft = commit_graft[mi];
  89                 int cmp = hashcmp(sha1, graft->sha1);
  90                 if (!cmp)
  91                         return mi;
  92                 if (cmp < 0)
  93                         hi = mi;
  94                 else
  95                         lo = mi + 1;
  96         }
  97         return -lo - 1;
  98 }
  99
 100 int register_commit_graft(struct commit_graft *graft, int ignore_dups)
 101 {
 102         int pos = commit_graft_pos(graft->sha1);
 103
 104         if (0 <= pos) {
 105                 if (ignore_dups)
 106                         free(graft);
 107                 else {
 108                         free(commit_graft[pos]);
 109                         commit_graft[pos] = graft;
 110                 }
 111                 return 1;
 112         }
 113         pos = -pos - 1;
 114         if (commit_graft_alloc <= ++commit_graft_nr) {
 115                 commit_graft_alloc = alloc_nr(commit_graft_alloc);
 116                 commit_graft = xrealloc(commit_graft,
 117                                         sizeof(*commit_graft) *
 118                                         commit_graft_alloc);
 119         }
 120         if (pos < commit_graft_nr)
 121                 memmove(commit_graft + pos + 1,
 122                         commit_graft + pos,
 123                         (commit_graft_nr - pos - 1) *
 124                         sizeof(*commit_graft));
 125         commit_graft[pos] = graft;
 126         return 0;
 127 }
 128
 129 struct commit_graft *read_graft_line(char *buf, int len)
 130 {
 131         /* The format is just "Commit Parent1 Parent2 ...\n" */
 132         int i;
 133         struct commit_graft *graft = NULL;
 134
 135         if (buf[len-1] == '\n')
 136                 buf[--len] = 0;
 137         if (buf[0] == '#' || buf[0] == '\0')
 138                 return NULL;
 139         if ((len + 1) % 41) {
 140         bad_graft_data:
 141                 error("bad graft data: %s", buf);
 142                 free(graft);
 143                 return NULL;
 144         }
 145         i = (len + 1) / 41 - 1;
 146         graft = xmalloc(sizeof(*graft) + 20 * i);
 147         graft->nr_parent = i;
 148         if (get_sha1_hex(buf, graft->sha1))
 149                 goto bad_graft_data;
 150         for (i = 40; i < len; i += 41) {
 151                 if (buf[i] != ' ')
 152                         goto bad_graft_data;
 153                 if (get_sha1_hex(buf + i + 1, graft->parent[i/41]))
 154                         goto bad_graft_data;
 155         }
 156         return graft;
 157 }
 158
 159 static int read_graft_file(const char *graft_file)
 160 {
 161         FILE *fp = fopen(graft_file, "r");
 162         char buf[1024];
 163         if (!fp)
 164                 return -1;
 165         while (fgets(buf, sizeof(buf), fp)) {
 166                 /* The format is just "Commit Parent1 Parent2 ...\n" */
 167                 int len = strlen(buf);
 168                 struct commit_graft *graft = read_graft_line(buf, len);
 169                 if (!graft)
 170                         continue;
 171                 if (register_commit_graft(graft, 1))
 172                         error("duplicate graft data: %s", buf);
 173         }
 174         fclose(fp);
 175         return 0;
 176 }
 177
 178 static void prepare_commit_graft(void)
 179 {
 180         static int commit_graft_prepared;
 181         char *graft_file;
 182
 183         if (commit_graft_prepared)
 184                 return;
 185         graft_file = get_graft_file();
 186         read_graft_file(graft_file);
 187         /* make sure shallows are read */
 188         is_repository_shallow();
 189         commit_graft_prepared = 1;
 190 }
 191
 192 struct commit_graft *lookup_commit_graft(const unsigned char *sha1)
 193 {
 194         int pos;
 195         prepare_commit_graft();
 196         pos = commit_graft_pos(sha1);
 197         if (pos < 0)
 198                 return NULL;
 199         return commit_graft[pos];
 200 }
 201
 202 int write_shallow_commits(int fd, int use_pack_protocol)
 203 {
 204         int i, count = 0;
 205         for (i = 0; i < commit_graft_nr; i++)
 206                 if (commit_graft[i]->nr_parent < 0) {
 207                         const char *hex =
 208                                 sha1_to_hex(commit_graft[i]->sha1);
 209                         count++;
 210                         if (use_pack_protocol)
 211                                 packet_write(fd, "shallow %s", hex);
 212                         else {
 213                                 if (write_in_full(fd, hex,  40) != 40)
 214                                         break;
 215                                 if (write_str_in_full(fd, "\n") != 1)
 216                                         break;
 217                         }
 218                 }
 219         return count;
 220 }
 221
 222 int unregister_shallow(const unsigned char *sha1)
 223 {
 224         int pos = commit_graft_pos(sha1);
 225         if (pos < 0)
 226                 return -1;
 227         if (pos + 1 < commit_graft_nr)
 228                 memcpy(commit_graft + pos, commit_graft + pos + 1,
 229                                 sizeof(struct commit_graft *)
 230                                 * (commit_graft_nr - pos - 1));
 231         commit_graft_nr--;
 232         return 0;
 233 }
 234
 235 int parse_commit_buffer(struct commit *item, void *buffer, unsigned long size)
 236 {
 237         char *tail = buffer;
 238         char *bufptr = buffer;
 239         unsigned char parent[20];
 240         struct commit_list **pptr;
 241         struct commit_graft *graft;
 242
 243         if (item->object.parsed)
 244                 return 0;
 245         item->object.parsed = 1;
 246         tail += size;
 247         if (tail <= bufptr + 46 || memcmp(bufptr, "tree ", 5) || bufptr[45] != '\n')
 248                 return error("bogus commit object %s", sha1_to_hex(item->object.sha1));
 249         if (get_sha1_hex(bufptr + 5, parent) < 0)
 250                 return error("bad tree pointer in commit %s",
 251                              sha1_to_hex(item->object.sha1));
 252         item->tree = lookup_tree(parent);
 253         bufptr += 46; /* "tree " + "hex sha1" + "\n" */
 254         pptr = &item->parents;
 255         while (pop_commit(pptr))
 256                 ; /* clear anything from cache */
 257
 258         graft = lookup_commit_graft(item->object.sha1);
 259         while (bufptr + 48 < tail && !memcmp(bufptr, "parent ", 7)) {
 260                 struct commit *new_parent;
 261
 262                 if (tail <= bufptr + 48 ||
 263                     get_sha1_hex(bufptr + 7, parent) ||
 264                     bufptr[47] != '\n')
 265                         return error("bad parents in commit %s", sha1_to_hex(item->object.sha1));
 266                 bufptr += 48;
 267                 /*
 268                  * The clone is shallow if nr_parent < 0, and we must
 269                  * not traverse its real parents even when we unhide them.
 270                  */
 271                 if (graft && (graft->nr_parent < 0 || grafts_replace_parents))
 272                         continue;
 273                 new_parent = lookup_commit(parent);
 274                 if (new_parent)
 275                         pptr = &commit_list_insert(new_parent, pptr)->next;
 276         }
 277         if (graft) {
 278                 int i;
 279                 struct commit *new_parent;
 280                 for (i = 0; i < graft->nr_parent; i++) {
 281                         new_parent = lookup_commit(graft->parent[i]);
 282                         if (!new_parent)
 283                                 continue;
 284                         pptr = &commit_list_insert(new_parent, pptr)->next;
 285                 }
 286         }
 287         item->date = parse_commit_date(bufptr, tail);
 288
 289         return 0;
 290 }
 291
 292 int parse_commit(struct commit *item)
 293 {
 294         enum object_type type;
 295         void *buffer;
 296         unsigned long size;
 297         int ret;
 298
 299         if (!item)
 300                 return -1;
 301         if (item->object.parsed)
 302                 return 0;
 303         buffer = read_sha1_file(item->object.sha1, &type, &size);
 304         if (!buffer)
 305                 return error("Could not read %s",
 306                              sha1_to_hex(item->object.sha1));
 307         if (type != OBJ_COMMIT) {
 308                 free(buffer);
 309                 return error("Object %s not a commit",
 310                              sha1_to_hex(item->object.sha1));
 311         }
 312         ret = parse_commit_buffer(item, buffer, size);
 313         if (save_commit_buffer && !ret) {
 314                 item->buffer = buffer;
 315                 return 0;
 316         }
 317         free(buffer);
 318         return ret;
 319 }
 320
 321 struct commit_list *commit_list_insert(struct commit *item, struct commit_list **list_p)
 322 {
 323         struct commit_list *new_list = xmalloc(sizeof(struct commit_list));
 324         new_list->item = item;
 325         new_list->next = *list_p;
 326         *list_p = new_list;
 327         return new_list;
 328 }
 329
 330 unsigned commit_list_count(const struct commit_list *l)
 331 {
 332         unsigned c = 0;
 333         for (; l; l = l->next )
 334                 c++;
 335         return c;
 336 }
 337
 338 void free_commit_list(struct commit_list *list)
 339 {
 340         while (list) {
 341                 struct commit_list *temp = list;
 342                 list = temp->next;
 343                 free(temp);
 344         }
 345 }
 346
 347 struct commit_list * insert_by_date(struct commit *item, struct commit_list **list)
 348 {
 349         struct commit_list **pp = list;
 350         struct commit_list *p;
 351         while ((p = *pp) != NULL) {
 352                 if (p->item->date < item->date) {
 353                         break;
 354                 }
 355                 pp = &p->next;
 356         }
 357         return commit_list_insert(item, pp);
 358 }
 359
 360
 361 void sort_by_date(struct commit_list **list)
 362 {
 363         struct commit_list *ret = NULL;
 364         while (*list) {
 365                 insert_by_date((*list)->item, &ret);
 366                 *list = (*list)->next;
 367         }
 368         *list = ret;
 369 }
 370
 371 struct commit *pop_most_recent_commit(struct commit_list **list,
 372                                       unsigned int mark)
 373 {
 374         struct commit *ret = (*list)->item;
 375         struct commit_list *parents = ret->parents;
 376         struct commit_list *old = *list;
 377
 378         *list = (*list)->next;
 379         free(old);
 380
 381         while (parents) {
 382                 struct commit *commit = parents->item;
 383                 if (!parse_commit(commit) && !(commit->object.flags & mark)) {
 384                         commit->object.flags |= mark;
 385                         insert_by_date(commit, list);
 386                 }
 387                 parents = parents->next;
 388         }
 389         return ret;
 390 }
 391
 392 void clear_commit_marks(struct commit *commit, unsigned int mark)
 393 {
 394         while (commit) {
 395                 struct commit_list *parents;
 396
 397                 if (!(mark & commit->object.flags))
 398                         return;
 399
 400                 commit->object.flags &= ~mark;
 401
 402                 parents = commit->parents;
 403                 if (!parents)
 404                         return;
 405
 406                 while ((parents = parents->next))
 407                         clear_commit_marks(parents->item, mark);
 408
 409                 commit = commit->parents->item;
 410         }
 411 }
 412
 413 struct commit *pop_commit(struct commit_list **stack)
 414 {
 415         struct commit_list *top = *stack;
 416         struct commit *item = top ? top->item : NULL;
 417
 418         if (top) {
 419                 *stack = top->next;
 420                 free(top);
 421         }
 422         return item;
 423 }
 424
 425 /*
 426  * Performs an in-place topological sort on the list supplied.
 427  */
 428 void sort_in_topological_order(struct commit_list ** list, int lifo)
 429 {
 430         struct commit_list *next, *orig = *list;
 431         struct commit_list *work, **insert;
 432         struct commit_list **pptr;
 433
 434         if (!orig)
 435                 return;
 436         *list = NULL;
 437
 438         /* Mark them and clear the indegree */
 439         for (next = orig; next; next = next->next) {
 440                 struct commit *commit = next->item;
 441                 commit->indegree = 1;
 442         }
 443
 444         /* update the indegree */
 445         for (next = orig; next; next = next->next) {
 446                 struct commit_list * parents = next->item->parents;
 447                 while (parents) {
 448                         struct commit *parent = parents->item;
 449
 450                         if (parent->indegree)
 451                                 parent->indegree++;
 452                         parents = parents->next;
 453                 }
 454         }
 455
 456         /*
 457          * find the tips
 458          *
 459          * tips are nodes not reachable from any other node in the list
 460          *
 461          * the tips serve as a starting set for the work queue.
 462          */
 463         work = NULL;
 464         insert = &work;
 465         for (next = orig; next; next = next->next) {
 466                 struct commit *commit = next->item;
 467
 468                 if (commit->indegree == 1)
 469                         insert = &commit_list_insert(commit, insert)->next;
 470         }
 471
 472         /* process the list in topological order */
 473         if (!lifo)
 474                 sort_by_date(&work);
 475
 476         pptr = list;
 477         *list = NULL;
 478         while (work) {
 479                 struct commit *commit;
 480                 struct commit_list *parents, *work_item;
 481
 482                 work_item = work;
 483                 work = work_item->next;
 484                 work_item->next = NULL;
 485
 486                 commit = work_item->item;
 487                 for (parents = commit->parents; parents ; parents = parents->next) {
 488                         struct commit *parent=parents->item;
 489
 490                         if (!parent->indegree)
 491                                 continue;
 492
 493                         /*
 494                          * parents are only enqueued for emission
 495                          * when all their children have been emitted thereby
 496                          * guaranteeing topological order.
 497                          */
 498                         if (--parent->indegree == 1) {
 499                                 if (!lifo)
 500                                         insert_by_date(parent, &work);
 501                                 else
 502                                         commit_list_insert(parent, &work);
 503                         }
 504                 }
 505                 /*
 506                  * work_item is a commit all of whose children
 507                  * have already been emitted. we can emit it now.
 508                  */
 509                 commit->indegree = 0;
 510                 *pptr = work_item;
 511                 pptr = &work_item->next;
 512         }
 513 }
 514
 515 /* merge-base stuff */
 516
 517 /* bits #0..15 in revision.h */
 518 #define PARENT1         (1u<<16)
 519 #define PARENT2         (1u<<17)
 520 #define STALE           (1u<<18)
 521 #define RESULT          (1u<<19)
 522
 523 static const unsigned all_flags = (PARENT1 | PARENT2 | STALE | RESULT);
 524
 525 static struct commit *interesting(struct commit_list *list)
 526 {
 527         while (list) {
 528                 struct commit *commit = list->item;
 529                 list = list->next;
 530                 if (commit->object.flags & STALE)
 531                         continue;
 532                 return commit;
 533         }
 534         return NULL;
 535 }
 536
 537 static struct commit_list *merge_bases_many(struct commit *one, int n, struct commit **twos)
 538 {
 539         struct commit_list *list = NULL;
 540         struct commit_list *result = NULL;
 541         int i;
 542
 543         for (i = 0; i < n; i++) {
 544                 if (one == twos[i])
 545                         /*
 546                          * We do not mark this even with RESULT so we do not
 547                          * have to clean it up.
 548                          */
 549                         return commit_list_insert(one, &result);
 550         }
 551
 552         if (parse_commit(one))
 553                 return NULL;
 554         for (i = 0; i < n; i++) {
 555                 if (parse_commit(twos[i]))
 556                         return NULL;
 557         }
 558
 559         one->object.flags |= PARENT1;
 560         insert_by_date(one, &list);
 561         for (i = 0; i < n; i++) {
 562                 twos[i]->object.flags |= PARENT2;
 563                 insert_by_date(twos[i], &list);
 564         }
 565
 566         while (interesting(list)) {
 567                 struct commit *commit;
 568                 struct commit_list *parents;
 569                 struct commit_list *next;
 570                 int flags;
 571
 572                 commit = list->item;
 573                 next = list->next;
 574                 free(list);
 575                 list = next;
 576
 577                 flags = commit->object.flags & (PARENT1 | PARENT2 | STALE);
 578                 if (flags == (PARENT1 | PARENT2)) {
 579                         if (!(commit->object.flags & RESULT)) {
 580                                 commit->object.flags |= RESULT;
 581                                 insert_by_date(commit, &result);
 582                         }
 583                         /* Mark parents of a found merge stale */
 584                         flags |= STALE;
 585                 }
 586                 parents = commit->parents;
 587                 while (parents) {
 588                         struct commit *p = parents->item;
 589                         parents = parents->next;
 590                         if ((p->object.flags & flags) == flags)
 591                                 continue;
 592                         if (parse_commit(p))
 593                                 return NULL;
 594                         p->object.flags |= flags;
 595                         insert_by_date(p, &list);
 596                 }
 597         }
 598
 599         /* Clean up the result to remove stale ones */
 600         free_commit_list(list);
 601         list = result; result = NULL;
 602         while (list) {
 603                 struct commit_list *next = list->next;
 604                 if (!(list->item->object.flags & STALE))
 605                         insert_by_date(list->item, &result);
 606                 free(list);
 607                 list = next;
 608         }
 609         return result;
 610 }
 611
 612 struct commit_list *get_octopus_merge_bases(struct commit_list *in)
 613 {
 614         struct commit_list *i, *j, *k, *ret = NULL;
 615         struct commit_list **pptr = &ret;
 616
 617         for (i = in; i; i = i->next) {
 618                 if (!ret)
 619                         pptr = &commit_list_insert(i->item, pptr)->next;
 620                 else {
 621                         struct commit_list *new = NULL, *end = NULL;
 622
 623                         for (j = ret; j; j = j->next) {
 624                                 struct commit_list *bases;
 625                                 bases = get_merge_bases(i->item, j->item, 1);
 626                                 if (!new)
 627                                         new = bases;
 628                                 else
 629                                         end->next = bases;
 630                                 for (k = bases; k; k = k->next)
 631                                         end = k;
 632                         }
 633                         ret = new;
 634                 }
 635         }
 636         return ret;
 637 }
 638
 639 struct commit_list *get_merge_bases_many(struct commit *one,
 640                                          int n,
 641                                          struct commit **twos,
 642                                          int cleanup)
 643 {
 644         struct commit_list *list;
 645         struct commit **rslt;
 646         struct commit_list *result;
 647         int cnt, i, j;
 648
 649         result = merge_bases_many(one, n, twos);
 650         for (i = 0; i < n; i++) {
 651                 if (one == twos[i])
 652                         return result;
 653         }
 654         if (!result || !result->next) {
 655                 if (cleanup) {
 656                         clear_commit_marks(one, all_flags);
 657                         for (i = 0; i < n; i++)
 658                                 clear_commit_marks(twos[i], all_flags);
 659                 }
 660                 return result;
 661         }
 662
 663         /* There are more than one */
 664         cnt = 0;
 665         list = result;
 666         while (list) {
 667                 list = list->next;
 668                 cnt++;
 669         }
 670         rslt = xcalloc(cnt, sizeof(*rslt));
 671         for (list = result, i = 0; list; list = list->next)
 672                 rslt[i++] = list->item;
 673         free_commit_list(result);
 674
 675         clear_commit_marks(one, all_flags);
 676         for (i = 0; i < n; i++)
 677                 clear_commit_marks(twos[i], all_flags);
 678         for (i = 0; i < cnt - 1; i++) {
 679                 for (j = i+1; j < cnt; j++) {
 680                         if (!rslt[i] || !rslt[j])
 681                                 continue;
 682                         result = merge_bases_many(rslt[i], 1, &rslt[j]);
 683                         clear_commit_marks(rslt[i], all_flags);
 684                         clear_commit_marks(rslt[j], all_flags);
 685                         for (list = result; list; list = list->next) {
 686                                 if (rslt[i] == list->item)
 687                                         rslt[i] = NULL;
 688                                 if (rslt[j] == list->item)
 689                                         rslt[j] = NULL;
 690                         }
 691                 }
 692         }
 693
 694         /* Surviving ones in rslt[] are the independent results */
 695         result = NULL;
 696         for (i = 0; i < cnt; i++) {
 697                 if (rslt[i])
 698                         insert_by_date(rslt[i], &result);
 699         }
 700         free(rslt);
 701         return result;
 702 }
 703
 704 struct commit_list *get_merge_bases(struct commit *one, struct commit *two,
 705                                     int cleanup)
 706 {
 707         return get_merge_bases_many(one, 1, &two, cleanup);
 708 }
 709
 710 int is_descendant_of(struct commit *commit, struct commit_list *with_commit)
 711 {
 712         if (!with_commit)
 713                 return 1;
 714         while (with_commit) {
 715                 struct commit *other;
 716
 717                 other = with_commit->item;
 718                 with_commit = with_commit->next;
 719                 if (in_merge_bases(other, &commit, 1))
 720                         return 1;
 721         }
 722         return 0;
 723 }
 724
 725 int in_merge_bases(struct commit *commit, struct commit **reference, int num)
 726 {
 727         struct commit_list *bases, *b;
 728         int ret = 0;
 729
 730         if (num == 1)
 731                 bases = get_merge_bases(commit, *reference, 1);
 732         else
 733                 die("not yet");
 734         for (b = bases; b; b = b->next) {
 735                 if (!hashcmp(commit->object.sha1, b->item->object.sha1)) {
 736                         ret = 1;
 737                         break;
 738                 }
 739         }
 740
 741         free_commit_list(bases);
 742         return ret;
 743 }
 744
 745 struct commit_list *reduce_heads(struct commit_list *heads)
 746 {
 747         struct commit_list *p;
 748         struct commit_list *result = NULL, **tail = &result;
 749         struct commit **other;
 750         size_t num_head, num_other;
 751
 752         if (!heads)
 753                 return NULL;
 754
 755         /* Avoid unnecessary reallocations */
 756         for (p = heads, num_head = 0; p; p = p->next)
 757                 num_head++;
 758         other = xcalloc(sizeof(*other), num_head);
 759
 760         /* For each commit, see if it can be reached by others */
 761         for (p = heads; p; p = p->next) {
 762                 struct commit_list *q, *base;
 763
 764                 /* Do we already have this in the result? */
 765                 for (q = result; q; q = q->next)
 766                         if (p->item == q->item)
 767                                 break;
 768                 if (q)
 769                         continue;
 770
 771                 num_other = 0;
 772                 for (q = heads; q; q = q->next) {
 773                         if (p->item == q->item)
 774                                 continue;
 775                         other[num_other++] = q->item;
 776                 }
 777                 if (num_other)
 778                         base = get_merge_bases_many(p->item, num_other, other, 1);
 779                 else
 780                         base = NULL;
 781                 /*
 782                  * If p->item does not have anything common with other
 783                  * commits, there won't be any merge base.  If it is
 784                  * reachable from some of the others, p->item will be
 785                  * the merge base.  If its history is connected with
 786                  * others, but p->item is not reachable by others, we
 787                  * will get something other than p->item back.
 788                  */
 789                 if (!base || (base->item != p->item))
 790                         tail = &(commit_list_insert(p->item, tail)->next);
 791                 free_commit_list(base);
 792         }
 793         free(other);
 794         return result;
 795 }