builtin-checkout.c

   1 #include "cache.h"
   2 #include "builtin.h"
   3 #include "parse-options.h"
   4 #include "refs.h"
   5 #include "commit.h"
   6 #include "tree.h"
   7 #include "tree-walk.h"
   8 #include "unpack-trees.h"
   9 #include "dir.h"
  10 #include "run-command.h"
  11 #include "merge-recursive.h"
  12 #include "branch.h"
  13 #include "diff.h"
  14 #include "revision.h"
  15 #include "remote.h"
  16 #include "blob.h"
  17 #include "xdiff-interface.h"
  18 #include "ll-merge.h"
  19
  20 static const char * const checkout_usage[] = {
  21         "git checkout [options] <branch>",
  22         "git checkout [options] [<branch>] -- <file>...",
  23         NULL,
  24 };
  25
  26 struct checkout_opts {
  27         int quiet;
  28         int merge;
  29         int force;
  30         int writeout_stage;
  31         int writeout_error;
  32
  33         const char *new_branch;
  34         int new_branch_log;
  35         enum branch_track track;
  36 };
  37
  38 static int post_checkout_hook(struct commit *old, struct commit *new,
  39                               int changed)
  40 {
  41         return run_hook(NULL, "post-checkout",
  42                         sha1_to_hex(old ? old->object.sha1 : null_sha1),
  43                         sha1_to_hex(new ? new->object.sha1 : null_sha1),
  44                         changed ? "1" : "0", NULL);
  45         /* "new" can be NULL when checking out from the index before
  46            a commit exists. */
  47
  48 }
  49
  50 static int update_some(const unsigned char *sha1, const char *base, int baselen,
  51                 const char *pathname, unsigned mode, int stage, void *context)
  52 {
  53         int len;
  54         struct cache_entry *ce;
  55
  56         if (S_ISGITLINK(mode))
  57                 return 0;
  58
  59         if (S_ISDIR(mode))
  60                 return READ_TREE_RECURSIVE;
  61
  62         len = baselen + strlen(pathname);
  63         ce = xcalloc(1, cache_entry_size(len));
  64         hashcpy(ce->sha1, sha1);
  65         memcpy(ce->name, base, baselen);
  66         memcpy(ce->name + baselen, pathname, len - baselen);
  67         ce->ce_flags = create_ce_flags(len, 0);
  68         ce->ce_mode = create_ce_mode(mode);
  69         add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
  70         return 0;
  71 }
  72
  73 static int read_tree_some(struct tree *tree, const char **pathspec)
  74 {
  75         read_tree_recursive(tree, "", 0, 0, pathspec, update_some, NULL);
  76
  77         /* update the index with the given tree's info
  78          * for all args, expanding wildcards, and exit
  79          * with any non-zero return code.
  80          */
  81         return 0;
  82 }
  83
  84 static int skip_same_name(struct cache_entry *ce, int pos)
  85 {
  86         while (++pos < active_nr &&
  87                !strcmp(active_cache[pos]->name, ce->name))
  88                 ; /* skip */
  89         return pos;
  90 }
  91
  92 static int check_stage(int stage, struct cache_entry *ce, int pos)
  93 {
  94         while (pos < active_nr &&
  95                !strcmp(active_cache[pos]->name, ce->name)) {
  96                 if (ce_stage(active_cache[pos]) == stage)
  97                         return 0;
  98                 pos++;
  99         }
 100         return error("path '%s' does not have %s version",
 101                      ce->name,
 102                      (stage == 2) ? "our" : "their");
 103 }
 104
 105 static int check_all_stages(struct cache_entry *ce, int pos)
 106 {
 107         if (ce_stage(ce) != 1 ||
 108             active_nr <= pos + 2 ||
 109             strcmp(active_cache[pos+1]->name, ce->name) ||
 110             ce_stage(active_cache[pos+1]) != 2 ||
 111             strcmp(active_cache[pos+2]->name, ce->name) ||
 112             ce_stage(active_cache[pos+2]) != 3)
 113                 return error("path '%s' does not have all three versions",
 114                              ce->name);
 115         return 0;
 116 }
 117
 118 static int checkout_stage(int stage, struct cache_entry *ce, int pos,
 119                           struct checkout *state)
 120 {
 121         while (pos < active_nr &&
 122                !strcmp(active_cache[pos]->name, ce->name)) {
 123                 if (ce_stage(active_cache[pos]) == stage)
 124                         return checkout_entry(active_cache[pos], state, NULL);
 125                 pos++;
 126         }
 127         return error("path '%s' does not have %s version",
 128                      ce->name,
 129                      (stage == 2) ? "our" : "their");
 130 }
 131
 132 /* NEEDSWORK: share with merge-recursive */
 133 static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
 134 {
 135         unsigned long size;
 136         enum object_type type;
 137
 138         if (!hashcmp(sha1, null_sha1)) {
 139                 mm->ptr = xstrdup("");
 140                 mm->size = 0;
 141                 return;
 142         }
 143
 144         mm->ptr = read_sha1_file(sha1, &type, &size);
 145         if (!mm->ptr || type != OBJ_BLOB)
 146                 die("unable to read blob object %s", sha1_to_hex(sha1));
 147         mm->size = size;
 148 }
 149
 150 static int checkout_merged(int pos, struct checkout *state)
 151 {
 152         struct cache_entry *ce = active_cache[pos];
 153         const char *path = ce->name;
 154         mmfile_t ancestor, ours, theirs;
 155         int status;
 156         unsigned char sha1[20];
 157         mmbuffer_t result_buf;
 158
 159         if (ce_stage(ce) != 1 ||
 160             active_nr <= pos + 2 ||
 161             strcmp(active_cache[pos+1]->name, path) ||
 162             ce_stage(active_cache[pos+1]) != 2 ||
 163             strcmp(active_cache[pos+2]->name, path) ||
 164             ce_stage(active_cache[pos+2]) != 3)
 165                 return error("path '%s' does not have all 3 versions", path);
 166
 167         fill_mm(active_cache[pos]->sha1, &ancestor);
 168         fill_mm(active_cache[pos+1]->sha1, &ours);
 169         fill_mm(active_cache[pos+2]->sha1, &theirs);
 170
 171         status = ll_merge(&result_buf, path, &ancestor,
 172                           &ours, "ours", &theirs, "theirs", 1);
 173         free(ancestor.ptr);
 174         free(ours.ptr);
 175         free(theirs.ptr);
 176         if (status < 0 || !result_buf.ptr) {
 177                 free(result_buf.ptr);
 178                 return error("path '%s': cannot merge", path);
 179         }
 180
 181         /*
 182          * NEEDSWORK:
 183          * There is absolutely no reason to write this as a blob object
 184          * and create a phoney cache entry just to leak.  This hack is
 185          * primarily to get to the write_entry() machinery that massages
 186          * the contents to work-tree format and writes out which only
 187          * allows it for a cache entry.  The code in write_entry() needs
 188          * to be refactored to allow us to feed a <buffer, size, mode>
 189          * instead of a cache entry.  Such a refactoring would help
 190          * merge_recursive as well (it also writes the merge result to the
 191          * object database even when it may contain conflicts).
 192          */
 193         if (write_sha1_file(result_buf.ptr, result_buf.size,
 194                             blob_type, sha1))
 195                 die("Unable to add merge result for '%s'", path);
 196         ce = make_cache_entry(create_ce_mode(active_cache[pos+1]->ce_mode),
 197                               sha1,
 198                               path, 2, 0);
 199         if (!ce)
 200                 die("make_cache_entry failed for path '%s'", path);
 201         status = checkout_entry(ce, state, NULL);
 202         return status;
 203 }
 204
 205 static int checkout_paths(struct tree *source_tree, const char **pathspec,
 206                           struct checkout_opts *opts)
 207 {
 208         int pos;
 209         struct checkout state;
 210         static char *ps_matched;
 211         unsigned char rev[20];
 212         int flag;
 213         struct commit *head;
 214         int errs = 0;
 215         int stage = opts->writeout_stage;
 216         int merge = opts->merge;
 217         int newfd;
 218         struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 219
 220         newfd = hold_locked_index(lock_file, 1);
 221         if (read_cache() < 0)
 222                 return error("corrupt index file");
 223
 224         if (source_tree)
 225                 read_tree_some(source_tree, pathspec);
 226
 227         for (pos = 0; pathspec[pos]; pos++)
 228                 ;
 229         ps_matched = xcalloc(1, pos);
 230
 231         for (pos = 0; pos < active_nr; pos++) {
 232                 struct cache_entry *ce = active_cache[pos];
 233                 match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, ps_matched);
 234         }
 235
 236         if (report_path_error(ps_matched, pathspec, 0))
 237                 return 1;
 238
 239         /* Any unmerged paths? */
 240         for (pos = 0; pos < active_nr; pos++) {
 241                 struct cache_entry *ce = active_cache[pos];
 242                 if (match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, NULL)) {
 243                         if (!ce_stage(ce))
 244                                 continue;
 245                         if (opts->force) {
 246                                 warning("path '%s' is unmerged", ce->name);
 247                         } else if (stage) {
 248                                 errs |= check_stage(stage, ce, pos);
 249                         } else if (opts->merge) {
 250                                 errs |= check_all_stages(ce, pos);
 251                         } else {
 252                                 errs = 1;
 253                                 error("path '%s' is unmerged", ce->name);
 254                         }
 255                         pos = skip_same_name(ce, pos) - 1;
 256                 }
 257         }
 258         if (errs)
 259                 return 1;
 260
 261         /* Now we are committed to check them out */
 262         memset(&state, 0, sizeof(state));
 263         state.force = 1;
 264         state.refresh_cache = 1;
 265         for (pos = 0; pos < active_nr; pos++) {
 266                 struct cache_entry *ce = active_cache[pos];
 267                 if (match_pathspec(pathspec, ce->name, ce_namelen(ce), 0, NULL)) {
 268                         if (!ce_stage(ce)) {
 269                                 errs |= checkout_entry(ce, &state, NULL);
 270                                 continue;
 271                         }
 272                         if (stage)
 273                                 errs |= checkout_stage(stage, ce, pos, &state);
 274                         else if (merge)
 275                                 errs |= checkout_merged(pos, &state);
 276                         pos = skip_same_name(ce, pos) - 1;
 277                 }
 278         }
 279
 280         if (write_cache(newfd, active_cache, active_nr) ||
 281             commit_locked_index(lock_file))
 282                 die("unable to write new index file");
 283
 284         resolve_ref("HEAD", rev, 0, &flag);
 285         head = lookup_commit_reference_gently(rev, 1);
 286
 287         errs |= post_checkout_hook(head, head, 0);
 288         return errs;
 289 }
 290
 291 static void show_local_changes(struct object *head)
 292 {
 293         struct rev_info rev;
 294         /* I think we want full paths, even if we're in a subdirectory. */
 295         init_revisions(&rev, NULL);
 296         rev.abbrev = 0;
 297         rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
 298         add_pending_object(&rev, head, NULL);
 299         run_diff_index(&rev, 0);
 300 }
 301
 302 static void describe_detached_head(char *msg, struct commit *commit)
 303 {
 304         struct strbuf sb = STRBUF_INIT;
 305         parse_commit(commit);
 306         pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, 0, NULL, NULL, 0, 0);
 307         fprintf(stderr, "%s %s... %s\n", msg,
 308                 find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
 309         strbuf_release(&sb);
 310 }
 311
 312 static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
 313 {
 314         struct unpack_trees_options opts;
 315         struct tree_desc tree_desc;
 316
 317         memset(&opts, 0, sizeof(opts));
 318         opts.head_idx = -1;
 319         opts.update = worktree;
 320         opts.skip_unmerged = !worktree;
 321         opts.reset = 1;
 322         opts.merge = 1;
 323         opts.fn = oneway_merge;
 324         opts.verbose_update = !o->quiet;
 325         opts.src_index = &the_index;
 326         opts.dst_index = &the_index;
 327         parse_tree(tree);
 328         init_tree_desc(&tree_desc, tree->buffer, tree->size);
 329         switch (unpack_trees(1, &tree_desc, &opts)) {
 330         case -2:
 331                 o->writeout_error = 1;
 332                 /*
 333                  * We return 0 nevertheless, as the index is all right
 334                  * and more importantly we have made best efforts to
 335                  * update paths in the work tree, and we cannot revert
 336                  * them.
 337                  */
 338         case 0:
 339                 return 0;
 340         default:
 341                 return 128;
 342         }
 343 }
 344
 345 struct branch_info {
 346         const char *name; /* The short name used */
 347         const char *path; /* The full name of a real branch */
 348         struct commit *commit; /* The named commit */
 349 };
 350
 351 static void setup_branch_path(struct branch_info *branch)
 352 {
 353         struct strbuf buf = STRBUF_INIT;
 354         int ret;
 355
 356         if ((ret = interpret_nth_last_branch(branch->name, &buf))
 357             && ret == strlen(branch->name)) {
 358                 branch->name = xstrdup(buf.buf);
 359                 strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
 360         } else {
 361                 strbuf_addstr(&buf, "refs/heads/");
 362                 strbuf_addstr(&buf, branch->name);
 363         }
 364         branch->path = strbuf_detach(&buf, NULL);
 365 }
 366
 367 static int merge_working_tree(struct checkout_opts *opts,
 368                               struct branch_info *old, struct branch_info *new)
 369 {
 370         int ret;
 371         struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 372         int newfd = hold_locked_index(lock_file, 1);
 373
 374         if (read_cache() < 0)
 375                 return error("corrupt index file");
 376
 377         if (opts->force) {
 378                 ret = reset_tree(new->commit->tree, opts, 1);
 379                 if (ret)
 380                         return ret;
 381         } else {
 382                 struct tree_desc trees[2];
 383                 struct tree *tree;
 384                 struct unpack_trees_options topts;
 385
 386                 memset(&topts, 0, sizeof(topts));
 387                 topts.head_idx = -1;
 388                 topts.src_index = &the_index;
 389                 topts.dst_index = &the_index;
 390
 391                 topts.msgs.not_uptodate_file = "You have local changes to '%s'; cannot switch branches.";
 392
 393                 refresh_cache(REFRESH_QUIET);
 394
 395                 if (unmerged_cache()) {
 396                         error("you need to resolve your current index first");
 397                         return 1;
 398                 }
 399
 400                 /* 2-way merge to the new branch */
 401                 topts.initial_checkout = is_cache_unborn();
 402                 topts.update = 1;
 403                 topts.merge = 1;
 404                 topts.gently = opts->merge;
 405                 topts.verbose_update = !opts->quiet;
 406                 topts.fn = twoway_merge;
 407                 topts.dir = xcalloc(1, sizeof(*topts.dir));
 408                 topts.dir->show_ignored = 1;
 409                 topts.dir->exclude_per_dir = ".gitignore";
 410                 tree = parse_tree_indirect(old->commit->object.sha1);
 411                 init_tree_desc(&trees[0], tree->buffer, tree->size);
 412                 tree = parse_tree_indirect(new->commit->object.sha1);
 413                 init_tree_desc(&trees[1], tree->buffer, tree->size);
 414
 415                 ret = unpack_trees(2, trees, &topts);
 416                 if (ret == -1) {
 417                         /*
 418                          * Unpack couldn't do a trivial merge; either
 419                          * give up or do a real merge, depending on
 420                          * whether the merge flag was used.
 421                          */
 422                         struct tree *result;
 423                         struct tree *work;
 424                         struct merge_options o;
 425                         if (!opts->merge)
 426                                 return 1;
 427                         parse_commit(old->commit);
 428
 429                         /* Do more real merge */
 430
 431                         /*
 432                          * We update the index fully, then write the
 433                          * tree from the index, then merge the new
 434                          * branch with the current tree, with the old
 435                          * branch as the base. Then we reset the index
 436                          * (but not the working tree) to the new
 437                          * branch, leaving the working tree as the
 438                          * merged version, but skipping unmerged
 439                          * entries in the index.
 440                          */
 441
 442                         add_files_to_cache(NULL, NULL, 0);
 443                         init_merge_options(&o);
 444                         o.verbosity = 0;
 445                         work = write_tree_from_memory(&o);
 446
 447                         ret = reset_tree(new->commit->tree, opts, 1);
 448                         if (ret)
 449                                 return ret;
 450                         o.branch1 = new->name;
 451                         o.branch2 = "local";
 452                         merge_trees(&o, new->commit->tree, work,
 453                                 old->commit->tree, &result);
 454                         ret = reset_tree(new->commit->tree, opts, 0);
 455                         if (ret)
 456                                 return ret;
 457                 }
 458         }
 459
 460         if (write_cache(newfd, active_cache, active_nr) ||
 461             commit_locked_index(lock_file))
 462                 die("unable to write new index file");
 463
 464         if (!opts->force && !opts->quiet)
 465                 show_local_changes(&new->commit->object);
 466
 467         return 0;
 468 }
 469
 470 static void report_tracking(struct branch_info *new)
 471 {
 472         struct strbuf sb = STRBUF_INIT;
 473         struct branch *branch = branch_get(new->name);
 474
 475         if (!format_tracking_info(branch, &sb))
 476                 return;
 477         fputs(sb.buf, stdout);
 478         strbuf_release(&sb);
 479 }
 480
 481 static void update_refs_for_switch(struct checkout_opts *opts,
 482                                    struct branch_info *old,
 483                                    struct branch_info *new)
 484 {
 485         struct strbuf msg = STRBUF_INIT;
 486         const char *old_desc;
 487         if (opts->new_branch) {
 488                 create_branch(old->name, opts->new_branch, new->name, 0,
 489                               opts->new_branch_log, opts->track);
 490                 new->name = opts->new_branch;
 491                 setup_branch_path(new);
 492         }
 493
 494         old_desc = old->name;
 495         if (!old_desc && old->commit)
 496                 old_desc = sha1_to_hex(old->commit->object.sha1);
 497         strbuf_addf(&msg, "checkout: moving from %s to %s",
 498                     old_desc ? old_desc : "(invalid)", new->name);
 499
 500         if (new->path) {
 501                 create_symref("HEAD", new->path, msg.buf);
 502                 if (!opts->quiet) {
 503                         if (old->path && !strcmp(new->path, old->path))
 504                                 fprintf(stderr, "Already on \"%s\"\n",
 505                                         new->name);
 506                         else
 507                                 fprintf(stderr, "Switched to%s branch \"%s\"\n",
 508                                         opts->new_branch ? " a new" : "",
 509                                         new->name);
 510                 }
 511         } else if (strcmp(new->name, "HEAD")) {
 512                 update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
 513                            REF_NODEREF, DIE_ON_ERR);
 514                 if (!opts->quiet) {
 515                         if (old->path)
 516                                 fprintf(stderr, "Note: moving to \"%s\" which isn't a local branch\nIf you want to create a new branch from this checkout, you may do so\n(now or later) by using -b with the checkout command again. Example:\n  git checkout -b <new_branch_name>\n", new->name);
 517                         describe_detached_head("HEAD is now at", new->commit);
 518                 }
 519         }
 520         remove_branch_state();
 521         strbuf_release(&msg);
 522         if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
 523                 report_tracking(new);
 524 }
 525
 526 static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 527 {
 528         int ret = 0;
 529         struct branch_info old;
 530         unsigned char rev[20];
 531         int flag;
 532         memset(&old, 0, sizeof(old));
 533         old.path = resolve_ref("HEAD", rev, 0, &flag);
 534         old.commit = lookup_commit_reference_gently(rev, 1);
 535         if (!(flag & REF_ISSYMREF))
 536                 old.path = NULL;
 537
 538         if (old.path && !prefixcmp(old.path, "refs/heads/"))
 539                 old.name = old.path + strlen("refs/heads/");
 540
 541         if (!new->name) {
 542                 new->name = "HEAD";
 543                 new->commit = old.commit;
 544                 if (!new->commit)
 545                         die("You are on a branch yet to be born");
 546                 parse_commit(new->commit);
 547         }
 548
 549         /*
 550          * If we were on a detached HEAD, but we are now moving to
 551          * a new commit, we want to mention the old commit once more
 552          * to remind the user that it might be lost.
 553          */
 554         if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
 555                 describe_detached_head("Previous HEAD position was", old.commit);
 556
 557         if (!old.commit && !opts->force) {
 558                 if (!opts->quiet) {
 559                         fprintf(stderr, "warning: You appear to be on a branch yet to be born.\n");
 560                         fprintf(stderr, "warning: Forcing checkout of %s.\n", new->name);
 561                 }
 562                 opts->force = 1;
 563         }
 564
 565         ret = merge_working_tree(opts, &old, new);
 566         if (ret)
 567                 return ret;
 568
 569         update_refs_for_switch(opts, &old, new);
 570
 571         ret = post_checkout_hook(old.commit, new->commit, 1);
 572         return ret || opts->writeout_error;
 573 }
 574
 575 static int git_checkout_config(const char *var, const char *value, void *cb)
 576 {
 577         return git_xmerge_config(var, value, cb);
 578 }
 579
 580 int cmd_checkout(int argc, const char **argv, const char *prefix)
 581 {
 582         struct checkout_opts opts;
 583         unsigned char rev[20];
 584         const char *arg;
 585         struct branch_info new;
 586         struct tree *source_tree = NULL;
 587         char *conflict_style = NULL;
 588         struct option options[] = {
 589                 OPT__QUIET(&opts.quiet),
 590                 OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
 591                 OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
 592                 OPT_SET_INT('t', "track",  &opts.track, "track",
 593                         BRANCH_TRACK_EXPLICIT),
 594                 OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
 595                             2),
 596                 OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
 597                             3),
 598                 OPT_BOOLEAN('f', NULL, &opts.force, "force"),
 599                 OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
 600                 OPT_STRING(0, "conflict", &conflict_style, "style",
 601                            "conflict style (merge or diff3)"),
 602                 OPT_END(),
 603         };
 604         int has_dash_dash;
 605
 606         memset(&opts, 0, sizeof(opts));
 607         memset(&new, 0, sizeof(new));
 608
 609         git_config(git_checkout_config, NULL);
 610
 611         opts.track = BRANCH_TRACK_UNSPECIFIED;
 612
 613         argc = parse_options(argc, argv, options, checkout_usage,
 614                              PARSE_OPT_KEEP_DASHDASH);
 615
 616         /* --track without -b should DWIM */
 617         if (0 < opts.track && !opts.new_branch) {
 618                 const char *argv0 = argv[0];
 619                 if (!argc || !strcmp(argv0, "--"))
 620                         die ("--track needs a branch name");
 621                 if (!prefixcmp(argv0, "refs/"))
 622                         argv0 += 5;
 623                 if (!prefixcmp(argv0, "remotes/"))
 624                         argv0 += 8;
 625                 argv0 = strchr(argv0, '/');
 626                 if (!argv0 || !argv0[1])
 627                         die ("Missing branch name; try -b");
 628                 opts.new_branch = argv0 + 1;
 629         }
 630
 631         if (opts.track == BRANCH_TRACK_UNSPECIFIED)
 632                 opts.track = git_branch_track;
 633         if (conflict_style) {
 634                 opts.merge = 1; /* implied */
 635                 git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
 636         }
 637
 638         if (opts.force && opts.merge)
 639                 die("git checkout: -f and -m are incompatible");
 640
 641         /*
 642          * case 1: git checkout <ref> -- [<paths>]
 643          *
 644          *   <ref> must be a valid tree, everything after the '--' must be
 645          *   a path.
 646          *
 647          * case 2: git checkout -- [<paths>]
 648          *
 649          *   everything after the '--' must be paths.
 650          *
 651          * case 3: git checkout <something> [<paths>]
 652          *
 653          *   With no paths, if <something> is a commit, that is to
 654          *   switch to the branch or detach HEAD at it.
 655          *
 656          *   Otherwise <something> shall not be ambiguous.
 657          *   - If it's *only* a reference, treat it like case (1).
 658          *   - If it's only a path, treat it like case (2).
 659          *   - else: fail.
 660          *
 661          */
 662         if (argc) {
 663                 if (!strcmp(argv[0], "--")) {       /* case (2) */
 664                         argv++;
 665                         argc--;
 666                         goto no_reference;
 667                 }
 668
 669                 arg = argv[0];
 670                 has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
 671
 672                 if (!strcmp(arg, "-"))
 673                         arg = "@{-1}";
 674
 675                 if (get_sha1(arg, rev)) {
 676                         if (has_dash_dash)          /* case (1) */
 677                                 die("invalid reference: %s", arg);
 678                         goto no_reference;          /* case (3 -> 2) */
 679                 }
 680
 681                 /* we can't end up being in (2) anymore, eat the argument */
 682                 argv++;
 683                 argc--;
 684
 685                 new.name = arg;
 686                 if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
 687                         setup_branch_path(&new);
 688                         if (resolve_ref(new.path, rev, 1, NULL))
 689                                 new.commit = lookup_commit_reference(rev);
 690                         else
 691                                 new.path = NULL;
 692                         parse_commit(new.commit);
 693                         source_tree = new.commit->tree;
 694                 } else
 695                         source_tree = parse_tree_indirect(rev);
 696
 697                 if (!source_tree)                   /* case (1): want a tree */
 698                         die("reference is not a tree: %s", arg);
 699                 if (!has_dash_dash) {/* case (3 -> 1) */
 700                         /*
 701                          * Do not complain the most common case
 702                          *      git checkout branch
 703                          * even if there happen to be a file called 'branch';
 704                          * it would be extremely annoying.
 705                          */
 706                         if (argc)
 707                                 verify_non_filename(NULL, arg);
 708                 }
 709                 else {
 710                         argv++;
 711                         argc--;
 712                 }
 713         }
 714
 715 no_reference:
 716         if (argc) {
 717                 const char **pathspec = get_pathspec(prefix, argv);
 718
 719                 if (!pathspec)
 720                         die("invalid path specification");
 721
 722                 /* Checkout paths */
 723                 if (opts.new_branch) {
 724                         if (argc == 1) {
 725                                 die("git checkout: updating paths is incompatible with switching branches.\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
 726                         } else {
 727                                 die("git checkout: updating paths is incompatible with switching branches.");
 728                         }
 729                 }
 730
 731                 if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
 732                         die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");
 733
 734                 return checkout_paths(source_tree, pathspec, &opts);
 735         }
 736
 737         if (opts.new_branch) {
 738                 struct strbuf buf = STRBUF_INIT;
 739                 strbuf_addstr(&buf, "refs/heads/");
 740                 strbuf_addstr(&buf, opts.new_branch);
 741                 if (!get_sha1(buf.buf, rev))
 742                         die("git checkout: branch %s already exists", opts.new_branch);
 743                 if (check_ref_format(buf.buf))
 744                         die("git checkout: we do not like '%s' as a branch name.", opts.new_branch);
 745                 strbuf_release(&buf);
 746         }
 747
 748         if (new.name && !new.commit) {
 749                 die("Cannot switch branch to a non-commit.");
 750         }
 751         if (opts.writeout_stage)
 752                 die("--ours/--theirs is incompatible with switching branches.");
 753
 754         return switch_branches(&opts, &new);
 755 }