git.oblomov.eu Git - git/blob - 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         if (diff_setup_done(&rev.diffopt) < 0)
 299                 die("diff_setup_done failed");
 300         add_pending_object(&rev, head, NULL);
 301         run_diff_index(&rev, 0);
 302 }
 303
 304 static void describe_detached_head(char *msg, struct commit *commit)
 305 {
 306         struct strbuf sb = STRBUF_INIT;
 307         parse_commit(commit);
 308         pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, 0, NULL, NULL, 0, 0);
 309         fprintf(stderr, "%s %s... %s\n", msg,
 310                 find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
 311         strbuf_release(&sb);
 312 }
 313
 314 static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
 315 {
 316         struct unpack_trees_options opts;
 317         struct tree_desc tree_desc;
 318
 319         memset(&opts, 0, sizeof(opts));
 320         opts.head_idx = -1;
 321         opts.update = worktree;
 322         opts.skip_unmerged = !worktree;
 323         opts.reset = 1;
 324         opts.merge = 1;
 325         opts.fn = oneway_merge;
 326         opts.verbose_update = !o->quiet;
 327         opts.src_index = &the_index;
 328         opts.dst_index = &the_index;
 329         parse_tree(tree);
 330         init_tree_desc(&tree_desc, tree->buffer, tree->size);
 331         switch (unpack_trees(1, &tree_desc, &opts)) {
 332         case -2:
 333                 o->writeout_error = 1;
 334                 /*
 335                  * We return 0 nevertheless, as the index is all right
 336                  * and more importantly we have made best efforts to
 337                  * update paths in the work tree, and we cannot revert
 338                  * them.
 339                  */
 340         case 0:
 341                 return 0;
 342         default:
 343                 return 128;
 344         }
 345 }
 346
 347 struct branch_info {
 348         const char *name; /* The short name used */
 349         const char *path; /* The full name of a real branch */
 350         struct commit *commit; /* The named commit */
 351 };
 352
 353 static void setup_branch_path(struct branch_info *branch)
 354 {
 355         struct strbuf buf = STRBUF_INIT;
 356         int ret;
 357
 358         if ((ret = interpret_nth_last_branch(branch->name, &buf))
 359             && ret == strlen(branch->name)) {
 360                 branch->name = xstrdup(buf.buf);
 361                 strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
 362         } else {
 363                 strbuf_addstr(&buf, "refs/heads/");
 364                 strbuf_addstr(&buf, branch->name);
 365         }
 366         branch->path = strbuf_detach(&buf, NULL);
 367 }
 368
 369 static int merge_working_tree(struct checkout_opts *opts,
 370                               struct branch_info *old, struct branch_info *new)
 371 {
 372         int ret;
 373         struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 374         int newfd = hold_locked_index(lock_file, 1);
 375
 376         if (read_cache() < 0)
 377                 return error("corrupt index file");
 378
 379         if (opts->force) {
 380                 ret = reset_tree(new->commit->tree, opts, 1);
 381                 if (ret)
 382                         return ret;
 383         } else {
 384                 struct tree_desc trees[2];
 385                 struct tree *tree;
 386                 struct unpack_trees_options topts;
 387
 388                 memset(&topts, 0, sizeof(topts));
 389                 topts.head_idx = -1;
 390                 topts.src_index = &the_index;
 391                 topts.dst_index = &the_index;
 392
 393                 topts.msgs.not_uptodate_file = "You have local changes to '%s'; cannot switch branches.";
 394
 395                 refresh_cache(REFRESH_QUIET);
 396
 397                 if (unmerged_cache()) {
 398                         error("you need to resolve your current index first");
 399                         return 1;
 400                 }
 401
 402                 /* 2-way merge to the new branch */
 403                 topts.initial_checkout = is_cache_unborn();
 404                 topts.update = 1;
 405                 topts.merge = 1;
 406                 topts.gently = opts->merge;
 407                 topts.verbose_update = !opts->quiet;
 408                 topts.fn = twoway_merge;
 409                 topts.dir = xcalloc(1, sizeof(*topts.dir));
 410                 topts.dir->flags |= DIR_SHOW_IGNORED;
 411                 topts.dir->exclude_per_dir = ".gitignore";
 412                 tree = parse_tree_indirect(old->commit->object.sha1);
 413                 init_tree_desc(&trees[0], tree->buffer, tree->size);
 414                 tree = parse_tree_indirect(new->commit->object.sha1);
 415                 init_tree_desc(&trees[1], tree->buffer, tree->size);
 416
 417                 ret = unpack_trees(2, trees, &topts);
 418                 if (ret == -1) {
 419                         /*
 420                          * Unpack couldn't do a trivial merge; either
 421                          * give up or do a real merge, depending on
 422                          * whether the merge flag was used.
 423                          */
 424                         struct tree *result;
 425                         struct tree *work;
 426                         struct merge_options o;
 427                         if (!opts->merge)
 428                                 return 1;
 429                         parse_commit(old->commit);
 430
 431                         /* Do more real merge */
 432
 433                         /*
 434                          * We update the index fully, then write the
 435                          * tree from the index, then merge the new
 436                          * branch with the current tree, with the old
 437                          * branch as the base. Then we reset the index
 438                          * (but not the working tree) to the new
 439                          * branch, leaving the working tree as the
 440                          * merged version, but skipping unmerged
 441                          * entries in the index.
 442                          */
 443
 444                         add_files_to_cache(NULL, NULL, 0);
 445                         init_merge_options(&o);
 446                         o.verbosity = 0;
 447                         work = write_tree_from_memory(&o);
 448
 449                         ret = reset_tree(new->commit->tree, opts, 1);
 450                         if (ret)
 451                                 return ret;
 452                         o.branch1 = new->name;
 453                         o.branch2 = "local";
 454                         merge_trees(&o, new->commit->tree, work,
 455                                 old->commit->tree, &result);
 456                         ret = reset_tree(new->commit->tree, opts, 0);
 457                         if (ret)
 458                                 return ret;
 459                 }
 460         }
 461
 462         if (write_cache(newfd, active_cache, active_nr) ||
 463             commit_locked_index(lock_file))
 464                 die("unable to write new index file");
 465
 466         if (!opts->force && !opts->quiet)
 467                 show_local_changes(&new->commit->object);
 468
 469         return 0;
 470 }
 471
 472 static void report_tracking(struct branch_info *new)
 473 {
 474         struct strbuf sb = STRBUF_INIT;
 475         struct branch *branch = branch_get(new->name);
 476
 477         if (!format_tracking_info(branch, &sb))
 478                 return;
 479         fputs(sb.buf, stdout);
 480         strbuf_release(&sb);
 481 }
 482
 483 static void update_refs_for_switch(struct checkout_opts *opts,
 484                                    struct branch_info *old,
 485                                    struct branch_info *new)
 486 {
 487         struct strbuf msg = STRBUF_INIT;
 488         const char *old_desc;
 489         if (opts->new_branch) {
 490                 create_branch(old->name, opts->new_branch, new->name, 0,
 491                               opts->new_branch_log, opts->track);
 492                 new->name = opts->new_branch;
 493                 setup_branch_path(new);
 494         }
 495
 496         old_desc = old->name;
 497         if (!old_desc && old->commit)
 498                 old_desc = sha1_to_hex(old->commit->object.sha1);
 499         strbuf_addf(&msg, "checkout: moving from %s to %s",
 500                     old_desc ? old_desc : "(invalid)", new->name);
 501
 502         if (new->path) {
 503                 create_symref("HEAD", new->path, msg.buf);
 504                 if (!opts->quiet) {
 505                         if (old->path && !strcmp(new->path, old->path))
 506                                 fprintf(stderr, "Already on \"%s\"\n",
 507                                         new->name);
 508                         else
 509                                 fprintf(stderr, "Switched to%s branch \"%s\"\n",
 510                                         opts->new_branch ? " a new" : "",
 511                                         new->name);
 512                 }
 513         } else if (strcmp(new->name, "HEAD")) {
 514                 update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
 515                            REF_NODEREF, DIE_ON_ERR);
 516                 if (!opts->quiet) {
 517                         if (old->path)
 518                                 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);
 519                         describe_detached_head("HEAD is now at", new->commit);
 520                 }
 521         }
 522         remove_branch_state();
 523         strbuf_release(&msg);
 524         if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
 525                 report_tracking(new);
 526 }
 527
 528 static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 529 {
 530         int ret = 0;
 531         struct branch_info old;
 532         unsigned char rev[20];
 533         int flag;
 534         memset(&old, 0, sizeof(old));
 535         old.path = resolve_ref("HEAD", rev, 0, &flag);
 536         old.commit = lookup_commit_reference_gently(rev, 1);
 537         if (!(flag & REF_ISSYMREF))
 538                 old.path = NULL;
 539
 540         if (old.path && !prefixcmp(old.path, "refs/heads/"))
 541                 old.name = old.path + strlen("refs/heads/");
 542
 543         if (!new->name) {
 544                 new->name = "HEAD";
 545                 new->commit = old.commit;
 546                 if (!new->commit)
 547                         die("You are on a branch yet to be born");
 548                 parse_commit(new->commit);
 549         }
 550
 551         /*
 552          * If we were on a detached HEAD, but we are now moving to
 553          * a new commit, we want to mention the old commit once more
 554          * to remind the user that it might be lost.
 555          */
 556         if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
 557                 describe_detached_head("Previous HEAD position was", old.commit);
 558
 559         if (!old.commit && !opts->force) {
 560                 if (!opts->quiet) {
 561                         fprintf(stderr, "warning: You appear to be on a branch yet to be born.\n");
 562                         fprintf(stderr, "warning: Forcing checkout of %s.\n", new->name);
 563                 }
 564                 opts->force = 1;
 565         }
 566
 567         ret = merge_working_tree(opts, &old, new);
 568         if (ret)
 569                 return ret;
 570
 571         update_refs_for_switch(opts, &old, new);
 572
 573         ret = post_checkout_hook(old.commit, new->commit, 1);
 574         return ret || opts->writeout_error;
 575 }
 576
 577 static int git_checkout_config(const char *var, const char *value, void *cb)
 578 {
 579         return git_xmerge_config(var, value, cb);
 580 }
 581
 582 int cmd_checkout(int argc, const char **argv, const char *prefix)
 583 {
 584         struct checkout_opts opts;
 585         unsigned char rev[20];
 586         const char *arg;
 587         struct branch_info new;
 588         struct tree *source_tree = NULL;
 589         char *conflict_style = NULL;
 590         struct option options[] = {
 591                 OPT__QUIET(&opts.quiet),
 592                 OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
 593                 OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
 594                 OPT_SET_INT('t', "track",  &opts.track, "track",
 595                         BRANCH_TRACK_EXPLICIT),
 596                 OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
 597                             2),
 598                 OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
 599                             3),
 600                 OPT_BOOLEAN('f', NULL, &opts.force, "force"),
 601                 OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
 602                 OPT_STRING(0, "conflict", &conflict_style, "style",
 603                            "conflict style (merge or diff3)"),
 604                 OPT_END(),
 605         };
 606         int has_dash_dash;
 607
 608         memset(&opts, 0, sizeof(opts));
 609         memset(&new, 0, sizeof(new));
 610
 611         git_config(git_checkout_config, NULL);
 612
 613         opts.track = BRANCH_TRACK_UNSPECIFIED;
 614
 615         argc = parse_options(argc, argv, options, checkout_usage,
 616                              PARSE_OPT_KEEP_DASHDASH);
 617
 618         /* --track without -b should DWIM */
 619         if (0 < opts.track && !opts.new_branch) {
 620                 const char *argv0 = argv[0];
 621                 if (!argc || !strcmp(argv0, "--"))
 622                         die ("--track needs a branch name");
 623                 if (!prefixcmp(argv0, "refs/"))
 624                         argv0 += 5;
 625                 if (!prefixcmp(argv0, "remotes/"))
 626                         argv0 += 8;
 627                 argv0 = strchr(argv0, '/');
 628                 if (!argv0 || !argv0[1])
 629                         die ("Missing branch name; try -b");
 630                 opts.new_branch = argv0 + 1;
 631         }
 632
 633         if (opts.track == BRANCH_TRACK_UNSPECIFIED)
 634                 opts.track = git_branch_track;
 635         if (conflict_style) {
 636                 opts.merge = 1; /* implied */
 637                 git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
 638         }
 639
 640         if (opts.force && opts.merge)
 641                 die("git checkout: -f and -m are incompatible");
 642
 643         /*
 644          * case 1: git checkout <ref> -- [<paths>]
 645          *
 646          *   <ref> must be a valid tree, everything after the '--' must be
 647          *   a path.
 648          *
 649          * case 2: git checkout -- [<paths>]
 650          *
 651          *   everything after the '--' must be paths.
 652          *
 653          * case 3: git checkout <something> [<paths>]
 654          *
 655          *   With no paths, if <something> is a commit, that is to
 656          *   switch to the branch or detach HEAD at it.
 657          *
 658          *   Otherwise <something> shall not be ambiguous.
 659          *   - If it's *only* a reference, treat it like case (1).
 660          *   - If it's only a path, treat it like case (2).
 661          *   - else: fail.
 662          *
 663          */
 664         if (argc) {
 665                 if (!strcmp(argv[0], "--")) {       /* case (2) */
 666                         argv++;
 667                         argc--;
 668                         goto no_reference;
 669                 }
 670
 671                 arg = argv[0];
 672                 has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
 673
 674                 if (!strcmp(arg, "-"))
 675                         arg = "@{-1}";
 676
 677                 if (get_sha1(arg, rev)) {
 678                         if (has_dash_dash)          /* case (1) */
 679                                 die("invalid reference: %s", arg);
 680                         goto no_reference;          /* case (3 -> 2) */
 681                 }
 682
 683                 /* we can't end up being in (2) anymore, eat the argument */
 684                 argv++;
 685                 argc--;
 686
 687                 new.name = arg;
 688                 if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
 689                         setup_branch_path(&new);
 690                         if (resolve_ref(new.path, rev, 1, NULL))
 691                                 new.commit = lookup_commit_reference(rev);
 692                         else
 693                                 new.path = NULL;
 694                         parse_commit(new.commit);
 695                         source_tree = new.commit->tree;
 696                 } else
 697                         source_tree = parse_tree_indirect(rev);
 698
 699                 if (!source_tree)                   /* case (1): want a tree */
 700                         die("reference is not a tree: %s", arg);
 701                 if (!has_dash_dash) {/* case (3 -> 1) */
 702                         /*
 703                          * Do not complain the most common case
 704                          *      git checkout branch
 705                          * even if there happen to be a file called 'branch';
 706                          * it would be extremely annoying.
 707                          */
 708                         if (argc)
 709                                 verify_non_filename(NULL, arg);
 710                 }
 711                 else {
 712                         argv++;
 713                         argc--;
 714                 }
 715         }
 716
 717 no_reference:
 718         if (argc) {
 719                 const char **pathspec = get_pathspec(prefix, argv);
 720
 721                 if (!pathspec)
 722                         die("invalid path specification");
 723
 724                 /* Checkout paths */
 725                 if (opts.new_branch) {
 726                         if (argc == 1) {
 727                                 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]);
 728                         } else {
 729                                 die("git checkout: updating paths is incompatible with switching branches.");
 730                         }
 731                 }
 732
 733                 if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
 734                         die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");
 735
 736                 return checkout_paths(source_tree, pathspec, &opts);
 737         }
 738
 739         if (opts.new_branch) {
 740                 struct strbuf buf = STRBUF_INIT;
 741                 strbuf_addstr(&buf, "refs/heads/");
 742                 strbuf_addstr(&buf, opts.new_branch);
 743                 if (!get_sha1(buf.buf, rev))
 744                         die("git checkout: branch %s already exists", opts.new_branch);
 745                 if (check_ref_format(buf.buf))
 746                         die("git checkout: we do not like '%s' as a branch name.", opts.new_branch);
 747                 strbuf_release(&buf);
 748         }
 749
 750         if (new.name && !new.commit) {
 751                 die("Cannot switch branch to a non-commit.");
 752         }
 753         if (opts.writeout_stage)
 754                 die("--ours/--theirs is incompatible with switching branches.");
 755
 756         return switch_branches(&opts, &new);
 757 }