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