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