Update COPYING with GPLv2 with new FSF address
[git] / 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         struct pretty_print_context ctx = {0};
306         parse_commit(commit);
307         pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, &ctx);
308         fprintf(stderr, "%s %s... %s\n", msg,
309                 find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
310         strbuf_release(&sb);
311 }
312
313 static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
314 {
315         struct unpack_trees_options opts;
316         struct tree_desc tree_desc;
317
318         memset(&opts, 0, sizeof(opts));
319         opts.head_idx = -1;
320         opts.update = worktree;
321         opts.skip_unmerged = !worktree;
322         opts.reset = 1;
323         opts.merge = 1;
324         opts.fn = oneway_merge;
325         opts.verbose_update = !o->quiet;
326         opts.src_index = &the_index;
327         opts.dst_index = &the_index;
328         parse_tree(tree);
329         init_tree_desc(&tree_desc, tree->buffer, tree->size);
330         switch (unpack_trees(1, &tree_desc, &opts)) {
331         case -2:
332                 o->writeout_error = 1;
333                 /*
334                  * We return 0 nevertheless, as the index is all right
335                  * and more importantly we have made best efforts to
336                  * update paths in the work tree, and we cannot revert
337                  * them.
338                  */
339         case 0:
340                 return 0;
341         default:
342                 return 128;
343         }
344 }
345
346 struct branch_info {
347         const char *name; /* The short name used */
348         const char *path; /* The full name of a real branch */
349         struct commit *commit; /* The named commit */
350 };
351
352 static void setup_branch_path(struct branch_info *branch)
353 {
354         struct strbuf buf = STRBUF_INIT;
355
356         strbuf_branchname(&buf, branch->name);
357         if (strcmp(buf.buf, branch->name))
358                 branch->name = xstrdup(buf.buf);
359         strbuf_splice(&buf, 0, 0, "refs/heads/", 11);
360         branch->path = strbuf_detach(&buf, NULL);
361 }
362
363 static int merge_working_tree(struct checkout_opts *opts,
364                               struct branch_info *old, struct branch_info *new)
365 {
366         int ret;
367         struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
368         int newfd = hold_locked_index(lock_file, 1);
369
370         if (read_cache_preload(NULL) < 0)
371                 return error("corrupt index file");
372
373         if (opts->force) {
374                 ret = reset_tree(new->commit->tree, opts, 1);
375                 if (ret)
376                         return ret;
377         } else {
378                 struct tree_desc trees[2];
379                 struct tree *tree;
380                 struct unpack_trees_options topts;
381
382                 memset(&topts, 0, sizeof(topts));
383                 topts.head_idx = -1;
384                 topts.src_index = &the_index;
385                 topts.dst_index = &the_index;
386
387                 topts.msgs.not_uptodate_file = "You have local changes to '%s'; cannot switch branches.";
388
389                 refresh_cache(REFRESH_QUIET);
390
391                 if (unmerged_cache()) {
392                         error("you need to resolve your current index first");
393                         return 1;
394                 }
395
396                 /* 2-way merge to the new branch */
397                 topts.initial_checkout = is_cache_unborn();
398                 topts.update = 1;
399                 topts.merge = 1;
400                 topts.gently = opts->merge && old->commit;
401                 topts.verbose_update = !opts->quiet;
402                 topts.fn = twoway_merge;
403                 topts.dir = xcalloc(1, sizeof(*topts.dir));
404                 topts.dir->flags |= DIR_SHOW_IGNORED;
405                 topts.dir->exclude_per_dir = ".gitignore";
406                 tree = parse_tree_indirect(old->commit ?
407                                            old->commit->object.sha1 :
408                                            (unsigned char *)EMPTY_TREE_SHA1_BIN);
409                 init_tree_desc(&trees[0], tree->buffer, tree->size);
410                 tree = parse_tree_indirect(new->commit->object.sha1);
411                 init_tree_desc(&trees[1], tree->buffer, tree->size);
412
413                 ret = unpack_trees(2, trees, &topts);
414                 if (ret == -1) {
415                         /*
416                          * Unpack couldn't do a trivial merge; either
417                          * give up or do a real merge, depending on
418                          * whether the merge flag was used.
419                          */
420                         struct tree *result;
421                         struct tree *work;
422                         struct merge_options o;
423                         if (!opts->merge)
424                                 return 1;
425
426                         /*
427                          * Without old->commit, the below is the same as
428                          * the two-tree unpack we already tried and failed.
429                          */
430                         if (!old->commit)
431                                 return 1;
432
433                         /* Do more real merge */
434
435                         /*
436                          * We update the index fully, then write the
437                          * tree from the index, then merge the new
438                          * branch with the current tree, with the old
439                          * branch as the base. Then we reset the index
440                          * (but not the working tree) to the new
441                          * branch, leaving the working tree as the
442                          * merged version, but skipping unmerged
443                          * entries in the index.
444                          */
445
446                         add_files_to_cache(NULL, NULL, 0);
447                         init_merge_options(&o);
448                         o.verbosity = 0;
449                         work = write_tree_from_memory(&o);
450
451                         ret = reset_tree(new->commit->tree, opts, 1);
452                         if (ret)
453                                 return ret;
454                         o.branch1 = new->name;
455                         o.branch2 = "local";
456                         merge_trees(&o, new->commit->tree, work,
457                                 old->commit->tree, &result);
458                         ret = reset_tree(new->commit->tree, opts, 0);
459                         if (ret)
460                                 return ret;
461                 }
462         }
463
464         if (write_cache(newfd, active_cache, active_nr) ||
465             commit_locked_index(lock_file))
466                 die("unable to write new index file");
467
468         if (!opts->force && !opts->quiet)
469                 show_local_changes(&new->commit->object);
470
471         return 0;
472 }
473
474 static void report_tracking(struct branch_info *new)
475 {
476         struct strbuf sb = STRBUF_INIT;
477         struct branch *branch = branch_get(new->name);
478
479         if (!format_tracking_info(branch, &sb))
480                 return;
481         fputs(sb.buf, stdout);
482         strbuf_release(&sb);
483 }
484
485 static void update_refs_for_switch(struct checkout_opts *opts,
486                                    struct branch_info *old,
487                                    struct branch_info *new)
488 {
489         struct strbuf msg = STRBUF_INIT;
490         const char *old_desc;
491         if (opts->new_branch) {
492                 create_branch(old->name, opts->new_branch, new->name, 0,
493                               opts->new_branch_log, opts->track);
494                 new->name = opts->new_branch;
495                 setup_branch_path(new);
496         }
497
498         old_desc = old->name;
499         if (!old_desc && old->commit)
500                 old_desc = sha1_to_hex(old->commit->object.sha1);
501         strbuf_addf(&msg, "checkout: moving from %s to %s",
502                     old_desc ? old_desc : "(invalid)", new->name);
503
504         if (new->path) {
505                 create_symref("HEAD", new->path, msg.buf);
506                 if (!opts->quiet) {
507                         if (old->path && !strcmp(new->path, old->path))
508                                 fprintf(stderr, "Already on '%s'\n",
509                                         new->name);
510                         else
511                                 fprintf(stderr, "Switched to%s branch '%s'\n",
512                                         opts->new_branch ? " a new" : "",
513                                         new->name);
514                 }
515         } else if (strcmp(new->name, "HEAD")) {
516                 update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
517                            REF_NODEREF, DIE_ON_ERR);
518                 if (!opts->quiet) {
519                         if (old->path)
520                                 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);
521                         describe_detached_head("HEAD is now at", new->commit);
522                 }
523         }
524         remove_branch_state();
525         strbuf_release(&msg);
526         if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
527                 report_tracking(new);
528 }
529
530 static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
531 {
532         int ret = 0;
533         struct branch_info old;
534         unsigned char rev[20];
535         int flag;
536         memset(&old, 0, sizeof(old));
537         old.path = resolve_ref("HEAD", rev, 0, &flag);
538         old.commit = lookup_commit_reference_gently(rev, 1);
539         if (!(flag & REF_ISSYMREF))
540                 old.path = NULL;
541
542         if (old.path && !prefixcmp(old.path, "refs/heads/"))
543                 old.name = old.path + strlen("refs/heads/");
544
545         if (!new->name) {
546                 new->name = "HEAD";
547                 new->commit = old.commit;
548                 if (!new->commit)
549                         die("You are on a branch yet to be born");
550                 parse_commit(new->commit);
551         }
552
553         ret = merge_working_tree(opts, &old, new);
554         if (ret)
555                 return ret;
556
557         /*
558          * If we were on a detached HEAD, but have now moved to
559          * a new commit, we want to mention the old commit once more
560          * to remind the user that it might be lost.
561          */
562         if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
563                 describe_detached_head("Previous HEAD position was", old.commit);
564
565         update_refs_for_switch(opts, &old, new);
566
567         ret = post_checkout_hook(old.commit, new->commit, 1);
568         return ret || opts->writeout_error;
569 }
570
571 static int git_checkout_config(const char *var, const char *value, void *cb)
572 {
573         return git_xmerge_config(var, value, cb);
574 }
575
576 static int interactive_checkout(const char *revision, const char **pathspec,
577                                 struct checkout_opts *opts)
578 {
579         return run_add_interactive(revision, "--patch=checkout", pathspec);
580 }
581
582 struct tracking_name_data {
583         const char *name;
584         char *remote;
585         int unique;
586 };
587
588 static int check_tracking_name(const char *refname, const unsigned char *sha1,
589                                int flags, void *cb_data)
590 {
591         struct tracking_name_data *cb = cb_data;
592         const char *slash;
593
594         if (prefixcmp(refname, "refs/remotes/"))
595                 return 0;
596         slash = strchr(refname + 13, '/');
597         if (!slash || strcmp(slash + 1, cb->name))
598                 return 0;
599         if (cb->remote) {
600                 cb->unique = 0;
601                 return 0;
602         }
603         cb->remote = xstrdup(refname);
604         return 0;
605 }
606
607 static const char *unique_tracking_name(const char *name)
608 {
609         struct tracking_name_data cb_data = { name, NULL, 1 };
610         for_each_ref(check_tracking_name, &cb_data);
611         if (cb_data.unique)
612                 return cb_data.remote;
613         free(cb_data.remote);
614         return NULL;
615 }
616
617 int cmd_checkout(int argc, const char **argv, const char *prefix)
618 {
619         struct checkout_opts opts;
620         unsigned char rev[20];
621         const char *arg;
622         struct branch_info new;
623         struct tree *source_tree = NULL;
624         char *conflict_style = NULL;
625         int patch_mode = 0;
626         int dwim_new_local_branch = 1;
627         struct option options[] = {
628                 OPT__QUIET(&opts.quiet),
629                 OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
630                 OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
631                 OPT_SET_INT('t', "track",  &opts.track, "track",
632                         BRANCH_TRACK_EXPLICIT),
633                 OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
634                             2),
635                 OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
636                             3),
637                 OPT_BOOLEAN('f', "force", &opts.force, "force"),
638                 OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
639                 OPT_STRING(0, "conflict", &conflict_style, "style",
640                            "conflict style (merge or diff3)"),
641                 OPT_BOOLEAN('p', "patch", &patch_mode, "select hunks interactively"),
642                 { OPTION_BOOLEAN, 0, "guess", &dwim_new_local_branch, NULL,
643                   "second guess 'git checkout no-such-branch'",
644                   PARSE_OPT_NOARG | PARSE_OPT_HIDDEN },
645                 OPT_END(),
646         };
647         int has_dash_dash;
648
649         memset(&opts, 0, sizeof(opts));
650         memset(&new, 0, sizeof(new));
651
652         git_config(git_checkout_config, NULL);
653
654         opts.track = BRANCH_TRACK_UNSPECIFIED;
655
656         argc = parse_options(argc, argv, prefix, options, checkout_usage,
657                              PARSE_OPT_KEEP_DASHDASH);
658
659         if (patch_mode && (opts.track > 0 || opts.new_branch
660                            || opts.new_branch_log || opts.merge || opts.force))
661                 die ("--patch is incompatible with all other options");
662
663         /* --track without -b should DWIM */
664         if (0 < opts.track && !opts.new_branch) {
665                 const char *argv0 = argv[0];
666                 if (!argc || !strcmp(argv0, "--"))
667                         die ("--track needs a branch name");
668                 if (!prefixcmp(argv0, "refs/"))
669                         argv0 += 5;
670                 if (!prefixcmp(argv0, "remotes/"))
671                         argv0 += 8;
672                 argv0 = strchr(argv0, '/');
673                 if (!argv0 || !argv0[1])
674                         die ("Missing branch name; try -b");
675                 opts.new_branch = argv0 + 1;
676         }
677
678         if (conflict_style) {
679                 opts.merge = 1; /* implied */
680                 git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
681         }
682
683         if (opts.force && opts.merge)
684                 die("git checkout: -f and -m are incompatible");
685
686         /*
687          * case 1: git checkout <ref> -- [<paths>]
688          *
689          *   <ref> must be a valid tree, everything after the '--' must be
690          *   a path.
691          *
692          * case 2: git checkout -- [<paths>]
693          *
694          *   everything after the '--' must be paths.
695          *
696          * case 3: git checkout <something> [<paths>]
697          *
698          *   With no paths, if <something> is a commit, that is to
699          *   switch to the branch or detach HEAD at it.  As a special case,
700          *   if <something> is A...B (missing A or B means HEAD but you can
701          *   omit at most one side), and if there is a unique merge base
702          *   between A and B, A...B names that merge base.
703          *
704          *   With no paths, if <something> is _not_ a commit, no -t nor -b
705          *   was given, and there is a tracking branch whose name is
706          *   <something> in one and only one remote, then this is a short-hand
707          *   to fork local <something> from that remote tracking branch.
708          *
709          *   Otherwise <something> shall not be ambiguous.
710          *   - If it's *only* a reference, treat it like case (1).
711          *   - If it's only a path, treat it like case (2).
712          *   - else: fail.
713          *
714          */
715         if (argc) {
716                 if (!strcmp(argv[0], "--")) {       /* case (2) */
717                         argv++;
718                         argc--;
719                         goto no_reference;
720                 }
721
722                 arg = argv[0];
723                 has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");
724
725                 if (!strcmp(arg, "-"))
726                         arg = "@{-1}";
727
728                 if (get_sha1_mb(arg, rev)) {
729                         if (has_dash_dash)          /* case (1) */
730                                 die("invalid reference: %s", arg);
731                         if (!patch_mode &&
732                             dwim_new_local_branch &&
733                             opts.track == BRANCH_TRACK_UNSPECIFIED &&
734                             !opts.new_branch &&
735                             !check_filename(NULL, arg) &&
736                             argc == 1) {
737                                 const char *remote = unique_tracking_name(arg);
738                                 if (!remote || get_sha1(remote, rev))
739                                         goto no_reference;
740                                 opts.new_branch = arg;
741                                 arg = remote;
742                                 /* DWIMmed to create local branch */
743                         }
744                         else
745                                 goto no_reference;
746                 }
747
748                 /* we can't end up being in (2) anymore, eat the argument */
749                 argv++;
750                 argc--;
751
752                 new.name = arg;
753                 if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
754                         setup_branch_path(&new);
755                         if (resolve_ref(new.path, rev, 1, NULL))
756                                 new.commit = lookup_commit_reference(rev);
757                         else
758                                 new.path = NULL;
759                         parse_commit(new.commit);
760                         source_tree = new.commit->tree;
761                 } else
762                         source_tree = parse_tree_indirect(rev);
763
764                 if (!source_tree)                   /* case (1): want a tree */
765                         die("reference is not a tree: %s", arg);
766                 if (!has_dash_dash) {/* case (3 -> 1) */
767                         /*
768                          * Do not complain the most common case
769                          *      git checkout branch
770                          * even if there happen to be a file called 'branch';
771                          * it would be extremely annoying.
772                          */
773                         if (argc)
774                                 verify_non_filename(NULL, arg);
775                 }
776                 else {
777                         argv++;
778                         argc--;
779                 }
780         }
781
782 no_reference:
783
784         if (opts.track == BRANCH_TRACK_UNSPECIFIED)
785                 opts.track = git_branch_track;
786
787         if (argc) {
788                 const char **pathspec = get_pathspec(prefix, argv);
789
790                 if (!pathspec)
791                         die("invalid path specification");
792
793                 if (patch_mode)
794                         return interactive_checkout(new.name, pathspec, &opts);
795
796                 /* Checkout paths */
797                 if (opts.new_branch) {
798                         if (argc == 1) {
799                                 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]);
800                         } else {
801                                 die("git checkout: updating paths is incompatible with switching branches.");
802                         }
803                 }
804
805                 if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
806                         die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");
807
808                 return checkout_paths(source_tree, pathspec, &opts);
809         }
810
811         if (patch_mode)
812                 return interactive_checkout(new.name, NULL, &opts);
813
814         if (opts.new_branch) {
815                 struct strbuf buf = STRBUF_INIT;
816                 if (strbuf_check_branch_ref(&buf, opts.new_branch))
817                         die("git checkout: we do not like '%s' as a branch name.",
818                             opts.new_branch);
819                 if (!get_sha1(buf.buf, rev))
820                         die("git checkout: branch %s already exists", opts.new_branch);
821                 strbuf_release(&buf);
822         }
823
824         if (new.name && !new.commit) {
825                 die("Cannot switch branch to a non-commit.");
826         }
827         if (opts.writeout_stage)
828                 die("--ours/--theirs is incompatible with switching branches.");
829
830         return switch_branches(&opts, &new);
831 }