summaryrefslogtreecommitdiff
path: root/builtin-checkout.c
blob: 57b94d282931187a0cb6394d4df707237038a7eb (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
#include "cache.h"
#include "builtin.h"
#include "parse-options.h"
#include "refs.h"
#include "commit.h"
#include "tree.h"
#include "tree-walk.h"
#include "unpack-trees.h"
#include "dir.h"
#include "run-command.h"
#include "merge-recursive.h"
#include "branch.h"
#include "diff.h"
#include "revision.h"
#include "remote.h"
#include "blob.h"
#include "xdiff-interface.h"
#include "ll-merge.h"

static const char * const checkout_usage[] = {
	"git checkout [options] <branch>",
	"git checkout [options] [<branch>] -- <file>...",
	NULL,
};

struct checkout_opts {
	int quiet;
	int merge;
	int force;
	int writeout_stage;
	int writeout_error;

	const char *new_branch;
	int new_branch_log;
	enum branch_track track;
};

static int post_checkout_hook(struct commit *old, struct commit *new,
			      int changed)
{
	struct child_process proc;
	const char *name = git_path("hooks/post-checkout");
	const char *argv[5];

	if (access(name, X_OK) < 0)
		return 0;

	memset(&proc, 0, sizeof(proc));
	argv[0] = name;
	argv[1] = xstrdup(sha1_to_hex(old->object.sha1));
	argv[2] = xstrdup(sha1_to_hex(new->object.sha1));
	argv[3] = changed ? "1" : "0";
	argv[4] = NULL;
	proc.argv = argv;
	proc.no_stdin = 1;
	proc.stdout_to_stderr = 1;
	return run_command(&proc);
}

static int update_some(const unsigned char *sha1, const char *base, int baselen,
		const char *pathname, unsigned mode, int stage, void *context)
{
	int len;
	struct cache_entry *ce;

	if (S_ISGITLINK(mode))
		return 0;

	if (S_ISDIR(mode))
		return READ_TREE_RECURSIVE;

	len = baselen + strlen(pathname);
	ce = xcalloc(1, cache_entry_size(len));
	hashcpy(ce->sha1, sha1);
	memcpy(ce->name, base, baselen);
	memcpy(ce->name + baselen, pathname, len - baselen);
	ce->ce_flags = create_ce_flags(len, 0);
	ce->ce_mode = create_ce_mode(mode);
	add_cache_entry(ce, ADD_CACHE_OK_TO_ADD | ADD_CACHE_OK_TO_REPLACE);
	return 0;
}

static int read_tree_some(struct tree *tree, const char **pathspec)
{
	read_tree_recursive(tree, "", 0, 0, pathspec, update_some, NULL);

	/* update the index with the given tree's info
	 * for all args, expanding wildcards, and exit
	 * with any non-zero return code.
	 */
	return 0;
}

static int skip_same_name(struct cache_entry *ce, int pos)
{
	while (++pos < active_nr &&
	       !strcmp(active_cache[pos]->name, ce->name))
		; /* skip */
	return pos;
}

static int check_stage(int stage, struct cache_entry *ce, int pos)
{
	while (pos < active_nr &&
	       !strcmp(active_cache[pos]->name, ce->name)) {
		if (ce_stage(active_cache[pos]) == stage)
			return 0;
		pos++;
	}
	return error("path '%s' does not have %s version",
		     ce->name,
		     (stage == 2) ? "our" : "their");
}

static int check_all_stages(struct cache_entry *ce, int pos)
{
	if (ce_stage(ce) != 1 ||
	    active_nr <= pos + 2 ||
	    strcmp(active_cache[pos+1]->name, ce->name) ||
	    ce_stage(active_cache[pos+1]) != 2 ||
	    strcmp(active_cache[pos+2]->name, ce->name) ||
	    ce_stage(active_cache[pos+2]) != 3)
		return error("path '%s' does not have all three versions",
			     ce->name);
	return 0;
}

static int checkout_stage(int stage, struct cache_entry *ce, int pos,
			  struct checkout *state)
{
	while (pos < active_nr &&
	       !strcmp(active_cache[pos]->name, ce->name)) {
		if (ce_stage(active_cache[pos]) == stage)
			return checkout_entry(active_cache[pos], state, NULL);
		pos++;
	}
	return error("path '%s' does not have %s version",
		     ce->name,
		     (stage == 2) ? "our" : "their");
}

/* NEEDSWORK: share with merge-recursive */
static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
{
	unsigned long size;
	enum object_type type;

	if (!hashcmp(sha1, null_sha1)) {
		mm->ptr = xstrdup("");
		mm->size = 0;
		return;
	}

	mm->ptr = read_sha1_file(sha1, &type, &size);
	if (!mm->ptr || type != OBJ_BLOB)
		die("unable to read blob object %s", sha1_to_hex(sha1));
	mm->size = size;
}

static int checkout_merged(int pos, struct checkout *state)
{
	struct cache_entry *ce = active_cache[pos];
	const char *path = ce->name;
	mmfile_t ancestor, ours, theirs;
	int status;
	unsigned char sha1[20];
	mmbuffer_t result_buf;

	if (ce_stage(ce) != 1 ||
	    active_nr <= pos + 2 ||
	    strcmp(active_cache[pos+1]->name, path) ||
	    ce_stage(active_cache[pos+1]) != 2 ||
	    strcmp(active_cache[pos+2]->name, path) ||
	    ce_stage(active_cache[pos+2]) != 3)
		return error("path '%s' does not have all 3 versions", path);

	fill_mm(active_cache[pos]->sha1, &ancestor);
	fill_mm(active_cache[pos+1]->sha1, &ours);
	fill_mm(active_cache[pos+2]->sha1, &theirs);

	status = ll_merge(&result_buf, path, &ancestor,
			  &ours, "ours", &theirs, "theirs", 1);
	free(ancestor.ptr);
	free(ours.ptr);
	free(theirs.ptr);
	if (status < 0 || !result_buf.ptr) {
		free(result_buf.ptr);
		return error("path '%s': cannot merge", path);
	}

	/*
	 * NEEDSWORK:
	 * There is absolutely no reason to write this as a blob object
	 * and create a phoney cache entry just to leak.  This hack is
	 * primarily to get to the write_entry() machinery that massages
	 * the contents to work-tree format and writes out which only
	 * allows it for a cache entry.  The code in write_entry() needs
	 * to be refactored to allow us to feed a <buffer, size, mode>
	 * instead of a cache entry.  Such a refactoring would help
	 * merge_recursive as well (it also writes the merge result to the
	 * object database even when it may contain conflicts).
	 */
	if (write_sha1_file(result_buf.ptr, result_buf.size,
			    blob_type, sha1))
		die("Unable to add merge result for '%s'", path);
	ce = make_cache_entry(create_ce_mode(active_cache[pos+1]->ce_mode),
			      sha1,
			      path, 2, 0);
	if (!ce)
		die("make_cache_entry failed for path '%s'", path);
	status = checkout_entry(ce, state, NULL);
	return status;
}

static int checkout_paths(struct tree *source_tree, const char **pathspec,
			  struct checkout_opts *opts)
{
	int pos;
	struct checkout state;
	static char *ps_matched;
	unsigned char rev[20];
	int flag;
	struct commit *head;
	int errs = 0;
	int stage = opts->writeout_stage;
	int merge = opts->merge;
	int newfd;
	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));

	newfd = hold_locked_index(lock_file, 1);
	read_cache();

	if (source_tree)
		read_tree_some(source_tree, pathspec);

	for (pos = 0; pathspec[pos]; pos++)
		;
	ps_matched = xcalloc(1, pos);

	for (pos = 0; pos < active_nr; pos++) {
		struct cache_entry *ce = active_cache[pos];
		pathspec_match(pathspec, ps_matched, ce->name, 0);
	}

	if (report_path_error(ps_matched, pathspec, 0))
		return 1;

	/* Any unmerged paths? */
	for (pos = 0; pos < active_nr; pos++) {
		struct cache_entry *ce = active_cache[pos];
		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
			if (!ce_stage(ce))
				continue;
			if (opts->force) {
				warning("path '%s' is unmerged", ce->name);
			} else if (stage) {
				errs |= check_stage(stage, ce, pos);
			} else if (opts->merge) {
				errs |= check_all_stages(ce, pos);
			} else {
				errs = 1;
				error("path '%s' is unmerged", ce->name);
			}
			pos = skip_same_name(ce, pos) - 1;
		}
	}
	if (errs)
		return 1;

	/* Now we are committed to check them out */
	memset(&state, 0, sizeof(state));
	state.force = 1;
	state.refresh_cache = 1;
	for (pos = 0; pos < active_nr; pos++) {
		struct cache_entry *ce = active_cache[pos];
		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
			if (!ce_stage(ce)) {
				errs |= checkout_entry(ce, &state, NULL);
				continue;
			}
			if (stage)
				errs |= checkout_stage(stage, ce, pos, &state);
			else if (merge)
				errs |= checkout_merged(pos, &state);
			pos = skip_same_name(ce, pos) - 1;
		}
	}

	if (write_cache(newfd, active_cache, active_nr) ||
	    commit_locked_index(lock_file))
		die("unable to write new index file");

	resolve_ref("HEAD", rev, 0, &flag);
	head = lookup_commit_reference_gently(rev, 1);

	errs |= post_checkout_hook(head, head, 0);
	return errs;
}

static void show_local_changes(struct object *head)
{
	struct rev_info rev;
	/* I think we want full paths, even if we're in a subdirectory. */
	init_revisions(&rev, NULL);
	rev.abbrev = 0;
	rev.diffopt.output_format |= DIFF_FORMAT_NAME_STATUS;
	add_pending_object(&rev, head, NULL);
	run_diff_index(&rev, 0);
}

static void describe_detached_head(char *msg, struct commit *commit)
{
	struct strbuf sb = STRBUF_INIT;
	parse_commit(commit);
	pretty_print_commit(CMIT_FMT_ONELINE, commit, &sb, 0, NULL, NULL, 0, 0);
	fprintf(stderr, "%s %s... %s\n", msg,
		find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV), sb.buf);
	strbuf_release(&sb);
}

static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
{
	struct unpack_trees_options opts;
	struct tree_desc tree_desc;

	memset(&opts, 0, sizeof(opts));
	opts.head_idx = -1;
	opts.update = worktree;
	opts.skip_unmerged = !worktree;
	opts.reset = 1;
	opts.merge = 1;
	opts.fn = oneway_merge;
	opts.verbose_update = !o->quiet;
	opts.src_index = &the_index;
	opts.dst_index = &the_index;
	parse_tree(tree);
	init_tree_desc(&tree_desc, tree->buffer, tree->size);
	switch (unpack_trees(1, &tree_desc, &opts)) {
	case -2:
		o->writeout_error = 1;
		/*
		 * We return 0 nevertheless, as the index is all right
		 * and more importantly we have made best efforts to
		 * update paths in the work tree, and we cannot revert
		 * them.
		 */
	case 0:
		return 0;
	default:
		return 128;
	}
}

struct branch_info {
	const char *name; /* The short name used */
	const char *path; /* The full name of a real branch */
	struct commit *commit; /* The named commit */
};

static void setup_branch_path(struct branch_info *branch)
{
	struct strbuf buf = STRBUF_INIT;
	strbuf_addstr(&buf, "refs/heads/");
	strbuf_addstr(&buf, branch->name);
	branch->path = strbuf_detach(&buf, NULL);
}

static int merge_working_tree(struct checkout_opts *opts,
			      struct branch_info *old, struct branch_info *new)
{
	int ret;
	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
	int newfd = hold_locked_index(lock_file, 1);
	read_cache();

	if (opts->force) {
		ret = reset_tree(new->commit->tree, opts, 1);
		if (ret)
			return ret;
	} else {
		struct tree_desc trees[2];
		struct tree *tree;
		struct unpack_trees_options topts;

		memset(&topts, 0, sizeof(topts));
		topts.head_idx = -1;
		topts.src_index = &the_index;
		topts.dst_index = &the_index;

		topts.msgs.not_uptodate_file = "You have local changes to '%s'; cannot switch branches.";

		refresh_cache(REFRESH_QUIET);

		if (unmerged_cache()) {
			error("you need to resolve your current index first");
			return 1;
		}

		/* 2-way merge to the new branch */
		topts.initial_checkout = (!active_nr &&
					  (old->commit == new->commit));
		topts.update = 1;
		topts.merge = 1;
		topts.gently = opts->merge;
		topts.verbose_update = !opts->quiet;
		topts.fn = twoway_merge;
		topts.dir = xcalloc(1, sizeof(*topts.dir));
		topts.dir->show_ignored = 1;
		topts.dir->exclude_per_dir = ".gitignore";
		tree = parse_tree_indirect(old->commit->object.sha1);
		init_tree_desc(&trees[0], tree->buffer, tree->size);
		tree = parse_tree_indirect(new->commit->object.sha1);
		init_tree_desc(&trees[1], tree->buffer, tree->size);

		ret = unpack_trees(2, trees, &topts);
		if (ret == -1) {
			/*
			 * Unpack couldn't do a trivial merge; either
			 * give up or do a real merge, depending on
			 * whether the merge flag was used.
			 */
			struct tree *result;
			struct tree *work;
			struct merge_options o;
			if (!opts->merge)
				return 1;
			parse_commit(old->commit);

			/* Do more real merge */

			/*
			 * We update the index fully, then write the
			 * tree from the index, then merge the new
			 * branch with the current tree, with the old
			 * branch as the base. Then we reset the index
			 * (but not the working tree) to the new
			 * branch, leaving the working tree as the
			 * merged version, but skipping unmerged
			 * entries in the index.
			 */

			add_files_to_cache(NULL, NULL, 0);
			init_merge_options(&o);
			o.verbosity = 0;
			work = write_tree_from_memory(&o);

			ret = reset_tree(new->commit->tree, opts, 1);
			if (ret)
				return ret;
			o.branch1 = new->name;
			o.branch2 = "local";
			merge_trees(&o, new->commit->tree, work,
				old->commit->tree, &result);
			ret = reset_tree(new->commit->tree, opts, 0);
			if (ret)
				return ret;
		}
	}

	if (write_cache(newfd, active_cache, active_nr) ||
	    commit_locked_index(lock_file))
		die("unable to write new index file");

	if (!opts->force && !opts->quiet)
		show_local_changes(&new->commit->object);

	return 0;
}

static void report_tracking(struct branch_info *new)
{
	struct strbuf sb = STRBUF_INIT;
	struct branch *branch = branch_get(new->name);

	if (!format_tracking_info(branch, &sb))
		return;
	fputs(sb.buf, stdout);
	strbuf_release(&sb);
}

static void update_refs_for_switch(struct checkout_opts *opts,
				   struct branch_info *old,
				   struct branch_info *new)
{
	struct strbuf msg = STRBUF_INIT;
	const char *old_desc;
	if (opts->new_branch) {
		create_branch(old->name, opts->new_branch, new->name, 0,
			      opts->new_branch_log, opts->track);
		new->name = opts->new_branch;
		setup_branch_path(new);
	}

	old_desc = old->name;
	if (!old_desc)
		old_desc = sha1_to_hex(old->commit->object.sha1);
	strbuf_addf(&msg, "checkout: moving from %s to %s",
		    old_desc, new->name);

	if (new->path) {
		create_symref("HEAD", new->path, msg.buf);
		if (!opts->quiet) {
			if (old->path && !strcmp(new->path, old->path))
				fprintf(stderr, "Already on \"%s\"\n",
					new->name);
			else
				fprintf(stderr, "Switched to%s branch \"%s\"\n",
					opts->new_branch ? " a new" : "",
					new->name);
		}
	} else if (strcmp(new->name, "HEAD")) {
		update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
			   REF_NODEREF, DIE_ON_ERR);
		if (!opts->quiet) {
			if (old->path)
				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);
			describe_detached_head("HEAD is now at", new->commit);
		}
	}
	remove_branch_state();
	strbuf_release(&msg);
	if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
		report_tracking(new);
}

static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
{
	int ret = 0;
	struct branch_info old;
	unsigned char rev[20];
	int flag;
	memset(&old, 0, sizeof(old));
	old.path = resolve_ref("HEAD", rev, 0, &flag);
	old.commit = lookup_commit_reference_gently(rev, 1);
	if (!(flag & REF_ISSYMREF))
		old.path = NULL;

	if (old.path && !prefixcmp(old.path, "refs/heads/"))
		old.name = old.path + strlen("refs/heads/");

	if (!new->name) {
		new->name = "HEAD";
		new->commit = old.commit;
		if (!new->commit)
			die("You are on a branch yet to be born");
		parse_commit(new->commit);
	}

	/*
	 * If we were on a detached HEAD, but we are now moving to
	 * a new commit, we want to mention the old commit once more
	 * to remind the user that it might be lost.
	 */
	if (!opts->quiet && !old.path && new->commit != old.commit)
		describe_detached_head("Previous HEAD position was", old.commit);

	if (!old.commit) {
		if (!opts->quiet) {
			fprintf(stderr, "warning: You appear to be on a branch yet to be born.\n");
			fprintf(stderr, "warning: Forcing checkout of %s.\n", new->name);
		}
		opts->force = 1;
	}

	ret = merge_working_tree(opts, &old, new);
	if (ret)
		return ret;

	update_refs_for_switch(opts, &old, new);

	ret = post_checkout_hook(old.commit, new->commit, 1);
	return ret || opts->writeout_error;
}

static int git_checkout_config(const char *var, const char *value, void *cb)
{
	return git_xmerge_config(var, value, cb);
}

int cmd_checkout(int argc, const char **argv, const char *prefix)
{
	struct checkout_opts opts;
	unsigned char rev[20];
	const char *arg;
	struct branch_info new;
	struct tree *source_tree = NULL;
	char *conflict_style = NULL;
	struct option options[] = {
		OPT__QUIET(&opts.quiet),
		OPT_STRING('b', NULL, &opts.new_branch, "new branch", "branch"),
		OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
		OPT_SET_INT('t', "track",  &opts.track, "track",
			BRANCH_TRACK_EXPLICIT),
		OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
			    2),
		OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
			    3),
		OPT_BOOLEAN('f', NULL, &opts.force, "force"),
		OPT_BOOLEAN('m', "merge", &opts.merge, "merge"),
		OPT_STRING(0, "conflict", &conflict_style, "style",
			   "conflict style (merge or diff3)"),
		OPT_END(),
	};
	int has_dash_dash;

	memset(&opts, 0, sizeof(opts));
	memset(&new, 0, sizeof(new));

	git_config(git_checkout_config, NULL);

	opts.track = BRANCH_TRACK_UNSPECIFIED;

	argc = parse_options(argc, argv, options, checkout_usage,
			     PARSE_OPT_KEEP_DASHDASH);

	/* --track without -b should DWIM */
	if (0 < opts.track && !opts.new_branch) {
		const char *argv0 = argv[0];
		if (!argc || !strcmp(argv0, "--"))
			die ("--track needs a branch name");
		if (!prefixcmp(argv0, "refs/"))
			argv0 += 5;
		if (!prefixcmp(argv0, "remotes/"))
			argv0 += 8;
		argv0 = strchr(argv0, '/');
		if (!argv0 || !argv0[1])
			die ("Missing branch name; try -b");
		opts.new_branch = argv0 + 1;
	}

	if (opts.track == BRANCH_TRACK_UNSPECIFIED)
		opts.track = git_branch_track;
	if (conflict_style) {
		opts.merge = 1; /* implied */
		git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
	}

	if (opts.force && opts.merge)
		die("git checkout: -f and -m are incompatible");

	/*
	 * case 1: git checkout <ref> -- [<paths>]
	 *
	 *   <ref> must be a valid tree, everything after the '--' must be
	 *   a path.
	 *
	 * case 2: git checkout -- [<paths>]
	 *
	 *   everything after the '--' must be paths.
	 *
	 * case 3: git checkout <something> [<paths>]
	 *
	 *   With no paths, if <something> is a commit, that is to
	 *   switch to the branch or detach HEAD at it.
	 *
	 *   Otherwise <something> shall not be ambiguous.
	 *   - If it's *only* a reference, treat it like case (1).
	 *   - If it's only a path, treat it like case (2).
	 *   - else: fail.
	 *
	 */
	if (argc) {
		if (!strcmp(argv[0], "--")) {       /* case (2) */
			argv++;
			argc--;
			goto no_reference;
		}

		arg = argv[0];
		has_dash_dash = (argc > 1) && !strcmp(argv[1], "--");

		if (get_sha1(arg, rev)) {
			if (has_dash_dash)          /* case (1) */
				die("invalid reference: %s", arg);
			goto no_reference;          /* case (3 -> 2) */
		}

		/* we can't end up being in (2) anymore, eat the argument */
		argv++;
		argc--;

		if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
			new.name = arg;
			setup_branch_path(&new);
			if (resolve_ref(new.path, rev, 1, NULL))
				new.commit = lookup_commit_reference(rev);
			else
				new.path = NULL;
			parse_commit(new.commit);
			source_tree = new.commit->tree;
		} else
			source_tree = parse_tree_indirect(rev);

		if (!source_tree)                   /* case (1): want a tree */
			die("reference is not a tree: %s", arg);
		if (!has_dash_dash) {/* case (3 -> 1) */
			/*
			 * Do not complain the most common case
			 *	git checkout branch
			 * even if there happen to be a file called 'branch';
			 * it would be extremely annoying.
			 */
			if (argc)
				verify_non_filename(NULL, arg);
		}
		else {
			argv++;
			argc--;
		}
	}

no_reference:
	if (argc) {
		const char **pathspec = get_pathspec(prefix, argv);

		if (!pathspec)
			die("invalid path specification");

		/* Checkout paths */
		if (opts.new_branch) {
			if (argc == 1) {
				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]);
			} else {
				die("git checkout: updating paths is incompatible with switching branches.");
			}
		}

		if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
			die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");

		return checkout_paths(source_tree, pathspec, &opts);
	}

	if (opts.new_branch) {
		struct strbuf buf = STRBUF_INIT;
		strbuf_addstr(&buf, "refs/heads/");
		strbuf_addstr(&buf, opts.new_branch);
		if (!get_sha1(buf.buf, rev))
			die("git checkout: branch %s already exists", opts.new_branch);
		if (check_ref_format(buf.buf))
			die("git checkout: we do not like '%s' as a branch name.", opts.new_branch);
		strbuf_release(&buf);
	}

	if (new.name && !new.commit) {
		die("Cannot switch branch to a non-commit.");
	}
	if (opts.writeout_stage)
		die("--ours/--theirs is incompatible with switching branches.");

	return switch_branches(&opts, &new);
}