1 /*
2  * Copyright (c) 2018, 2019, 2020 Stefan Sperling <stsp@openbsd.org>
3  *
4  * Permission to use, copy, modify, and distribute this software for any
5  * purpose with or without fee is hereby granted, provided that the above
6  * copyright notice and this permission notice appear in all copies.
7  *
8  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15  */
16 
17 #include <sys/types.h>
18 #include <sys/stat.h>
19 #include <sys/queue.h>
20 
21 #include <limits.h>
22 #include <stdio.h>
23 #include <stdlib.h>
24 #include <string.h>
25 #include <zlib.h>
26 #include <ctype.h>
27 
28 #include "got_compat.h"
29 
30 #include "got_error.h"
31 #include "got_object.h"
32 #include "got_cancel.h"
33 #include "got_commit_graph.h"
34 #include "got_path.h"
35 
36 #include "got_lib_delta.h"
37 #include "got_lib_inflate.h"
38 #include "got_lib_object.h"
39 #include "got_lib_object_idset.h"
40 #include "got_lib_object_qid.h"
41 
42 #ifndef nitems
43 #define nitems(_a)	(sizeof((_a)) / sizeof((_a)[0]))
44 #endif
45 
46 struct got_commit_graph_node {
47 	struct got_object_id id;
48 
49 	/* Used for topological sorting. */
50 	struct got_commit_graph_node *parents[2];
51 	struct got_commit_graph_node **more_parents;
52 	int nparents;
53 	int indegree;
54 
55 	/* Used only during iteration. */
56 	time_t timestamp;
57 	TAILQ_ENTRY(got_commit_graph_node) entry;
58 };
59 
60 TAILQ_HEAD(got_commit_graph_iter_list, got_commit_graph_node);
61 
62 struct got_commit_graph_branch_tip {
63 	struct got_object_id *commit_id;
64 	struct got_commit_object *commit;
65 	struct got_commit_graph_node *new_node;
66 };
67 
68 struct got_commit_graph {
69 	/* The set of all commits we have traversed. */
70 	struct got_object_idset *node_ids;
71 
72 	int flags;
73 #define GOT_COMMIT_GRAPH_FIRST_PARENT_TRAVERSAL		0x01
74 #define GOT_COMMIT_GRAPH_TOPOSORT			0x02
75 
76 	/*
77 	 * A set of object IDs of known parent commits which we have not yet
78 	 * traversed. Each commit ID in this set represents a branch in commit
79 	 * history: Either the first-parent branch of the head node, or another
80 	 * branch corresponding to a traversed merge commit for which we have
81 	 * not traversed a branch point commit yet.
82 	 *
83 	 * Whenever we add a commit with a matching ID to the graph, we remove
84 	 * its corresponding element from this set, and add new elements for
85 	 * each of that commit's parent commits which were not traversed yet.
86 	 *
87 	 * When API users ask us to fetch more commits, we fetch commits from
88 	 * all currently open branches. This allows API users to process
89 	 * commits in linear order even though the history contains branches.
90 	 */
91 	struct got_object_idset *open_branches;
92 
93 	/* Array of branch tips for fetch_commits_from_open_branches(). */
94 	struct got_commit_graph_branch_tip *tips;
95 	int ntips;
96 
97 	/* Path of tree entry of interest to the API user. */
98 	char *path;
99 
100 	/*
101 	 * Nodes which will be passed to the API user next, sorted by
102 	 * commit timestamp. Sorted in topological order only if topological
103 	 * sorting was requested.
104 	 */
105 	struct got_commit_graph_iter_list iter_list;
106 };
107 
108 static const struct got_error *
109 detect_changed_path(int *changed, struct got_commit_object *commit,
110     struct got_object_id *commit_id, const char *path,
111     struct got_repository *repo)
112 {
113 	const struct got_error *err = NULL;
114 	struct got_commit_object *pcommit = NULL;
115 	struct got_tree_object *tree = NULL, *ptree = NULL;
116 	struct got_object_qid *pid;
117 
118 	if (got_path_is_root_dir(path)) {
119 		*changed = 1;
120 		return NULL;
121 	}
122 
123 	*changed = 0;
124 
125 	pid = STAILQ_FIRST(&commit->parent_ids);
126 	if (pid == NULL) {
127 		struct got_object_id *obj_id;
128 		err = got_object_id_by_path(&obj_id, repo, commit, path);
129 		if (err) {
130 			if (err->code == GOT_ERR_NO_TREE_ENTRY)
131 				err = NULL;
132 		} else
133 			*changed = 1; /* The path was created in this commit. */
134 		free(obj_id);
135 		return err;
136 	}
137 
138 	err = got_object_open_as_tree(&tree, repo, commit->tree_id);
139 	if (err)
140 		return err;
141 
142 	err = got_object_open_as_commit(&pcommit, repo, &pid->id);
143 	if (err)
144 		goto done;
145 
146 	err = got_object_open_as_tree(&ptree, repo, pcommit->tree_id);
147 	if (err)
148 		goto done;
149 
150 	err = got_object_tree_path_changed(changed, tree, ptree, path, repo);
151 done:
152 	if (tree)
153 		got_object_tree_close(tree);
154 	if (ptree)
155 		got_object_tree_close(ptree);
156 	if (pcommit)
157 		got_object_commit_close(pcommit);
158 	return err;
159 }
160 
161 static void
162 add_node_to_iter_list(struct got_commit_graph *graph,
163     struct got_commit_graph_node *node, time_t committer_time)
164 {
165 	struct got_commit_graph_node *n, *next;
166 
167 	node->timestamp = committer_time;
168 
169 	n = TAILQ_FIRST(&graph->iter_list);
170 	while (n) {
171 		next = TAILQ_NEXT(n, entry);
172 		if (next && node->timestamp >= next->timestamp) {
173 			TAILQ_INSERT_BEFORE(next, node, entry);
174 			return;
175 		}
176 		n = next;
177 	}
178 	TAILQ_INSERT_TAIL(&graph->iter_list, node, entry);
179 }
180 
181 static const struct got_error *
182 add_node(struct got_commit_graph_node **new_node,
183     struct got_commit_graph *graph, struct got_object_id *commit_id,
184     struct got_repository *repo)
185 {
186 	const struct got_error *err = NULL;
187 	struct got_commit_graph_node *node;
188 
189 	*new_node = NULL;
190 
191 	node = calloc(1, sizeof(*node));
192 	if (node == NULL)
193 		return got_error_from_errno("calloc");
194 
195 	memcpy(&node->id, commit_id, sizeof(node->id));
196 	node->nparents = -1;
197 	err = got_object_idset_add(graph->node_ids, &node->id, node);
198 	if (err)
199 		free(node);
200 	else
201 		*new_node = node;
202 	return err;
203 }
204 
205 /*
206  * Ask got-read-pack to traverse first-parent history until a commit is
207  * encountered which modified graph->path, or until the pack file runs
208  * out of relevant commits. This is faster than sending an individual
209  * request for each commit stored in the pack file.
210  */
211 static const struct got_error *
212 packed_first_parent_traversal(int *ncommits_traversed,
213     struct got_commit_graph *graph, struct got_object_id *commit_id,
214     struct got_repository *repo)
215 {
216 	const struct got_error *err = NULL;
217 	struct got_object_id_queue traversed_commits;
218 	struct got_object_qid *qid;
219 
220 	STAILQ_INIT(&traversed_commits);
221 	*ncommits_traversed = 0;
222 
223 	err = got_traverse_packed_commits(&traversed_commits,
224 	    commit_id, graph->path, repo);
225 	if (err)
226 		return err;
227 
228 	/* Add all traversed commits to the graph... */
229 	STAILQ_FOREACH(qid, &traversed_commits, entry) {
230 		if (got_object_idset_contains(graph->open_branches, &qid->id))
231 			continue;
232 		if (got_object_idset_contains(graph->node_ids, &qid->id))
233 			continue;
234 
235 		(*ncommits_traversed)++;
236 
237 		/* ... except the last commit is the new branch tip. */
238 		if (STAILQ_NEXT(qid, entry) == NULL) {
239 			err = got_object_idset_add(graph->open_branches,
240 			    &qid->id, NULL);
241 			break;
242 		}
243 
244 		err = got_object_idset_add(graph->node_ids, &qid->id, NULL);
245 		if (err)
246 			break;
247 	}
248 
249 	got_object_id_queue_free(&traversed_commits);
250 	return err;
251 }
252 
253 static const struct got_error *
254 close_branch(struct got_commit_graph *graph, struct got_object_id *commit_id)
255 {
256 	const struct got_error *err;
257 
258 	err = got_object_idset_remove(NULL, graph->open_branches, commit_id);
259 	if (err && err->code != GOT_ERR_NO_OBJ)
260 		return err;
261 	return NULL;
262 }
263 
264 static const struct got_error *
265 advance_branch(struct got_commit_graph *graph, struct got_object_id *commit_id,
266     struct got_commit_object *commit, struct got_repository *repo)
267 {
268 	const struct got_error *err;
269 	struct got_object_qid *qid;
270 	struct got_object_id *merged_id = NULL;
271 
272 	err = close_branch(graph, commit_id);
273 	if (err)
274 		return err;
275 
276 	if (graph->flags & GOT_COMMIT_GRAPH_FIRST_PARENT_TRAVERSAL) {
277 		qid = STAILQ_FIRST(&commit->parent_ids);
278 		if (qid == NULL ||
279 		    got_object_idset_contains(graph->open_branches, &qid->id))
280 			return NULL;
281 		/*
282 		 * The root directory always changes by definition, and when
283 		 * logging the root we want to traverse consecutive commits
284 		 * even if they point at the same tree.
285 		 * But if we are looking for a specific path then we can avoid
286 		 * fetching packed commits which did not modify the path and
287 		 * only fetch their IDs. This speeds up 'got blame'.
288 		 */
289 		if (!got_path_is_root_dir(graph->path) &&
290 		    (commit->flags & GOT_COMMIT_FLAG_PACKED)) {
291 			int ncommits = 0;
292 			err = packed_first_parent_traversal(&ncommits,
293 			    graph, &qid->id, repo);
294 			if (err || ncommits > 0)
295 				return err;
296 		}
297 		return got_object_idset_add(graph->open_branches,
298 		    &qid->id, NULL);
299 	}
300 
301 	/*
302 	 * If we are graphing commits for a specific path, skip branches
303 	 * which do not contribute any content to this path.
304 	 */
305 	if (commit->nparents > 1 && !got_path_is_root_dir(graph->path)) {
306 		err = got_object_id_by_path(&merged_id, repo, commit, graph->path);
307 		if (err && err->code != GOT_ERR_NO_TREE_ENTRY)
308 			return err;
309 		/* The requested path does not exist in this merge commit. */
310 	}
311 	if (commit->nparents > 1 && !got_path_is_root_dir(graph->path) &&
312 	    merged_id != NULL) {
313 		struct got_object_id *prev_id = NULL;
314 		int branches_differ = 0;
315 
316 
317 		STAILQ_FOREACH(qid, &commit->parent_ids, entry) {
318 			struct got_object_id *id = NULL;
319 			struct got_commit_object *pcommit = NULL;
320 
321 			if (got_object_idset_contains(graph->open_branches,
322 			    &qid->id))
323 				continue;
324 
325 			err = got_object_open_as_commit(&pcommit, repo,
326 			    &qid->id);
327 			if (err) {
328 				free(merged_id);
329 				free(prev_id);
330 				return err;
331 			}
332 			err = got_object_id_by_path(&id, repo, pcommit,
333 			    graph->path);
334 			got_object_commit_close(pcommit);
335 			pcommit = NULL;
336 			if (err) {
337 				if (err->code == GOT_ERR_NO_TREE_ENTRY) {
338 					branches_differ = 1;
339 					continue;
340 				}
341 				free(merged_id);
342 				free(prev_id);
343 				return err;
344 			}
345 
346 			if (prev_id) {
347 				if (!branches_differ &&
348 				    got_object_id_cmp(id, prev_id) != 0)
349 					branches_differ = 1;
350 				free(prev_id);
351 			}
352 			prev_id = id;
353 
354 			/*
355 			 * If a branch has created the merged content we can
356 			 * skip any other branches.
357 			 */
358 			if (got_object_id_cmp(merged_id, id) == 0) {
359 				err = got_object_idset_add(graph->open_branches,
360 				    &qid->id, NULL);
361 				free(merged_id);
362 				free(id);
363 				return err;
364 			}
365 		}
366 
367 		free(prev_id);
368 		prev_id = NULL;
369 		free(merged_id);
370 		merged_id = NULL;
371 
372 		/*
373 		 * If the path's content is the same on all branches,
374 		 * follow the first parent only.
375 		 */
376 		if (!branches_differ) {
377 			qid = STAILQ_FIRST(&commit->parent_ids);
378 			if (qid == NULL)
379 				return NULL;
380 			if (got_object_idset_contains(graph->open_branches,
381 			    &qid->id))
382 				return NULL;
383 			if (got_object_idset_contains(graph->node_ids,
384 			    &qid->id))
385 				return NULL; /* parent already traversed */
386 			return got_object_idset_add(graph->open_branches,
387 			    &qid->id, NULL);
388 		}
389 	}
390 
391 	STAILQ_FOREACH(qid, &commit->parent_ids, entry) {
392 		if (got_object_idset_contains(graph->open_branches, &qid->id))
393 			continue;
394 		if (got_object_idset_contains(graph->node_ids, &qid->id))
395 			continue; /* parent already traversed */
396 		err = got_object_idset_add(graph->open_branches, &qid->id,
397 		    NULL);
398 		if (err)
399 			return err;
400 	}
401 
402 	return NULL;
403 }
404 
405 const struct got_error *
406 got_commit_graph_open(struct got_commit_graph **graph,
407     const char *path, int first_parent_traversal)
408 {
409 	const struct got_error *err = NULL;
410 
411 	*graph = calloc(1, sizeof(**graph));
412 	if (*graph == NULL)
413 		return got_error_from_errno("calloc");
414 
415 	TAILQ_INIT(&(*graph)->iter_list);
416 
417 	(*graph)->path = strdup(path);
418 	if ((*graph)->path == NULL) {
419 		err = got_error_from_errno("strdup");
420 		goto done;
421 	}
422 
423 	(*graph)->node_ids = got_object_idset_alloc();
424 	if ((*graph)->node_ids == NULL) {
425 		err = got_error_from_errno("got_object_idset_alloc");
426 		goto done;
427 	}
428 
429 	(*graph)->open_branches = got_object_idset_alloc();
430 	if ((*graph)->open_branches == NULL) {
431 		err = got_error_from_errno("got_object_idset_alloc");
432 		goto done;
433 	}
434 
435 	if (first_parent_traversal)
436 		(*graph)->flags |= GOT_COMMIT_GRAPH_FIRST_PARENT_TRAVERSAL;
437 done:
438 	if (err) {
439 		got_commit_graph_close(*graph);
440 		*graph = NULL;
441 	}
442 	return err;
443 }
444 
445 struct add_branch_tip_arg {
446 	struct got_commit_graph_branch_tip *tips;
447 	int ntips;
448 	struct got_repository *repo;
449 	struct got_commit_graph *graph;
450 };
451 
452 static const struct got_error *
453 add_branch_tip(struct got_object_id *commit_id, void *data, void *arg)
454 {
455 	const struct got_error *err;
456 	struct add_branch_tip_arg *a = arg;
457 	struct got_commit_graph_node *new_node;
458 	struct got_commit_object *commit;
459 
460 	err = got_object_open_as_commit(&commit, a->repo, commit_id);
461 	if (err)
462 		return err;
463 
464 	err = add_node(&new_node, a->graph, commit_id, a->repo);
465 	if (err) {
466 		got_object_commit_close(commit);
467 		return err;
468 	}
469 
470 	a->tips[a->ntips].commit_id = &new_node->id;
471 	a->tips[a->ntips].commit = commit;
472 	a->tips[a->ntips].new_node = new_node;
473 	a->ntips++;
474 
475 	return NULL;
476 }
477 
478 static const struct got_error *
479 fetch_commits_from_open_branches(struct got_commit_graph *graph,
480     struct got_repository *repo, got_cancel_cb cancel_cb, void *cancel_arg)
481 {
482 	const struct got_error *err;
483 	struct add_branch_tip_arg arg;
484 	int i, ntips;
485 
486 	ntips = got_object_idset_num_elements(graph->open_branches);
487 	if (ntips == 0)
488 		return NULL;
489 
490 	/* (Re-)allocate branch tips array if necessary. */
491 	if (graph->ntips < ntips) {
492 		struct got_commit_graph_branch_tip *tips;
493 		tips = recallocarray(graph->tips, graph->ntips, ntips,
494 		    sizeof(*tips));
495 		if (tips == NULL)
496 			return got_error_from_errno("recallocarray");
497 		graph->tips = tips;
498 		graph->ntips = ntips;
499 	}
500 	arg.tips = graph->tips;
501 	arg.ntips = 0; /* add_branch_tip() will increment */
502 	arg.repo = repo;
503 	arg.graph = graph;
504 	err = got_object_idset_for_each(graph->open_branches, add_branch_tip,
505 	    &arg);
506 	if (err)
507 		goto done;
508 
509 	for (i = 0; i < arg.ntips; i++) {
510 		struct got_object_id *commit_id;
511 		struct got_commit_object *commit;
512 		struct got_commit_graph_node *new_node;
513 		int changed;
514 
515 		if (cancel_cb) {
516 			err = (*cancel_cb)(cancel_arg);
517 			if (err)
518 				break;
519 		}
520 
521 		commit_id = arg.tips[i].commit_id;
522 		commit = arg.tips[i].commit;
523 		new_node = arg.tips[i].new_node;
524 
525 		err = detect_changed_path(&changed, commit, commit_id,
526 		    graph->path, repo);
527 		if (err) {
528 			if (err->code != GOT_ERR_NO_OBJ)
529 				break;
530 			/*
531 			 * History of the path stops here on the current
532 			 * branch. Keep going on other branches.
533 			 */
534 			err = close_branch(graph, commit_id);
535 			if (err)
536 				break;
537 			continue;
538 		}
539 		if (changed) {
540 			add_node_to_iter_list(graph, new_node,
541 			    got_object_commit_get_committer_time(commit));
542 			arg.tips[i].new_node = NULL;
543 		}
544 		err = advance_branch(graph, commit_id, commit, repo);
545 		if (err)
546 			break;
547 	}
548 done:
549 	for (i = 0; i < arg.ntips; i++) {
550 		got_object_commit_close(arg.tips[i].commit);
551 		free(arg.tips[i].new_node);
552 	}
553 	return err;
554 }
555 
556 void
557 got_commit_graph_close(struct got_commit_graph *graph)
558 {
559 	struct got_commit_graph_node *node;
560 
561 	while ((node = TAILQ_FIRST(&graph->iter_list))) {
562 		TAILQ_REMOVE(&graph->iter_list, node, entry);
563 		free(node->more_parents);
564 		free(node);
565 	}
566 
567 	if (graph->open_branches)
568 		got_object_idset_free(graph->open_branches);
569 	if (graph->node_ids)
570 		got_object_idset_free(graph->node_ids);
571 	free(graph->tips);
572 	free(graph->path);
573 	free(graph);
574 }
575 
576 static const struct got_error *
577 remove_branch_tip(struct got_object_id *commit_id, void *data, void *arg)
578 {
579 	struct got_object_idset *open_branches = arg;
580 
581 	return got_object_idset_remove(NULL, open_branches, commit_id);
582 }
583 
584 const struct got_error *
585 got_commit_graph_bfsort(struct got_commit_graph *graph,
586     struct got_object_id *id, struct got_repository *repo,
587     got_cancel_cb cancel_cb, void *cancel_arg)
588 {
589 	const struct got_error *err = NULL;
590 	struct got_commit_graph_node *node;
591 
592 	graph->flags &= ~GOT_COMMIT_GRAPH_TOPOSORT;
593 
594 	/* Clear left-over state from previous iteration attempts. */
595 	while ((node = TAILQ_FIRST(&graph->iter_list)))
596 		TAILQ_REMOVE(&graph->iter_list, node, entry);
597 	err = got_object_idset_for_each(graph->open_branches,
598 	    remove_branch_tip, graph->open_branches);
599 	if (err)
600 		return err;
601 
602 	err = got_object_idset_add(graph->open_branches, id, NULL);
603 	if (err)
604 		return err;
605 
606 	/* Locate first commit which changed graph->path. */
607 	while (TAILQ_EMPTY(&graph->iter_list) &&
608 	    got_object_idset_num_elements(graph->open_branches) > 0) {
609 		err = fetch_commits_from_open_branches(graph, repo,
610 		    cancel_cb, cancel_arg);
611 		if (err)
612 			return err;
613 	}
614 
615 	if (TAILQ_EMPTY(&graph->iter_list)) {
616 		const char *path;
617 		if (got_path_is_root_dir(graph->path))
618 			return got_error_no_obj(id);
619 		path = graph->path;
620 		while (path[0] == '/')
621 			path++;
622 		return got_error_path(path, GOT_ERR_NO_TREE_ENTRY);
623 	}
624 
625 	return NULL;
626 }
627 
628 const struct got_error *
629 got_commit_graph_iter_next(struct got_object_id *id,
630     struct got_commit_graph *graph, struct got_repository *repo,
631     got_cancel_cb cancel_cb, void *cancel_arg)
632 {
633 	const struct got_error *err = NULL;
634 	struct got_commit_graph_node *node, *pnode;
635 	int i;
636 
637 	node = TAILQ_FIRST(&graph->iter_list);
638 	if (node == NULL) {
639 		/* We are done iterating, or iteration was not started. */
640 		return got_error(GOT_ERR_ITER_COMPLETED);
641 	}
642 
643 	if (graph->flags & GOT_COMMIT_GRAPH_TOPOSORT) {
644 		/* At least one node with in-degree zero must exist. */
645 		while (node->indegree != 0)
646 			node = TAILQ_NEXT(node, entry);
647 	} else {
648 		while (TAILQ_NEXT(node, entry) == NULL &&
649 		    got_object_idset_num_elements(graph->open_branches) > 0) {
650 			err = fetch_commits_from_open_branches(graph, repo,
651 			    cancel_cb, cancel_arg);
652 			if (err)
653 				return err;
654 		}
655 	}
656 
657 	memcpy(id, &node->id, sizeof(*id));
658 
659 	TAILQ_REMOVE(&graph->iter_list, node, entry);
660 	if (graph->flags & GOT_COMMIT_GRAPH_TOPOSORT) {
661 		/* When visiting a commit decrement in-degree of all parents. */
662 		for (i = 0; i < node->nparents; i++) {
663 			if (i < nitems(node->parents))
664 				pnode = node->parents[i];
665 			else
666 				pnode = node->more_parents[i];
667 			pnode->indegree--;
668 		}
669 	}
670 	free(node);
671 	return NULL;
672 }
673 
674 static const struct got_error *
675 find_yca_add_id(struct got_object_id **yca_id, struct got_commit_graph *graph,
676     struct got_object_idset *commit_ids, struct got_repository *repo,
677     got_cancel_cb cancel_cb, void *cancel_arg)
678 {
679 	const struct got_error *err = NULL;
680 	struct got_object_id id;
681 
682 	err = got_commit_graph_iter_next(&id, graph, repo, cancel_cb,
683 	    cancel_arg);
684 	if (err)
685 		return err;
686 
687 	if (got_object_idset_contains(commit_ids, &id)) {
688 		*yca_id = got_object_id_dup(&id);
689 		if (*yca_id == NULL)
690 			err = got_error_from_errno("got_object_id_dup");
691 		return err;
692 	}
693 
694 	return got_object_idset_add(commit_ids, &id, NULL);
695 }
696 
697 /*
698  * Sets *yca_id to the youngest common ancestor of commit_id and
699  * commit_id2. Returns got_error(GOT_ERR_ANCESTRY) if they have no
700  * common ancestors.
701  *
702  * If first_parent_traversal is nonzero, only linear history is considered.
703  * If toposort is set then sort commits in topological order before
704  * traversing them.
705  */
706 const struct got_error *
707 got_commit_graph_find_youngest_common_ancestor(struct got_object_id **yca_id,
708     struct got_object_id *commit_id, struct got_object_id *commit_id2,
709     int first_parent_traversal, int toposort, struct got_repository *repo,
710     got_cancel_cb cancel_cb, void *cancel_arg)
711 {
712 	const struct got_error *err = NULL;
713 	struct got_commit_graph *graph = NULL, *graph2 = NULL;
714 	int completed = 0, completed2 = 0;
715 	struct got_object_idset *commit_ids;
716 
717 	*yca_id = NULL;
718 
719 	commit_ids = got_object_idset_alloc();
720 	if (commit_ids == NULL)
721 		return got_error_from_errno("got_object_idset_alloc");
722 
723 	err = got_commit_graph_open(&graph, "/", first_parent_traversal);
724 	if (err)
725 		goto done;
726 
727 	err = got_commit_graph_open(&graph2, "/", first_parent_traversal);
728 	if (err)
729 		goto done;
730 
731 	if (toposort) {
732 		err = got_commit_graph_toposort(graph, commit_id, repo,
733 		    cancel_cb, cancel_arg);
734 		if (err)
735 			goto done;
736 
737 		err = got_commit_graph_toposort(graph2, commit_id2, repo,
738 		    cancel_cb, cancel_arg);
739 		if (err)
740 			goto done;
741 	} else {
742 		err = got_commit_graph_bfsort(graph, commit_id, repo,
743 		    cancel_cb, cancel_arg);
744 		if (err)
745 			goto done;
746 
747 		err = got_commit_graph_bfsort(graph2, commit_id2, repo,
748 		    cancel_cb, cancel_arg);
749 		if (err)
750 			goto done;
751 	}
752 
753 	for (;;) {
754 		if (cancel_cb) {
755 			err = (*cancel_cb)(cancel_arg);
756 			if (err)
757 				break;
758 		}
759 
760 		if (!completed) {
761 			err = find_yca_add_id(yca_id, graph, commit_ids, repo,
762 			    cancel_cb, cancel_arg);
763 			if (err) {
764 				if (err->code != GOT_ERR_ITER_COMPLETED)
765 					break;
766 				err = NULL;
767 				completed = 1;
768 			}
769 			if (*yca_id)
770 				break;
771 		}
772 
773 		if (!completed2) {
774 			err = find_yca_add_id(yca_id, graph2, commit_ids, repo,
775 			    cancel_cb, cancel_arg);
776 			if (err) {
777 				if (err->code != GOT_ERR_ITER_COMPLETED)
778 					break;
779 				err = NULL;
780 				completed2 = 1;
781 			}
782 			if (*yca_id)
783 				break;
784 		}
785 
786 		if (completed && completed2) {
787 			err = got_error(GOT_ERR_ANCESTRY);
788 			break;
789 		}
790 	}
791 done:
792 	got_object_idset_free(commit_ids);
793 	if (graph)
794 		got_commit_graph_close(graph);
795 	if (graph2)
796 		got_commit_graph_close(graph2);
797 	return err;
798 }
799 
800 /*
801  * Sort the graph for traversal in topological order.
802  *
803  * This implementation is based on the description of topological sorting
804  * of git commits by Derrick Stolee at
805  * https://github.blog/2022-08-30-gits-database-internals-ii-commit-history-queries/#topological-sorting
806  * which reads as follows:
807  *
808  * The basic algorithm for topological sorting is Kahn’s algorithm which
809  * follows two big steps:
810  * 1. Walk all reachable commits, counting the number of times a commit appears
811  * as a parent of another commit. Call these numbers the in-degree of the
812  * commit, referencing the number of incoming edges.
813  * 2. Walk the reachable commits, but only visit a commit if its in-degree
814  * value is zero. When visiting a commit, decrement the in-degree value of
815  * each parent.
816  * 
817  * This algorithm works because at least one of our starting points will
818  * have in-degree zero, and then decrementing the in-degree value is similar
819  * to deleting the commit from the graph, always having at least one commit
820  * with in-degree zero.
821  */
822 const struct got_error *
823 got_commit_graph_toposort(struct got_commit_graph *graph,
824     struct got_object_id *id, struct got_repository *repo,
825     got_cancel_cb cancel_cb, void *cancel_arg)
826 {
827 	const struct got_error *err = NULL;
828 	struct got_commit_graph_node *node = NULL, *pnode = NULL;
829 	struct got_commit_object *commit = NULL;
830 	struct got_object_id_queue commits;
831 	const struct got_object_id_queue *parent_ids;
832 	struct got_object_qid *qid = NULL, *pid;
833 	int i;
834 
835 	STAILQ_INIT(&commits);
836 
837 	if (graph->flags & GOT_COMMIT_GRAPH_FIRST_PARENT_TRAVERSAL)
838 		return got_commit_graph_bfsort(graph, id, repo,
839 		    cancel_cb, cancel_arg);
840 
841 	/* Clear left-over state from previous iteration attempts. */
842 	while ((node = TAILQ_FIRST(&graph->iter_list)))
843 		TAILQ_REMOVE(&graph->iter_list, node, entry);
844 	err = got_object_idset_for_each(graph->open_branches,
845 	    remove_branch_tip, graph->open_branches);
846 	if (err)
847 		return err;
848 
849 	graph->flags |= GOT_COMMIT_GRAPH_TOPOSORT;
850 
851 	/*
852 	 * Sorting the commit graph in topological order requires visiting
853 	 * every reachable commit. This is very expensive but there are
854 	 * ways to speed this up significantly in the future:
855 	 * 1) Run this loop in got-read-pack if possible.
856 	 * 2) Use Git's commit-graph file to compute the result incrementally.
857 	 *    See the blog post linked above for details.
858 	 */
859 	err = got_object_qid_alloc_partial(&qid);
860 	if (err)
861 		return err;
862 	memcpy(&qid->id, id, sizeof(qid->id));
863 	STAILQ_INSERT_TAIL(&commits, qid, entry);
864 	while (!STAILQ_EMPTY(&commits)) {
865 		if (cancel_cb) {
866 			err = (*cancel_cb)(cancel_arg);
867 			if (err)
868 				break;
869 		}
870 
871 		qid = STAILQ_FIRST(&commits);
872 		STAILQ_REMOVE_HEAD(&commits, entry);
873 		err = got_object_open_as_commit(&commit, repo, &qid->id);
874 		if (err)
875 			break;
876 
877 		node = got_object_idset_get(graph->node_ids, &qid->id);
878 		if (node == NULL) {
879 			err = add_node(&node, graph, id, repo);
880 			if (err)
881 				break;
882 			TAILQ_INSERT_TAIL(&graph->iter_list, node, entry);
883 		}
884 
885 		got_object_qid_free(qid);
886 		qid = NULL;
887 
888 		if (node->timestamp != 0) /* already traversed once */
889 			continue;
890 
891 		if (node->nparents == -1) {
892 			node->nparents = got_object_commit_get_nparents(commit);
893 			if (node->nparents > nitems(node->parents)) {
894 				node->more_parents = calloc(node->nparents,
895 				    sizeof(*node->more_parents));
896 				if (node->more_parents == NULL) {
897 					err = got_error_from_errno("calloc");
898 					break;
899 				}
900 			}
901 
902 		}
903 
904 		node->timestamp = got_object_commit_get_committer_time(commit);
905 		parent_ids = got_object_commit_get_parent_ids(commit);
906 		i = 0;
907 		STAILQ_FOREACH(pid, parent_ids, entry) {
908 			if (cancel_cb) {
909 				err = (*cancel_cb)(cancel_arg);
910 				if (err)
911 					goto done;
912 			}
913 
914 			/*
915 			 * Increment the in-degree counter every time a given
916 			 * commit appears as the parent of another commit.
917 			 */
918 			pnode = got_object_idset_get(graph->node_ids, &pid->id);
919 			if (pnode == NULL) {
920 				err = add_node(&pnode, graph, &pid->id, repo);
921 				if (err)
922 					goto done;
923 				TAILQ_INSERT_TAIL(&graph->iter_list, pnode,
924 				    entry);
925 			}
926 			pnode->indegree++;
927 
928 			/*
929 			 * Cache parent pointers on the node to make future
930 			 * in-degree updates easier.
931 			 */
932 			if (node->nparents <= nitems(node->parents)) {
933 				node->parents[i] = pnode;
934 			} else {
935 				node->more_parents[i] = pnode;
936 				if (i < nitems(node->parents))
937 					node->parents[i] = pnode;
938 			}
939 			i++;
940 
941 			/* Keep traversing through all parent commits. */
942 			err = got_object_qid_alloc_partial(&qid);
943 			if (err)
944 				goto done;
945 			memcpy(&qid->id, &pid->id, sizeof(qid->id));
946 			STAILQ_INSERT_TAIL(&commits, qid, entry);
947 			qid = NULL;
948 		}
949 
950 		got_object_commit_close(commit);
951 		commit = NULL;
952 	}
953 done:
954 	if (commit)
955 		got_object_commit_close(commit);
956 	got_object_qid_free(qid);
957 	got_object_id_queue_free(&commits);
958 	return err;
959 }