t: add test functions to translate hash-related values

[git.git] / commit.c

#include "cache.h"
#include "tag.h"
#include "commit.h"
#include "commit-graph.h"
#include "repository.h"
#include "object-store.h"
#include "pkt-line.h"
#include "utf8.h"
#include "diff.h"
#include "revision.h"
#include "notes.h"
#include "alloc.h"
#include "gpg-interface.h"
#include "mergesort.h"
#include "commit-slab.h"
#include "prio-queue.h"
#include "sha1-lookup.h"
#include "wt-status.h"
#include "advice.h"

static struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);

int save_commit_buffer = 1;

const char *commit_type = "commit";

struct commit *lookup_commit_reference_gently(struct repository *r,
                const struct object_id *oid, int quiet)
{
        struct object *obj = deref_tag(r,
                                       parse_object(r, oid),
                                       NULL, 0);

        if (!obj)
                return NULL;
        return object_as_type(r, obj, OBJ_COMMIT, quiet);
}

struct commit *lookup_commit_reference(struct repository *r, const struct object_id *oid)
{
        return lookup_commit_reference_gently(r, oid, 0);
}

struct commit *lookup_commit_or_die(const struct object_id *oid, const char *ref_name)
{
        struct commit *c = lookup_commit_reference(the_repository, oid);
        if (!c)
                die(_("could not parse %s"), ref_name);
        if (oidcmp(oid, &c->object.oid)) {
                warning(_("%s %s is not a commit!"),
                        ref_name, oid_to_hex(oid));
        }
        return c;
}

struct commit *lookup_commit(struct repository *r, const struct object_id *oid)
{
        struct object *obj = lookup_object(r, oid->hash);
        if (!obj)
                return create_object(r, oid->hash,
                                     alloc_commit_node(r));
        return object_as_type(r, obj, OBJ_COMMIT, 0);
}

struct commit *lookup_commit_reference_by_name(const char *name)
{
        struct object_id oid;
        struct commit *commit;

        if (get_oid_committish(name, &oid))
                return NULL;
        commit = lookup_commit_reference(the_repository, &oid);
        if (parse_commit(commit))
                return NULL;
        return commit;
}

static timestamp_t parse_commit_date(const char *buf, const char *tail)
{
        const char *dateptr;

        if (buf + 6 >= tail)
                return 0;
        if (memcmp(buf, "author", 6))
                return 0;
        while (buf < tail && *buf++ != '\n')
                /* nada */;
        if (buf + 9 >= tail)
                return 0;
        if (memcmp(buf, "committer", 9))
                return 0;
        while (buf < tail && *buf++ != '>')
                /* nada */;
        if (buf >= tail)
                return 0;
        dateptr = buf;
        while (buf < tail && *buf++ != '\n')
                /* nada */;
        if (buf >= tail)
                return 0;
        /* dateptr < buf && buf[-1] == '\n', so parsing will stop at buf-1 */
        return parse_timestamp(dateptr, NULL, 10);
}

static const unsigned char *commit_graft_sha1_access(size_t index, void *table)
{
        struct commit_graft **commit_graft_table = table;
        return commit_graft_table[index]->oid.hash;
}

static int commit_graft_pos(struct repository *r, const unsigned char *sha1)
{
        return sha1_pos(sha1, r->parsed_objects->grafts,
                        r->parsed_objects->grafts_nr,
                        commit_graft_sha1_access);
}

int register_commit_graft(struct repository *r, struct commit_graft *graft,
                          int ignore_dups)
{
        int pos = commit_graft_pos(r, graft->oid.hash);

        if (0 <= pos) {
                if (ignore_dups)
                        free(graft);
                else {
                        free(r->parsed_objects->grafts[pos]);
                        r->parsed_objects->grafts[pos] = graft;
                }
                return 1;
        }
        pos = -pos - 1;
        ALLOC_GROW(r->parsed_objects->grafts,
                   r->parsed_objects->grafts_nr + 1,
                   r->parsed_objects->grafts_alloc);
        r->parsed_objects->grafts_nr++;
        if (pos < r->parsed_objects->grafts_nr)
                memmove(r->parsed_objects->grafts + pos + 1,
                        r->parsed_objects->grafts + pos,
                        (r->parsed_objects->grafts_nr - pos - 1) *
                        sizeof(*r->parsed_objects->grafts));
        r->parsed_objects->grafts[pos] = graft;
        return 0;
}

struct commit_graft *read_graft_line(struct strbuf *line)
{
        /* The format is just "Commit Parent1 Parent2 ...\n" */
        int i, phase;
        const char *tail = NULL;
        struct commit_graft *graft = NULL;
        struct object_id dummy_oid, *oid;

        strbuf_rtrim(line);
        if (!line->len || line->buf[0] == '#')
                return NULL;
        /*
         * phase 0 verifies line, counts hashes in line and allocates graft
         * phase 1 fills graft
         */
        for (phase = 0; phase < 2; phase++) {
                oid = graft ? &graft->oid : &dummy_oid;
                if (parse_oid_hex(line->buf, oid, &tail))
                        goto bad_graft_data;
                for (i = 0; *tail != '\0'; i++) {
                        oid = graft ? &graft->parent[i] : &dummy_oid;
                        if (!isspace(*tail++) || parse_oid_hex(tail, oid, &tail))
                                goto bad_graft_data;
                }
                if (!graft) {
                        graft = xmalloc(st_add(sizeof(*graft),
                                               st_mult(sizeof(struct object_id), i)));
                        graft->nr_parent = i;
                }
        }
        return graft;

bad_graft_data:
        error("bad graft data: %s", line->buf);
        assert(!graft);
        return NULL;
}

static int read_graft_file(struct repository *r, const char *graft_file)
{
        FILE *fp = fopen_or_warn(graft_file, "r");
        struct strbuf buf = STRBUF_INIT;
        if (!fp)
                return -1;
        if (advice_graft_file_deprecated)
                advise(_("Support for <GIT_DIR>/info/grafts is deprecated\n"
                         "and will be removed in a future Git version.\n"
                         "\n"
                         "Please use \"git replace --convert-graft-file\"\n"
                         "to convert the grafts into replace refs.\n"
                         "\n"
                         "Turn this message off by running\n"
                         "\"git config advice.graftFileDeprecated false\""));
        while (!strbuf_getwholeline(&buf, fp, '\n')) {
                /* The format is just "Commit Parent1 Parent2 ...\n" */
                struct commit_graft *graft = read_graft_line(&buf);
                if (!graft)
                        continue;
                if (register_commit_graft(r, graft, 1))
                        error("duplicate graft data: %s", buf.buf);
        }
        fclose(fp);
        strbuf_release(&buf);
        return 0;
}

static void prepare_commit_graft(struct repository *r)
{
        char *graft_file;

        if (r->parsed_objects->commit_graft_prepared)
                return;
        if (!startup_info->have_repository)
                return;

        graft_file = get_graft_file(r);
        read_graft_file(r, graft_file);
        /* make sure shallows are read */
        is_repository_shallow(r);
        r->parsed_objects->commit_graft_prepared = 1;
}

struct commit_graft *lookup_commit_graft(struct repository *r, const struct object_id *oid)
{
        int pos;
        prepare_commit_graft(r);
        pos = commit_graft_pos(r, oid->hash);
        if (pos < 0)
                return NULL;
        return r->parsed_objects->grafts[pos];
}

int for_each_commit_graft(each_commit_graft_fn fn, void *cb_data)
{
        int i, ret;
        for (i = ret = 0; i < the_repository->parsed_objects->grafts_nr && !ret; i++)
                ret = fn(the_repository->parsed_objects->grafts[i], cb_data);
        return ret;
}

int unregister_shallow(const struct object_id *oid)
{
        int pos = commit_graft_pos(the_repository, oid->hash);
        if (pos < 0)
                return -1;
        if (pos + 1 < the_repository->parsed_objects->grafts_nr)
                MOVE_ARRAY(the_repository->parsed_objects->grafts + pos,
                           the_repository->parsed_objects->grafts + pos + 1,
                           the_repository->parsed_objects->grafts_nr - pos - 1);
        the_repository->parsed_objects->grafts_nr--;
        return 0;
}

struct commit_buffer {
        void *buffer;
        unsigned long size;
};
define_commit_slab(buffer_slab, struct commit_buffer);

struct buffer_slab *allocate_commit_buffer_slab(void)
{
        struct buffer_slab *bs = xmalloc(sizeof(*bs));
        init_buffer_slab(bs);
        return bs;
}

void free_commit_buffer_slab(struct buffer_slab *bs)
{
        clear_buffer_slab(bs);
        free(bs);
}

void set_commit_buffer(struct repository *r, struct commit *commit, void *buffer, unsigned long size)
{
        struct commit_buffer *v = buffer_slab_at(
                r->parsed_objects->buffer_slab, commit);
        v->buffer = buffer;
        v->size = size;
}

const void *get_cached_commit_buffer(struct repository *r, const struct commit *commit, unsigned long *sizep)
{
        struct commit_buffer *v = buffer_slab_peek(
                r->parsed_objects->buffer_slab, commit);
        if (!v) {
                if (sizep)
                        *sizep = 0;
                return NULL;
        }
        if (sizep)
                *sizep = v->size;
        return v->buffer;
}

const void *get_commit_buffer(const struct commit *commit, unsigned long *sizep)
{
        const void *ret = get_cached_commit_buffer(the_repository, commit, sizep);
        if (!ret) {
                enum object_type type;
                unsigned long size;
                ret = read_object_file(&commit->object.oid, &type, &size);
                if (!ret)
                        die("cannot read commit object %s",
                            oid_to_hex(&commit->object.oid));
                if (type != OBJ_COMMIT)
                        die("expected commit for %s, got %s",
                            oid_to_hex(&commit->object.oid), type_name(type));
                if (sizep)
                        *sizep = size;
        }
        return ret;
}

void unuse_commit_buffer(const struct commit *commit, const void *buffer)
{
        struct commit_buffer *v = buffer_slab_peek(
                the_repository->parsed_objects->buffer_slab, commit);
        if (!(v && v->buffer == buffer))
                free((void *)buffer);
}

void free_commit_buffer(struct commit *commit)
{
        struct commit_buffer *v = buffer_slab_peek(
                the_repository->parsed_objects->buffer_slab, commit);
        if (v) {
                FREE_AND_NULL(v->buffer);
                v->size = 0;
        }
}

struct tree *get_commit_tree(const struct commit *commit)
{
        if (commit->maybe_tree || !commit->object.parsed)
                return commit->maybe_tree;

        if (commit->graph_pos == COMMIT_NOT_FROM_GRAPH)
                BUG("commit has NULL tree, but was not loaded from commit-graph");

        return get_commit_tree_in_graph(the_repository, commit);
}

struct object_id *get_commit_tree_oid(const struct commit *commit)
{
        return &get_commit_tree(commit)->object.oid;
}

void release_commit_memory(struct commit *c)
{
        c->maybe_tree = NULL;
        c->index = 0;
        free_commit_buffer(c);
        free_commit_list(c->parents);
        /* TODO: what about commit->util? */

        c->object.parsed = 0;
}

const void *detach_commit_buffer(struct commit *commit, unsigned long *sizep)
{
        struct commit_buffer *v = buffer_slab_peek(
                the_repository->parsed_objects->buffer_slab, commit);
        void *ret;

        if (!v) {
                if (sizep)
                        *sizep = 0;
                return NULL;
        }
        ret = v->buffer;
        if (sizep)
                *sizep = v->size;

        v->buffer = NULL;
        v->size = 0;
        return ret;
}

int parse_commit_buffer(struct repository *r, struct commit *item, const void *buffer, unsigned long size, int check_graph)
{
        const char *tail = buffer;
        const char *bufptr = buffer;
        struct object_id parent;
        struct commit_list **pptr;
        struct commit_graft *graft;
        const int tree_entry_len = the_hash_algo->hexsz + 5;
        const int parent_entry_len = the_hash_algo->hexsz + 7;

        if (item->object.parsed)
                return 0;
        item->object.parsed = 1;
        tail += size;
        if (tail <= bufptr + tree_entry_len + 1 || memcmp(bufptr, "tree ", 5) ||
                        bufptr[tree_entry_len] != '\n')
                return error("bogus commit object %s", oid_to_hex(&item->object.oid));
        if (get_oid_hex(bufptr + 5, &parent) < 0)
                return error("bad tree pointer in commit %s",
                             oid_to_hex(&item->object.oid));
        item->maybe_tree = lookup_tree(r, &parent);
        bufptr += tree_entry_len + 1; /* "tree " + "hex sha1" + "\n" */
        pptr = &item->parents;

        graft = lookup_commit_graft(r, &item->object.oid);
        while (bufptr + parent_entry_len < tail && !memcmp(bufptr, "parent ", 7)) {
                struct commit *new_parent;

                if (tail <= bufptr + parent_entry_len + 1 ||
                    get_oid_hex(bufptr + 7, &parent) ||
                    bufptr[parent_entry_len] != '\n')
                        return error("bad parents in commit %s", oid_to_hex(&item->object.oid));
                bufptr += parent_entry_len + 1;
                /*
                 * The clone is shallow if nr_parent < 0, and we must
                 * not traverse its real parents even when we unhide them.
                 */
                if (graft && (graft->nr_parent < 0 || grafts_replace_parents))
                        continue;
                new_parent = lookup_commit(r, &parent);
                if (new_parent)
                        pptr = &commit_list_insert(new_parent, pptr)->next;
        }
        if (graft) {
                int i;
                struct commit *new_parent;
                for (i = 0; i < graft->nr_parent; i++) {
                        new_parent = lookup_commit(r,
                                                   &graft->parent[i]);
                        if (!new_parent)
                                continue;
                        pptr = &commit_list_insert(new_parent, pptr)->next;
                }
        }
        item->date = parse_commit_date(bufptr, tail);

        if (check_graph)
                load_commit_graph_info(the_repository, item);

        return 0;
}

int parse_commit_internal(struct commit *item, int quiet_on_missing, int use_commit_graph)
{
        enum object_type type;
        void *buffer;
        unsigned long size;
        int ret;

        if (!item)
                return -1;
        if (item->object.parsed)
                return 0;
        if (use_commit_graph && parse_commit_in_graph(the_repository, item))
                return 0;
        buffer = read_object_file(&item->object.oid, &type, &size);
        if (!buffer)
                return quiet_on_missing ? -1 :
                        error("Could not read %s",
                             oid_to_hex(&item->object.oid));
        if (type != OBJ_COMMIT) {
                free(buffer);
                return error("Object %s not a commit",
                             oid_to_hex(&item->object.oid));
        }

        ret = parse_commit_buffer(the_repository, item, buffer, size, 0);
        if (save_commit_buffer && !ret) {
                set_commit_buffer(the_repository, item, buffer, size);
                return 0;
        }
        free(buffer);
        return ret;
}

int parse_commit_gently(struct commit *item, int quiet_on_missing)
{
        return parse_commit_internal(item, quiet_on_missing, 1);
}

void parse_commit_or_die(struct commit *item)
{
        if (parse_commit(item))
                die("unable to parse commit %s",
                    item ? oid_to_hex(&item->object.oid) : "(null)");
}

int find_commit_subject(const char *commit_buffer, const char **subject)
{
        const char *eol;
        const char *p = commit_buffer;

        while (*p && (*p != '\n' || p[1] != '\n'))
                p++;
        if (*p) {
                p = skip_blank_lines(p + 2);
                eol = strchrnul(p, '\n');
        } else
                eol = p;

        *subject = p;

        return eol - p;
}

struct commit_list *commit_list_insert(struct commit *item, struct commit_list **list_p)
{
        struct commit_list *new_list = xmalloc(sizeof(struct commit_list));
        new_list->item = item;
        new_list->next = *list_p;
        *list_p = new_list;
        return new_list;
}

unsigned commit_list_count(const struct commit_list *l)
{
        unsigned c = 0;
        for (; l; l = l->next )
                c++;
        return c;
}

struct commit_list *copy_commit_list(struct commit_list *list)
{
        struct commit_list *head = NULL;
        struct commit_list **pp = &head;
        while (list) {
                pp = commit_list_append(list->item, pp);
                list = list->next;
        }
        return head;
}

void free_commit_list(struct commit_list *list)
{
        while (list)
                pop_commit(&list);
}

struct commit_list * commit_list_insert_by_date(struct commit *item, struct commit_list **list)
{
        struct commit_list **pp = list;
        struct commit_list *p;
        while ((p = *pp) != NULL) {
                if (p->item->date < item->date) {
                        break;
                }
                pp = &p->next;
        }
        return commit_list_insert(item, pp);
}

static int commit_list_compare_by_date(const void *a, const void *b)
{
        timestamp_t a_date = ((const struct commit_list *)a)->item->date;
        timestamp_t b_date = ((const struct commit_list *)b)->item->date;
        if (a_date < b_date)
                return 1;
        if (a_date > b_date)
                return -1;
        return 0;
}

static void *commit_list_get_next(const void *a)
{
        return ((const struct commit_list *)a)->next;
}

static void commit_list_set_next(void *a, void *next)
{
        ((struct commit_list *)a)->next = next;
}

void commit_list_sort_by_date(struct commit_list **list)
{
        *list = llist_mergesort(*list, commit_list_get_next, commit_list_set_next,
                                commit_list_compare_by_date);
}

struct commit *pop_most_recent_commit(struct commit_list **list,
                                      unsigned int mark)
{
        struct commit *ret = pop_commit(list);
        struct commit_list *parents = ret->parents;

        while (parents) {
                struct commit *commit = parents->item;
                if (!parse_commit(commit) && !(commit->object.flags & mark)) {
                        commit->object.flags |= mark;
                        commit_list_insert_by_date(commit, list);
                }
                parents = parents->next;
        }
        return ret;
}

static void clear_commit_marks_1(struct commit_list **plist,
                                 struct commit *commit, unsigned int mark)
{
        while (commit) {
                struct commit_list *parents;

                if (!(mark & commit->object.flags))
                        return;

                commit->object.flags &= ~mark;

                parents = commit->parents;
                if (!parents)
                        return;

                while ((parents = parents->next))
                        commit_list_insert(parents->item, plist);

                commit = commit->parents->item;
        }
}

void clear_commit_marks_many(int nr, struct commit **commit, unsigned int mark)
{
        struct commit_list *list = NULL;

        while (nr--) {
                clear_commit_marks_1(&list, *commit, mark);
                commit++;
        }
        while (list)
                clear_commit_marks_1(&list, pop_commit(&list), mark);
}

void clear_commit_marks(struct commit *commit, unsigned int mark)
{
        clear_commit_marks_many(1, &commit, mark);
}

struct commit *pop_commit(struct commit_list **stack)
{
        struct commit_list *top = *stack;
        struct commit *item = top ? top->item : NULL;

        if (top) {
                *stack = top->next;
                free(top);
        }
        return item;
}

/*
 * Topological sort support
 */

/* count number of children that have not been emitted */
define_commit_slab(indegree_slab, int);

/* record author-date for each commit object */
define_commit_slab(author_date_slab, timestamp_t);

static void record_author_date(struct author_date_slab *author_date,
                               struct commit *commit)
{
        const char *buffer = get_commit_buffer(commit, NULL);
        struct ident_split ident;
        const char *ident_line;
        size_t ident_len;
        char *date_end;
        timestamp_t date;

        ident_line = find_commit_header(buffer, "author", &ident_len);
        if (!ident_line)
                goto fail_exit; /* no author line */
        if (split_ident_line(&ident, ident_line, ident_len) ||
            !ident.date_begin || !ident.date_end)
                goto fail_exit; /* malformed "author" line */

        date = parse_timestamp(ident.date_begin, &date_end, 10);
        if (date_end != ident.date_end)
                goto fail_exit; /* malformed date */
        *(author_date_slab_at(author_date, commit)) = date;

fail_exit:
        unuse_commit_buffer(commit, buffer);
}

static int compare_commits_by_author_date(const void *a_, const void *b_,
                                          void *cb_data)
{
        const struct commit *a = a_, *b = b_;
        struct author_date_slab *author_date = cb_data;
        timestamp_t a_date = *(author_date_slab_at(author_date, a));
        timestamp_t b_date = *(author_date_slab_at(author_date, b));

        /* newer commits with larger date first */
        if (a_date < b_date)
                return 1;
        else if (a_date > b_date)
                return -1;
        return 0;
}

int compare_commits_by_gen_then_commit_date(const void *a_, const void *b_, void *unused)
{
        const struct commit *a = a_, *b = b_;

        /* newer commits first */
        if (a->generation < b->generation)
                return 1;
        else if (a->generation > b->generation)
                return -1;

        /* use date as a heuristic when generations are equal */
        if (a->date < b->date)
                return 1;
        else if (a->date > b->date)
                return -1;
        return 0;
}

int compare_commits_by_commit_date(const void *a_, const void *b_, void *unused)
{
        const struct commit *a = a_, *b = b_;
        /* newer commits with larger date first */
        if (a->date < b->date)
                return 1;
        else if (a->date > b->date)
                return -1;
        return 0;
}

/*
 * Performs an in-place topological sort on the list supplied.
 */
void sort_in_topological_order(struct commit_list **list, enum rev_sort_order sort_order)
{
        struct commit_list *next, *orig = *list;
        struct commit_list **pptr;
        struct indegree_slab indegree;
        struct prio_queue queue;
        struct commit *commit;
        struct author_date_slab author_date;

        if (!orig)
                return;
        *list = NULL;

        init_indegree_slab(&indegree);
        memset(&queue, '\0', sizeof(queue));

        switch (sort_order) {
        default: /* REV_SORT_IN_GRAPH_ORDER */
                queue.compare = NULL;
                break;
        case REV_SORT_BY_COMMIT_DATE:
                queue.compare = compare_commits_by_commit_date;
                break;
        case REV_SORT_BY_AUTHOR_DATE:
                init_author_date_slab(&author_date);
                queue.compare = compare_commits_by_author_date;
                queue.cb_data = &author_date;
                break;
        }

        /* Mark them and clear the indegree */
        for (next = orig; next; next = next->next) {
                struct commit *commit = next->item;
                *(indegree_slab_at(&indegree, commit)) = 1;
                /* also record the author dates, if needed */
                if (sort_order == REV_SORT_BY_AUTHOR_DATE)
                        record_author_date(&author_date, commit);
        }

        /* update the indegree */
        for (next = orig; next; next = next->next) {
                struct commit_list *parents = next->item->parents;
                while (parents) {
                        struct commit *parent = parents->item;
                        int *pi = indegree_slab_at(&indegree, parent);

                        if (*pi)
                                (*pi)++;
                        parents = parents->next;
                }
        }

        /*
         * find the tips
         *
         * tips are nodes not reachable from any other node in the list
         *
         * the tips serve as a starting set for the work queue.
         */
        for (next = orig; next; next = next->next) {
                struct commit *commit = next->item;

                if (*(indegree_slab_at(&indegree, commit)) == 1)
                        prio_queue_put(&queue, commit);
        }

        /*
         * This is unfortunate; the initial tips need to be shown
         * in the order given from the revision traversal machinery.
         */
        if (sort_order == REV_SORT_IN_GRAPH_ORDER)
                prio_queue_reverse(&queue);

        /* We no longer need the commit list */
        free_commit_list(orig);

        pptr = list;
        *list = NULL;
        while ((commit = prio_queue_get(&queue)) != NULL) {
                struct commit_list *parents;

                for (parents = commit->parents; parents ; parents = parents->next) {
                        struct commit *parent = parents->item;
                        int *pi = indegree_slab_at(&indegree, parent);

                        if (!*pi)
                                continue;

                        /*
                         * parents are only enqueued for emission
                         * when all their children have been emitted thereby
                         * guaranteeing topological order.
                         */
                        if (--(*pi) == 1)
                                prio_queue_put(&queue, parent);
                }
                /*
                 * all children of commit have already been
                 * emitted. we can emit it now.
                 */
                *(indegree_slab_at(&indegree, commit)) = 0;

                pptr = &commit_list_insert(commit, pptr)->next;
        }

        clear_indegree_slab(&indegree);
        clear_prio_queue(&queue);
        if (sort_order == REV_SORT_BY_AUTHOR_DATE)
                clear_author_date_slab(&author_date);
}

/* merge-base stuff */

/* Remember to update object flag allocation in object.h */
#define PARENT1         (1u<<16)
#define PARENT2         (1u<<17)
#define STALE           (1u<<18)
#define RESULT          (1u<<19)

static const unsigned all_flags = (PARENT1 | PARENT2 | STALE | RESULT);

static int queue_has_nonstale(struct prio_queue *queue)
{
        int i;
        for (i = 0; i < queue->nr; i++) {
                struct commit *commit = queue->array[i].data;
                if (!(commit->object.flags & STALE))
                        return 1;
        }
        return 0;
}

/* all input commits in one and twos[] must have been parsed! */
static struct commit_list *paint_down_to_common(struct commit *one, int n,
                                                struct commit **twos,
                                                int min_generation)
{
        struct prio_queue queue = { compare_commits_by_gen_then_commit_date };
        struct commit_list *result = NULL;
        int i;
        uint32_t last_gen = GENERATION_NUMBER_INFINITY;

        if (!min_generation)
                queue.compare = compare_commits_by_commit_date;

        one->object.flags |= PARENT1;
        if (!n) {
                commit_list_append(one, &result);
                return result;
        }
        prio_queue_put(&queue, one);

        for (i = 0; i < n; i++) {
                twos[i]->object.flags |= PARENT2;
                prio_queue_put(&queue, twos[i]);
        }

        while (queue_has_nonstale(&queue)) {
                struct commit *commit = prio_queue_get(&queue);
                struct commit_list *parents;
                int flags;

                if (min_generation && commit->generation > last_gen)
                        BUG("bad generation skip %8x > %8x at %s",
                            commit->generation, last_gen,
                            oid_to_hex(&commit->object.oid));
                last_gen = commit->generation;

                if (commit->generation < min_generation)
                        break;

                flags = commit->object.flags & (PARENT1 | PARENT2 | STALE);
                if (flags == (PARENT1 | PARENT2)) {
                        if (!(commit->object.flags & RESULT)) {
                                commit->object.flags |= RESULT;
                                commit_list_insert_by_date(commit, &result);
                        }
                        /* Mark parents of a found merge stale */
                        flags |= STALE;
                }
                parents = commit->parents;
                while (parents) {
                        struct commit *p = parents->item;
                        parents = parents->next;
                        if ((p->object.flags & flags) == flags)
                                continue;
                        if (parse_commit(p))
                                return NULL;
                        p->object.flags |= flags;
                        prio_queue_put(&queue, p);
                }
        }

        clear_prio_queue(&queue);
        return result;
}

static struct commit_list *merge_bases_many(struct commit *one, int n, struct commit **twos)
{
        struct commit_list *list = NULL;
        struct commit_list *result = NULL;
        int i;

        for (i = 0; i < n; i++) {
                if (one == twos[i])
                        /*
                         * We do not mark this even with RESULT so we do not
                         * have to clean it up.
                         */
                        return commit_list_insert(one, &result);
        }

        if (parse_commit(one))
                return NULL;
        for (i = 0; i < n; i++) {
                if (parse_commit(twos[i]))
                        return NULL;
        }

        list = paint_down_to_common(one, n, twos, 0);

        while (list) {
                struct commit *commit = pop_commit(&list);
                if (!(commit->object.flags & STALE))
                        commit_list_insert_by_date(commit, &result);
        }
        return result;
}

struct commit_list *get_octopus_merge_bases(struct commit_list *in)
{
        struct commit_list *i, *j, *k, *ret = NULL;

        if (!in)
                return ret;

        commit_list_insert(in->item, &ret);

        for (i = in->next; i; i = i->next) {
                struct commit_list *new_commits = NULL, *end = NULL;

                for (j = ret; j; j = j->next) {
                        struct commit_list *bases;
                        bases = get_merge_bases(i->item, j->item);
                        if (!new_commits)
                                new_commits = bases;
                        else
                                end->next = bases;
                        for (k = bases; k; k = k->next)
                                end = k;
                }
                ret = new_commits;
        }
        return ret;
}

static int remove_redundant(struct commit **array, int cnt)
{
        /*
         * Some commit in the array may be an ancestor of
         * another commit.  Move such commit to the end of
         * the array, and return the number of commits that
         * are independent from each other.
         */
        struct commit **work;
        unsigned char *redundant;
        int *filled_index;
        int i, j, filled;

        work = xcalloc(cnt, sizeof(*work));
        redundant = xcalloc(cnt, 1);
        ALLOC_ARRAY(filled_index, cnt - 1);

        for (i = 0; i < cnt; i++)
                parse_commit(array[i]);
        for (i = 0; i < cnt; i++) {
                struct commit_list *common;
                uint32_t min_generation = array[i]->generation;

                if (redundant[i])
                        continue;
                for (j = filled = 0; j < cnt; j++) {
                        if (i == j || redundant[j])
                                continue;
                        filled_index[filled] = j;
                        work[filled++] = array[j];

                        if (array[j]->generation < min_generation)
                                min_generation = array[j]->generation;
                }
                common = paint_down_to_common(array[i], filled, work,
                                              min_generation);
                if (array[i]->object.flags & PARENT2)
                        redundant[i] = 1;
                for (j = 0; j < filled; j++)
                        if (work[j]->object.flags & PARENT1)
                                redundant[filled_index[j]] = 1;
                clear_commit_marks(array[i], all_flags);
                clear_commit_marks_many(filled, work, all_flags);
                free_commit_list(common);
        }

        /* Now collect the result */
        COPY_ARRAY(work, array, cnt);
        for (i = filled = 0; i < cnt; i++)
                if (!redundant[i])
                        array[filled++] = work[i];
        for (j = filled, i = 0; i < cnt; i++)
                if (redundant[i])
                        array[j++] = work[i];
        free(work);
        free(redundant);
        free(filled_index);
        return filled;
}

static struct commit_list *get_merge_bases_many_0(struct commit *one,
                                                  int n,
                                                  struct commit **twos,
                                                  int cleanup)
{
        struct commit_list *list;
        struct commit **rslt;
        struct commit_list *result;
        int cnt, i;

        result = merge_bases_many(one, n, twos);
        for (i = 0; i < n; i++) {
                if (one == twos[i])
                        return result;
        }
        if (!result || !result->next) {
                if (cleanup) {
                        clear_commit_marks(one, all_flags);
                        clear_commit_marks_many(n, twos, all_flags);
                }
                return result;
        }

        /* There are more than one */
        cnt = commit_list_count(result);
        rslt = xcalloc(cnt, sizeof(*rslt));
        for (list = result, i = 0; list; list = list->next)
                rslt[i++] = list->item;
        free_commit_list(result);

        clear_commit_marks(one, all_flags);
        clear_commit_marks_many(n, twos, all_flags);

        cnt = remove_redundant(rslt, cnt);
        result = NULL;
        for (i = 0; i < cnt; i++)
                commit_list_insert_by_date(rslt[i], &result);
        free(rslt);
        return result;
}

struct commit_list *get_merge_bases_many(struct commit *one,
                                         int n,
                                         struct commit **twos)
{
        return get_merge_bases_many_0(one, n, twos, 1);
}

struct commit_list *get_merge_bases_many_dirty(struct commit *one,
                                               int n,
                                               struct commit **twos)
{
        return get_merge_bases_many_0(one, n, twos, 0);
}

struct commit_list *get_merge_bases(struct commit *one, struct commit *two)
{
        return get_merge_bases_many_0(one, 1, &two, 1);
}

/*
 * Is "commit" a descendant of one of the elements on the "with_commit" list?
 */
int is_descendant_of(struct commit *commit, struct commit_list *with_commit)
{
        if (!with_commit)
                return 1;
        while (with_commit) {
                struct commit *other;

                other = with_commit->item;
                with_commit = with_commit->next;
                if (in_merge_bases(other, commit))
                        return 1;
        }
        return 0;
}

/*
 * Is "commit" an ancestor of one of the "references"?
 */
int in_merge_bases_many(struct commit *commit, int nr_reference, struct commit **reference)
{
        struct commit_list *bases;
        int ret = 0, i;
        uint32_t min_generation = GENERATION_NUMBER_INFINITY;

        if (parse_commit(commit))
                return ret;
        for (i = 0; i < nr_reference; i++) {
                if (parse_commit(reference[i]))
                        return ret;
                if (reference[i]->generation < min_generation)
                        min_generation = reference[i]->generation;
        }

        if (commit->generation > min_generation)
                return ret;

        bases = paint_down_to_common(commit, nr_reference, reference, commit->generation);
        if (commit->object.flags & PARENT2)
                ret = 1;
        clear_commit_marks(commit, all_flags);
        clear_commit_marks_many(nr_reference, reference, all_flags);
        free_commit_list(bases);
        return ret;
}

/*
 * Is "commit" an ancestor of (i.e. reachable from) the "reference"?
 */
int in_merge_bases(struct commit *commit, struct commit *reference)
{
        return in_merge_bases_many(commit, 1, &reference);
}

struct commit_list *reduce_heads(struct commit_list *heads)
{
        struct commit_list *p;
        struct commit_list *result = NULL, **tail = &result;
        struct commit **array;
        int num_head, i;

        if (!heads)
                return NULL;

        /* Uniquify */
        for (p = heads; p; p = p->next)
                p->item->object.flags &= ~STALE;
        for (p = heads, num_head = 0; p; p = p->next) {
                if (p->item->object.flags & STALE)
                        continue;
                p->item->object.flags |= STALE;
                num_head++;
        }
        array = xcalloc(num_head, sizeof(*array));
        for (p = heads, i = 0; p; p = p->next) {
                if (p->item->object.flags & STALE) {
                        array[i++] = p->item;
                        p->item->object.flags &= ~STALE;
                }
        }
        num_head = remove_redundant(array, num_head);
        for (i = 0; i < num_head; i++)
                tail = &commit_list_insert(array[i], tail)->next;
        free(array);
        return result;
}

void reduce_heads_replace(struct commit_list **heads)
{
        struct commit_list *result = reduce_heads(*heads);
        free_commit_list(*heads);
        *heads = result;
}

static const char gpg_sig_header[] = "gpgsig";
static const int gpg_sig_header_len = sizeof(gpg_sig_header) - 1;

static int do_sign_commit(struct strbuf *buf, const char *keyid)
{
        struct strbuf sig = STRBUF_INIT;
        int inspos, copypos;
        const char *eoh;

        /* find the end of the header */
        eoh = strstr(buf->buf, "\n\n");
        if (!eoh)
                inspos = buf->len;
        else
                inspos = eoh - buf->buf + 1;

        if (!keyid || !*keyid)
                keyid = get_signing_key();
        if (sign_buffer(buf, &sig, keyid)) {
                strbuf_release(&sig);
                return -1;
        }

        for (copypos = 0; sig.buf[copypos]; ) {
                const char *bol = sig.buf + copypos;
                const char *eol = strchrnul(bol, '\n');
                int len = (eol - bol) + !!*eol;

                if (!copypos) {
                        strbuf_insert(buf, inspos, gpg_sig_header, gpg_sig_header_len);
                        inspos += gpg_sig_header_len;
                }
                strbuf_insert(buf, inspos++, " ", 1);
                strbuf_insert(buf, inspos, bol, len);
                inspos += len;
                copypos += len;
        }
        strbuf_release(&sig);
        return 0;
}

int parse_signed_commit(const struct commit *commit,
                        struct strbuf *payload, struct strbuf *signature)
{

        unsigned long size;
        const char *buffer = get_commit_buffer(commit, &size);
        int in_signature, saw_signature = -1;
        const char *line, *tail;

        line = buffer;
        tail = buffer + size;
        in_signature = 0;
        saw_signature = 0;
        while (line < tail) {
                const char *sig = NULL;
                const char *next = memchr(line, '\n', tail - line);

                next = next ? next + 1 : tail;
                if (in_signature && line[0] == ' ')
                        sig = line + 1;
                else if (starts_with(line, gpg_sig_header) &&
                         line[gpg_sig_header_len] == ' ')
                        sig = line + gpg_sig_header_len + 1;
                if (sig) {
                        strbuf_add(signature, sig, next - sig);
                        saw_signature = 1;
                        in_signature = 1;
                } else {
                        if (*line == '\n')
                                /* dump the whole remainder of the buffer */
                                next = tail;
                        strbuf_add(payload, line, next - line);
                        in_signature = 0;
                }
                line = next;
        }
        unuse_commit_buffer(commit, buffer);
        return saw_signature;
}

int remove_signature(struct strbuf *buf)
{
        const char *line = buf->buf;
        const char *tail = buf->buf + buf->len;
        int in_signature = 0;
        const char *sig_start = NULL;
        const char *sig_end = NULL;

        while (line < tail) {
                const char *next = memchr(line, '\n', tail - line);
                next = next ? next + 1 : tail;

                if (in_signature && line[0] == ' ')
                        sig_end = next;
                else if (starts_with(line, gpg_sig_header) &&
                         line[gpg_sig_header_len] == ' ') {
                        sig_start = line;
                        sig_end = next;
                        in_signature = 1;
                } else {
                        if (*line == '\n')
                                /* dump the whole remainder of the buffer */
                                next = tail;
                        in_signature = 0;
                }
                line = next;
        }

        if (sig_start)
                strbuf_remove(buf, sig_start - buf->buf, sig_end - sig_start);

        return sig_start != NULL;
}

static void handle_signed_tag(struct commit *parent, struct commit_extra_header ***tail)
{
        struct merge_remote_desc *desc;
        struct commit_extra_header *mergetag;
        char *buf;
        unsigned long size, len;
        enum object_type type;

        desc = merge_remote_util(parent);
        if (!desc || !desc->obj)
                return;
        buf = read_object_file(&desc->obj->oid, &type, &size);
        if (!buf || type != OBJ_TAG)
                goto free_return;
        len = parse_signature(buf, size);
        if (size == len)
                goto free_return;
        /*
         * We could verify this signature and either omit the tag when
         * it does not validate, but the integrator may not have the
         * public key of the signer of the tag he is merging, while a
         * later auditor may have it while auditing, so let's not run
         * verify-signed-buffer here for now...
         *
         * if (verify_signed_buffer(buf, len, buf + len, size - len, ...))
         *      warn("warning: signed tag unverified.");
         */
        mergetag = xcalloc(1, sizeof(*mergetag));
        mergetag->key = xstrdup("mergetag");
        mergetag->value = buf;
        mergetag->len = size;

        **tail = mergetag;
        *tail = &mergetag->next;
        return;

free_return:
        free(buf);
}

int check_commit_signature(const struct commit *commit, struct signature_check *sigc)
{
        struct strbuf payload = STRBUF_INIT;
        struct strbuf signature = STRBUF_INIT;
        int ret = 1;

        sigc->result = 'N';

        if (parse_signed_commit(commit, &payload, &signature) <= 0)
                goto out;
        ret = check_signature(payload.buf, payload.len, signature.buf,
                signature.len, sigc);

 out:
        strbuf_release(&payload);
        strbuf_release(&signature);

        return ret;
}



void append_merge_tag_headers(struct commit_list *parents,
                              struct commit_extra_header ***tail)
{
        while (parents) {
                struct commit *parent = parents->item;
                handle_signed_tag(parent, tail);
                parents = parents->next;
        }
}

static void add_extra_header(struct strbuf *buffer,
                             struct commit_extra_header *extra)
{
        strbuf_addstr(buffer, extra->key);
        if (extra->len)
                strbuf_add_lines(buffer, " ", extra->value, extra->len);
        else
                strbuf_addch(buffer, '\n');
}

struct commit_extra_header *read_commit_extra_headers(struct commit *commit,
                                                      const char **exclude)
{
        struct commit_extra_header *extra = NULL;
        unsigned long size;
        const char *buffer = get_commit_buffer(commit, &size);
        extra = read_commit_extra_header_lines(buffer, size, exclude);
        unuse_commit_buffer(commit, buffer);
        return extra;
}

int for_each_mergetag(each_mergetag_fn fn, struct commit *commit, void *data)
{
        struct commit_extra_header *extra, *to_free;
        int res = 0;

        to_free = read_commit_extra_headers(commit, NULL);
        for (extra = to_free; !res && extra; extra = extra->next) {
                if (strcmp(extra->key, "mergetag"))
                        continue; /* not a merge tag */
                res = fn(commit, extra, data);
        }
        free_commit_extra_headers(to_free);
        return res;
}

static inline int standard_header_field(const char *field, size_t len)
{
        return ((len == 4 && !memcmp(field, "tree", 4)) ||
                (len == 6 && !memcmp(field, "parent", 6)) ||
                (len == 6 && !memcmp(field, "author", 6)) ||
                (len == 9 && !memcmp(field, "committer", 9)) ||
                (len == 8 && !memcmp(field, "encoding", 8)));
}

static int excluded_header_field(const char *field, size_t len, const char **exclude)
{
        if (!exclude)
                return 0;

        while (*exclude) {
                size_t xlen = strlen(*exclude);
                if (len == xlen && !memcmp(field, *exclude, xlen))
                        return 1;
                exclude++;
        }
        return 0;
}

static struct commit_extra_header *read_commit_extra_header_lines(
        const char *buffer, size_t size,
        const char **exclude)
{
        struct commit_extra_header *extra = NULL, **tail = &extra, *it = NULL;
        const char *line, *next, *eof, *eob;
        struct strbuf buf = STRBUF_INIT;

        for (line = buffer, eob = line + size;
             line < eob && *line != '\n';
             line = next) {
                next = memchr(line, '\n', eob - line);
                next = next ? next + 1 : eob;
                if (*line == ' ') {
                        /* continuation */
                        if (it)
                                strbuf_add(&buf, line + 1, next - (line + 1));
                        continue;
                }
                if (it)
                        it->value = strbuf_detach(&buf, &it->len);
                strbuf_reset(&buf);
                it = NULL;

                eof = memchr(line, ' ', next - line);
                if (!eof)
                        eof = next;
                else if (standard_header_field(line, eof - line) ||
                         excluded_header_field(line, eof - line, exclude))
                        continue;

                it = xcalloc(1, sizeof(*it));
                it->key = xmemdupz(line, eof-line);
                *tail = it;
                tail = &it->next;
                if (eof + 1 < next)
                        strbuf_add(&buf, eof + 1, next - (eof + 1));
        }
        if (it)
                it->value = strbuf_detach(&buf, &it->len);
        return extra;
}

void free_commit_extra_headers(struct commit_extra_header *extra)
{
        while (extra) {
                struct commit_extra_header *next = extra->next;
                free(extra->key);
                free(extra->value);
                free(extra);
                extra = next;
        }
}

int commit_tree(const char *msg, size_t msg_len, const struct object_id *tree,
                struct commit_list *parents, struct object_id *ret,
                const char *author, const char *sign_commit)
{
        struct commit_extra_header *extra = NULL, **tail = &extra;
        int result;

        append_merge_tag_headers(parents, &tail);
        result = commit_tree_extended(msg, msg_len, tree, parents, ret,
                                      author, sign_commit, extra);
        free_commit_extra_headers(extra);
        return result;
}

static int find_invalid_utf8(const char *buf, int len)
{
        int offset = 0;
        static const unsigned int max_codepoint[] = {
                0x7f, 0x7ff, 0xffff, 0x10ffff
        };

        while (len) {
                unsigned char c = *buf++;
                int bytes, bad_offset;
                unsigned int codepoint;
                unsigned int min_val, max_val;

                len--;
                offset++;

                /* Simple US-ASCII? No worries. */
                if (c < 0x80)
                        continue;

                bad_offset = offset-1;

                /*
                 * Count how many more high bits set: that's how
                 * many more bytes this sequence should have.
                 */
                bytes = 0;
                while (c & 0x40) {
                        c <<= 1;
                        bytes++;
                }

                /*
                 * Must be between 1 and 3 more bytes.  Longer sequences result in
                 * codepoints beyond U+10FFFF, which are guaranteed never to exist.
                 */
                if (bytes < 1 || 3 < bytes)
                        return bad_offset;

                /* Do we *have* that many bytes? */
                if (len < bytes)
                        return bad_offset;

                /*
                 * Place the encoded bits at the bottom of the value and compute the
                 * valid range.
                 */
                codepoint = (c & 0x7f) >> bytes;
                min_val = max_codepoint[bytes-1] + 1;
                max_val = max_codepoint[bytes];

                offset += bytes;
                len -= bytes;

                /* And verify that they are good continuation bytes */
                do {
                        codepoint <<= 6;
                        codepoint |= *buf & 0x3f;
                        if ((*buf++ & 0xc0) != 0x80)
                                return bad_offset;
                } while (--bytes);

                /* Reject codepoints that are out of range for the sequence length. */
                if (codepoint < min_val || codepoint > max_val)
                        return bad_offset;
                /* Surrogates are only for UTF-16 and cannot be encoded in UTF-8. */
                if ((codepoint & 0x1ff800) == 0xd800)
                        return bad_offset;
                /* U+xxFFFE and U+xxFFFF are guaranteed non-characters. */
                if ((codepoint & 0xfffe) == 0xfffe)
                        return bad_offset;
                /* So are anything in the range U+FDD0..U+FDEF. */
                if (codepoint >= 0xfdd0 && codepoint <= 0xfdef)
                        return bad_offset;
        }
        return -1;
}

/*
 * This verifies that the buffer is in proper utf8 format.
 *
 * If it isn't, it assumes any non-utf8 characters are Latin1,
 * and does the conversion.
 */
static int verify_utf8(struct strbuf *buf)
{
        int ok = 1;
        long pos = 0;

        for (;;) {
                int bad;
                unsigned char c;
                unsigned char replace[2];

                bad = find_invalid_utf8(buf->buf + pos, buf->len - pos);
                if (bad < 0)
                        return ok;
                pos += bad;
                ok = 0;
                c = buf->buf[pos];
                strbuf_remove(buf, pos, 1);

                /* We know 'c' must be in the range 128-255 */
                replace[0] = 0xc0 + (c >> 6);
                replace[1] = 0x80 + (c & 0x3f);
                strbuf_insert(buf, pos, replace, 2);
                pos += 2;
        }
}

static const char commit_utf8_warn[] =
N_("Warning: commit message did not conform to UTF-8.\n"
   "You may want to amend it after fixing the message, or set the config\n"
   "variable i18n.commitencoding to the encoding your project uses.\n");

int commit_tree_extended(const char *msg, size_t msg_len,
                         const struct object_id *tree,
                         struct commit_list *parents, struct object_id *ret,
                         const char *author, const char *sign_commit,
                         struct commit_extra_header *extra)
{
        int result;
        int encoding_is_utf8;
        struct strbuf buffer;

        assert_oid_type(tree, OBJ_TREE);

        if (memchr(msg, '\0', msg_len))
                return error("a NUL byte in commit log message not allowed.");

        /* Not having i18n.commitencoding is the same as having utf-8 */
        encoding_is_utf8 = is_encoding_utf8(git_commit_encoding);

        strbuf_init(&buffer, 8192); /* should avoid reallocs for the headers */
        strbuf_addf(&buffer, "tree %s\n", oid_to_hex(tree));

        /*
         * NOTE! This ordering means that the same exact tree merged with a
         * different order of parents will be a _different_ changeset even
         * if everything else stays the same.
         */
        while (parents) {
                struct commit *parent = pop_commit(&parents);
                strbuf_addf(&buffer, "parent %s\n",
                            oid_to_hex(&parent->object.oid));
        }

        /* Person/date information */
        if (!author)
                author = git_author_info(IDENT_STRICT);
        strbuf_addf(&buffer, "author %s\n", author);
        strbuf_addf(&buffer, "committer %s\n", git_committer_info(IDENT_STRICT));
        if (!encoding_is_utf8)
                strbuf_addf(&buffer, "encoding %s\n", git_commit_encoding);

        while (extra) {
                add_extra_header(&buffer, extra);
                extra = extra->next;
        }
        strbuf_addch(&buffer, '\n');

        /* And add the comment */
        strbuf_add(&buffer, msg, msg_len);

        /* And check the encoding */
        if (encoding_is_utf8 && !verify_utf8(&buffer))
                fprintf(stderr, _(commit_utf8_warn));

        if (sign_commit && do_sign_commit(&buffer, sign_commit)) {
                result = -1;
                goto out;
        }

        result = write_object_file(buffer.buf, buffer.len, commit_type, ret);
out:
        strbuf_release(&buffer);
        return result;
}

define_commit_slab(merge_desc_slab, struct merge_remote_desc *);
static struct merge_desc_slab merge_desc_slab = COMMIT_SLAB_INIT(1, merge_desc_slab);

struct merge_remote_desc *merge_remote_util(struct commit *commit)
{
        return *merge_desc_slab_at(&merge_desc_slab, commit);
}

void set_merge_remote_desc(struct commit *commit,
                           const char *name, struct object *obj)
{
        struct merge_remote_desc *desc;
        FLEX_ALLOC_STR(desc, name, name);
        desc->obj = obj;
        *merge_desc_slab_at(&merge_desc_slab, commit) = desc;
}

struct commit *get_merge_parent(const char *name)
{
        struct object *obj;
        struct commit *commit;
        struct object_id oid;
        if (get_oid(name, &oid))
                return NULL;
        obj = parse_object(the_repository, &oid);
        commit = (struct commit *)peel_to_type(name, 0, obj, OBJ_COMMIT);
        if (commit && !merge_remote_util(commit))
                set_merge_remote_desc(commit, name, obj);
        return commit;
}

/*
 * Append a commit to the end of the commit_list.
 *
 * next starts by pointing to the variable that holds the head of an
 * empty commit_list, and is updated to point to the "next" field of
 * the last item on the list as new commits are appended.
 *
 * Usage example:
 *
 *     struct commit_list *list;
 *     struct commit_list **next = &list;
 *
 *     next = commit_list_append(c1, next);
 *     next = commit_list_append(c2, next);
 *     assert(commit_list_count(list) == 2);
 *     return list;
 */
struct commit_list **commit_list_append(struct commit *commit,
                                        struct commit_list **next)
{
        struct commit_list *new_commit = xmalloc(sizeof(struct commit_list));
        new_commit->item = commit;
        *next = new_commit;
        new_commit->next = NULL;
        return &new_commit->next;
}

const char *find_commit_header(const char *msg, const char *key, size_t *out_len)
{
        int key_len = strlen(key);
        const char *line = msg;

        while (line) {
                const char *eol = strchrnul(line, '\n');

                if (line == eol)
                        return NULL;

                if (eol - line > key_len &&
                    !strncmp(line, key, key_len) &&
                    line[key_len] == ' ') {
                        *out_len = eol - line - key_len - 1;
                        return line + key_len + 1;
                }
                line = *eol ? eol + 1 : NULL;
        }
        return NULL;
}

/*
 * Inspect the given string and determine the true "end" of the log message, in
 * order to find where to put a new Signed-off-by: line.  Ignored are
 * trailing comment lines and blank lines.  To support "git commit -s
 * --amend" on an existing commit, we also ignore "Conflicts:".  To
 * support "git commit -v", we truncate at cut lines.
 *
 * Returns the number of bytes from the tail to ignore, to be fed as
 * the second parameter to append_signoff().
 */
int ignore_non_trailer(const char *buf, size_t len)
{
        int boc = 0;
        int bol = 0;
        int in_old_conflicts_block = 0;
        size_t cutoff = wt_status_locate_end(buf, len);

        while (bol < cutoff) {
                const char *next_line = memchr(buf + bol, '\n', len - bol);

                if (!next_line)
                        next_line = buf + len;
                else
                        next_line++;

                if (buf[bol] == comment_line_char || buf[bol] == '\n') {
                        /* is this the first of the run of comments? */
                        if (!boc)
                                boc = bol;
                        /* otherwise, it is just continuing */
                } else if (starts_with(buf + bol, "Conflicts:\n")) {
                        in_old_conflicts_block = 1;
                        if (!boc)
                                boc = bol;
                } else if (in_old_conflicts_block && buf[bol] == '\t') {
                        ; /* a pathname in the conflicts block */
                } else if (boc) {
                        /* the previous was not trailing comment */
                        boc = 0;
                        in_old_conflicts_block = 0;
                }
                bol = next_line - buf;
        }
        return boc ? len - boc : len - cutoff;
}