/*
 * Builtin "git pull"
 *
 * Based on git-pull.sh by Junio C Hamano
 *
 * Fetch one or more remote refs and merge it/them into the current HEAD.
 */
#include "cache.h"
#include "config.h"
#include "builtin.h"
#include "parse-options.h"
#include "exec_cmd.h"
#include "run-command.h"
#include "sha1-array.h"
#include "remote.h"
#include "dir.h"
#include "refs.h"
#include "revision.h"
#include "submodule.h"
#include "submodule-config.h"
#include "tempfile.h"
#include "lockfile.h"
#include "wt-status.h"

enum rebase_type {
        REBASE_INVALID = -1,
        REBASE_FALSE = 0,
        REBASE_TRUE,
        REBASE_PRESERVE,
        REBASE_RECREATE,
        REBASE_INTERACTIVE
};

/**
 * Parses the value of --rebase. If value is a false value, returns
 * REBASE_FALSE. If value is a true value, returns REBASE_TRUE. If value is
 * "recreate", returns REBASE_RECREATE. If value is "preserve", returns
 * REBASE_PRESERVE. If value is a invalid value, dies with a fatal error if
 * fatal is true, otherwise returns REBASE_INVALID.
 */
static enum rebase_type parse_config_rebase(const char *key, const char *value,
                int fatal)
{
        int v = git_parse_maybe_bool(value);

        if (!v)
                return REBASE_FALSE;
        else if (v > 0)
                return REBASE_TRUE;
        else if (!strcmp(value, "preserve"))
                return REBASE_PRESERVE;
        else if (!strcmp(value, "recreate"))
                return REBASE_RECREATE;
        else if (!strcmp(value, "interactive"))
                return REBASE_INTERACTIVE;

        if (fatal)
                die(_("Invalid value for %s: %s"), key, value);
        else
                error(_("Invalid value for %s: %s"), key, value);

        return REBASE_INVALID;
}

/**
 * Callback for --rebase, which parses arg with parse_config_rebase().
 */
static int parse_opt_rebase(const struct option *opt, const char *arg, int unset)
{
        enum rebase_type *value = opt->value;

        if (arg)
                *value = parse_config_rebase("--rebase", arg, 0);
        else
                *value = unset ? REBASE_FALSE : REBASE_TRUE;
        return *value == REBASE_INVALID ? -1 : 0;
}

static const char * const pull_usage[] = {
        N_("git pull [<options>] [<repository> [<refspec>...]]"),
        NULL
};

/* Shared options */
static int opt_verbosity;
static char *opt_progress;
static int recurse_submodules = RECURSE_SUBMODULES_DEFAULT;

/* Options passed to git-merge or git-rebase */
static enum rebase_type opt_rebase = -1;
static char *opt_diffstat;
static char *opt_log;
static char *opt_signoff;
static char *opt_squash;
static char *opt_commit;
static char *opt_edit;
static char *opt_ff;
static char *opt_verify_signatures;
static int opt_autostash = -1;
static int config_autostash;
static struct argv_array opt_strategies = ARGV_ARRAY_INIT;
static struct argv_array opt_strategy_opts = ARGV_ARRAY_INIT;
static char *opt_gpg_sign;
static int opt_allow_unrelated_histories;

/* Options passed to git-fetch */
static char *opt_all;
static char *opt_append;
static char *opt_upload_pack;
static int opt_force;
static char *opt_tags;
static char *opt_prune;
static char *max_children;
static int opt_dry_run;
static char *opt_keep;
static char *opt_depth;
static char *opt_unshallow;
static char *opt_update_shallow;
static char *opt_refmap;
static char *opt_ipv4;
static char *opt_ipv6;

static struct option pull_options[] = {
        /* Shared options */
        OPT__VERBOSITY(&opt_verbosity),
        OPT_PASSTHRU(0, "progress", &opt_progress, NULL,
                N_("force progress reporting"),
                PARSE_OPT_NOARG),
        { OPTION_CALLBACK, 0, "recurse-submodules",
                   &recurse_submodules, N_("on-demand"),
                   N_("control for recursive fetching of submodules"),
                   PARSE_OPT_OPTARG, option_fetch_parse_recurse_submodules },

        /* Options passed to git-merge or git-rebase */
        OPT_GROUP(N_("Options related to merging")),
        { OPTION_CALLBACK, 'r', "rebase", &opt_rebase,
          "false|true|recreate|preserve|interactive",
          N_("incorporate changes by rebasing rather than merging"),
          PARSE_OPT_OPTARG, parse_opt_rebase },
        OPT_PASSTHRU('n', NULL, &opt_diffstat, NULL,
                N_("do not show a diffstat at the end of the merge"),
                PARSE_OPT_NOARG | PARSE_OPT_NONEG),
        OPT_PASSTHRU(0, "stat", &opt_diffstat, NULL,
                N_("show a diffstat at the end of the merge"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "summary", &opt_diffstat, NULL,
                N_("(synonym to --stat)"),
                PARSE_OPT_NOARG | PARSE_OPT_HIDDEN),
        OPT_PASSTHRU(0, "log", &opt_log, N_("n"),
                N_("add (at most <n>) entries from shortlog to merge commit message"),
                PARSE_OPT_OPTARG),
        OPT_PASSTHRU(0, "signoff", &opt_signoff, NULL,
                N_("add Signed-off-by:"),
                PARSE_OPT_OPTARG),
        OPT_PASSTHRU(0, "squash", &opt_squash, NULL,
                N_("create a single commit instead of doing a merge"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "commit", &opt_commit, NULL,
                N_("perform a commit if the merge succeeds (default)"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "edit", &opt_edit, NULL,
                N_("edit message before committing"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "ff", &opt_ff, NULL,
                N_("allow fast-forward"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "ff-only", &opt_ff, NULL,
                N_("abort if fast-forward is not possible"),
                PARSE_OPT_NOARG | PARSE_OPT_NONEG),
        OPT_PASSTHRU(0, "verify-signatures", &opt_verify_signatures, NULL,
                N_("verify that the named commit has a valid GPG signature"),
                PARSE_OPT_NOARG),
        OPT_BOOL(0, "autostash", &opt_autostash,
                N_("automatically stash/stash pop before and after rebase")),
        OPT_PASSTHRU_ARGV('s', "strategy", &opt_strategies, N_("strategy"),
                N_("merge strategy to use"),
                0),
        OPT_PASSTHRU_ARGV('X', "strategy-option", &opt_strategy_opts,
                N_("option=value"),
                N_("option for selected merge strategy"),
                0),
        OPT_PASSTHRU('S', "gpg-sign", &opt_gpg_sign, N_("key-id"),
                N_("GPG sign commit"),
                PARSE_OPT_OPTARG),
        OPT_SET_INT(0, "allow-unrelated-histories",
                    &opt_allow_unrelated_histories,
                    N_("allow merging unrelated histories"), 1),

        /* Options passed to git-fetch */
        OPT_GROUP(N_("Options related to fetching")),
        OPT_PASSTHRU(0, "all", &opt_all, NULL,
                N_("fetch from all remotes"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU('a', "append", &opt_append, NULL,
                N_("append to .git/FETCH_HEAD instead of overwriting"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "upload-pack", &opt_upload_pack, N_("path"),
                N_("path to upload pack on remote end"),
                0),
        OPT__FORCE(&opt_force, N_("force overwrite of local branch")),
        OPT_PASSTHRU('t', "tags", &opt_tags, NULL,
                N_("fetch all tags and associated objects"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU('p', "prune", &opt_prune, NULL,
                N_("prune remote-tracking branches no longer on remote"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU('j', "jobs", &max_children, N_("n"),
                N_("number of submodules pulled in parallel"),
                PARSE_OPT_OPTARG),
        OPT_BOOL(0, "dry-run", &opt_dry_run,
                N_("dry run")),
        OPT_PASSTHRU('k', "keep", &opt_keep, NULL,
                N_("keep downloaded pack"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "depth", &opt_depth, N_("depth"),
                N_("deepen history of shallow clone"),
                0),
        OPT_PASSTHRU(0, "unshallow", &opt_unshallow, NULL,
                N_("convert to a complete repository"),
                PARSE_OPT_NONEG | PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "update-shallow", &opt_update_shallow, NULL,
                N_("accept refs that update .git/shallow"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU(0, "refmap", &opt_refmap, N_("refmap"),
                N_("specify fetch refmap"),
                PARSE_OPT_NONEG),
        OPT_PASSTHRU('4',  "ipv4", &opt_ipv4, NULL,
                N_("use IPv4 addresses only"),
                PARSE_OPT_NOARG),
        OPT_PASSTHRU('6',  "ipv6", &opt_ipv6, NULL,
                N_("use IPv6 addresses only"),
                PARSE_OPT_NOARG),

        OPT_END()
};

/**
 * Pushes "-q" or "-v" switches into arr to match the opt_verbosity level.
 */
static void argv_push_verbosity(struct argv_array *arr)
{
        int verbosity;

        for (verbosity = opt_verbosity; verbosity > 0; verbosity--)
                argv_array_push(arr, "-v");

        for (verbosity = opt_verbosity; verbosity < 0; verbosity++)
                argv_array_push(arr, "-q");
}

/**
 * Pushes "-f" switches into arr to match the opt_force level.
 */
static void argv_push_force(struct argv_array *arr)
{
        int force = opt_force;
        while (force-- > 0)
                argv_array_push(arr, "-f");
}

/**
 * Sets the GIT_REFLOG_ACTION environment variable to the concatenation of argv
 */
static void set_reflog_message(int argc, const char **argv)
{
        int i;
        struct strbuf msg = STRBUF_INIT;

        for (i = 0; i < argc; i++) {
                if (i)
                        strbuf_addch(&msg, ' ');
                strbuf_addstr(&msg, argv[i]);
        }

        setenv("GIT_REFLOG_ACTION", msg.buf, 0);

        strbuf_release(&msg);
}

/**
 * If pull.ff is unset, returns NULL. If pull.ff is "true", returns "--ff". If
 * pull.ff is "false", returns "--no-ff". If pull.ff is "only", returns
 * "--ff-only". Otherwise, if pull.ff is set to an invalid value, die with an
 * error.
 */
static const char *config_get_ff(void)
{
        const char *value;

        if (git_config_get_value("pull.ff", &value))
                return NULL;

        switch (git_parse_maybe_bool(value)) {
        case 0:
                return "--no-ff";
        case 1:
                return "--ff";
        }

        if (!strcmp(value, "only"))
                return "--ff-only";

        die(_("Invalid value for pull.ff: %s"), value);
}

/**
 * Returns the default configured value for --rebase. It first looks for the
 * value of "branch.$curr_branch.rebase", where $curr_branch is the current
 * branch, and if HEAD is detached or the configuration key does not exist,
 * looks for the value of "pull.rebase". If both configuration keys do not
 * exist, returns REBASE_FALSE.
 */
static enum rebase_type config_get_rebase(void)
{
        struct branch *curr_branch = branch_get("HEAD");
        const char *value;

        if (curr_branch) {
                char *key = xstrfmt("branch.%s.rebase", curr_branch->name);

                if (!git_config_get_value(key, &value)) {
                        enum rebase_type ret = parse_config_rebase(key, value, 1);
                        free(key);
                        return ret;
                }

                free(key);
        }

        if (!git_config_get_value("pull.rebase", &value))
                return parse_config_rebase("pull.rebase", value, 1);

        return REBASE_FALSE;
}

/**
 * Read config variables.
 */
static int git_pull_config(const char *var, const char *value, void *cb)
{
        if (!strcmp(var, "rebase.autostash")) {
                config_autostash = git_config_bool(var, value);
                return 0;
        } else if (!strcmp(var, "submodule.recurse")) {
                recurse_submodules = git_config_bool(var, value) ?
                        RECURSE_SUBMODULES_ON : RECURSE_SUBMODULES_OFF;
                return 0;
        }
        return git_default_config(var, value, cb);
}

/**
 * Appends merge candidates from FETCH_HEAD that are not marked not-for-merge
 * into merge_heads.
 */
static void get_merge_heads(struct oid_array *merge_heads)
{
        const char *filename = git_path_fetch_head();
        FILE *fp;
        struct strbuf sb = STRBUF_INIT;
        struct object_id oid;

        fp = xfopen(filename, "r");
        while (strbuf_getline_lf(&sb, fp) != EOF) {
                if (get_oid_hex(sb.buf, &oid))
                        continue;  /* invalid line: does not start with SHA1 */
                if (starts_with(sb.buf + GIT_SHA1_HEXSZ, "\tnot-for-merge\t"))
                        continue;  /* ref is not-for-merge */
                oid_array_append(merge_heads, &oid);
        }
        fclose(fp);
        strbuf_release(&sb);
}

/**
 * Used by die_no_merge_candidates() as a for_each_remote() callback to
 * retrieve the name of the remote if the repository only has one remote.
 */
static int get_only_remote(struct remote *remote, void *cb_data)
{
        const char **remote_name = cb_data;

        if (*remote_name)
                return -1;

        *remote_name = remote->name;
        return 0;
}

/**
 * Dies with the appropriate reason for why there are no merge candidates:
 *
 * 1. We fetched from a specific remote, and a refspec was given, but it ended
 *    up not fetching anything. This is usually because the user provided a
 *    wildcard refspec which had no matches on the remote end.
 *
 * 2. We fetched from a non-default remote, but didn't specify a branch to
 *    merge. We can't use the configured one because it applies to the default
 *    remote, thus the user must specify the branches to merge.
 *
 * 3. We fetched from the branch's or repo's default remote, but:
 *
 *    a. We are not on a branch, so there will never be a configured branch to
 *       merge with.
 *
 *    b. We are on a branch, but there is no configured branch to merge with.
 *
 * 4. We fetched from the branch's or repo's default remote, but the configured
 *    branch to merge didn't get fetched. (Either it doesn't exist, or wasn't
 *    part of the configured fetch refspec.)
 */
static void NORETURN die_no_merge_candidates(const char *repo, const char **refspecs)
{
        struct branch *curr_branch = branch_get("HEAD");
        const char *remote = curr_branch ? curr_branch->remote_name : NULL;

        if (*refspecs) {
                if (opt_rebase)
                        fprintf_ln(stderr, _("There is no candidate for rebasing against among the refs that you just fetched."));
                else
                        fprintf_ln(stderr, _("There are no candidates for merging among the refs that you just fetched."));
                fprintf_ln(stderr, _("Generally this means that you provided a wildcard refspec which had no\n"
                                        "matches on the remote end."));
        } else if (repo && curr_branch && (!remote || strcmp(repo, remote))) {
                fprintf_ln(stderr, _("You asked to pull from the remote '%s', but did not specify\n"
                        "a branch. Because this is not the default configured remote\n"
                        "for your current branch, you must specify a branch on the command line."),
                        repo);
        } else if (!curr_branch) {
                fprintf_ln(stderr, _("You are not currently on a branch."));
                if (opt_rebase)
                        fprintf_ln(stderr, _("Please specify which branch you want to rebase against."));
                else
                        fprintf_ln(stderr, _("Please specify which branch you want to merge with."));
                fprintf_ln(stderr, _("See git-pull(1) for details."));
                fprintf(stderr, "\n");
                fprintf_ln(stderr, "    git pull %s %s", _("<remote>"), _("<branch>"));
                fprintf(stderr, "\n");
        } else if (!curr_branch->merge_nr) {
                const char *remote_name = NULL;

                if (for_each_remote(get_only_remote, &remote_name) || !remote_name)
                        remote_name = _("<remote>");

                fprintf_ln(stderr, _("There is no tracking information for the current branch."));
                if (opt_rebase)
                        fprintf_ln(stderr, _("Please specify which branch you want to rebase against."));
                else
                        fprintf_ln(stderr, _("Please specify which branch you want to merge with."));
                fprintf_ln(stderr, _("See git-pull(1) for details."));
                fprintf(stderr, "\n");
                fprintf_ln(stderr, "    git pull %s %s", _("<remote>"), _("<branch>"));
                fprintf(stderr, "\n");
                fprintf_ln(stderr, _("If you wish to set tracking information for this branch you can do so with:"));
                fprintf(stderr, "\n");
                fprintf_ln(stderr, "    git branch --set-upstream-to=%s/%s %s\n",
                                remote_name, _("<branch>"), curr_branch->name);
        } else
                fprintf_ln(stderr, _("Your configuration specifies to merge with the ref '%s'\n"
                        "from the remote, but no such ref was fetched."),
                        *curr_branch->merge_name);
        exit(1);
}

/**
 * Parses argv into [<repo> [<refspecs>...]], returning their values in `repo`
 * as a string and `refspecs` as a null-terminated array of strings. If `repo`
 * is not provided in argv, it is set to NULL.
 */
static void parse_repo_refspecs(int argc, const char **argv, const char **repo,
                const char ***refspecs)
{
        if (argc > 0) {
                *repo = *argv++;
                argc--;
        } else
                *repo = NULL;
        *refspecs = argv;
}

/**
 * Runs git-fetch, returning its exit status. `repo` and `refspecs` are the
 * repository and refspecs to fetch, or NULL if they are not provided.
 */
static int run_fetch(const char *repo, const char **refspecs)
{
        struct argv_array args = ARGV_ARRAY_INIT;
        int ret;

        argv_array_pushl(&args, "fetch", "--update-head-ok", NULL);

        /* Shared options */
        argv_push_verbosity(&args);
        if (opt_progress)
                argv_array_push(&args, opt_progress);

        /* Options passed to git-fetch */
        if (opt_all)
                argv_array_push(&args, opt_all);
        if (opt_append)
                argv_array_push(&args, opt_append);
        if (opt_upload_pack)
                argv_array_push(&args, opt_upload_pack);
        argv_push_force(&args);
        if (opt_tags)
                argv_array_push(&args, opt_tags);
        if (opt_prune)
                argv_array_push(&args, opt_prune);
        if (recurse_submodules != RECURSE_SUBMODULES_DEFAULT)
                switch (recurse_submodules) {
                case RECURSE_SUBMODULES_ON:
                        argv_array_push(&args, "--recurse-submodules=on");
                        break;
                case RECURSE_SUBMODULES_OFF:
                        argv_array_push(&args, "--recurse-submodules=no");
                        break;
                case RECURSE_SUBMODULES_ON_DEMAND:
                        argv_array_push(&args, "--recurse-submodules=on-demand");
                        break;
                default:
                        BUG("submodule recursion option not understood");
                }
        if (max_children)
                argv_array_push(&args, max_children);
        if (opt_dry_run)
                argv_array_push(&args, "--dry-run");
        if (opt_keep)
                argv_array_push(&args, opt_keep);
        if (opt_depth)
                argv_array_push(&args, opt_depth);
        if (opt_unshallow)
                argv_array_push(&args, opt_unshallow);
        if (opt_update_shallow)
                argv_array_push(&args, opt_update_shallow);
        if (opt_refmap)
                argv_array_push(&args, opt_refmap);
        if (opt_ipv4)
                argv_array_push(&args, opt_ipv4);
        if (opt_ipv6)
                argv_array_push(&args, opt_ipv6);

        if (repo) {
                argv_array_push(&args, repo);
                argv_array_pushv(&args, refspecs);
        } else if (*refspecs)
                die("BUG: refspecs without repo?");
        ret = run_command_v_opt(args.argv, RUN_GIT_CMD);
        argv_array_clear(&args);
        return ret;
}

/**
 * "Pulls into void" by branching off merge_head.
 */
static int pull_into_void(const struct object_id *merge_head,
                const struct object_id *curr_head)
{
        /*
         * Two-way merge: we treat the index as based on an empty tree,
         * and try to fast-forward to HEAD. This ensures we will not lose
         * index/worktree changes that the user already made on the unborn
         * branch.
         */
        if (checkout_fast_forward(the_hash_algo->empty_tree, merge_head, 0))
                return 1;

        if (update_ref("initial pull", "HEAD", merge_head, curr_head, 0, UPDATE_REFS_DIE_ON_ERR))
                return 1;

        return 0;
}

static int rebase_submodules(void)
{
        struct child_process cp = CHILD_PROCESS_INIT;

        cp.git_cmd = 1;
        cp.no_stdin = 1;
        argv_array_pushl(&cp.args, "submodule", "update",
                                   "--recursive", "--rebase", NULL);

        return run_command(&cp);
}

static int update_submodules(void)
{
        struct child_process cp = CHILD_PROCESS_INIT;

        cp.git_cmd = 1;
        cp.no_stdin = 1;
        argv_array_pushl(&cp.args, "submodule", "update",
                                   "--recursive", "--checkout", NULL);

        return run_command(&cp);
}

/**
 * Runs git-merge, returning its exit status.
 */
static int run_merge(void)
{
        int ret;
        struct argv_array args = ARGV_ARRAY_INIT;

        argv_array_pushl(&args, "merge", NULL);

        /* Shared options */
        argv_push_verbosity(&args);
        if (opt_progress)
                argv_array_push(&args, opt_progress);

        /* Options passed to git-merge */
        if (opt_diffstat)
                argv_array_push(&args, opt_diffstat);
        if (opt_log)
                argv_array_push(&args, opt_log);
        if (opt_signoff)
                argv_array_push(&args, opt_signoff);
        if (opt_squash)
                argv_array_push(&args, opt_squash);
        if (opt_commit)
                argv_array_push(&args, opt_commit);
        if (opt_edit)
                argv_array_push(&args, opt_edit);
        if (opt_ff)
                argv_array_push(&args, opt_ff);
        if (opt_verify_signatures)
                argv_array_push(&args, opt_verify_signatures);
        argv_array_pushv(&args, opt_strategies.argv);
        argv_array_pushv(&args, opt_strategy_opts.argv);
        if (opt_gpg_sign)
                argv_array_push(&args, opt_gpg_sign);
        if (opt_allow_unrelated_histories > 0)
                argv_array_push(&args, "--allow-unrelated-histories");

        argv_array_push(&args, "FETCH_HEAD");
        ret = run_command_v_opt(args.argv, RUN_GIT_CMD);
        argv_array_clear(&args);
        return ret;
}

/**
 * Returns remote's upstream branch for the current branch. If remote is NULL,
 * the current branch's configured default remote is used. Returns NULL if
 * `remote` does not name a valid remote, HEAD does not point to a branch,
 * remote is not the branch's configured remote or the branch does not have any
 * configured upstream branch.
 */
static const char *get_upstream_branch(const char *remote)
{
        struct remote *rm;
        struct branch *curr_branch;
        const char *curr_branch_remote;

        rm = remote_get(remote);
        if (!rm)
                return NULL;

        curr_branch = branch_get("HEAD");
        if (!curr_branch)
                return NULL;

        curr_branch_remote = remote_for_branch(curr_branch, NULL);
        assert(curr_branch_remote);

        if (strcmp(curr_branch_remote, rm->name))
                return NULL;

        return branch_get_upstream(curr_branch, NULL);
}

/**
 * Derives the remote tracking branch from the remote and refspec.
 *
 * FIXME: The current implementation assumes the default mapping of
 * refs/heads/<branch_name> to refs/remotes/<remote_name>/<branch_name>.
 */
static const char *get_tracking_branch(const char *remote, const char *refspec)
{
        struct refspec *spec;
        const char *spec_src;
        const char *merge_branch;

        spec = parse_fetch_refspec(1, &refspec);
        spec_src = spec->src;
        if (!*spec_src || !strcmp(spec_src, "HEAD"))
                spec_src = "HEAD";
        else if (skip_prefix(spec_src, "heads/", &spec_src))
                ;
        else if (skip_prefix(spec_src, "refs/heads/", &spec_src))
                ;
        else if (starts_with(spec_src, "refs/") ||
                starts_with(spec_src, "tags/") ||
                starts_with(spec_src, "remotes/"))
                spec_src = "";

        if (*spec_src) {
                if (!strcmp(remote, "."))
                        merge_branch = mkpath("refs/heads/%s", spec_src);
                else
                        merge_branch = mkpath("refs/remotes/%s/%s", remote, spec_src);
        } else
                merge_branch = NULL;

        free_refspec(1, spec);
        return merge_branch;
}

/**
 * Given the repo and refspecs, sets fork_point to the point at which the
 * current branch forked from its remote tracking branch. Returns 0 on success,
 * -1 on failure.
 */
static int get_rebase_fork_point(struct object_id *fork_point, const char *repo,
                const char *refspec)
{
        int ret;
        struct branch *curr_branch;
        const char *remote_branch;
        struct child_process cp = CHILD_PROCESS_INIT;
        struct strbuf sb = STRBUF_INIT;

        curr_branch = branch_get("HEAD");
        if (!curr_branch)
                return -1;

        if (refspec)
                remote_branch = get_tracking_branch(repo, refspec);
        else
                remote_branch = get_upstream_branch(repo);

        if (!remote_branch)
                return -1;

        argv_array_pushl(&cp.args, "merge-base", "--fork-point",
                        remote_branch, curr_branch->name, NULL);
        cp.no_stdin = 1;
        cp.no_stderr = 1;
        cp.git_cmd = 1;

        ret = capture_command(&cp, &sb, GIT_SHA1_HEXSZ);
        if (ret)
                goto cleanup;

        ret = get_oid_hex(sb.buf, fork_point);
        if (ret)
                goto cleanup;

cleanup:
        strbuf_release(&sb);
        return ret ? -1 : 0;
}

/**
 * Sets merge_base to the octopus merge base of curr_head, merge_head and
 * fork_point. Returns 0 if a merge base is found, 1 otherwise.
 */
static int get_octopus_merge_base(struct object_id *merge_base,
                const struct object_id *curr_head,
                const struct object_id *merge_head,
                const struct object_id *fork_point)
{
        struct commit_list *revs = NULL, *result;

        commit_list_insert(lookup_commit_reference(curr_head), &revs);
        commit_list_insert(lookup_commit_reference(merge_head), &revs);
        if (!is_null_oid(fork_point))
                commit_list_insert(lookup_commit_reference(fork_point), &revs);

        result = get_octopus_merge_bases(revs);
        free_commit_list(revs);
        reduce_heads_replace(&result);

        if (!result)
                return 1;

        oidcpy(merge_base, &result->item->object.oid);
        free_commit_list(result);
        return 0;
}

/**
 * Given the current HEAD SHA1, the merge head returned from git-fetch and the
 * fork point calculated by get_rebase_fork_point(), runs git-rebase with the
 * appropriate arguments and returns its exit status.
 */
static int run_rebase(const struct object_id *curr_head,
                const struct object_id *merge_head,
                const struct object_id *fork_point)
{
        int ret;
        struct object_id oct_merge_base;
        struct argv_array args = ARGV_ARRAY_INIT;

        if (!get_octopus_merge_base(&oct_merge_base, curr_head, merge_head, fork_point))
                if (!is_null_oid(fork_point) && !oidcmp(&oct_merge_base, fork_point))
                        fork_point = NULL;

        argv_array_push(&args, "rebase");

        /* Shared options */
        argv_push_verbosity(&args);

        /* Options passed to git-rebase */
        if (opt_rebase == REBASE_RECREATE)
                argv_array_push(&args, "--recreate-merges");
        else if (opt_rebase == REBASE_PRESERVE)
                argv_array_push(&args, "--preserve-merges");
        else if (opt_rebase == REBASE_INTERACTIVE)
                argv_array_push(&args, "--interactive");
        if (opt_diffstat)
                argv_array_push(&args, opt_diffstat);
        argv_array_pushv(&args, opt_strategies.argv);
        argv_array_pushv(&args, opt_strategy_opts.argv);
        if (opt_gpg_sign)
                argv_array_push(&args, opt_gpg_sign);
        if (opt_autostash == 0)
                argv_array_push(&args, "--no-autostash");
        else if (opt_autostash == 1)
                argv_array_push(&args, "--autostash");
        if (opt_verify_signatures &&
            !strcmp(opt_verify_signatures, "--verify-signatures"))
                warning(_("ignoring --verify-signatures for rebase"));

        argv_array_push(&args, "--onto");
        argv_array_push(&args, oid_to_hex(merge_head));

        if (fork_point && !is_null_oid(fork_point))
                argv_array_push(&args, oid_to_hex(fork_point));
        else
                argv_array_push(&args, oid_to_hex(merge_head));

        ret = run_command_v_opt(args.argv, RUN_GIT_CMD);
        argv_array_clear(&args);
        return ret;
}

int cmd_pull(int argc, const char **argv, const char *prefix)
{
        const char *repo, **refspecs;
        struct oid_array merge_heads = OID_ARRAY_INIT;
        struct object_id orig_head, curr_head;
        struct object_id rebase_fork_point;
        int autostash;

        if (!getenv("GIT_REFLOG_ACTION"))
                set_reflog_message(argc, argv);

        git_config(git_pull_config, NULL);

        argc = parse_options(argc, argv, prefix, pull_options, pull_usage, 0);

        parse_repo_refspecs(argc, argv, &repo, &refspecs);

        if (!opt_ff)
                opt_ff = xstrdup_or_null(config_get_ff());

        if (opt_rebase < 0)
                opt_rebase = config_get_rebase();

        if (read_cache_unmerged())
                die_resolve_conflict("pull");

        if (file_exists(git_path_merge_head()))
                die_conclude_merge();

        if (get_oid("HEAD", &orig_head))
                oidclr(&orig_head);

        if (!opt_rebase && opt_autostash != -1)
                die(_("--[no-]autostash option is only valid with --rebase."));

        autostash = config_autostash;
        if (opt_rebase) {
                if (opt_autostash != -1)
                        autostash = opt_autostash;

                if (is_null_oid(&orig_head) && !is_cache_unborn())
                        die(_("Updating an unborn branch with changes added to the index."));

                if (!autostash)
                        require_clean_work_tree(N_("pull with rebase"),
                                _("please commit or stash them."), 1, 0);

                if (get_rebase_fork_point(&rebase_fork_point, repo, *refspecs))
                        oidclr(&rebase_fork_point);
        }

        if (run_fetch(repo, refspecs))
                return 1;

        if (opt_dry_run)
                return 0;

        if (get_oid("HEAD", &curr_head))
                oidclr(&curr_head);

        if (!is_null_oid(&orig_head) && !is_null_oid(&curr_head) &&
                        oidcmp(&orig_head, &curr_head)) {
                /*
                 * The fetch involved updating the current branch.
                 *
                 * The working tree and the index file are still based on
                 * orig_head commit, but we are merging into curr_head.
                 * Update the working tree to match curr_head.
                 */

                warning(_("fetch updated the current branch head.\n"
                        "fast-forwarding your working tree from\n"
                        "commit %s."), oid_to_hex(&orig_head));

                if (checkout_fast_forward(&orig_head, &curr_head, 0))
                        die(_("Cannot fast-forward your working tree.\n"
                                "After making sure that you saved anything precious from\n"
                                "$ git diff %s\n"
                                "output, run\n"
                                "$ git reset --hard\n"
                                "to recover."), oid_to_hex(&orig_head));
        }

        get_merge_heads(&merge_heads);

        if (!merge_heads.nr)
                die_no_merge_candidates(repo, refspecs);

        if (is_null_oid(&orig_head)) {
                if (merge_heads.nr > 1)
                        die(_("Cannot merge multiple branches into empty head."));
                return pull_into_void(merge_heads.oid, &curr_head);
        }
        if (opt_rebase && merge_heads.nr > 1)
                die(_("Cannot rebase onto multiple branches."));

        if (opt_rebase) {
                int ret = 0;
                if ((recurse_submodules == RECURSE_SUBMODULES_ON ||
                     recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND) &&
                    submodule_touches_in_range(&rebase_fork_point, &curr_head))
                        die(_("cannot rebase with locally recorded submodule modifications"));
                if (!autostash) {
                        struct commit_list *list = NULL;
                        struct commit *merge_head, *head;

                        head = lookup_commit_reference(&orig_head);
                        commit_list_insert(head, &list);
                        merge_head = lookup_commit_reference(&merge_heads.oid[0]);
                        if (is_descendant_of(merge_head, list)) {
                                /* we can fast-forward this without invoking rebase */
                                opt_ff = "--ff-only";
                                ret = run_merge();
                        }
                }
                ret = run_rebase(&curr_head, merge_heads.oid, &rebase_fork_point);

                if (!ret && (recurse_submodules == RECURSE_SUBMODULES_ON ||
                             recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND))
                        ret = rebase_submodules();

                return ret;
        } else {
                int ret = run_merge();
                if (!ret && (recurse_submodules == RECURSE_SUBMODULES_ON ||
                             recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND))
                        ret = update_submodules();
                return ret;
        }
}