+/**
+ * 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(unsigned char *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_sha1_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(unsigned char *merge_base,
+ const unsigned char *curr_head,
+ const unsigned char *merge_head,
+ const unsigned char *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_sha1(fork_point))
+ commit_list_insert(lookup_commit_reference(fork_point), &revs);
+
+ result = reduce_heads(get_octopus_merge_bases(revs));
+ free_commit_list(revs);
+ if (!result)
+ return 1;
+
+ hashcpy(merge_base, result->item->object.sha1);
+ 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 unsigned char *curr_head,
+ const unsigned char *merge_head,
+ const unsigned char *fork_point)
+{
+ int ret;
+ unsigned char oct_merge_base[GIT_SHA1_RAWSZ];
+ struct argv_array args = ARGV_ARRAY_INIT;
+
+ if (!get_octopus_merge_base(oct_merge_base, curr_head, merge_head, fork_point))
+ if (!is_null_sha1(fork_point) && !hashcmp(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_PRESERVE)
+ argv_array_push(&args, "--preserve-merges");
+ 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);
+
+ argv_array_push(&args, "--onto");
+ argv_array_push(&args, sha1_to_hex(merge_head));
+
+ if (fork_point && !is_null_sha1(fork_point))
+ argv_array_push(&args, sha1_to_hex(fork_point));
+ else
+ argv_array_push(&args, sha1_to_hex(merge_head));
+
+ ret = run_command_v_opt(args.argv, RUN_GIT_CMD);
+ argv_array_clear(&args);
+ return ret;
+}
+