refs/files: fix NULL pointer deref when releasing ref store

[git/gitster.git] / path.c

/*
 * Utilities for paths and pathnames
 */
#include "git-compat-util.h"
#include "abspath.h"
#include "environment.h"
#include "gettext.h"
#include "hex.h"
#include "repository.h"
#include "strbuf.h"
#include "string-list.h"
#include "dir.h"
#include "worktree.h"
#include "setup.h"
#include "submodule-config.h"
#include "path.h"
#include "packfile.h"
#include "object-store-ll.h"
#include "lockfile.h"
#include "exec-cmd.h"

static int get_st_mode_bits(const char *path, int *mode)
{
        struct stat st;
        if (lstat(path, &st) < 0)
                return -1;
        *mode = st.st_mode;
        return 0;
}

static struct strbuf *get_pathname(void)
{
        static struct strbuf pathname_array[4] = {
                STRBUF_INIT, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT
        };
        static int index;
        struct strbuf *sb = &pathname_array[index];
        index = (index + 1) % ARRAY_SIZE(pathname_array);
        strbuf_reset(sb);
        return sb;
}

static const char *cleanup_path(const char *path)
{
        /* Clean it up */
        if (skip_prefix(path, "./", &path)) {
                while (*path == '/')
                        path++;
        }
        return path;
}

static void strbuf_cleanup_path(struct strbuf *sb)
{
        const char *path = cleanup_path(sb->buf);
        if (path > sb->buf)
                strbuf_remove(sb, 0, path - sb->buf);
}

static int dir_prefix(const char *buf, const char *dir)
{
        int len = strlen(dir);
        return !strncmp(buf, dir, len) &&
                (is_dir_sep(buf[len]) || buf[len] == '\0');
}

/* $buf =~ m|$dir/+$file| but without regex */
static int is_dir_file(const char *buf, const char *dir, const char *file)
{
        int len = strlen(dir);
        if (strncmp(buf, dir, len) || !is_dir_sep(buf[len]))
                return 0;
        while (is_dir_sep(buf[len]))
                len++;
        return !strcmp(buf + len, file);
}

static void replace_dir(struct strbuf *buf, int len, const char *newdir)
{
        int newlen = strlen(newdir);
        int need_sep = (buf->buf[len] && !is_dir_sep(buf->buf[len])) &&
                !is_dir_sep(newdir[newlen - 1]);
        if (need_sep)
                len--;   /* keep one char, to be replaced with '/'  */
        strbuf_splice(buf, 0, len, newdir, newlen);
        if (need_sep)
                buf->buf[newlen] = '/';
}

struct common_dir {
        /* Not considered garbage for report_linked_checkout_garbage */
        unsigned ignore_garbage:1;
        unsigned is_dir:1;
        /* Belongs to the common dir, though it may contain paths that don't */
        unsigned is_common:1;
        const char *path;
};

static struct common_dir common_list[] = {
        { 0, 1, 1, "branches" },
        { 0, 1, 1, "common" },
        { 0, 1, 1, "hooks" },
        { 0, 1, 1, "info" },
        { 0, 0, 0, "info/sparse-checkout" },
        { 1, 1, 1, "logs" },
        { 1, 0, 0, "logs/HEAD" },
        { 0, 1, 0, "logs/refs/bisect" },
        { 0, 1, 0, "logs/refs/rewritten" },
        { 0, 1, 0, "logs/refs/worktree" },
        { 0, 1, 1, "lost-found" },
        { 0, 1, 1, "objects" },
        { 0, 1, 1, "refs" },
        { 0, 1, 0, "refs/bisect" },
        { 0, 1, 0, "refs/rewritten" },
        { 0, 1, 0, "refs/worktree" },
        { 0, 1, 1, "remotes" },
        { 0, 1, 1, "worktrees" },
        { 0, 1, 1, "rr-cache" },
        { 0, 1, 1, "svn" },
        { 0, 0, 1, "config" },
        { 1, 0, 1, "gc.pid" },
        { 0, 0, 1, "packed-refs" },
        { 0, 0, 1, "shallow" },
        { 0, 0, 0, NULL }
};

/*
 * A compressed trie.  A trie node consists of zero or more characters that
 * are common to all elements with this prefix, optionally followed by some
 * children.  If value is not NULL, the trie node is a terminal node.
 *
 * For example, consider the following set of strings:
 * abc
 * def
 * definite
 * definition
 *
 * The trie would look like:
 * root: len = 0, children a and d non-NULL, value = NULL.
 *    a: len = 2, contents = bc, value = (data for "abc")
 *    d: len = 2, contents = ef, children i non-NULL, value = (data for "def")
 *       i: len = 3, contents = nit, children e and i non-NULL, value = NULL
 *           e: len = 0, children all NULL, value = (data for "definite")
 *           i: len = 2, contents = on, children all NULL,
 *              value = (data for "definition")
 */
struct trie {
        struct trie *children[256];
        int len;
        char *contents;
        void *value;
};

static struct trie *make_trie_node(const char *key, void *value)
{
        struct trie *new_node = xcalloc(1, sizeof(*new_node));
        new_node->len = strlen(key);
        if (new_node->len) {
                new_node->contents = xmalloc(new_node->len);
                memcpy(new_node->contents, key, new_node->len);
        }
        new_node->value = value;
        return new_node;
}

/*
 * Add a key/value pair to a trie.  The key is assumed to be \0-terminated.
 * If there was an existing value for this key, return it.
 */
static void *add_to_trie(struct trie *root, const char *key, void *value)
{
        struct trie *child;
        void *old;
        int i;

        if (!*key) {
                /* we have reached the end of the key */
                old = root->value;
                root->value = value;
                return old;
        }

        for (i = 0; i < root->len; i++) {
                if (root->contents[i] == key[i])
                        continue;

                /*
                 * Split this node: child will contain this node's
                 * existing children.
                 */
                child = xmalloc(sizeof(*child));
                memcpy(child->children, root->children, sizeof(root->children));

                child->len = root->len - i - 1;
                if (child->len) {
                        child->contents = xstrndup(root->contents + i + 1,
                                                   child->len);
                }
                child->value = root->value;
                root->value = NULL;
                root->len = i;

                memset(root->children, 0, sizeof(root->children));
                root->children[(unsigned char)root->contents[i]] = child;

                /* This is the newly-added child. */
                root->children[(unsigned char)key[i]] =
                        make_trie_node(key + i + 1, value);
                return NULL;
        }

        /* We have matched the entire compressed section */
        if (key[i]) {
                child = root->children[(unsigned char)key[root->len]];
                if (child) {
                        return add_to_trie(child, key + root->len + 1, value);
                } else {
                        child = make_trie_node(key + root->len + 1, value);
                        root->children[(unsigned char)key[root->len]] = child;
                        return NULL;
                }
        }

        old = root->value;
        root->value = value;
        return old;
}

typedef int (*match_fn)(const char *unmatched, void *value, void *baton);

/*
 * Search a trie for some key.  Find the longest /-or-\0-terminated
 * prefix of the key for which the trie contains a value.  If there is
 * no such prefix, return -1.  Otherwise call fn with the unmatched
 * portion of the key and the found value.  If fn returns 0 or
 * positive, then return its return value.  If fn returns negative,
 * then call fn with the next-longest /-terminated prefix of the key
 * (i.e. a parent directory) for which the trie contains a value, and
 * handle its return value the same way.  If there is no shorter
 * /-terminated prefix with a value left, then return the negative
 * return value of the most recent fn invocation.
 *
 * The key is partially normalized: consecutive slashes are skipped.
 *
 * For example, consider the trie containing only [logs,
 * logs/refs/bisect], both with values, but not logs/refs.
 *
 * | key                | unmatched      | prefix to node   | return value |
 * |--------------------|----------------|------------------|--------------|
 * | a                  | not called     | n/a              | -1           |
 * | logstore           | not called     | n/a              | -1           |
 * | logs               | \0             | logs             | as per fn    |
 * | logs/              | /              | logs             | as per fn    |
 * | logs/refs          | /refs          | logs             | as per fn    |
 * | logs/refs/         | /refs/         | logs             | as per fn    |
 * | logs/refs/b        | /refs/b        | logs             | as per fn    |
 * | logs/refs/bisected | /refs/bisected | logs             | as per fn    |
 * | logs/refs/bisect   | \0             | logs/refs/bisect | as per fn    |
 * | logs/refs/bisect/  | /              | logs/refs/bisect | as per fn    |
 * | logs/refs/bisect/a | /a             | logs/refs/bisect | as per fn    |
 * | (If fn in the previous line returns -1, then fn is called once more:) |
 * | logs/refs/bisect/a | /refs/bisect/a | logs             | as per fn    |
 * |--------------------|----------------|------------------|--------------|
 */
static int trie_find(struct trie *root, const char *key, match_fn fn,
                     void *baton)
{
        int i;
        int result;
        struct trie *child;

        if (!*key) {
                /* we have reached the end of the key */
                if (root->value && !root->len)
                        return fn(key, root->value, baton);
                else
                        return -1;
        }

        for (i = 0; i < root->len; i++) {
                /* Partial path normalization: skip consecutive slashes. */
                if (key[i] == '/' && key[i+1] == '/') {
                        key++;
                        continue;
                }
                if (root->contents[i] != key[i])
                        return -1;
        }

        /* Matched the entire compressed section */
        key += i;
        if (!*key) {
                /* End of key */
                if (root->value)
                        return fn(key, root->value, baton);
                else
                        return -1;
        }

        /* Partial path normalization: skip consecutive slashes */
        while (key[0] == '/' && key[1] == '/')
                key++;

        child = root->children[(unsigned char)*key];
        if (child)
                result = trie_find(child, key + 1, fn, baton);
        else
                result = -1;

        if (result >= 0 || (*key != '/' && *key != 0))
                return result;
        if (root->value)
                return fn(key, root->value, baton);
        else
                return -1;
}

static struct trie common_trie;
static int common_trie_done_setup;

static void init_common_trie(void)
{
        struct common_dir *p;

        if (common_trie_done_setup)
                return;

        for (p = common_list; p->path; p++)
                add_to_trie(&common_trie, p->path, p);

        common_trie_done_setup = 1;
}

/*
 * Helper function for update_common_dir: returns 1 if the dir
 * prefix is common.
 */
static int check_common(const char *unmatched, void *value,
                        void *baton UNUSED)
{
        struct common_dir *dir = value;

        if (dir->is_dir && (unmatched[0] == 0 || unmatched[0] == '/'))
                return dir->is_common;

        if (!dir->is_dir && unmatched[0] == 0)
                return dir->is_common;

        return 0;
}

static void update_common_dir(struct strbuf *buf, int git_dir_len,
                              const char *common_dir)
{
        char *base = buf->buf + git_dir_len;
        int has_lock_suffix = strbuf_strip_suffix(buf, LOCK_SUFFIX);

        init_common_trie();
        if (trie_find(&common_trie, base, check_common, NULL) > 0)
                replace_dir(buf, git_dir_len, common_dir);

        if (has_lock_suffix)
                strbuf_addstr(buf, LOCK_SUFFIX);
}

void report_linked_checkout_garbage(void)
{
        struct strbuf sb = STRBUF_INIT;
        const struct common_dir *p;
        int len;

        if (!the_repository->different_commondir)
                return;
        strbuf_addf(&sb, "%s/", get_git_dir());
        len = sb.len;
        for (p = common_list; p->path; p++) {
                const char *path = p->path;
                if (p->ignore_garbage)
                        continue;
                strbuf_setlen(&sb, len);
                strbuf_addstr(&sb, path);
                if (file_exists(sb.buf))
                        report_garbage(PACKDIR_FILE_GARBAGE, sb.buf);
        }
        strbuf_release(&sb);
}

static void adjust_git_path(const struct repository *repo,
                            struct strbuf *buf, int git_dir_len)
{
        const char *base = buf->buf + git_dir_len;
        if (is_dir_file(base, "info", "grafts"))
                strbuf_splice(buf, 0, buf->len,
                              repo->graft_file, strlen(repo->graft_file));
        else if (!strcmp(base, "index"))
                strbuf_splice(buf, 0, buf->len,
                              repo->index_file, strlen(repo->index_file));
        else if (dir_prefix(base, "objects"))
                replace_dir(buf, git_dir_len + 7, repo->objects->odb->path);
        else if (git_hooks_path && dir_prefix(base, "hooks"))
                replace_dir(buf, git_dir_len + 5, git_hooks_path);
        else if (repo->different_commondir)
                update_common_dir(buf, git_dir_len, repo->commondir);
}

static void strbuf_worktree_gitdir(struct strbuf *buf,
                                   const struct repository *repo,
                                   const struct worktree *wt)
{
        if (!wt)
                strbuf_addstr(buf, repo->gitdir);
        else if (!wt->id)
                strbuf_addstr(buf, repo->commondir);
        else
                strbuf_git_common_path(buf, repo, "worktrees/%s", wt->id);
}

static void do_git_path(const struct repository *repo,
                        const struct worktree *wt, struct strbuf *buf,
                        const char *fmt, va_list args)
{
        int gitdir_len;
        strbuf_worktree_gitdir(buf, repo, wt);
        if (buf->len && !is_dir_sep(buf->buf[buf->len - 1]))
                strbuf_addch(buf, '/');
        gitdir_len = buf->len;
        strbuf_vaddf(buf, fmt, args);
        if (!wt)
                adjust_git_path(repo, buf, gitdir_len);
        strbuf_cleanup_path(buf);
}

char *repo_git_path(const struct repository *repo,
                    const char *fmt, ...)
{
        struct strbuf path = STRBUF_INIT;
        va_list args;
        va_start(args, fmt);
        do_git_path(repo, NULL, &path, fmt, args);
        va_end(args);
        return strbuf_detach(&path, NULL);
}

void strbuf_repo_git_path(struct strbuf *sb,
                          const struct repository *repo,
                          const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        do_git_path(repo, NULL, sb, fmt, args);
        va_end(args);
}

char *git_path_buf(struct strbuf *buf, const char *fmt, ...)
{
        va_list args;
        strbuf_reset(buf);
        va_start(args, fmt);
        do_git_path(the_repository, NULL, buf, fmt, args);
        va_end(args);
        return buf->buf;
}

void strbuf_git_path(struct strbuf *sb, const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        do_git_path(the_repository, NULL, sb, fmt, args);
        va_end(args);
}

const char *git_path(const char *fmt, ...)
{
        struct strbuf *pathname = get_pathname();
        va_list args;
        va_start(args, fmt);
        do_git_path(the_repository, NULL, pathname, fmt, args);
        va_end(args);
        return pathname->buf;
}

char *git_pathdup(const char *fmt, ...)
{
        struct strbuf path = STRBUF_INIT;
        va_list args;
        va_start(args, fmt);
        do_git_path(the_repository, NULL, &path, fmt, args);
        va_end(args);
        return strbuf_detach(&path, NULL);
}

char *mkpathdup(const char *fmt, ...)
{
        struct strbuf sb = STRBUF_INIT;
        va_list args;
        va_start(args, fmt);
        strbuf_vaddf(&sb, fmt, args);
        va_end(args);
        strbuf_cleanup_path(&sb);
        return strbuf_detach(&sb, NULL);
}

const char *mkpath(const char *fmt, ...)
{
        va_list args;
        struct strbuf *pathname = get_pathname();
        va_start(args, fmt);
        strbuf_vaddf(pathname, fmt, args);
        va_end(args);
        return cleanup_path(pathname->buf);
}

const char *worktree_git_path(const struct worktree *wt, const char *fmt, ...)
{
        struct strbuf *pathname = get_pathname();
        va_list args;
        va_start(args, fmt);
        do_git_path(the_repository, wt, pathname, fmt, args);
        va_end(args);
        return pathname->buf;
}

static void do_worktree_path(const struct repository *repo,
                             struct strbuf *buf,
                             const char *fmt, va_list args)
{
        strbuf_addstr(buf, repo->worktree);
        if(buf->len && !is_dir_sep(buf->buf[buf->len - 1]))
                strbuf_addch(buf, '/');

        strbuf_vaddf(buf, fmt, args);
        strbuf_cleanup_path(buf);
}

char *repo_worktree_path(const struct repository *repo, const char *fmt, ...)
{
        struct strbuf path = STRBUF_INIT;
        va_list args;

        if (!repo->worktree)
                return NULL;

        va_start(args, fmt);
        do_worktree_path(repo, &path, fmt, args);
        va_end(args);

        return strbuf_detach(&path, NULL);
}

void strbuf_repo_worktree_path(struct strbuf *sb,
                               const struct repository *repo,
                               const char *fmt, ...)
{
        va_list args;

        if (!repo->worktree)
                return;

        va_start(args, fmt);
        do_worktree_path(repo, sb, fmt, args);
        va_end(args);
}

/* Returns 0 on success, negative on failure. */
static int do_submodule_path(struct strbuf *buf, const char *path,
                             const char *fmt, va_list args)
{
        struct strbuf git_submodule_common_dir = STRBUF_INIT;
        struct strbuf git_submodule_dir = STRBUF_INIT;
        int ret;

        ret = submodule_to_gitdir(&git_submodule_dir, path);
        if (ret)
                goto cleanup;

        strbuf_complete(&git_submodule_dir, '/');
        strbuf_addbuf(buf, &git_submodule_dir);
        strbuf_vaddf(buf, fmt, args);

        if (get_common_dir_noenv(&git_submodule_common_dir, git_submodule_dir.buf))
                update_common_dir(buf, git_submodule_dir.len, git_submodule_common_dir.buf);

        strbuf_cleanup_path(buf);

cleanup:
        strbuf_release(&git_submodule_dir);
        strbuf_release(&git_submodule_common_dir);
        return ret;
}

char *git_pathdup_submodule(const char *path, const char *fmt, ...)
{
        int err;
        va_list args;
        struct strbuf buf = STRBUF_INIT;
        va_start(args, fmt);
        err = do_submodule_path(&buf, path, fmt, args);
        va_end(args);
        if (err) {
                strbuf_release(&buf);
                return NULL;
        }
        return strbuf_detach(&buf, NULL);
}

int strbuf_git_path_submodule(struct strbuf *buf, const char *path,
                              const char *fmt, ...)
{
        int err;
        va_list args;
        va_start(args, fmt);
        err = do_submodule_path(buf, path, fmt, args);
        va_end(args);

        return err;
}

static void do_git_common_path(const struct repository *repo,
                               struct strbuf *buf,
                               const char *fmt,
                               va_list args)
{
        strbuf_addstr(buf, repo->commondir);
        if (buf->len && !is_dir_sep(buf->buf[buf->len - 1]))
                strbuf_addch(buf, '/');
        strbuf_vaddf(buf, fmt, args);
        strbuf_cleanup_path(buf);
}

const char *git_common_path(const char *fmt, ...)
{
        struct strbuf *pathname = get_pathname();
        va_list args;
        va_start(args, fmt);
        do_git_common_path(the_repository, pathname, fmt, args);
        va_end(args);
        return pathname->buf;
}

void strbuf_git_common_path(struct strbuf *sb,
                            const struct repository *repo,
                            const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        do_git_common_path(repo, sb, fmt, args);
        va_end(args);
}

int validate_headref(const char *path)
{
        struct stat st;
        char buffer[256];
        const char *refname;
        struct object_id oid;
        int fd;
        ssize_t len;

        if (lstat(path, &st) < 0)
                return -1;

        /* Make sure it is a "refs/.." symlink */
        if (S_ISLNK(st.st_mode)) {
                len = readlink(path, buffer, sizeof(buffer)-1);
                if (len >= 5 && !memcmp("refs/", buffer, 5))
                        return 0;
                return -1;
        }

        /*
         * Anything else, just open it and try to see if it is a symbolic ref.
         */
        fd = open(path, O_RDONLY);
        if (fd < 0)
                return -1;
        len = read_in_full(fd, buffer, sizeof(buffer)-1);
        close(fd);

        if (len < 0)
                return -1;
        buffer[len] = '\0';

        /*
         * Is it a symbolic ref?
         */
        if (skip_prefix(buffer, "ref:", &refname)) {
                while (isspace(*refname))
                        refname++;
                if (starts_with(refname, "refs/"))
                        return 0;
        }

        /*
         * Is this a detached HEAD?
         */
        if (!get_oid_hex(buffer, &oid))
                return 0;

        return -1;
}

static struct passwd *getpw_str(const char *username, size_t len)
{
        struct passwd *pw;
        char *username_z = xmemdupz(username, len);
        pw = getpwnam(username_z);
        free(username_z);
        return pw;
}

/*
 * Return a string with ~ and ~user expanded via getpw*. Returns NULL on getpw
 * failure or if path is NULL.
 *
 * If real_home is true, strbuf_realpath($HOME) is used in the `~/` expansion.
 *
 * If the path starts with `%(prefix)/`, the remainder is interpreted as
 * relative to where Git is installed, and expanded to the absolute path.
 */
char *interpolate_path(const char *path, int real_home)
{
        struct strbuf user_path = STRBUF_INIT;
        const char *to_copy = path;

        if (!path)
                goto return_null;

        if (skip_prefix(path, "%(prefix)/", &path))
                return system_path(path);

        if (path[0] == '~') {
                const char *first_slash = strchrnul(path, '/');
                const char *username = path + 1;
                size_t username_len = first_slash - username;
                if (username_len == 0) {
                        const char *home = getenv("HOME");
                        if (!home)
                                goto return_null;
                        if (real_home)
                                strbuf_add_real_path(&user_path, home);
                        else
                                strbuf_addstr(&user_path, home);
#ifdef GIT_WINDOWS_NATIVE
                        convert_slashes(user_path.buf);
#endif
                } else {
                        struct passwd *pw = getpw_str(username, username_len);
                        if (!pw)
                                goto return_null;
                        strbuf_addstr(&user_path, pw->pw_dir);
                }
                to_copy = first_slash;
        }
        strbuf_addstr(&user_path, to_copy);
        return strbuf_detach(&user_path, NULL);
return_null:
        strbuf_release(&user_path);
        return NULL;
}

/*
 * First, one directory to try is determined by the following algorithm.
 *
 * (0) If "strict" is given, the path is used as given and no DWIM is
 *     done. Otherwise:
 * (1) "~/path" to mean path under the running user's home directory;
 * (2) "~user/path" to mean path under named user's home directory;
 * (3) "relative/path" to mean cwd relative directory; or
 * (4) "/absolute/path" to mean absolute directory.
 *
 * Unless "strict" is given, we check "%s/.git", "%s", "%s.git/.git", "%s.git"
 * in this order. We select the first one that is a valid git repository, and
 * chdir() to it. If none match, or we fail to chdir, we return NULL.
 *
 * If all goes well, we return the directory we used to chdir() (but
 * before ~user is expanded), avoiding getcwd() resolving symbolic
 * links.  User relative paths are also returned as they are given,
 * except DWIM suffixing.
 */
const char *enter_repo(const char *path, int strict)
{
        static struct strbuf validated_path = STRBUF_INIT;
        static struct strbuf used_path = STRBUF_INIT;

        if (!path)
                return NULL;

        if (!strict) {
                static const char *suffix[] = {
                        "/.git", "", ".git/.git", ".git", NULL,
                };
                const char *gitfile;
                int len = strlen(path);
                int i;
                while ((1 < len) && (path[len-1] == '/'))
                        len--;

                /*
                 * We can handle arbitrary-sized buffers, but this remains as a
                 * sanity check on untrusted input.
                 */
                if (PATH_MAX <= len)
                        return NULL;

                strbuf_reset(&used_path);
                strbuf_reset(&validated_path);
                strbuf_add(&used_path, path, len);
                strbuf_add(&validated_path, path, len);

                if (used_path.buf[0] == '~') {
                        char *newpath = interpolate_path(used_path.buf, 0);
                        if (!newpath)
                                return NULL;
                        strbuf_attach(&used_path, newpath, strlen(newpath),
                                      strlen(newpath));
                }
                for (i = 0; suffix[i]; i++) {
                        struct stat st;
                        size_t baselen = used_path.len;
                        strbuf_addstr(&used_path, suffix[i]);
                        if (!stat(used_path.buf, &st) &&
                            (S_ISREG(st.st_mode) ||
                            (S_ISDIR(st.st_mode) && is_git_directory(used_path.buf)))) {
                                strbuf_addstr(&validated_path, suffix[i]);
                                break;
                        }
                        strbuf_setlen(&used_path, baselen);
                }
                if (!suffix[i])
                        return NULL;
                gitfile = read_gitfile(used_path.buf);
                die_upon_dubious_ownership(gitfile, NULL, used_path.buf);
                if (gitfile) {
                        strbuf_reset(&used_path);
                        strbuf_addstr(&used_path, gitfile);
                }
                if (chdir(used_path.buf))
                        return NULL;
                path = validated_path.buf;
        }
        else {
                const char *gitfile = read_gitfile(path);
                die_upon_dubious_ownership(gitfile, NULL, path);
                if (gitfile)
                        path = gitfile;
                if (chdir(path))
                        return NULL;
        }

        if (is_git_directory(".")) {
                set_git_dir(".", 0);
                check_repository_format(NULL);
                return path;
        }

        return NULL;
}

int calc_shared_perm(int mode)
{
        int tweak;

        if (get_shared_repository() < 0)
                tweak = -get_shared_repository();
        else
                tweak = get_shared_repository();

        if (!(mode & S_IWUSR))
                tweak &= ~0222;
        if (mode & S_IXUSR)
                /* Copy read bits to execute bits */
                tweak |= (tweak & 0444) >> 2;
        if (get_shared_repository() < 0)
                mode = (mode & ~0777) | tweak;
        else
                mode |= tweak;

        return mode;
}


int adjust_shared_perm(const char *path)
{
        int old_mode, new_mode;

        if (!get_shared_repository())
                return 0;
        if (get_st_mode_bits(path, &old_mode) < 0)
                return -1;

        new_mode = calc_shared_perm(old_mode);
        if (S_ISDIR(old_mode)) {
                /* Copy read bits to execute bits */
                new_mode |= (new_mode & 0444) >> 2;

                /*
                 * g+s matters only if any extra access is granted
                 * based on group membership.
                 */
                if (FORCE_DIR_SET_GID && (new_mode & 060))
                        new_mode |= FORCE_DIR_SET_GID;
        }

        if (((old_mode ^ new_mode) & ~S_IFMT) &&
                        chmod(path, (new_mode & ~S_IFMT)) < 0)
                return -2;
        return 0;
}

void safe_create_dir(const char *dir, int share)
{
        if (mkdir(dir, 0777) < 0) {
                if (errno != EEXIST) {
                        perror(dir);
                        exit(1);
                }
        }
        else if (share && adjust_shared_perm(dir))
                die(_("Could not make %s writable by group"), dir);
}

static int have_same_root(const char *path1, const char *path2)
{
        int is_abs1, is_abs2;

        is_abs1 = is_absolute_path(path1);
        is_abs2 = is_absolute_path(path2);
        return (is_abs1 && is_abs2 && tolower(path1[0]) == tolower(path2[0])) ||
               (!is_abs1 && !is_abs2);
}

/*
 * Give path as relative to prefix.
 *
 * The strbuf may or may not be used, so do not assume it contains the
 * returned path.
 */
const char *relative_path(const char *in, const char *prefix,
                          struct strbuf *sb)
{
        int in_len = in ? strlen(in) : 0;
        int prefix_len = prefix ? strlen(prefix) : 0;
        int in_off = 0;
        int prefix_off = 0;
        int i = 0, j = 0;

        if (!in_len)
                return "./";
        else if (!prefix_len)
                return in;

        if (have_same_root(in, prefix))
                /* bypass dos_drive, for "c:" is identical to "C:" */
                i = j = has_dos_drive_prefix(in);
        else {
                return in;
        }

        while (i < prefix_len && j < in_len && prefix[i] == in[j]) {
                if (is_dir_sep(prefix[i])) {
                        while (is_dir_sep(prefix[i]))
                                i++;
                        while (is_dir_sep(in[j]))
                                j++;
                        prefix_off = i;
                        in_off = j;
                } else {
                        i++;
                        j++;
                }
        }

        if (
            /* "prefix" seems like prefix of "in" */
            i >= prefix_len &&
            /*
             * but "/foo" is not a prefix of "/foobar"
             * (i.e. prefix not end with '/')
             */
            prefix_off < prefix_len) {
                if (j >= in_len) {
                        /* in="/a/b", prefix="/a/b" */
                        in_off = in_len;
                } else if (is_dir_sep(in[j])) {
                        /* in="/a/b/c", prefix="/a/b" */
                        while (is_dir_sep(in[j]))
                                j++;
                        in_off = j;
                } else {
                        /* in="/a/bbb/c", prefix="/a/b" */
                        i = prefix_off;
                }
        } else if (
                   /* "in" is short than "prefix" */
                   j >= in_len &&
                   /* "in" not end with '/' */
                   in_off < in_len) {
                if (is_dir_sep(prefix[i])) {
                        /* in="/a/b", prefix="/a/b/c/" */
                        while (is_dir_sep(prefix[i]))
                                i++;
                        in_off = in_len;
                }
        }
        in += in_off;
        in_len -= in_off;

        if (i >= prefix_len) {
                if (!in_len)
                        return "./";
                else
                        return in;
        }

        strbuf_reset(sb);
        strbuf_grow(sb, in_len);

        while (i < prefix_len) {
                if (is_dir_sep(prefix[i])) {
                        strbuf_addstr(sb, "../");
                        while (is_dir_sep(prefix[i]))
                                i++;
                        continue;
                }
                i++;
        }
        if (!is_dir_sep(prefix[prefix_len - 1]))
                strbuf_addstr(sb, "../");

        strbuf_addstr(sb, in);

        return sb->buf;
}

/*
 * A simpler implementation of relative_path
 *
 * Get relative path by removing "prefix" from "in". This function
 * first appears in v1.5.6-1-g044bbbc, and makes git_dir shorter
 * to increase performance when traversing the path to work_tree.
 */
const char *remove_leading_path(const char *in, const char *prefix)
{
        static struct strbuf buf = STRBUF_INIT;
        int i = 0, j = 0;

        if (!prefix || !prefix[0])
                return in;
        while (prefix[i]) {
                if (is_dir_sep(prefix[i])) {
                        if (!is_dir_sep(in[j]))
                                return in;
                        while (is_dir_sep(prefix[i]))
                                i++;
                        while (is_dir_sep(in[j]))
                                j++;
                        continue;
                } else if (in[j] != prefix[i]) {
                        return in;
                }
                i++;
                j++;
        }
        if (
            /* "/foo" is a prefix of "/foo" */
            in[j] &&
            /* "/foo" is not a prefix of "/foobar" */
            !is_dir_sep(prefix[i-1]) && !is_dir_sep(in[j])
           )
                return in;
        while (is_dir_sep(in[j]))
                j++;

        strbuf_reset(&buf);
        if (!in[j])
                strbuf_addstr(&buf, ".");
        else
                strbuf_addstr(&buf, in + j);
        return buf.buf;
}

/*
 * It is okay if dst == src, but they should not overlap otherwise.
 * The "dst" buffer must be at least as long as "src"; normalizing may shrink
 * the size of the path, but will never grow it.
 *
 * Performs the following normalizations on src, storing the result in dst:
 * - Ensures that components are separated by '/' (Windows only)
 * - Squashes sequences of '/' except "//server/share" on Windows
 * - Removes "." components.
 * - Removes ".." components, and the components the precede them.
 * Returns failure (non-zero) if a ".." component appears as first path
 * component anytime during the normalization. Otherwise, returns success (0).
 *
 * Note that this function is purely textual.  It does not follow symlinks,
 * verify the existence of the path, or make any system calls.
 *
 * prefix_len != NULL is for a specific case of prefix_pathspec():
 * assume that src == dst and src[0..prefix_len-1] is already
 * normalized, any time "../" eats up to the prefix_len part,
 * prefix_len is reduced. In the end prefix_len is the remaining
 * prefix that has not been overridden by user pathspec.
 *
 * NEEDSWORK: This function doesn't perform normalization w.r.t. trailing '/'.
 * For everything but the root folder itself, the normalized path should not
 * end with a '/', then the callers need to be fixed up accordingly.
 *
 */
int normalize_path_copy_len(char *dst, const char *src, int *prefix_len)
{
        char *dst0;
        const char *end;

        /*
         * Copy initial part of absolute path: "/", "C:/", "//server/share/".
         */
        end = src + offset_1st_component(src);
        while (src < end) {
                char c = *src++;
                if (is_dir_sep(c))
                        c = '/';
                *dst++ = c;
        }
        dst0 = dst;

        while (is_dir_sep(*src))
                src++;

        for (;;) {
                char c = *src;

                /*
                 * A path component that begins with . could be
                 * special:
                 * (1) "." and ends   -- ignore and terminate.
                 * (2) "./"           -- ignore them, eat slash and continue.
                 * (3) ".." and ends  -- strip one and terminate.
                 * (4) "../"          -- strip one, eat slash and continue.
                 */
                if (c == '.') {
                        if (!src[1]) {
                                /* (1) */
                                src++;
                        } else if (is_dir_sep(src[1])) {
                                /* (2) */
                                src += 2;
                                while (is_dir_sep(*src))
                                        src++;
                                continue;
                        } else if (src[1] == '.') {
                                if (!src[2]) {
                                        /* (3) */
                                        src += 2;
                                        goto up_one;
                                } else if (is_dir_sep(src[2])) {
                                        /* (4) */
                                        src += 3;
                                        while (is_dir_sep(*src))
                                                src++;
                                        goto up_one;
                                }
                        }
                }

                /* copy up to the next '/', and eat all '/' */
                while ((c = *src++) != '\0' && !is_dir_sep(c))
                        *dst++ = c;
                if (is_dir_sep(c)) {
                        *dst++ = '/';
                        while (is_dir_sep(c))
                                c = *src++;
                        src--;
                } else if (!c)
                        break;
                continue;

        up_one:
                /*
                 * dst0..dst is prefix portion, and dst[-1] is '/';
                 * go up one level.
                 */
                dst--;  /* go to trailing '/' */
                if (dst <= dst0)
                        return -1;
                /* Windows: dst[-1] cannot be backslash anymore */
                while (dst0 < dst && dst[-1] != '/')
                        dst--;
                if (prefix_len && *prefix_len > dst - dst0)
                        *prefix_len = dst - dst0;
        }
        *dst = '\0';
        return 0;
}

int normalize_path_copy(char *dst, const char *src)
{
        return normalize_path_copy_len(dst, src, NULL);
}

int strbuf_normalize_path(struct strbuf *src)
{
        struct strbuf dst = STRBUF_INIT;

        strbuf_grow(&dst, src->len);
        if (normalize_path_copy(dst.buf, src->buf) < 0) {
                strbuf_release(&dst);
                return -1;
        }

        /*
         * normalize_path does not tell us the new length, so we have to
         * compute it by looking for the new NUL it placed
         */
        strbuf_setlen(&dst, strlen(dst.buf));
        strbuf_swap(src, &dst);
        strbuf_release(&dst);
        return 0;
}

/*
 * path = Canonical absolute path
 * prefixes = string_list containing normalized, absolute paths without
 * trailing slashes (except for the root directory, which is denoted by "/").
 *
 * Determines, for each path in prefixes, whether the "prefix"
 * is an ancestor directory of path.  Returns the length of the longest
 * ancestor directory, excluding any trailing slashes, or -1 if no prefix
 * is an ancestor.  (Note that this means 0 is returned if prefixes is
 * ["/"].) "/foo" is not considered an ancestor of "/foobar".  Directories
 * are not considered to be their own ancestors.  path must be in a
 * canonical form: empty components, or "." or ".." components are not
 * allowed.
 */
int longest_ancestor_length(const char *path, struct string_list *prefixes)
{
        int i, max_len = -1;

        if (!strcmp(path, "/"))
                return -1;

        for (i = 0; i < prefixes->nr; i++) {
                const char *ceil = prefixes->items[i].string;
                int len = strlen(ceil);

                /*
                 * For root directories (`/`, `C:/`, `//server/share/`)
                 * adjust the length to exclude the trailing slash.
                 */
                if (len > 0 && ceil[len - 1] == '/')
                        len--;

                if (strncmp(path, ceil, len) ||
                    path[len] != '/' || !path[len + 1])
                        continue; /* no match */

                if (len > max_len)
                        max_len = len;
        }

        return max_len;
}

/* strip arbitrary amount of directory separators at end of path */
static inline int chomp_trailing_dir_sep(const char *path, int len)
{
        while (len && is_dir_sep(path[len - 1]))
                len--;
        return len;
}

/*
 * If path ends with suffix (complete path components), returns the offset of
 * the last character in the path before the suffix (sans trailing directory
 * separators), and -1 otherwise.
 */
static ssize_t stripped_path_suffix_offset(const char *path, const char *suffix)
{
        int path_len = strlen(path), suffix_len = strlen(suffix);

        while (suffix_len) {
                if (!path_len)
                        return -1;

                if (is_dir_sep(path[path_len - 1])) {
                        if (!is_dir_sep(suffix[suffix_len - 1]))
                                return -1;
                        path_len = chomp_trailing_dir_sep(path, path_len);
                        suffix_len = chomp_trailing_dir_sep(suffix, suffix_len);
                }
                else if (path[--path_len] != suffix[--suffix_len])
                        return -1;
        }

        if (path_len && !is_dir_sep(path[path_len - 1]))
                return -1;
        return chomp_trailing_dir_sep(path, path_len);
}

/*
 * Returns true if the path ends with components, considering only complete path
 * components, and false otherwise.
 */
int ends_with_path_components(const char *path, const char *components)
{
        return stripped_path_suffix_offset(path, components) != -1;
}

/*
 * If path ends with suffix (complete path components), returns the
 * part before suffix (sans trailing directory separators).
 * Otherwise returns NULL.
 */
char *strip_path_suffix(const char *path, const char *suffix)
{
        ssize_t offset = stripped_path_suffix_offset(path, suffix);

        return offset == -1 ? NULL : xstrndup(path, offset);
}

int daemon_avoid_alias(const char *p)
{
        int sl, ndot;

        /*
         * This resurrects the belts and suspenders paranoia check by HPA
         * done in <435560F7.4080006@zytor.com> thread, now enter_repo()
         * does not do getcwd() based path canonicalization.
         *
         * sl becomes true immediately after seeing '/' and continues to
         * be true as long as dots continue after that without intervening
         * non-dot character.
         */
        if (!p || (*p != '/' && *p != '~'))
                return -1;
        sl = 1; ndot = 0;
        p++;

        while (1) {
                char ch = *p++;
                if (sl) {
                        if (ch == '.')
                                ndot++;
                        else if (ch == '/') {
                                if (ndot < 3)
                                        /* reject //, /./ and /../ */
                                        return -1;
                                ndot = 0;
                        }
                        else if (ch == 0) {
                                if (0 < ndot && ndot < 3)
                                        /* reject /.$ and /..$ */
                                        return -1;
                                return 0;
                        }
                        else
                                sl = ndot = 0;
                }
                else if (ch == 0)
                        return 0;
                else if (ch == '/') {
                        sl = 1;
                        ndot = 0;
                }
        }
}

/*
 * On NTFS, we need to be careful to disallow certain synonyms of the `.git/`
 * directory:
 *
 * - For historical reasons, file names that end in spaces or periods are
 *   automatically trimmed. Therefore, `.git . . ./` is a valid way to refer
 *   to `.git/`.
 *
 * - For other historical reasons, file names that do not conform to the 8.3
 *   format (up to eight characters for the basename, three for the file
 *   extension, certain characters not allowed such as `+`, etc) are associated
 *   with a so-called "short name", at least on the `C:` drive by default.
 *   Which means that `git~1/` is a valid way to refer to `.git/`.
 *
 *   Note: Technically, `.git/` could receive the short name `git~2` if the
 *   short name `git~1` were already used. In Git, however, we guarantee that
 *   `.git` is the first item in a directory, therefore it will be associated
 *   with the short name `git~1` (unless short names are disabled).
 *
 * - For yet other historical reasons, NTFS supports so-called "Alternate Data
 *   Streams", i.e. metadata associated with a given file, referred to via
 *   `<filename>:<stream-name>:<stream-type>`. There exists a default stream
 *   type for directories, allowing `.git/` to be accessed via
 *   `.git::$INDEX_ALLOCATION/`.
 *
 * When this function returns 1, it indicates that the specified file/directory
 * name refers to a `.git` file or directory, or to any of these synonyms, and
 * Git should therefore not track it.
 *
 * For performance reasons, _all_ Alternate Data Streams of `.git/` are
 * forbidden, not just `::$INDEX_ALLOCATION`.
 *
 * This function is intended to be used by `git fsck` even on platforms where
 * the backslash is a regular filename character, therefore it needs to handle
 * backlash characters in the provided `name` specially: they are interpreted
 * as directory separators.
 */
int is_ntfs_dotgit(const char *name)
{
        char c;

        /*
         * Note that when we don't find `.git` or `git~1` we end up with `name`
         * advanced partway through the string. That's okay, though, as we
         * return immediately in those cases, without looking at `name` any
         * further.
         */
        c = *(name++);
        if (c == '.') {
                /* .git */
                if (((c = *(name++)) != 'g' && c != 'G') ||
                    ((c = *(name++)) != 'i' && c != 'I') ||
                    ((c = *(name++)) != 't' && c != 'T'))
                        return 0;
        } else if (c == 'g' || c == 'G') {
                /* git ~1 */
                if (((c = *(name++)) != 'i' && c != 'I') ||
                    ((c = *(name++)) != 't' && c != 'T') ||
                    *(name++) != '~' ||
                    *(name++) != '1')
                        return 0;
        } else
                return 0;

        for (;;) {
                c = *(name++);
                if (!c || is_xplatform_dir_sep(c) || c == ':')
                        return 1;
                if (c != '.' && c != ' ')
                        return 0;
        }
}

static int is_ntfs_dot_generic(const char *name,
                               const char *dotgit_name,
                               size_t len,
                               const char *dotgit_ntfs_shortname_prefix)
{
        int saw_tilde;
        size_t i;

        if ((name[0] == '.' && !strncasecmp(name + 1, dotgit_name, len))) {
                i = len + 1;
only_spaces_and_periods:
                for (;;) {
                        char c = name[i++];
                        if (!c || c == ':')
                                return 1;
                        if (c != ' ' && c != '.')
                                return 0;
                }
        }

        /*
         * Is it a regular NTFS short name, i.e. shortened to 6 characters,
         * followed by ~1, ... ~4?
         */
        if (!strncasecmp(name, dotgit_name, 6) && name[6] == '~' &&
            name[7] >= '1' && name[7] <= '4') {
                i = 8;
                goto only_spaces_and_periods;
        }

        /*
         * Is it a fall-back NTFS short name (for details, see
         * https://en.wikipedia.org/wiki/8.3_filename?
         */
        for (i = 0, saw_tilde = 0; i < 8; i++)
                if (name[i] == '\0')
                        return 0;
                else if (saw_tilde) {
                        if (name[i] < '0' || name[i] > '9')
                                return 0;
                } else if (name[i] == '~') {
                        if (name[++i] < '1' || name[i] > '9')
                                return 0;
                        saw_tilde = 1;
                } else if (i >= 6)
                        return 0;
                else if (name[i] & 0x80) {
                        /*
                         * We know our needles contain only ASCII, so we clamp
                         * here to make the results of tolower() sane.
                         */
                        return 0;
                } else if (tolower(name[i]) != dotgit_ntfs_shortname_prefix[i])
                        return 0;

        goto only_spaces_and_periods;
}

/*
 * Inline helper to make sure compiler resolves strlen() on literals at
 * compile time.
 */
static inline int is_ntfs_dot_str(const char *name, const char *dotgit_name,
                                  const char *dotgit_ntfs_shortname_prefix)
{
        return is_ntfs_dot_generic(name, dotgit_name, strlen(dotgit_name),
                                   dotgit_ntfs_shortname_prefix);
}

int is_ntfs_dotgitmodules(const char *name)
{
        return is_ntfs_dot_str(name, "gitmodules", "gi7eba");
}

int is_ntfs_dotgitignore(const char *name)
{
        return is_ntfs_dot_str(name, "gitignore", "gi250a");
}

int is_ntfs_dotgitattributes(const char *name)
{
        return is_ntfs_dot_str(name, "gitattributes", "gi7d29");
}

int is_ntfs_dotmailmap(const char *name)
{
        return is_ntfs_dot_str(name, "mailmap", "maba30");
}

int looks_like_command_line_option(const char *str)
{
        return str && str[0] == '-';
}

char *xdg_config_home_for(const char *subdir, const char *filename)
{
        const char *home, *config_home;

        assert(subdir);
        assert(filename);
        config_home = getenv("XDG_CONFIG_HOME");
        if (config_home && *config_home)
                return mkpathdup("%s/%s/%s", config_home, subdir, filename);

        home = getenv("HOME");
        if (home)
                return mkpathdup("%s/.config/%s/%s", home, subdir, filename);

        return NULL;
}

char *xdg_config_home(const char *filename)
{
        return xdg_config_home_for("git", filename);
}

char *xdg_cache_home(const char *filename)
{
        const char *home, *cache_home;

        assert(filename);
        cache_home = getenv("XDG_CACHE_HOME");
        if (cache_home && *cache_home)
                return mkpathdup("%s/git/%s", cache_home, filename);

        home = getenv("HOME");
        if (home)
                return mkpathdup("%s/.cache/git/%s", home, filename);
        return NULL;
}

REPO_GIT_PATH_FUNC(squash_msg, "SQUASH_MSG")
REPO_GIT_PATH_FUNC(merge_msg, "MERGE_MSG")
REPO_GIT_PATH_FUNC(merge_rr, "MERGE_RR")
REPO_GIT_PATH_FUNC(merge_mode, "MERGE_MODE")
REPO_GIT_PATH_FUNC(merge_head, "MERGE_HEAD")
REPO_GIT_PATH_FUNC(fetch_head, "FETCH_HEAD")
REPO_GIT_PATH_FUNC(shallow, "shallow")