无竞争目录遍历 (C++)

Question

我需要遍历目录树并获取每个文件的统计值。我想在修改文件系统时安全地执行此操作。

在 Python 中，最好的选择是 os.fwalk, which gives access to the fd for the directory being traversed; I can then os.stat with the dir_fd (fstatat) and get current stat values. This is as race-free as it can be made on Linux (if the contents of this directory are being modified, I may have to rescan it). In C, there is nftw, which is implemented similarly, and fts，它在 glibc 中使用普通 (l)stat，因此很活泼（它通过更改目录减少竞争 window，这很不方便）。

C++ 有一个新的 filesystem API graduated from boost, which caches stat values but doesn't expose them（我需要访问 st_dev）。这不仅仅是一个头文件库，所以我无法解决这个问题。

我是否错过了一个不错的 C++ 选项，它使用 fstatat 并且不受 Boost 不公开特定于平台的调用的理想的约束？或者我最好的选择是包装 nftw（甚至 find）？

Answer 1

事实证明它很容易实现。

我使用了 dryproject 中的 libposix。

#include <posix++.h>

class Walker {
public:
    void walk(posix::directory dir) {
        dir.for_each([this, dir](auto& dirent) {
            if (dirent.name == "." or dirent.name == "..")
                    return;
            if (!handle_dirent(dirent))
                return;
            struct stat stat;
            if (dirent.type == DT_DIR || dirent.type == DT_UNKNOWN) {
                int fd = openat(
                    dir.fd(), dirent.name.c_str(), O_DIRECTORY|O_NOFOLLOW|O_NOATIME);
                if (fd < 0) {
                    // ELOOP when O_NOFOLLOW is used on a symlink
                    if (errno == ENOTDIR || errno == ELOOP)
                        goto enotdir;
                    if (errno == ENOENT)
                        goto enoent;
                    posix::throw_error(
                        "openat", "%d, \"%s\"", dir.fd(), dirent.name);
                }
                posix::directory dir1(fd);
                fstat(fd, &stat);
                if (handle_directory(dirent, fd, stat))
                    walk(dir1);
                close(fd);
                return;
            }
enotdir:
            try {
                dir.stat(dirent.name.c_str(), stat, AT_SYMLINK_NOFOLLOW);
            } catch (const posix::runtime_error &error) {
                if (error.number() == ENOENT)
                    goto enoent;
                throw;
            }
            handle_file(dirent, stat);
            return;
enoent:
            handle_missing(dirent);
        });
    }
protected:
    /* return value: whether to stat */
    virtual bool handle_dirent(const posix::directory::entry&) { return true; }
    /* return value: whether to recurse
     * stat will refer to a directory, dirent info may be obsolete */
    virtual bool handle_directory(
            const posix::directory::entry &dirent,
            const int fd, const struct stat&) { return true; }
    /* stat might refer to a directory in case of a race;
     * it still won't be recursed into.  dirent may be obsolete. */
    virtual void handle_file(
            const posix::directory::entry &dirent,
            const struct stat&) {}
    /* in case of a race */
    virtual void handle_missing(
            const posix::directory::entry &dirent) {}
};

性能与 GNU find 相同（与基础 class 比较时，使用 -size $RANDOM 抑制输出并强制 find 到 stat 所有文件，而不仅仅是DT_DIR 候选人）。

无竞争目录遍历 (C++)

race-free directory walk (C++)

c++

linux

filesystems

stat

race-condition