fs-util: add new helper linkat_replace()

src/basic/fs-util.c

@@ -1256,3 +1256,79 @@ int link_fd(int fd, int newdirfd, const char *newpath) {
 
         return RET_NERRNO(linkat(fd, "", newdirfd, newpath, AT_EMPTY_PATH));
 }
+
+int linkat_replace(int olddirfd, const char *oldpath, int newdirfd, const char *newpath) {
+        _cleanup_close_ int old_fd = -EBADF;
+        int r;
+
+        assert(olddirfd >= 0 || olddirfd == AT_FDCWD);
+        assert(newdirfd >= 0 || newdirfd == AT_FDCWD);
+        assert(!isempty(newpath)); /* source path is optional, but the target path is not */
+
+        /* Like linkat() but replaces the target if needed. Is a NOP if source and target already share the
+         * same inode. */
+
+        if (olddirfd == AT_FDCWD && isempty(oldpath)) /* Refuse operating on the cwd (which is a dir, and dirs can't be hardlinked) */
+                return -EISDIR;
+
+        if (path_implies_directory(oldpath)) /* Refuse these definite directories early */
+                return -EISDIR;
+
+        if (path_implies_directory(newpath))
+                return -EISDIR;
+
+        /* First, try to link this directly */
+        if (oldpath)
+                r = RET_NERRNO(linkat(olddirfd, oldpath, newdirfd, newpath, 0));
+        else
+                r = link_fd(olddirfd, newdirfd, newpath);
+        if (r >= 0)
+                return 0;
+        if (r != -EEXIST)
+                return r;
+
+        old_fd = xopenat(olddirfd, oldpath, O_PATH|O_CLOEXEC);
+        if (old_fd < 0)
+                return old_fd;
+
+        struct stat old_st;
+        if (fstat(old_fd, &old_st) < 0)
+                return -errno;
+
+        if (S_ISDIR(old_st.st_mode)) /* Don't bother if we are operating on a directory */
+                return -EISDIR;
+
+        struct stat new_st;
+        if (fstatat(newdirfd, newpath, &new_st, AT_SYMLINK_NOFOLLOW) < 0)
+                return -errno;
+
+        if (S_ISDIR(new_st.st_mode)) /* Refuse replacing directories */
+                return -EEXIST;
+
+        if (stat_inode_same(&old_st, &new_st)) /* Already the same inode? Then shortcut this */
+                return 0;
+
+        _cleanup_free_ char *tmp_path = NULL;
+        r = tempfn_random(newpath, /* extra= */ NULL, &tmp_path);
+        if (r < 0)
+                return r;
+
+        r = link_fd(old_fd, newdirfd, tmp_path);
+        if (r < 0) {
+                if (!ERRNO_IS_PRIVILEGE(r))
+                        return r;
+
+                /* If that didn't work due to permissions then go via the path of the dentry */
+                r = RET_NERRNO(linkat(olddirfd, oldpath, newdirfd, tmp_path, 0));
+                if (r < 0)
+                        return r;
+        }
+
+        r = RET_NERRNO(renameat(newdirfd, tmp_path, newdirfd, newpath));
+        if (r < 0) {
+                (void) unlinkat(newdirfd, tmp_path, /* flags= */ 0);
+                return r;
+        }
+
+        return 0;
+}

src/basic/fs-util.h

@@ -148,3 +148,5 @@ static inline int xopenat_lock(int dir_fd, const char *path, int open_flags, Loc
 }
 
 int link_fd(int fd, int newdirfd, const char *newpath);
+
+int linkat_replace(int olddirfd, const char *oldpath, int newdirfd, const char *newpath);