dirCreateFileAtomic

fn dirCreateFileAtomic(
    userdata: ?*anyopaque,
    dir: Dir,
    dest_path: []const u8,
    options: Dir.CreateFileAtomicOptions,
) Dir.CreateFileAtomicError!File.Atomic {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    // Linux has O_TMPFILE, but linkat() does not support AT_REPLACE, so it's
    // useless when we have to make up a bogus path name to do the rename()
    // anyway.
    if (!options.replace) tmpfile: {
        const flags: linux.O = if (@hasField(linux.O, "TMPFILE")) .{
            .ACCMODE = .RDWR,
            .TMPFILE = true,
            .DIRECTORY = true,
            .CLOEXEC = true,
        } else if (@hasField(linux.O, "TMPFILE0") and !@hasField(linux.O, "TMPFILE2")) .{
            .ACCMODE = .RDWR,
            .TMPFILE0 = true,
            .TMPFILE1 = true,
            .DIRECTORY = true,
            .CLOEXEC = true,
        } else break :tmpfile;

        const dest_dirname = Dir.path.dirname(dest_path);
        if (dest_dirname) |dirname| {
            // This has a nice side effect of preemptively triggering EISDIR or
            // ENOENT, avoiding the ambiguity below.
            _ = dirCreateDirPath(ev, dir, dirname, .default_dir) catch |err| switch (err) {
                // None of these make sense in this context.
                error.IsDir,
                error.Streaming,
                error.DiskQuota,
                error.PathAlreadyExists,
                error.LinkQuotaExceeded,
                error.PipeBusy,
                error.FileTooBig,
                error.DeviceBusy,
                error.FileLocksUnsupported,
                error.FileBusy,
                => return error.Unexpected,

                else => |e| return e,
            };
        }

        var path_buffer: [PATH_MAX]u8 = undefined;
        const sub_path_posix = try pathToPosix(dest_dirname orelse ".", &path_buffer);

        var cancel_region: CancelRegion = .init();
        defer cancel_region.deinit();
        return .{
            .file = .{
                .handle = ev.openat(
                    &cancel_region,
                    dir.handle,
                    sub_path_posix,
                    flags,
                    options.permissions.toMode(),
                ) catch |err| switch (err) {
                    error.IsDir, error.FileNotFound, error.OperationUnsupported => {
                        // Ambiguous error code. It might mean the file system
                        // does not support O_TMPFILE. Therefore, we must fall
                        // back to not using O_TMPFILE.
                        break :tmpfile;
                    },
                    error.FileTooBig => return errnoBug(.FBIG),
                    error.DeviceBusy => return errnoBug(.BUSY), // O_EXCL not passed
                    error.PathAlreadyExists => return errnoBug(.EXIST), // Not creating.
                    else => |e| return e,
                },
                .flags = .{ .nonblocking = false },
            },
            .file_basename_hex = 0,
            .dest_sub_path = dest_path,
            .file_open = true,
            .file_exists = false,
            .close_dir_on_deinit = false,
            .dir = dir,
        };
    }

    if (Dir.path.dirname(dest_path)) |dirname| {
        const new_dir = if (options.make_path)
            dirCreateDirPathOpen(ev, dir, dirname, .default_dir, .{}) catch |err| switch (err) {
                // None of these make sense in this context.
                error.IsDir,
                error.Streaming,
                error.DiskQuota,
                error.PathAlreadyExists,
                error.LinkQuotaExceeded,
                error.PipeBusy,
                error.FileTooBig,
                error.FileLocksUnsupported,
                error.DeviceBusy,
                => return error.Unexpected,

                else => |e| return e,
            }
        else
            try dirOpenDir(ev, dir, dirname, .{});

        return ev.atomicFileInit(Dir.path.basename(dest_path), options.permissions, new_dir, true);
    }

    return ev.atomicFileInit(dest_path, options.permissions, dir, false);
}