JuiceFS vs. Local file system: setattr on Unlinked Files behave different!

[bigwhite]

What happened:

I had an example：

// example.c

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <string.h>

void print_error(const char* op) {
    printf("%s failed: %s (errno: %d)\n", op, strerror(errno), errno);
}

int main() {
    const char* filename = "test.file";
    int fd;
    struct stat st;
    
    // 创建测试文件
    fd = open(filename, O_RDWR | O_CREAT | O_TRUNC, 0600);
    if (fd == -1) {
        print_error("open");
        return 1;
    }
    printf("File created with fd: %d\n", fd);

    // 写入一些初始数据
    if (write(fd, "test data", 9) == -1) {
        print_error("write");
        close(fd);
        return 1;
    }

    // 获取并打印初始文件状态
    if (fstat(fd, &st) == 0) {
        printf("Initial file size: %ld\n", st.st_size);
    }

    // 执行 unlink
    printf("Unlinking file...\n");
    if (unlink(filename) == -1) {
        print_error("unlink");
        close(fd);
        return 1;
    }
    printf("File unlinked but still open\n");

    // unlink 后尝试用 ftruncate 改变文件大小
    printf("Attempting to resize file after unlink...\n");
    if (ftruncate(fd, 16384) == -1) {
        print_error("ftruncate");
    } else {
        printf("Successfully resized file to 16384 bytes\n");
    }

    // 再次获取文件状态
    if (fstat(fd, &st) == 0) {
        printf("Final file size: %ld\n", st.st_size);
    }

    // 关闭文件
    close(fd);
    printf("File closed\n");

    return 0;
}

When I run it on local filesystem, it runs well：

$gcc example.c

[root@xx~/tonybai/setattr-after-unlink]#  ./a.out
File created with fd: 3
Initial file size: 9
Unlinking file...
File unlinked but still open
Attempting to resize file after unlink...
Successfully resized file to 16384 bytes
Final file size: 16384
File closed

but when I run it on directory mounted by juicefs, I got error below:

#  /mnt/jfs-debug/tonybai ./a.out
File created with fd: 3
Initial file size: 9
Unlinking file...
File unlinked but still open
Attempting to resize file after unlink...
ftruncate failed: Operation not permitted (errno: 1) --- here!!!!
Final file size: 9
File closed

What you expected to happen:

The results of above two runs should be the same！

How to reproduce it (as minimally and precisely as possible):

Compiile and Run the example program on linux /ext4 file system

Anything else we need to know?

Environment:

• JuiceFS version (use juicefs --version) or Hadoop Java SDK version:

juicefs --version
juicefs version 1.2.1+2024-08-30.cd871d1

• Cloud provider or hardware configuration running JuiceFS:
• OS (e.g cat /etc/os-release):

NAME="BigCloud Enterprise Linux"
VERSION="22.10U1 LTS"
ID="bclinux"
VERSION_ID="22.10U1"
PRETTY_NAME="BigCloud Enterprise Linux For Euler 22.10U1 LTS"
ANSI_COLOR="0;31"

• Kernel (e.g. uname -a): linux 5.10+
• Object storage (cloud provider and region, or self maintained): ecloud eos
• Metadata engine info (version, cloud provider managed or self maintained): pg
• Network connectivity (JuiceFS to metadata engine, JuiceFS to object storage):
• Others: