Node.js、Windows、fs.open() - 新创建的文件上不需要的只读标志

Question

I'm having trouble working out why Windows is creating a new file with the read-only attribute set. What I want is exclusive random-access write permissions to a file that I'll be using as a simple file-based data store. I'm fine with other processes reading from it, but I only want a single process to be able to write to it.

I've tried several different variations of the open flags, but no matter what I do, Windows seems to be setting a read-only attribute on the file . As such, after closing the file, attempts to reopen it for writing fail . I understand that I could manually try to manipulate the attribute after the fact, but this is backwards. I don't want the attribute to appear in the first place. When I close the file, it should be safe for someone else to open for writing.

FS.open(filePath + '.lock', 'w+', 6, (err, fd) => {
  if (err) console.log('failed to open lock file', err);
  else {
    console.log('successfully opened lock file');
    setTimeout(() => {
      FS.close(fd, () => {
        console.log(`closed lock file`);
        setTimeout(() => {
          console.log(`attempt reopening the file...`);
          FS.open(filePath + '.lock', 'w+', 6, (err, fd) => {
            if (err) console.log('failed to open lock file', err);
            else {
              console.log(`file closed successfully`);
              FS.closeSync(fd);
            }
          });
        }, 500);
      });
    }, 500);
  }
});

Output:

successfully opened lock file
closed lock file
attempt reopening the file...
failed to open lock file [Error: EPERM: operation not permitted, open 'D:\salix\test.data.lock'] {
  errno: -4048,
  code: 'EPERM',
  syscall: 'open',
  path: 'D:\\dev\\test.data.lock'
}

As you can see, the file seems to have the read-only attribute set, even after the file is closed.

Answer 1

The file mode argument 6 is creating a read only file.

Use 0o600 if you want read/write for the user.

Answer 2

TLDR: Set the file mode to 0o644 so you can read and write to it, but others can only read. Setting the value to just 6 does not give you (the owner of the file) any read or write permissions and thus the EPERM issue you run into when you try to open it yourself after creating it.

It looks to me like your file mode values are not correct. When you set it to 6 , that defines read and write for others, but does not define read and write for yourself.

There's a pretty good description of how they work here .

You basically want three octal values. The left most one is for you (the owner). The middle one is for the group. The right one is for others. So, 0o644 gives you the owner read and write and group and others only read.

As a test harness to try different modes, I used this program:

const fs = require('fs');
const fsp = fs.promises;

function delay(t) {
    return new Promise(resolve => {
        setTimeout(resolve, t);
    });
}

async function run() {
    const mode = 0o644;
    const fileHandle1 = await fsp.open("test.lock", "w+", mode);
    console.log("file open first time");
    await fileHandle1.close();
    console.log("file closed first time");

    await delay(500);

    const fileHandle2 = await fsp.open("test.lock", "w+", mode);
    console.log("file open second time");
    await fileHandle2.close();
    console.log("file closed second time");
}

run().then(result => {
    console.log("done");
}).catch(err => {
    console.log(err);
});

FYI, node.js has named constants for the different permissions which might make the code more readable in the long run than just using octal digits.